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OVERVIEW & ARCHITECTURE OF CORDECT WLL

Introduction

The corDECT Wireless Access System (WAS) is designed, to provide simultaneous circuit- switched voice and medium-rate Internet connectivity at homes and offices. The Access System model, which corDECT emulates.

Conceptual Access System

In this conceptual model, there is a Subscriber Unit (SU) located at the subscriber premises. The SU has a standard two-wire interface to connect to a telephone, fax machine, PCO (Public Call Office), speakerphone, cordless phone, or modem. it also provides direct (without a modern) Internet connectivity to a standard PC, using either a serial port (RS-232 or USB) or Ethernet. The Access System allows simultaneous telephone and Internet connectivity. The SU's are connected to an Access Centre (AC) using any convenient technology like wireless, plain old copper, DSL on copper, coaxial cable, optical fibre, or even power lines.

The AC must be scalable, serving as few as 200 subscribers and as many as 2000 subscribers. In urban areas, the AC could be located at a street corner, serving a radius of 7OO m to 1 km. This small radius in urban areas is important for wireless access, in order to enable efficient re- use of spectrum. When cable is used, the small radius ensures lower cost and higher bit rate connectivity However in rural areas, the distance between the AC and the SU could easily be 10 km and even go up to 25 km in certain situations.

The AC is thus a shared system catering to multiple subscribers. The voice and Internet traffic to and from subscribers can be concentrated here and then carried on any appropriate backhaul transport network to the telephone and lnternet networks respectively.

At the AC, the telephone and Internet traffic is separated. The telephone traffic is carried to the telephone network on El links using access protocols such as V5.2. The Internet traffic from multiple subscribers is statistically multiplexed, taking advantage of the bursty nature of Internet traffic, and carried to the Internet network. As use of Voice-over-IP (VOIP) grows, voice traffic from SU's could also be sent to the Internet, gradually making connectivity to the telephone network redundant. However, for connecting to the legacy telephone network.

The AC may be required for some time to come. An AC could also incorporate switching and maintenance functions when required. Futher, It is possible to co- locate internet servers with the AC.

corDECT Wireless Access System

Following the conceptual model, the corDECT Wireless Access System uses a similar architecture to provide telephone and Internet service to a subscriber, as shown The subscriber premises equipment, Wallset IP (WS-IP)

could also incorporate switching maintenance functions when required. Further, is possible to co-locate lnternet servers with Ea AC.

The subscriber premises equipment, Wallset IP (WS-IP) or Wallset (WS), bias a wireiess Connection through a Compact Base Station CBS) to an Access Switch, called a DECT Interface Unit (DIU). The air interface is compliant c) the DECT standard specified by ETSI. The )IU switches the voice traffic to the telephone I network using the V5.2 protocol to connect to in exchange. The DIU also switches the Internet built-in Remote Access Switch(which then routes the traffic to the Int network. The Ras has an Ethernet into which is connected to the Internet using suitable routing device.

The CBS is normally connected to the DIU three twisted-pair wires, which carry sign well as power from the DIU to the Alternatively, it can be connected to through a Base Station Distributor (BSD BSD is remote unit connected to the DIU a standard E1 interface ( on radio, fibre, A BSD can sup to four CBS’s.

The long range communication, a WS is can also be connected to the CBS using hop DECT wireless link, one between V WS and a Relay Base Station (RBS) and between the RBS and CBS, The wireless range supported be WS 0 Ip or WS and the CBS or RBS is

line-of-Sight (LOS)'conditions. The range supported between a CBS and RBS is 25 km in LOS conditions.

A typical system consists of one DIU with one or two RAS units, up to 20 CBS'S, and up to a 1 000 WS-IP's or WS's. The BSD and RBS units are used as required by the deployment scenario.

Sub-systems of the corDECT Wireless Access System

Before we get into more details at the system level, we take a breif look at each of the subsystems.

Wallset IP and Wallset

The Wallset with Internet Port (WS-IP) provides voice connectivity to the subscriber using a RJ-11 interface, enabling one to connect a standard DTMF or decadic telephone, G3 tax machine, PCO (battery reversal and 12/16 kHz metering are standard features), speakerphone, cordless phone, or modem. In addition, the WS-IP has a RS-232 port to directly connect to a PC (obviating the need for a telephone modem). The PC establishes a dial-up PPP (Point-to-Point Protocol) Internet connection using a standard dial-up utility. Internet access is supported at 35 or 70 kbps. In fact, the WS-IP can support simultaneous voice and 35 kbps Internet connections.

Besides these two user interfaces, the WS-IP has an antenna port where either a whip antenna, or an externally mounted antenna (through cable), can be connected. The power to the WS-IP is provided by a 12V adaptor connected to the AC mains and optionally by a solar panel which can be connected in parallel. The WS-IP has a built-in battery and battery charger. The built-in-battery provides 16 hours stand-by time and more than 3 hours talk time for voice calls.

A Wallset (WS) is a similar terminal without the Internet Port.

Multiwallset

The Multiwallset (MWS), provides simultaneous voice service to four subscribers. It has all the features of the WS, but at a significantly lower per-line cost.

The Multiwallset has a DECT Transceiver Module (DTM), which is an outdoor unit with a built-in antenna with 7.5 dB gain. It is connected to an indoor Subscriber Interface Module (SIM), which has four RJ-11 ports for telephones. Each port supports all the terminals a WS supports.

The connection between the DTM and the SIM uses a single twisted-pair wire, obviating the need for RF cable and connectors. The MWS has a built-in-battery for backup and is powered through the AC mains.

Multiwallset IP

The Multiwallset with Internet Port (MWS-IP) is a MWS with four telephones and an additional Ethernet Interface to provide dial-up Internet connectivity. Multiple PC’s can be connected to the Ethernet port and provide a shared 35/70 kbps Internet connection. The PPP-over-Ethernet protocol is used to set up individual connections it is to be noted that at any time, either four simultaneous telephone calls with no Internet connection, or three telephone calls and a 35kbps shared Internet connection, or two telephone calls and a shared 70 kbps Internet connection, can be made. Depending on usage, this may introduce some blocking for voice calls.

Compact Base Station

The Compact Base Station (CBS), provides the radio interface between the DIU and the corDECT subscriber terminal. It supports up to 12 simultaneous voice calls. It is a small, unobtrusive, weatherproof unit that is remotely powered from the DIU or a BSD.

The CBS has two antennas for diversity. A directional antenna with significant gain can be used when coverage is required to be confined to certain directions. For example, if the coverage area is divided into sectors, each sector can be covered by a different Base Station with directional antennas. For 360 degrees coverage using a single CBS, omni-directional antennas are used. More than one CBS can be deployed to serve a single sector or a cell.

The maximum LOS range between a subscriber unit and a CBS is 10 km. An isolated CBS supports approximately 5.8 E of traffic with a Grade of Service (GOS) of 1%, typically serving 30-70 subscribers. Multiple CBS’s serving the same sector or cell increase the traffic handled by each CBS (see Chapter 6).

The CBS is conencted to a DIU or a Base Station Distributor (BSD) with three twisted-pair copper wires, each of which carry voice/data traffic, signalling and power. The maximum loop length, with a 0.4 mm diameter wire, can be 4 km between the DIU and the CBS and 1km between the BDS and the CBS.

DECT Interface Unit

The DECT Interface Unit (DIU) shown in 3.8, implements the functions of a Switch (or a Remote Line Unit), Base Station Controller, and the Operation and Maintenance Console (OMC). System reliability is guaranteed by redundant, hot stand-by architecture. The OMC allows exhaustive real-time monitoring and management of the entire corDECT system. A fully cond DIU with an in-built Remote Access Switch (RAS) only occupies a single 28U, 19’’ cabinet and consumes less than 600w.

Up to 20 CBS’s can be supported by a DIU, directly or through the BSD. The DIU provides up to eight E1 links to the telephone network and/or RAS. The signaling protocol used is either V5.2, which parents the DIU (as a RUL) to an exchange, or R2-MF, in which case the DIU acts as a 1000-line exchange. There is a third option, wherein the corDECT system, using additional equipment, appears to an exchange simply as a number of twisted-pair lines.

Multiple DIU’s are managed through a centralized Network Management System (NMS).

iKON Remote Access Switch

The iKON Remote Access Switch (RAS),, is a 19” 1U unit normally integrated within the DIU cabinet. It terminates the PPP connections from Internet subscribers using corDECT WS-IP or MWS-IP. It is connected to the DIU using up to two E1 ports and does IP-based routing for up to 60 simultaneous corDECT Internet calls. The RAS has a 10Base T Ethernet port to connect to the Internet. It supports RADIUS for accounting and authentication, PAP for user authentication and is managed using SNMP.

Base Station Distributor

The Base Station Distributor (BSD) is a compact, remotely located, locally powered, rack-mountable unit that supports up to four CBS’s (with power feed). The E1 interface between DIU and the BSD can be on copper, fibre, or and link distance depends only on the linkd.

The BSD is designed to extend cord coverage to ppockets of subscribers located away from the DIU.

Relay Base Station

A Relay Base Station (RBS), extends the range of the corDECT system by relaying DECT packets between the CBS subscriber units. The RBS can handle 11 simultaneouslys.

The RBS consists of two units. The RBS A is typically monted on a tower/mast and on the baseband and the RF sub-system. The Ground Unit supplies power and produce maintenance support to the Air Unit at mounted at the bottom of the tower/mast.

The RBS uses three antennas. One and (usually a directional antenna with high ) refererd to as the RBSWS antenna, towards the CBS with which the RBS communicating. The other two antennas RBSBS antennas are used for communication with the subscriber units (two antennas are used for diversity). These antennas are similar to those used by the CBS.

The maximum LOS range between a CBS and a RBS is 25 km, while the maximum LOS range between the RBS and corDECT subscribers is 10 km.

Network Management

CorDECT provides comprehensive operation and maintenance through the corView OMC console. Its repertoire includes hardware and software configuration, subscriber administration, accounting, fault notification, and traffic management. 3.12 depicts the corView GUI for configuring the DIU. Commands range from a bird’s-eye view of the operational status of a network of corDECT systems to probing the internals of an individual Wallset.

This easy-to use, menu-driven console can be run either locally or remotely. When used remotely, a single corView workstation serves as the NMS for a number of corDECT systems. CorView can also be used with the CygNetNMS to provide integrated management of a network of corDECT and other systems.

CorView supports the SNMP protocol and can be connected to the corDECT system by any IP network. In the future, corView will also supportTMN/Q3. When the DFIU is used as a Switch, detailed billing records are maintained and can be exported to the billing centre via several media.

corDECT Access Centre Functionality and Interfaces

The corDECT Access Centre, consisting of a DIU and iKON RAS, is designed to provide interfaces to the telephone network and to the Internet.

The Telephone Connection

The telephone connection provided to a corDECT subscriber is a circuit-switched one. The DIU switches the connection to the telephone network. The interface to the telephone network is provided in three different ways:

1. RLU mode, with V5.2 protocol on E1 interfaces to a parent exchange and

2. Transparent mode, with two-wire interface to a parent exchange and

3. Switch mode, with R2-MF protocol on E1 interfaces to the telephone network.

RLU Mode

The DIU has up to six E1’s that can be connected to a parent exchange using V5.2 signaling. The DIU in this case works as a 1000-line RLU of the parent exchange, as shown in 3.13. Even calls between two corDECT subscribers belonging to the same DIU are switched by the parent exchange.

The numbering and all subscriber facilities are provided by the exchange and billing too is carried out at the exchange. The DIU does some limited subscriber administration, such as authenticating a subscriber (as per the DECT standard). The DIU console, however, provides management functions for managing the DIU, CBS, RBS, BSD, WS, WS-IP, MWS and MWS-IP, and also carries out wireless traffic monitoring. The management functions canalso be carried out centrally for multiple DIU’s,.

Transparent Mode

In this mode, the DIU is parented to an exchange using two-wire interfaces. Each subscriber line is mapped to an unique two-wire port on the exchange. Hook status and digits dealed at the WS/WS-IP/MWS are mapped by the DIU to reflect at the corresponding exchange port. All services of the exchange are available to the subscriber. Billing is carried out at the exchange. However, as in the RLU mode, the DIU carries out subscriber authentication and system management functions.

To provide two-wire interfaces at the DIU, a Concentrating Subscriber Multiplexer (CSMUX) is used. Each SMUX, housed in one 6U 19” rack, can provide up to 240 two-wire ports (grouped as 2 x 120 two-wire ports). The CSMUX is connected to the DIU typically using two E1 ports, providing 4:1 concentration. Thus, using eight E1’s and four CSMUX units and a DIU integrated in two cabinets, one can serve up to

Subscribers in transparent mode a concentration of 4:1 is normally acceptable since wireless channels are anyway shared. Sharing an E1 port among 120 subsceibers, one can serve nearly 0.2 erlang per subscriber at 1% GOS. However, it is to avoid concentration at the CSMUX connect eight E1’s to a signle CSMUX rack. This case, one DIU will be limited to serve a minimum of 240 subscribers.

The transparent mode is the quickset way to reconnect corDECT to an existing telephone work. However, it is not a preferred mode for concetration. In order to serve 960 subscribers, 960 wire ports are required on the exchange side connected to four CSMUX units. In contrast, only to six E1 ports are required at the exchange the RLU mode and use of the CSMUX is sided. Thus, in the RLU mode, the sixe of the exchange as well as the DIU is much smaller the power required is also less when prepared to the transparent mode.

A more serious problem in the transparent mode comes froma signalling anomaly that can emerge in some specific situations. For example, when an incoming call comes to the exchange for a subscriber, the exchange signals ring-back to the calling subscriber if it finds from its database that the called subscriber if free. The exchange simultaneously feeds ring to the corresponding two-wire port. This is detected by the CSMUX in the DIU and the DIU then attempts to page the corresponding WS/WS-IP and ring the subscriber. However as wireless channels are shared, it is possible that sometimes the DIU finds no free channel and fails to feed ring to the subscriber. The anomaly develops when the called port gets ring-back tone, but the called party does not get a ring. Such a situation can sometimes become problematic. The transparent mode is therefore not the most desirable mode of operation. Nevertheless, it is the quickest way to integrate a wireless system to the existing telephone network anywhere in the world.

Switch Mode

The DIU is designed to be a 1000-line, full-fledged, medium-sized exchange for corDECT wireless subscribers. It interfaces to the telephone network on up to six E1-lines using R2-MF protocol. all the exchange functions, including subscriber administration, billing, and management, are carried out at the DIU itself. The advantage of this mode is that the cost of an exchange is totally saved.

The DIU can also serve as a Direct In-Dialing (DID) PBX.

Internet Connection

A corDECT subscriber connects to the WS-IP using a PPP dial-up connection on the RS-232 port. The port is programmed at 38.4 kbps rate for a 35kbps Internet connection and at 115.2kbps rate for a 70kbps Internet connection. The PC connected to the RS-232 port on the WS-IP dials a pre-designated number using a standard dial-up routine. The DIU sets up a circuit-switched connection between the Ws-IP and the iKON RAS connected to the DIU on an E1 port.

The Internet connection employs the wireless link between the WS-IP and the CBS and the wired links between the CBS and the DIU and between the DIU and the RAS. Since the BER on the wireless link could occasionally be high, the PPP packet is fragmented and transmitted with an error detection code on the link from the WS-IP to the DIU. ARQ is performed on this link to obtain error-free fragment transmission. The PPP packets are re-assembled from these fragments before transmitting it to the PC (on the WS-IP side) and to the RAS (on the DIU side).

The connection between the WS-IP and the DIU is at 32kbps or 64kbps (using one or two DECT slots on air). The start/stop bits received at the RS-232 port are stripped before transmission on air. This enables 35kbps Internet throughput between the user PC and the RAS on the 32 kbps connection in an error-free situation. Similarly, 70kbps Internet throughput is possible between the user pC and the RAS on the 64kbps connection. Bit errors on the link will temporarily bring down the throughput.

Each RAS has tow E1 ports for connecting to the DIU and thus can support Internet connections for up to 60 subscribers at a time. The PPP connections are terminated at the RAS and IP packets are routed to the Ethernet port of the RAS for onward transmission to the Internet. The Ethernet ports from multiple RAS’s would normally be connected to an Ethernet switch. The Ethernet switch in turn would be connected to an Internet router, completing the connection to the Internet.

3 G Mobile communication

Introduction

• Wireless Generations
• What is IMT-2000
• What IMT-2000 offers
• Key features and objectives
• Spectrum for IMT-2000
• Technologies for IMT-2000
• Migration paths
• Future Trends

1946- 1960s 1980s 1990s 2000s

Appeared 1G 2G 3G

Analog Digital Digital

Multi Multi Unified

Standard Standard Standard

Terrestrial Terrestrial Terr. & Sat

WIRELESS GENERATIONS

1 G -analog (cellular revolution) only mobile voice services

2 G - digital (breaking digital barrier) -mostly for voice services & data delivery possible

3 G - Voice & data (breaking data barrier) Mainly for data services where voice services will also be possible

Beyond 3G Wide band OFDM ?But surely higher data rates

LIMITATIONS OF 2nd GENERATION SYSTEMS

• No Global standards
• No common frequency band
• Low information bit rates
• Low voice quality
• No support of Video
• Various categories of systems to meet specific requirements

THIRD GENERATION (3 G ) STANDARD

• International mobile telecom 2000. imt-2000
• ITU’s vision for third generation mobile system
• a future standard in which a single inexpensive mobile terminal can truly provide communications any time and any where

• Provisioning of these services over wide range of user densities and coverage areas.(in-building , urban , sub-urban, global)
• Efficient use of radio spectrum consistent with providing service at acceptable costly.
• IMT-2000 shall cover application areas presently provided by seperately systems i.e cellular, cordless and paging etc.
• A high degree of commonality of design worldwide.
• A modular structure which will allow the system to grow in size and complexity.
• Single unified standard (data & multimedia services)
• Anywhere, anytime communication
• Across networks, across technologies, seamless operation using a small pocket terminal worldwide.
• High speed access 144kb/s, 384 kb/s & 2mb/s fast wireless access to internet
• Full motion videophone

• Terrestrial & satellite components
• Enhanced voice quality, ubiquitous coverage and enable operators to provide service at reasonable cost
• Increased network efficiency and capacity
• New voice and data services and capabilities
• An orderly evolution path from 2G to 3G systems to protect investments.

IMT TECHNOLOGIES

ITU has finally narrowed down technology options to the following five:

1. IMT -DS (Direct Spread) : W-CDMA UTRA FDD
2. IMT -MC (Multi Carrier) : CDMA 2000
3. IMT-TC ( Time Code) : TD -SCDMA UTRA TDD
4. IMT -SC ( Single Carrier ) : UWC - 136
5. IMT-FT (Frequency Time) : DECT

IMT-2000 HARMONIZATION IS ON-GOING

I

MT standards development involves extensive collaboration between many different organizations

• Today’s operators need seamless 2G 3G
• Many Focus groups have been established by industry
• 2 G operators GSM ; CDG ,UWCC, DECT forum
• 3 G Groups UMTS Forum , OHG
• Focus group for IP-based 3G architecture (3G. IP)
• SDOs created 3G PP (Partnership Projects)SDO Standards Development Organizations

Migration Path

• While a multiplicity of 2G standards have been developed and deployed, the ITU wanted to avoid a similar situation to develop for 3G.
• Hence, the ITU Radio communication Sector (ITU-R) has elaborated on a framework for a global set of 3G standards, which will facilitate global roaming by operating in a common core spectrum and providing migration path from all the major existing 2G technologies. 
• The major 2G Radio access networks are based on either CDMA One or GSM technologies and different migration path is proposed for each of these technologies.

Evolution from GSM to 3G

GSM Evolution

EDGE (Enhanced Data for GSM Evolution)

• Next step towards 3G for GSM/GPRS Networks
• Increased data rated up to 384 Kbps by bundling up to 8 channels of 48 Kbps/channel
• GPRS is based on modulation technique known as GMSK
• EDGE is based on a new modulation scheme that allows a much higher bit rate across the air-interface called 8PSK modulation.
• Since 8PSK will be used for UMTS, network operators will be required to introduce this at some stage before migration to 3G.

GSM to UMTS

GSM to GPRS to EDGE to 3G

• GSM can be upgraded for higher data rate upto 115 Kbps through deploying GPRS (General Packet Radio Service) network.This requires addition of two core modules
• SGSN (Serving GPRS Service Node)
• GGSN (Gateway GPRS Service Node)
• GSM radio access network is connected to SGSN through suitable interfaces.
• GPRS phase-II will support higher data rates up to 384 Kbps through incorporating EDGE
• ( Enhanced Data Rate for GSM Evolution).

GSM to 3G

• Further, to support data rates up to 2 Mbps, Third Generation radio access network (3G RAN)
• W-CDMA is deployed. 3G RAN is connected to GSM MSC for circuit oriented services and to SGSN for packet oriented services (internet access). Therefore the migration path can be represented as : 

GSM GPRS W-CDMA.

Migration Summarized

• In terms of migration of major 2G system to 3G capabilities, there would finally be 3 modes of CDMA-based radio interfaces (MC-CDMA, W-CDMA and CDMA-TDD) and two `TDMA based radio interfaces (UWC-136 and DECT).
• Considerable work is being carried out in respect of W-CDMA and CDMA 2000 worldwide. All European countries are expected to deploy W-CDMA as they have GSM based networks. While other countries such as Japan, Korea, USA etc. are likely to use CDMA-2000 or W-CDMA.

FUTURE TRENDS (3 G to 4G ONWARDS)

New data services, interactive TV and evolving Internet behavior will influence mobile data usage. Long sessions in always-on mode will force a re-think of radio access technology to achieve the required but not easy to attain capacity (Gbit/s/km) at low cost. The ideas presented in this article can increase capacity by a factor of 500 with regard to expected cellular deployments. Coverage will be based on large umbrella cells (3G, WiMAX) and numerous Pico cells interconnected to provide the user with seamless high data rate (several Mbs) sessions. Scalable and progressive deployments are possible while protecting the operator’s long-term investment. The 4G infrastructure operator will mix several technologies, each of which has its optimal usage. The connection to one of them will result in a real-time trade-off which will offer the user the best possible service. Some tools that genuinely improve the user’s multimedia quality of experience (availability, response time, definition, etc) are also presented in this article.

4G Mobile

4G will deliver low cost multi-megabit/s sessions any time, any place, using any terminal.

Operational Excellence

Voice was the driver for second generation mobile and has been a considerable success. Today, video and TV services are driving forward third generation (3G) deployment and in the future, low cost, high speed data will drive forward the fourth generation (4G) as short-range communication emerges. Service and application ubiquity, with a high degree of personalization and synchronization between various user appliances, will be another driver. At the same time, it is probable that the radio

access network will evolve from a central-ized architecture to a distributed one.

Service Evolution

The evolution from 3G to 4G will be driven by services that offer better quality (e.g. video and sound) thanks to greater bandwidth, more sophistication in the association of a large quantity of information, and improved personalization. Convergence with other network (enterprise,fixed) services will come about through the high session data rate. It will require an always-on connection and a revenue model based on a fixed monthly fee. The impact on network capacity is expected to be significant. Machine-to-machine transmission will involve two basic equipment types: sensors (which measure parameters) and tags (which are generally read/write equipment). It is expected that users will require high data rates, similar to those on fixed networks, for data and streaming applications. Mobile terminal usage (laptops, Personal digital assistants, hand-helds) is expected to grow rapidly as they become more user friendly. Fluid high quality video and network reactivity are important user requirements. Key infrastructure design requirements include: fast response, high session rate, high capacity, low user charges, rapid return on investment for operators, investment that is in line with the growth in demand, and simple autonomous terminals. The infrastructure will be much more distributed than in current deployments, facilitating the introduction of a new source of local traffic: machine-to-machine. Figure 1 shows one vision of how services are likely to evolve; most such visions are similar. Dimensioning targets A simple calculation illustrates the order of magnitude. The design target in terms of radio performance is to achieve a scalable capacity from 50 to 500 bit/s/Hz/km 2 (including capacity for indoor use), as shown in Figure 2. As a comparison, the expected best performance of 3G is around 10 bit/s/Hz/km2 using High Speed Down link Packet Access (HSDPA), Multiple-Input Multiple-Output (MIMO), etc. No current technology is capable of such performance. Dimensioning objectives Based on various traffic analyses, the Wireless World Initiative (WWI) has issued target air interface performance figures. A consensus has been reached around peak rates of 100 Mbit/s in mobile situations and 1 Gbit/s in nomadic and pedestrian situations, at least as targets. So far, in a 10 MHz spec-trum, a carrier rate of 20 Mbit/s has been achieved when the user is moving at high speed, and 40 Mbit/s in nomadic use. These values will double when MIMO is introduced. Clearly, the bit rate should be associated with an amount of spectrum. For mobile use, a good target is a network performance of 5 bit/s/Hz, rising to 8 bit/s/Hz in nomadic use.

Figure 1

Figure 2:Dimensioning examples

Multi-Technology Approach

Many technologies are competing on the road to 4G, as can be seen in Figure 3. Three paths are possible, even if they are more or less specialized. The first is the 3G-centric path, in which Code Division Multiple Access (CDMA) will be progressively pushed to the point at which terminal manufacturers will give up. When this point is reached, another technology will be needed to realize the requi-red increases in capacity and data rates. The second path is the radio LAN one. Wide-spread deployment of WiFi is expected to start in 2005 for PCs, laptops and PDAs. In enterprises, voice may start to be car-ried by Voice over Wireless LAN (VoWLAN). However, it is not clear what the next successful technology will be. Reaching a consensus on a 200 Mbit/s (and more) technology will be a lengthy task, with too many proprietary solutions on offer. A third path is IEEE 802.16e and 802.20, which are simpler than 3G for the equivalent performance. A core network evolution towards a broadband Next Generation Network (NGN) will facilitate the introduction of new access network technologies through standard access gateways, based on ETSI-TISPAN, ITU-T, 3GPP, China Communication Standards Association (CCSA) and other standards. How can an operator provide a large number of users with high session data rates using its existing infrastructure? At least two technologies are needed. The first (called “parent coverage”) is dedicated to large coverage and real-time services. Legacy technologies, such as 2G/3G and their evolutions will be complemented by WiFi and WiMAX. A second set of technologies is needed to increase capacity, and can be designed without any constraints on coverage continuity. This is known as picocell coverage. Only the use of both technologies can achieve both targets (Figure 4). Handover between parent coverage and pico cell coverage is different from a classical roaming process, but similar to classical handover. Parent coverage can also be used as a back-up when service delivery in the pico cell becomes too difficult.

Key 4G Technologies

Some of the key technologies required for 4G are briefly described below:

OFDMA

Orthogonal Frequency Division Multiplexing (OFDM) not only provides clear advantages for physical layer performance, but also a framework for improving layer 2 performance by proposing an additional degree of freedom (Pico cell). A good example of a pico cell is a WiFi coverage. By extension, a pico cell has a radius around 50 m and the associated base station is similar to a WiFi access point. It can be deployed indoors or outdoors.

Figure 4:Coverage performance trends

Using ODFM, it is possible to exploit the time domain, the space domain, the frequency domain and even the code domain to optimize radio channel usage. It ensures very robust transmission in multi-path environments with reduced receiver com-plexity. As shown in Figure 5, the signal is split into orthogonal sub carriers, on each of which the signal is “narrow band” (a few kHz) and therefore immune to multi-path effects, provided a guard interval is inserted between each OFDM symbol. OFDM also provides a frequency diversity gain, improving the physical layer performance. It is also compatible with other enhancement technologies, such as smart antennas and MIMO. OFDM modulation can also be employed as a multiple access technology (Orthogonal Frequency Division Multiple Access; OFDMA). In this case, each OFDM symbol can transmit information to/from several users using a different set of subcarriers (subchannels). This not only provides additional flexibility for resource allocation (increasing the capacity), but also enables cross-layer optimization of radio link usage.

Software Defined Radio

Software Defined Radio (SDR) benefits from today’s high processing power to develop multi-band, multi-standard base stations and terminals. Although in future the terminals will adapt the air interface to the available radio access technology, at present this is done by the infra-structure. Several infrastructure gains are expected from SDR. For example, to increase network capacity at a specific time (e.g. during

a sports event), an operator will reconfigure its net-work adding several modems at a given Base Transceiver Station (BTS). SDR makes this reconfiguration easy. In the context of 4G systems, SDR will become an enabler for the aggregation of multi-standard pico/micro cells. For a manufacturer, this can be a powerful aid to providing multi-standard, multi-band equipment with reduced development effort and costs through simultaneous multi-channel processing.

Multiple-Input Multiple-Output

MIMO uses signal multiplexing between multiple transmitting antennas (space multiplex) and time or frequency. It is well suited to OFDM, as it is possible to process independent time symbols as soon as the OFDM waveform is correctly designed for the channel. This aspect of OFDM greatly simplifies processing. The signal transmitted by m antennas is received by n antennas. Processing of the received signals may deliver several performance improvements: range, quality of received signal and spectrum efficiency. In principle, MIMO is more efficient when many multiple path signals are received. The performance in cellular deployments is still subject to research and simulations. However, it is generally admitted that the gain in spectrum efficiency is directly related to the minimum number of antennas in the link.

Software Defined Radio

Software Defined Radio (SDR) benefits from today’s high processing power to develop multi-band, multi-standard base stations and terminals. Although in future the terminals will adapt the air interface to the available radio access technology, at present this is done by the infra-structure. Several infrastructure gains are expected from SDR. For example, to increase network capacity at a specific time (e.g. during

a sports event), an operator will reconfigure its net-work adding several modems at a given Base Transceiver Station (BTS). SDR makes this reconfiguration easy. In the context of 4G systems, SDR will become an enabler for the aggregation of multi-standard pico/micro cells. For a manufacturer, this can be a powerful aid to providing multi-standard, multi-band equipment with reduced development effort and costs through simultaneous multi-channel processing.

Multiple-Input Multiple-Output

MIMO uses signal multiplexing between multiple transmitting antennas (space multiplex) and time or frequency. It is well suited to OFDM, as it is possible to process independent time symbols as soon as the OFDM waveform is correctly designed for the channel. This aspect of OFDM greatly simplifies processing. The signal transmitted by m antennas is received by n antennas. Processing of the received signals may deliver several performance improvements: range, quality of received signal and spectrum efficiency. In principle, MIMO is more efficient when many multiple path signals are received. The performance in cellular deployments is still subject to research and simulations . However, it is generally admitted that the gain in spectrum efficiency is directly related to the minimum number of antennas in the link.

Interlayer Optimization

The most obvious interaction is the one between MIMO and the MAC layer. Other interactions have been identified

Handover and Mobility

Handover technologies based on mobile IP technology have been considered for data and voice. Mobile IP techniques are slow but can be accelerated with classical methods (hierarchical, fast mobile IP). These methods are applicable to data and probably also voice. In single-frequency networks, it is necessary to reconsider the handover methods. Several techniques can be used when the carrier to interference ratio is negative (e.g. VSF-OFDM, bit repetition), but the drawback of these techniques is capacity. In OFDM, the same alternative exists as in CDMA, which is to use macro-diversity. In the case of OFDM, MIMO allows macro-diversity processing with performance gains. However, the implementation of macro-diversity implies that MIMO processing is centralized and transmissions are synchronous. This is not as complex as in CDMA, but such a technique should only be used in situations where spectrum is very scarce.

Figure 5:OFDM principles

Caching and Pico Cells

Memory in the network and terminals facilitates service delivery. In cellular systems, this extends the capabilities of the MAC scheduler, as it facilitates the delivery of real-time services. Resources can be assigned to data only when the radio conditions are favorable. This method can double the capacity of a classical cellular system. In Pico cellular coverage, high data rate (non-real-time) services can be delivered even when reception/transmission is interrupted for a few seconds. Consequently, the coverage zone within which data can be received/transmitted can be designed with no constraints other than limiting interference. Data delivery is preferred in places where the bit rate is a maximum. Between these areas, the coverage is not used most of the time, creating an apparent discontinuity. In these areas, content is sent to the terminal cache at the high data rate and read at the service rate. Coverages are “discontinuous”. The advantage of coverage, especially when designed with caching technology, is high spectrum efficiency, high scalability (from 50 to 500 bit/s/Hz), high capacity and lower cost. A specific architecture is needed to intro-duce cache memory in the net-work. An example is shown in Figure 8. At the entrance of the access network, lines of cache at the destination of a terminal are built and stored. When a terminal enters an area in which a transfer is possible, it simply asks for the line of cache following the last received. Between the terminal and the cache. A simple, robust and reliable protocol is used between the terminal and the cache for every service delivered in this type of coverage.

Multimedia Service Delivery, Service Adaptation and Robust Transmission

Audio and video coding are scalable. For instance, a video flow can be split into three flows which can be transported independently: one base layer (30 kbit/s), which is a robust flow but of limited quality (e.g. 5 images/s), and two enhancement flows (50 kbs and 200 kbs). The first flow provides availability, the other two quality and definition. In a streaming situation, the terminal will have three caches. In Pico cellular coverage, the parent coverage establishes the service dialog and service start-up (with the base layer). As soon as the terminal enters pico cell coverage, the terminal caches are filled, starting with the base cache. Video (and audio) transmissions are cur-rently transmitted without error and without packet loss. However, it is possible to allow error rates of about 10 -5 /10 –6 and a packet loss around 10 –2 /10 -3 . Coded images still contain enough redundancy for error correction. It is possible to gain about 10 dB in transmission with a reasonable increase in complexity. Using the described technologies, multimedia transmission can provide a good quality user experience.

Coverage

Coverage is achieved by adding new technologies (possibly in overlay mode) and progressively enhancing density. Take a WiMAX deployment, for example: first the parent coverage is deployed; it is then made denser by adding discontinuous Pico cells, after which the Pico cell is made denser but still discontinuously. Finally the pico cell cover-age is made continuous either by using MIMO or by deploying another Pico cell coverage in a different frequency band .Parent coverage performance may vary from 1 to 20 bit/s/Hz/km?, while Pico cell technology can achieve from 100 to 500 bit/s/Hz/km?, depending on the complexity of the terminal hardware and software. These performances only refer to outdoor coverage; not all the issues associated with indoor coverage have yet been resolved. However, indoor coverage can be obtained by:

• Direct penetration; this is only possible in low frequency bands (significantly below 1 GHz) and requires an excess of power, which may raise significant interference issues.

• Indoor short range radio connected to the fixed network.

• Connection via a relay to a Pico cellular access point.

Integration in a Broadband NGN

The focus is now on deploying an architecture realizing convergence between the fixed and mobile networks (ITU-T Broad-band NGN and ETSI- TISPAN). This generic architecture integrates all service enablers (e.g. IMS, network selection, middle ware for applications providers), and offers a unique inter-face to application service providers.

Conclusion

The provision of megabit/s data rates to thousands of radio and mobile terminals per square kilometer presents several challenges. Some key technologies permit the progressive introduction of such networks without jeopardizing existing investment. Disruptive technologies are needed to achieve high capacity at low cost, but it can still be done in a progressive manner. The key enablers are:

• Sufficient spectrum, with associated sharing mechanisms.

• Coverage with two technologies: parent (2G, 3G, WiMAX) for real-time delivery,

and discontinuous Pico cell for high data rate delivery.

• Caching technology in the network and terminals.

• OFDM and MIMO.

• IP mobility.

• Multi-technology distributed architecture.

• Fixed-mobile convergence (for indoor service).

• Network selection mechanisms.

Many other features, such as robust transmission and cross-layer optimization, will contribute to optimizing the performance, which can reach between 100 and 500 bit/s/Hz/km The distributed, full IP architecture can be deployed using two main products: base stations and the associated controllers. Terminal complexity depends on the number of technologies they can work with. The minimum number of technologies is two: one for the radio coverage and one for short range use (e.g. PANs). However, the presence of legacy networks will increase this to six or seven.

• Distributed architecture.

Architecture with a large number of decentralized connections to the core network.

GSM Services

It is important to note that all the GSM services were not introduced since the appearance of GSM but they have been introduced in a regular way. The GSM Memorandum of Understanding (MoU) defined four classes for the introduction of the different GSM services:

• E1: introduced at the start of the service.
• E2: introduced at the end of 1991.
• Eh: introduced on availability of half-rate channels.
• A: these services are optional.

Three categories of services can be distinguished:

• Teleservices.
• Bearer services.
• Supplementary Services.

Teleservices

- Telephony (E1® Eh).

- Facsimile group 3 (E1).

- Emergency calls (E1® Eh).

- Teletex.

Short Message Services (E1, E2, A) Using these services, a message of a maximum of 160 alphanumeric characters can be sent to or from a mobile station. If the mobile is powered off, the message is stored. With the SMS Cell Broadcast (SMS-CB), a message of a maximum of 93 characters can be broadcast to all mobiles in a certain geographical area.

- Fax mail. Thanks to this service, the subscriber can receive fax messages at any fax machine.

- Voice mail. This service corresponds to an answering machine.

Bearer Services

A bearer service is used for transporting user data. Some of the bearer services are listed below:

• Asynchronous and synchronous data, 300-9600 bps (E1).
• Alternate speech and data, 300-9600 bps (E1).
• Asynchronous PAD (packet-switched, packet assembler/dissembler) access, 300-9600 bps (E1).
• Synchronous dedicated packet data access, 2400-9600 bps (E2).

Supplementary Services

- Call Forwarding (E1). The subscriber can forward incoming calls to another number if the called mobile is busy (CFB), unreachable (CFNRc) or if there is no reply (CFNRy). Call forwarding can also be applied unconditionally (CFU).

- Call Barring. There are different types of `call barring' services:

• Barring of All Outgoing Calls, BAOC (E1).
• Barring of Outgoing International Calls, BOIC (E1).
• Barring of Outgoing International Calls except those directed toward the Home PLMN Country, BOIC-exHC (E1).
• Barring of All Incoming Calls, BAIC (E1)
• Barring of incoming calls when roaming (A).

- Call holds (E2) puts an active call on hold.

- Call Waiting, CW (E2) informs the user, during a conversation, about another incoming call. The user can answer, reject or ignore this incoming call.

- Advice of Charge, AoC (E2) provides the user with online charge information.

- Multiparty service (E2) Possibility of establishing a multiparty conversation.

- Closed User Group, CUG (A). It corresponds to a group of users with limited possibilities of calling (only the people of the group and certain numbers).

- Calling Line Identification Presentation, CLIP (A). It supplies the called user with the ISDN of the calling user.

- Calling Line Identification Restriction, CLIR (A). It enables the calling user to restrict the presentation.

- Connected Line identification Presentation, CoLP (A). It supplies the calling user with the directory number he gets if his call is forwarded.

- Connected Line identification Restriction, CoLR (A). It enables the called user to restrict the presentation.

- Operator determined barring (A).Restriction of different services and call types by the operator.

Conclusion

The aim of this paper was to give an overview of the GSM system and not to provide a complete and exhaustive guide.

As it is shown in this chapter, GSM is a very complex standard. It can be considered as the first serious attempt to fulfill the requirements for a universal personal communication system. GSM is then used as a basis for the development of the Universal Mobile Telecommunication System (UMTS).

architecture

The Global System for Mobile communications is a digital cellular communications system. It was developed in order to create a common European mobile telephone standard but it has been rapidly accepted worldwide. GSM was designed to be compatible with ISDN services.

History of the Cellular Mobile Radio and GSM

The idea of cell-based mobile radio systems appeared at Bell Laboratories (in USA) in the early 1970s. However, mobile cellular systems were not introduced for commercial use until the 1980s. During the early 1980s, analog cellular telephone systems experienced a very rapid growth in Europe, particularly in Scandinavia and the United Kingdom. Today cellular systems still represent one of the fastest growing telecommunications systems.

But in the beginnings of cellular systems, each country developed its own system, which was an undesirable situation for the following reasons:  

• The equipment was limited to operate only within the boundaries of each country.
• The market for each mobile equipment was limited.

In order to overcome these problems, the Conference of European Posts and Telecommunications (CEPT) formed, in 1982, the Group Special Mobile (GSM) in order to develop a pan-European mobile cellular radio system (the GSM acronym became later the acronym for Global System for Mobile communications). The standardized system had to meet certain criteria:

• Spectrum efficiency
• International roaming
• Low mobile and base stations costs
• Good subjective voice quality
• Compatibility with other systems such as ISDN (Integrated Services Digital Network)
• Ability to support new services

Unlike the existing cellular systems, which were developed using an analog technology, the GSM system was developed using a digital technology.

In 1989 the responsibility for the GSM specifications passed from the CEPT to the European Telecommunications Standards Institute (ETSI). The aim of the GSM specifications is to describe the functionality and the interface for each component of the system, and to provide guidance on the design of the system. These specifications will then standardize the system in order to guarantee the proper inter-working between the different elements of the GSM system. In 1990, the phase I of the GSM specifications was published but the commercial use of GSM did not start until mid-1991. The most important events in the development of the GSM system are presented in the table 1.

Year	Events
1982	CEPT establishes a GSM group in order to develop the standards for a pan-European cellular mobile system
1985	Adoption of a list of recommendations to be generated by the group
1986	Field tests were performed in order to test the different radio techniques proposed for the air interface
1987	TDMA is chosen as access method (in fact, it will be used with FDMA) Initial Memorandum of Understanding (MoU) signed by telecommunication operators (representing 12 countries)
1988	Validation of the GSM system
1989	The responsibility of the GSM specifications is passed to the ETSI
1990	Appearance of the phase 1 of the GSM specifications
1991	Commercial launch of the GSM service
1992	Enlargement of the countries that signed the GSM- MoU> Coverage of larger cities/airports
1993	Coverage of main roads GSM services start outside Europe
1995	Phase 2 of the GSM specifications Coverage of rural areas

Table 1: Events in the development of GSM

From the evolution of GSM, it is clear that GSM is not anymore only a European standard. GSM networks are operational or planned in over 80 countries around the world. The rapid and increasing acceptance of the GSM system is illustrated with the following figures:  

• 1.3 million GSM subscribers worldwide in the beginning of 1994.
• Over 5 million GSM subscribers worldwide in the beginning of 1995.
• Over 10 million GSM subscribers only in Europe by December 1995.

Since the appearance of GSM, other digital mobile systems have been developed. The table 2 charts the different mobile cellular systems developed since the commercial launch of cellular systems.

Year	Mobile Cellular System
1981	Nordic Mobile Telephony (NMT), 450>
1983	American Mobile Phone System (AMPS)
1985	Total Access Communication System (TACS) Radiocom 2000 C-Netz
1986	Nordic Mobile Telephony (NMT), 900>
1991	Global System for Mobile communications> North American Digital Cellular (NADC)
1992	Digital Cellular System (DCS) 1800
1994	Personal Digital Cellular (PDC) or Japanese Digital Cellular (JDC)
1995	Personal Communications Systems (PCS) 1900- Canada>
1996	PCS-United States of America>

Table 2: Mobile cellular systems

Cellular Systems

The Cellular Structure

In a cellular system, the covering area of an operator is divided into cells. A cell corresponds to the covering area of one transmitter or a small collection of transmitters. The size of a cell is determined by the transmitter's power.

The concept of cellular systems is the use of low power transmitters in order to enable the efficient reuse of the frequencies. In fact, if the transmitters used are very powerful, the frequencies can not be reused for hundred of kilometers as they are limited to the covering area of the transmitter.

The frequency band allocated to a cellular mobile radio system is distributed over a group of cells and this distribution is repeated in all the covering area of an operator. The whole number of radio channels available can then be used in each group of cells that form the covering area of an operator. Frequencies used in a cell will be reused several cells away. The distance between the cells using the same frequency must be sufficient to avoid interference. The frequency reuse will increase considerably the capacity in number of users.

In order to work properly, a cellular system must verify the following two main conditions:

• The power level of a transmitter within a single cell must be limited in order to reduce the interference with the transmitters of neighboring cells. The interference will not produce any damage to the system if a distance of about 2.5 to 3 times the diameter of a cell is reserved between transmitters. The receiver filters must also be very performant.
• Neighboring cells can not share the same channels. In order to reduce the interference, the frequencies must be reused only within a certain pattern.

In order to exchange the information needed to maintain the communication links within the cellular network, several radio channels are reserved for the signaling information.

Cluster

The cells are grouped into clusters. The number of cells in a cluster must be determined so that the cluster can be repeated continuously within the covering area of an operator. The typical clusters contain 4, 7, 12 or 21 cells. The number of cells in each cluster is very important. The smaller the number of cells per cluster is, the bigger the number of channels per cell will be. The capacity of each cell will be therefore increased. However a balance must be found in order to avoid the interference that could occur between neighboring clusters. This interference is produced by the small size of the clusters (the size of the cluster is defined by the number of cells per cluster). The total number of channels per cell depends on the number of available channels and the type of cluster used.

Types Of Cells

The density of population in a country is so varied that different types of cells are used:

Macro cells

The macro cells are large cells for remote and sparsely populated areas

Micro cells

These cells are used for densely populated areas. By splitting the existing areas into smaller cells, the number of channels available is increased as well as the capacity of the cells. The power level of the transmitters used in these cells is then decreased, reducing the possibility of interference between neighboring cells.

Selective cells

It is not always useful to define a cell with a full coverage of 360 degrees. In some cases, cells with a particular shape and coverage are needed. These cells are called selective cells. Typical examples of selective cells are the cells that may be located at the entrances of tunnels where coverage of 360 degrees is not needed. In this case, a selective cell with coverage of 120 degrees is used. 

Umbrella cells

A freeway crossing very small cells produces an important number of handovers among the different small neighboring cells. In order to solve this problem, the concept of umbrella cells is introduced. An umbrella cell covers several micro cells. The power level inside an umbrella cell is increased comparing to the power levels used in the micro cells that form the umbrella cell. When the speed of the mobile is too high, the mobile is handed off to the umbrella cell. The mobile will then stay longer in the same cell (in this case the umbrella cell). This will reduce the number of handovers and the work of the network.

A too important number of handover demands and the propagation characteristics of a mobile can help to detect its high speed.

The Transition From Analog To Digital Technology

In the 1980s most mobile cellular systems were based on analog systems. The GSM system can be considered as the first digital cellular system. The different reasons that explain this transition from analog to digital technology are presented in this section.

The Capacity of the System

As it is explained in section 1, cellular systems have experienced a very important growth. Analog systems were not able to cope with this increasing demand. In order to overcome this problem, new frequency bands and new technologies were proposed. But the possibility of using new frequency bands was rejected by a big number of countries because of the restricted spectrum (even if later on, other frequency bands have been allocated for the development of mobile cellular radio). The new analog technologies proposed were able to overcome the problem to a certain degree but the costs were too important.

The digital radio was, therefore, the best option (but not the perfect one) to handle the capacity needs in a cost-efficiency way.

Compatibility with other Systems such as ISDN

The decision of adopting a digital technology for GSM was made in the course of developing the standard. During the development of GSM, the telecommunications industry converted to digital methods. The ISDN network is an example of this evolution. In order to make GSM compatible with the services offered by ISDN, it was decide that the digital technology was the best option.

Additionally, a digital system allows, easily than an analog one, the implementation of future improvements and the change of its own characteristics. 

Aspects of Quality

The quality of the service can be considerably improved using a digital technology rather than an analog one. In fact, analog systems pass the physical disturbances in radio transmission (such as fades, multi-path reception, spurious signals or interferences) to the receiver. These disturbances decrease the quality of the communication because they produce effects such as fadeouts, cross-talks, hisses, etc. On the other hand, digital systems avoid these effects transforming the signal into bits. These transformations combined with other techniques, such as digital coding, improve the quality of the transmission. The improvement of digital systems comparing to analog systems is more noticeable under difficult reception conditions than under good reception conditions.

The GSM Network

Architecture of the GSM Network

The GSM technical specifications define the different entities that form the GSM network by defining their functions and interface requirements.

The GSM network can be divided into four main parts:

The architecture of the GSM network is presented in figure 1.

1. Architecture of the GSM network

Mobile Station

A Mobile Station consists of two main elements:

The Terminal

There are different types of terminals distinguished principally by their power and application:

• The `fixed' terminals are the ones installed in cars. Their maximum allowed output power is 20 W.
• The GSM portable terminals can also be installed in vehicles. Their maximum allowed output power is 8W.
• The handheld terminals have experienced the biggest success thanks to the weight and volume, which are continuously decreasing. These terminals can emit up to 2 W. The evolution of technologies allows decreasing the maximum allowed power to 0.8 W.

The SIM

The SIM is a smart card that identifies the terminal. By inserting the SIM card into the terminal, the user can have access to all the subscribed services. Without the SIM card, the terminal is not operational.

The SIM card is protected by a four-digit Personal Identification Number (PIN). In order to identify the subscriber to the system, the SIM card contains some parameters of the user such as its International Mobile Subscriber Identity (IMSI).

Another advantage of the SIM card is the mobility of the users. In fact, the only element that personalizes a terminal is the SIM card. Therefore, the user can have access to its subscribed services in any terminal using its SIM card.

The geographical areas of the GSM network

The figure 2 presents the different areas that form a GSM network.

2. GSM network areas

As it has already been explained a cell, identified by its Cell Global Identity number (CGI), corresponds to the radio coverage of a base transceiver station. A Location Area (LA), identified by its Location Area Identity (LAI) number, is a group of cells served by a single MSC/VLR. A group of location areas under the control of the same MSC/VLR defines the MSC/VLR area. A Public Land Mobile Network (PLMN) is the area served by one network operator.

The GSM functions

In this paragraph, the description of the GSM network is focused on the different functions to fulfill by the network and not on its physical components. In GSM, five main functions can be defined:

Transmission

The transmission function includes two sub-functions:

• The first one is related to the means needed for the transmission of user information.
• The second one is related to the means needed for the transmission of signaling information.

Not all the components of the GSM network are strongly related with the transmission functions. The MS, the BTS and the BSC, among others, are deeply concerned with transmission. But other components, such as the registers HLR, VLR or EIR, are only concerned with the transmission for their signaling needs with other components of the GSM network. Some of the most important aspects of the transmission are described in section 5.

Radio Resources management (RR)

The role of the RR function is to establish, maintain and release communication links between mobile stations and the MSC. The elements that are mainly concerned with the RR function are the mobile station and the base station. However, as the RR function is also in charge of maintaining a connection even if the user moves from one cell to another, the MSC, in charge of handovers, is also concerned with the RR functions.

The RR is also responsible for the management of the frequency spectrum and the reaction of the network to changing radio environment conditions. Some of the main RR procedures that assure its responsibilities are:

• Channel assignment, change and release.
• Handover.
• Frequency hopping.
• Power-level control.
• Discontinuous transmission and reception.
• Timing advance.

Some of these procedures are described in section 5. In this paragraph only the handover, which represents one of the most important responsibilities of the RR, is described.

Handover

The user movements can produce the need to change the channel or cell, especially when the quality of the communication is decreasing. This procedure of changing the resources is called handover. Four different types of handovers can be distinguished:

• Handover of channels in the same cell.
• Handover of cells controlled by the same BSC.
• Handover of cells belonging to the same MSC but controlled by different BSCs.
• Handover of cells controlled by different MSCs.

Handovers are mainly controlled by the MSC. However in order to avoid unnecessary signaling information, the first two types of handovers are managed by the concerned BSC (in this case, the MSC is only notified of the handover).

The mobile station is the active participant in this procedure. In order to perform the handover, the mobile station controls continuously its own signal strength and the signal strength of the neighboring cells. The list of cells that must be monitored by the mobile station is given by the base station. The power measurements allow deciding which the best cell is in order to maintain the quality of the communication link. Two basic algorithms are used for the handover:

• The `minimum acceptable performance' algorithm. When the quality of the transmission decreases (i.e. the signal is deteriorated), the power level of the mobile is increased. This is done until the increase of the power level has no effect on the quality of the signal. When this happens, a handover is performed.
• The `power budget' algorithm. This algorithm performs a handover, instead of continuously increasing the power level, in order to obtain a good communication quality.

Mobility Management

The MM function is in charge of all the aspects related with the mobility of the user, specially the location management and the authentication and security.

Location management

When a mobile station is powered on, it performs a location update procedure by indicating its IMSI to the network. The first location update procedure is called the IMSI attach procedure.

The mobile station also performs location updating, in order to indicate its current location, when it moves to a new Location Area or a different PLMN. This location updating message is sent to the new MSC/VLR, which gives the location information to the subscriber's HLR. If the mobile station is authorized in the new MSC/VLR, the subscriber's HLR cancels the registration of the mobile station with the old MSC/VLR.

A location updating is also performed periodically. If after the updating time period, the mobile station has not registered, it is then deregistered.

When a mobile station is powered off, it performs an IMSI detach procedure in order to tell the network that it is no longer connected.

Authentication and security

The authentication procedure involves the SIM card and the Authentication Center. A secret key, stored in the SIM card and the AuC, and a ciphering algorithm called A3 are used in order to verify the authenticity of the user. The mobile station and the AuC compute a SRES using the secret key, the algorithm A3 and a random number generated by the AuC. If the two computed SRES are the same, the subscriber is authenticated. The different services to which the subscriber has access are also checked.

Another security procedure is to check the equipment identity. If the IMEI number of the mobile is authorized in the EIR, the mobile station is allowed to connect the network.

In order to assure user confidentiality, the user is registered with a Temporary Mobile Subscriber Identity (TMSI) after its first location update procedure.

Enciphering is another option to guarantee a very strong security but this procedure is going to be described in section 5.

Communication Management (CM)

The CM function is responsible for:

• Call control.
• Supplementary Services management.
• Short Message Services management.

Call Control (CC)

The CC is responsible for call establishing, maintaining and releasing as well as for selecting the type of service. One of the most important functions of the CC is the call routing. In order to reach a mobile subscriber, a user dials the Mobile Subscriber ISDN (MSISDN) number, which includes:

• a country code
• a national destination code identifying the subscriber's operator
• a code corresponding to the subscriber's HLR

The call is then passed to the GMSC (if the call is originated from a fixed network) which knows the HLR corresponding to a certain MISDN number. The GMSC asks the HLR for information helping to the call routing. The HLR requests this information from the subscriber's current VLR. This VLR allocates temporarily a Mobile Station Roaming Number (MSRN) for the call. The MSRN number is the information returned by the HLR to the GMSC. Thanks to the MSRN number, the call is routed to subscriber's current MSC/VLR. In the subscriber's current LA, the mobile is paged.

Supplementary Services management

The mobile station and the HLR are the only components of the GSM network involved with this function. The different Supplementary Services (SS) to which the users have access are presented in section 6.3.

Short Message Services management

In order to support these services, a GSM network is in contact with a Short Message Service Center through the two following interfaces:

• The SMS-GMSC for Mobile Terminating Short Messages (SMS-MT/PP). It has the same role as the GMSC.
• The SMS-IWMSC for Mobile Originating Short Messages (SMS-MO/PP).

Operation, Administration and Maintenance (OAM)

The OAM function allows the operator to monitor and control the system as well as to modify the configuration of the elements of the system. Not only the OSS is part of the OAM, also the BSS and NSS participate in its functions as it is shown in the following examples:

• The components of the BSS and NSS provide the operator with all the information it needs. This information is then passed to the OSS which is in charge of analyzing it and control the network.
• The self test tasks, usually incorporated in the components of the BSS and NSS, also contribute to the OAM functions.
• The BSC, in charge of controlling several BTSs, is another example of an OAM function performed outside the OSS.

The GSM Radio Interface

The radio interface is the interface between the mobile stations and the fixed infrastructure. It is one of the most important interfaces of the GSM system.

One of the main objectives of GSM is roaming. Therefore, in order to obtain a complete compatibility between mobile stations and networks of different manufacturers and operators, the radio interface must be completely defined.

The spectrum efficiency depends on the radio interface and the transmission, more particularly in aspects such as the capacity of the system and the techniques used in order to decrease the interference and to improve the frequency reuse scheme. The specification of the radio interface has then an important influence on the spectrum efficiency.

Frequency Allocation

Two frequency bands, of 25 MHz each one, have been allocated for the GSM system:

• The band 890-915 MHz has been allocated for the uplink direction (transmitting from the mobile station to the base station).
• The band 935-960 MHz has been allocated for the downlink direction (transmitting from the base station to the mobile station).

But not all the countries can use the whole GSM frequency bands. This is due principally to military reasons and to the existence of previous analog systems using part of the two 25 MHz frequency bands.

From source information to radio waves

The figure 4 presents the different operations that have to be performed in order to pass from the speech source to radio waves and vice versa.

Speech coding

The transmission of speech is, at the moment, the most important service of a mobile cellular system. The GSM speech codec, which will transform the analog signal (voice) into a digital representation, has to meet the following criteria:

3. From speech source to radio waves

 

If the source of information is data and not speech, the speech coding will not be performed 

• A good speech quality, at least as good as the one obtained with previous cellular systems.
• To reduce the redundancy in the sounds of the voice. This reduction is essential due to the limited capacity of transmission of a radio channel.
• The speech codec must not be very complex because complexity is equivalent to high costs.

The final choice for the GSM speech codec is a codec named RPE-LTP (Regular Pulse Excitation Long-Term Prediction). This codec uses the information from previous samples (this information does not change very quickly) in order to predict the current sample. The speech signal is divided into blocks of 20 ms. These blocks are then passed to the speech codec, which has a rate of 13 kbps, in order to obtain blocks of 260 bits.

Channel coding

Channel coding adds redundancy bits to the original information in order to detect and correct, if possible, errors occurred during the transmission.

Channel coding for the GSM data TCH channels

The channel coding is performed using two codes: a block code and a convolution code.

The block code corresponds to the block code defined in the GSM Recommendations 05.03. The block code receives an input block of 240 bits and adds four zero tail bits at the end of the input block. The output of the block code is consequently a block of 244 bits.

A convolution code adds redundancy bits in order to protect the information. A convolution encoder contains memory. This property differentiates a convolution code from a block code. A convolution code can be defined by three variables: n, k and K. The value n corresponds to the number of bits at the output of the encoder, k to the number of bits at the input of the block and K to the memory of the encoder. The ratio, R, of the code is defined as follows: R = k/n. Let's consider a convolution code with the following values: k is equal to 1, n to 2 and K to 5. This convolution code uses then a rate of R = 1/2 and a delay of K = 5, which means that it will add a redundant bit for each input bit. The convolution code uses 5 consecutive bits in order to compute the redundancy bit. As the convolution code is a 1/2 rate convolution code, a block of 488 bits is generated. These 488 bits are punctured in order to produce a block of 456 bits. Thirty two bits, obtained as follows, are not transmitted:

    C (11 + 15 j) for j = 0, 1, 31

The block of 456 bits produced by the convolution code is then passed to the interleaver.

Channel coding for the GSM speech channels

Before applying the channel coding, the 260 bits of a GSM speech frame are divided in three different classes according to their function and importance. The most important class is the class Ia containing 50 bits. Next in importance is the class Ib, which contains 132 bits. The least important is the class II, which contains the remaining 78 bits. The different classes are coded differently. First of all, the class Ia bits are block-coded. Three parity bits, used for error detection, are added to the 50 class Ia bits. The resultant 53 bits are added to the class Ib bits. Four zero bits are added to this block of 185 bits (50+3+132). A convolution code, with r = 1/2 and K = 5, is then applied, obtaining an output block of 378 bits. The class II bits are added, without any protection, to the output block of the convolution coder. An output block of 456 bits is finally obtained.

Channel coding for the GSM control channels

In GSM the signaling information is just contained in 184 bits. Forty parity bits, obtained using a fire code, and four zero bits are added to the 184 bits before applying the convolution code (r = 1/2 and K = 5). The output of the convolution code is then a block of 456 bits, which does not need to be punctured.

Interleaving

An interleaving rearranges a group of bits in a particular way. It is used in combination with FEC codes in order to improve the performance of the error correction mechanisms. The interleaving decreases the possibility of losing whole bursts during the transmission, by dispersing the errors. Being the errors less concentrated, it is then easier to correct them.

Interleaving for the GSM control channels

A burst in GSM transmits two blocks of 57 data bits each. Therefore the 456 bits corresponding to the output of the channel coder fit into four bursts (4*114 = 456). The 456 bits are divided into eight blocks of 57 bits. The first block of 57 bits contains the bit numbers (0, 8, 16,448), the second one the bit numbers (1, 9, 17,455). The first four blocks of 57 bits are placed in the even-numbered bits of four bursts. The other four blocks of 57 bits are placed in the odd-numbered bits of the same four bursts. Therefore the interleaving depth of the GSM interleaving for control channels is four and a new data block starts every four bursts. The interleaver for control channels is called a block rectangular interleaver.

Interleaving for the GSM speech channels

The block of 456 bits, obtained after the channel coding, is then divided in eight blocks of 57 bits in the same way as it is explained in the previous paragraph. But these eight blocks of 57 bits are distributed differently. The first four blocks of 57 bits are placed in the even-numbered bits of four consecutive bursts. The other four blocks of 57 bits are placed in the odd-numbered bits of the next four bursts. The interleaving depth of the GSM interleaving for speech channels is then eight. A new data block also starts every four bursts. The interleaver for speech channels is called a block diagonal interleaver.

Interleaving for the GSM data TCH channels

A particular interleaving scheme, with an interleaving depth equal to 22, is applied to the block of 456 bits obtained after the channel coding. The block is divided into 16 blocks of 24 bits each, 2 blocks of 18 bits each, 2 blocks of 12 bits each and 2 blocks of 6 bits each. It is spread over 22 bursts in the following way:

• the first and the twenty-second bursts carry one block of 6 bits each
• the second and the twenty-first bursts carry one block of 12 bits each
• the third and the twentieth bursts carry one block of 18 bits each
• from the fourth to the nineteenth burst, a block of 24 bits is placed in each burst

A burst will then carry information from five or six consecutive data blocks. The data blocks are said to be interleaved diagonally. A new data block starts every four bursts.

Burst assembling

The burst assembling procedure is in charge of grouping the bits into bursts. Section 5.2.3 presents the different bursts structures and describes in detail the structure of the normal burst 

Ciphering 

Ciphering is used to protect signaling and user data. First of all, a ciphering key is computed using the algorithm A8 stored on the SIM card, the subscriber key and a random number delivered by the network (this random number is the same as the one used for the authentication procedure). Secondly, a 114 bit sequence is produced using the ciphering key, an algorithm called A5 and the burst numbers. This bit sequence is then XORed with the two 57 bit blocks of data included in a normal burst.

In order to decipher correctly, the receiver has to use the same algorithm A5 for the deciphering procedure.

Modulation

The modulation chosen for the GSM system is the Gaussian Modulation Shift Keying (GMSK).

The aim of this section is not to describe precisely the GMSK modulation as it is too long and it implies the presentation of too many mathematical concepts. Therefore, only brief aspects of the GMSK modulation are presented in this section.

The GMSK modulation has been chosen as a compromise between spectrum efficiency, complexity and low spurious radiations (that reduce the possibilities of adjacent channel interference). The GMSK modulation has a rate of 270 5/6 kbauds and a BT product equal to 0.3. Figure 5 presents the principle of a GMSK modulator.

4. GMSK modulator

Discontinuous transmission (DTX)

This is another aspect of GSM that could have been included as one of the requirements of the GSM speech codec. The function of the DTX is to suspend the radio transmission during the silence periods. This can become quite interesting if we take into consideration the fact that a person speaks less than 40 or 50 percent during a conversation. The DTX helps then to reduce interference between different cells and to increase the capacity of the system. It also extends the life of a mobile's battery. The DTX function is performed thanks to two main features:

• The Voice Activity Detection (VAD), which has to determine whether the sound represents speech or noise, even if the background noise is very important. If the voice signal is considered as noise, the transmitter is turned off producing then, an unpleasant effect called clipping.
• The comfort noise. An inconvenient of the DTX function is that when the signal is considered as noise, the transmitter is turned off and therefore, a total silence is heard at the receiver. This can be very annoying to the user at the reception because it seems that the connection is dead. In order to overcome this problem, the receiver creates a minimum of background noise called comfort noise. The comfort noise eliminates the impression that the connection is dead.

Timing advance

The timing of the bursts transmissions is very important. Mobiles are at different distances from the base stations. Their delay depends, consequently, on their distance. The aim of the timing advance is that the signals coming from the different mobile stations arrive to the base station at the right time. The base station measures the timing delay of the mobile stations. If the bursts corresponding to a mobile station arrive too late and overlap with other bursts, the base station tells, this mobile, to advance the transmission of its bursts.

Power Control

At the same time the base stations perform the timing measurements, they also perform measurements on the power level of the different mobile stations. These power levels are adjusted so that the power is nearly the same for each burst.

A base station also controls its power level. The mobile station measures the strength and the quality of the signal between itself and the base station. If the mobile station does not receive correctly the signal, the base station changes its power level.

Discontinuous Reception

It is a method used to conserve the mobile station's power. The paging channel is divided into sub channels corresponding to single mobile stations. Each mobile station will then only 'listen' to its sub channel and will stay in the sleep mode during the other sub channels of the paging channel.

Multipath and Equalisation

At the GSM frequency bands, radio waves reflect from buildings, cars, hills, etc. So not only the 'right' signal (the output signal of the emitter) is received by an antenna, but also many reflected signals, which corrupt the information, with different phases.

An equalizer is in charge of extracting the 'right' signal from the received signal. It estimates the channel impulse response of the GSM system and then constructs an inverse filter. The receiver knows which training sequence it must wait for. The equalizer will then, comparing the received training sequence with the training sequence it was expecting, compute the coefficients of the channel impulse response. In order to extract the 'right' signal, the received signal is passed through the inverse filter.

GSM Reference Model

System entities

The GSM system entities represent groupings of specific wireless functionality.
The following figure shows the GSM reference Model.

A Public Land Mobile Network (PLMN), as represented by the GSM reference model on the opposite page, includes the following system entities:

Mapping Model to Network

Example of a GSM network is shown.

Conclusion

The aim of this paper was to give an overview of the GSM system and not to provide a complete and exhaustive guide.

As it is shown in this chapter, GSM is a very complex standard. It can be considered as the first serious attempt to fulfill the requirements for a universal personal communication system. GSM is then used as a basis for the development of the Universal Mobile Telecommunication System (UMTS).