Speech coding and channel coding

The speech coder is a regular pulse excited linear predictive coder (RPE-LPC) with long term prediction. This provides a net bit rate of 13kbit/s. It is a block based coder where the input samples are analysed in blocks with a 20ms duration. Work is also being carried out to specify a half rate speech coder which will effectively double the system capacity of GSM.

Before being assembled into the timeslots and frames, the digital speech and signalling data is encoded and interleaved. The speech coder output is divided up into three classes of bits and the most sensitive bits are encoded by adding parity check bits followed by a convolutional coder. Signalling data is encoded using a FIRE code. A process of interleaving is then used to spread the data blocks over a number of bursts.

For speech, an interleaving degree of 8 is used, i.e the speech block is spread over 8 bursts, whilst an interleaving degree of 4 is used for signalling. This overall process is shown in Figure 47.11, and the combined use of coding and interleaving provides good protection of channel data from the fading, dispersion and inter­ference effects on the radio path. With the addition of frequency hopping and diversity techniques, the GSM air interface is particu­larly robust.

One of the penalties to be paid for this is the overall transmission delay. The speech coder contributes about 25ms and the channel coding and interleaving a further 37ms. The rest of the transmission delay budget allows for analogue to digital conversions, 16kbit/s transmission and switching in various parts of the network. The overall one way transmission delay thus amounts to around 90ms. Such a delay means that echo control is necessary even on short national calls.

Table 47.4 GSM logical channels

Figure 47.11GSM channel coding and interleaving

GSM signalling

Figure 47.12 shows the overall signalling model. The Air Interface uses LAPDm Layer 2 signalling protocol and this is also used for the A-bis, BTS to BSC interface.

The layer 3 protocol consists of three sublayers, dealing with radio resource management (RR), mobility management (MM), and con­nection management (CM). Radio resource management is con­cerned with managing the logical channels, including paging, channel assignments, handover, measurement reporting, and other functions.

The mobility management layer contains functions necessary to support the mobility of the user which include authentication, loca­tion updating, attach and detach of IMSI (International Mobile Subscriber Identity), and registration. The connection management layer is concerned with call control, establishing and clearing cir­cuits, management of supplementary services and the short message service.

The BSC to MSC A-interface, and the various MSC to Register interfaces employ CCITT No.7 signalling using the Message Trans­fer Part (MTP), Signalling Connection Control Part (SCCP), Trans­action Capabilities Part (TCAP) and Mobile Application Part (MAP).

An example of the signalling messaging for establishing a mobile originated call is shown in Figure 47.13. The key events are:

1. Request and assignment of a channel, between MS and BSS.

2. A service request procedure which accesses the VLR.

3. An authentication and ciphering exchange which validates the mobile user and sets the encryption cipher.

4. Call set up which includes sending of dialled digits and estab­lishing the connection.

Location updating is shown in Figure 47.14 An update request is indicated by the mobile and passed to the VLR in the new location area. The new VLR requests the IMSI from the old VLR and then signals the new location to the HLR. The HLR provides the subscriber data to the new VLR and cancels the subscriber entry in the old VLR. Finally a confirmation message is set back to the mobile. There are, of course, many other signalling exchanges, dealing with mobile terminating calls, supplementary services, and short message service. There is not space in this chapter to deal with the detailed signalling for these cases; the examples above describe the general principle and illustrate the roles of the MS, BSS, MSC, VLR and HLR.

Figure 47.12GSM signalling model

Security features

The information on the air interface needs to be protected, to provide user data (including speech) confidentiality and to prevent fraudulent use of subscriber and mobile identities. The basic mech­anisms employed are user authentication and user data encryption. Each mobile user is provided with a Subscriber Identity Module (SIM) which contains the IMSI, the individual subscriber authenti­cation key (K_i) and the authentication algorithm (A3). After the mobile user has made an access and service request, the network checks the identity of the user by sending a random number (RAND) to the mobile. The mobile uses the RAND, K_i and A3 algorithm to produce a signed response SRES. This response is compared with a similar response calculated by the network, and access only continues if the two responses match.

The SIM also contains a cipher key generating algorithm (A8). The MS uses the RAND and A8 to calculate a ciphering key (Kc) which is used to encrypt and decrypt signalling and user data information.

The authentication centre (AUC) is responsible for all security aspects and its function is closely linked with the HLR. The AUC generates the Ki's and associates them with IMSIs, and provides the HLR with sets of RAND, SRES and Kc for each IMSI. The HLR then provides the appropriate VLR with these sets and it is the VLR which carries out the authentication check. Authentication of mobile users can be carried out on call set up, both mobile originated and mobile terminated, on location updating, and on activation of supplementary services. As the authentication sets are used up in the VLR, further sets are requested from the HLR.

An additional security feature of GSM is the equipment identity register (E1R). This enables monitoring ofthe mobile equipment 1MEI (International Mobile Equipment Identity) which is used to validate mobile equipments thus preventing non-approved, faulty or stolen equipment from using the system. This range ofsecurity features provide a high degree of protection to the user and the network operator.

Figure 47.13 Mobile originating call

GSM services and features

In addition to speech, GSM offers a wide range of data bearer services up to 9.6kbit/s suitable for connection to circuit switched or packet switched data networks. GSM also supports Group 111 facsimile as a data service by use ofan appropriate converter.

A comprehensive range of supplementary services are offered by GSM, including call forwarding, call barring, multi-party service, advice ofcharge and others. A full description is provided in the GSM Recommendations, and further detail of cellular services is provided later in this chapter.

An important feature ofGSM is the short message service (SMS). This allows transmission of alphanumeric messages of up to 160 characters to or from a mobile via a service centre. If the message cannot be delivered due to mobile being switched off, or outside of the coverage area, the message is stored at a service centre and re-transmitted when the mobile registers again. Received messages can be displayed on the mobile and stored in the SIM forfuture reference. A related service is cell broadcast which allows mess­ages ofup to 93 characters to be sent to all mobiles within a specific geographical area, for example to deliver traffic or weather reports.

Roaming

Naturally, with a pan-European system, roaming of subscribers between networks is specified by GSM. When a mobile first swit­ches on in a foreign PLMN (Public Land Mobile Network), the local MSC/VLR will determine the identity ofthe home PLMN from the mobile network code which is part of the IMSI. The home HLR will be interrogated to establish whether roaming is permitted and for authentication. The home HLR then passes the subscriber data to the local (foreign) VLR and registers the foreign location of the mobile. Calls to and from the roamed mobile can then take place.

Exercise 1 Learn the words and word combinations

Exercise 2 Read the text

Exercise 3 Find the Russian equivalents for the following English words and word combinations

Exercise 4 Speak on the problems:

Part IV (47.6 – 47.8)

Services

The primary purpose ofall cellular radio networks is to offer speech telephony service to its customers. In addition most networks offer a range of supplementary and value added services to enhance the basic product.

In analogue systems, basic telephony is provided directly by the audio path between mobile and network. Other than some linear speech processing to increase the channel's signal to noise performance, the audio path is transparent across the speech band, allow­ing other sounds (tones, non-voice signals etc) to pass through undistorted. By contrast GSM (and other digital systems) use a speech coder tailored to voice characteristics. They therefore pro­vide a fully acceptable telephony service, but non-voice signals can suffer distortion across the non-audio transparent path.

Figure 47.14Location updating

Supplementary services

Supplementary services are provided by means of enhancements to the basic call processing software in the MSCs. Many of these services have specific relevance to the cellular radio user, and in the main they parallel services which are becoming increasingly avail­able on the fixed telephone networks (such as BT's Star Services in the UK). Typical services are as follows:

1. Call divert, where all calls are diverted to the specified number, which may be another mobile or a termination on another network. This is of use if the user wishes to make calls but not receive them.

2. Divert on no answer, where calls are diverted to the specified number when the user does not answer within (for instance) 20 seconds. This is of use if a mobile is left switched on in an unattended vehicle.

3. Divert on mobile unavailable, where calls are diverted to the specified number if the network cannot contact the mobile owing to its being turned off or out of range. This is of particular use in a cellular system where, in general, users are not avail­ able at all times, and where coverage is not universal. This service is often combined with the "divert on no answer" service.

4. Divert on busy, where calls are diverted to the specified number when the mobile is already engaged on a call. As an alternative, networks also provide call waiting.

5. Call waiting, where if a call is received when the mobile is already engaged on a call, the user is informed that a second call is waiting, and can choose to place the first call on hold whilst dealing with the second caller.

6. Three party calling, where the mobile user may set up calls to two other parties and connect them in a three way conference. This service can also be used to make enquiry calls whilst holding the original call.

Value added services

Value added services are normally provided by means of peripheral units attached to the cellular network, or to the fixed network with which to cellular network interconnects. In some countries, the prevailing regulatory regime will influence what services may be offered and in what manner, however, the following are typical.

47.6.2.1 Messaging services

Voice messaging is commonly available in association with cellular networks. Used in conjunction with the call divert supplementary services, the messaging service can pick up calls when the user cannot, and the caller can leave a message for later retrieval by the user. Some services allow the user to be alerted to the receipt of messages by means of a radiopaging service, or in some cases by a ringback on the cellular network itself.

In addition to voice messaging, GSM networks will incorporate the 'Short Message Service' which effectively turns a GSM mobile into a two way alphanumeric pager with forced message delivery and message delivery confirmation.

47.6.2.2 1 nformation services

Voice information services are commonly available on fixed net­works, normally carrying some premium call charge. Some services (such as travel and weather information) are of particular value to a mobile user and some networks make these more readily accessible, for instance by using the mobile's current location to select the appropriate information for that area.

47.6.2.3 Private interconnect

A large user of cellular can often gain economies by leasing a direct connection between the cellular network and their company's pri­vate network, thus bypassing the PSTN. Call charges for such direct connections are tariffed by the cellular operator at a level substan­tially less than that for PSTN calls. An extra benefit of private interconnect are that calls can be delivered direct to extensions on a company's network without having to be handled by the switch­board operator, saving time and labour.

Data services

In analogue cellular systems, the transparent audio path between the network and mobile can be used not only for voice communication, but also for non-voice communication such as data using in-band modems, and facsimile. In order to be used in conjunction with a mobile, data modems and fax machines which are designed for PSTN use have to be adapted for connection to the mobile by means of a special interface. Such interfaces are available for a range of mobiles, and often permit automatic call establishment and clear down under the control of the modem or fax machine.

The data rate achievable over a cellular radio channel will often be less than that over a direct PSTN path, mainly due to the more limited frequency response of the channel, and the delay spread characteristic which is affected by the audio processing in both mobile and base station. However data transmission at 120()bit/s (using CCITT V.22) and 48(K)bit/s (using V32) can be achieved quite commonly on cellular networks, as well as fax up to 7200 or 9600bit/s.

The radio link between a cellular network's base stations and a mobile station is a notoriously hostile environment for data trans­mission. Disturbance and interruptions come from a variety of sources, such as variability of the radio signal strength, noise and interference, and 'intentional' breaks due to signalling interchanges between base station and mobile for handover and power control. In order to transmit data reliably over such a path, error control of some form is essential.

The simplest form of error control is a layer 2 protocol, and the emergence of the CCITT V.42 standard has led to error correcting modems becoming readily available. Although V.42 (which con­tains two protocols, the 'open' LAP-M and 'proprietary' MNP4) was designed for fixed PSTN use, it has proved to perform suffi­ciently well over cellular paths, particularly to static mobiles, for the user to receive good service.

Many proprietary protocols have been specifically developed to cope with the errors experienced over cellular radio channels. One such protocol is called Cellular Data Link Control (CDLC), and was developed in the UK by Racal Vodata. CDLC uses two levels of error correction with dynamic switching, and techniques such as forward error correction, bit interleaving and BCH block coding with a basic HDLC protocol to give a highly robust data trans­mission path, even over poor quality channels.

Facsimile transmission over cellular has benefitted by the increas­ingly widespread adoption of Group 3 error correcting (ECM) fax machines and the availability of portable machines suitable for vehicle use.

The GSM system does not provide a transparent audio path due to the voice coding techniques used, so data transmission in GSM is dealt with differently. When the data mode is selected, the speech coder is replaced by a rate adaptor and channel coder which apply forward error correction to the data bits, and the resulting bit stream is then transmitted across the radio path in the same burst structure as for voice transmission. At the receive end the bit stream is extracted and errors are corrected up to the limit of the forward error correction scheme. If there are any errors remaining, a higher layer protocol is needed to detect and correct them.

GSM has defined two families of data services, termed transpar­ent and non-transparent. The transparent service applies only for­ward error correction as described above, and the user application must be able to cope with the residual error rate. The characteristics of the transparent service are constant delay and throughput but variable error rate. The transparent service is of particular use in synchronous applications (eg X.25, IBM SDLC) where the higher layer protocol inherent in the application will correct the errors. Asynchronous applications may also use the transparent service, particularly at low bit rates where the forward error correction applied by GSM is stronger.

The non-transparent service applies a GSM specific layer 2 proto­col between the mobile and the network in order to correct all residual errors, resulting in a near zero error rate. The penalty, however, is variable throughput and delay, dependent upon the prevailing radio conditions. The non-transparent service is of par­ticular application to simple asynchronous terminals, although pro­vision in the standards is also made for protocol conversion to allow X.25 packets to be carried.

Facsimile transmission over GSM is complicated by the use in the Group 3 standard of a number of data transmission rates and modem types (V.21, V.29, V.27). In order to carry the fax signals, GSM mobiles need a special adaptor to convert the multiple standards into a synchronous bit stream for transmission between mobile and network. A similar converter in the network then converts the signal back into the Group 3 protocol to interwork with fax machines in the fixed network.

Future developments

The technology of cellular radio systems continues to develop very rapidly. The early 1980s saw the introduction of the first commer­cial analogue systems and by the end of the decade trials of second generation digital systems were already under way. Systems such as GSM are now entering into service and work is already starting on the specification of a third generation world wide standard system. These developments are not introducing technology for its own sake, but are aimed at improving the quality, capacity, and availa­bility, and reducing the cost of mobile communications. In addition to these step changes in 'generations' of system there are technical advances which are applicable to current systems. These include techniques such as microcellular and intelligent networks.

Microcells

As the capacity of cellular systems has increased, cell sizes have decreased, in some networks to as small as 0.5km radius, such that controlling co-channel interface becomes a major problem. The use of microcells, that is, very small cells, is a way of increasing capacity still further. In a microcellular layout, base station antennas are placed below the building height in urban areas, and low power is used such that the propagation characteristics between base sta­tion and mobile are dominated by the street layout. Interference from adjacent cells is blocked by buildings.

Microcellular techniques allow significantly higher traffic den­sities to be achieved, and also enable smaller, lower power mobiles to be used. The use of microcells requires improved handover techniques, which allow for fast and reliable handoff, for example when turning a street corner. One way of easing handover problems is to employ an 'umbrella cell' arrangement using conventional cells overlaying the microcells such that handover can be made into the umbrella cell where no suitable adjacent microcell can be identified. This also avoids the need to plan a contiguous coverage of micro-cells in an urban area.

New technology is now enabling the use of more compact and cheaper base stations. Conventional base sites have generally re­quired a purpose built building, or rented space within an existing building for installation of base station racks of equipment. Now, base stations can be housed in small roadside or roof top mounted cabinets, and further reductions in size can be expected. Small base station equipment, and antennas, are essential to enable microcells to be built cost effectively.

Intelligent networks

Intelligent Network techniques (IN), are not, of course unique to cellular systems and have already become well established in fixed networks for the provision of 'free fone' or 'toll-free' type services, for example. However, the ability of an IN architecture to provide customised services is particularly valuable to a mobile user, who can have improved control over the handling of incoming calls. IN techniques also provide the ability to create a wide variety of advanced services.

Second generation cellular systems such as GSM are already designed around an architecture which can support IN type applica­tions. In particular, the HLR function is closely related to the IN service control point. We can expect further developments in the near future which will bring a range of IN features to both the mobile user and the service provider.

Personal communications

The term PCN, Personal Communications Network, is used widely in the UK, whilst PCS, Personal Communications Services is used in the USA. Both aim at the same objective of serving the mass consumer market with mobile communications. The key challenge is to provide a very high capacity network to support a large number of users at low cost. Microcellular techniques will certainly be needed, and in order to keep costs down, the concept of regional service, and local access to the PSTN is being considered. IN techniques may offer personal numbering across a variety of net­works.

PCN is dealt with in detail in Chapter 48. The standard in Europe, known as DCS1800 is based on the GSM standard but operating at 1800MHz. There is therefore unlikely to be a significant technical difference between Cellular GSM and PCN, with microcellular techniques being equally applicable to either system.

In the USA, the use of CDMA, code division multiple access, is being trialled for PCS. CDMA works on the principle of transmit­ting unique (orthogonal) codes to identify different users. Detection of signals is achieved by using correlating receivers such that other users appear as pseudonoise. CDMA thus allows a large number of users to share the same (wideband) radio channel.

There is considerable debate about the advantages and disadvant­ages of CDMA, in particular how to control near/far user inter­ference; the extent to which this can be achieved is crucial to the ultimate capacity of CDMA. One of the key benefits of CDMA is the potential to share spectrum with other users, forexample fixed links, and for this reason it is particularly attractive where additional spectrum for mobile systems cannot be made available.

Conclusion

Cellular radio is a comparatively young technology. Networks em­ploying analogue systems have developed rapidly and now provide high quality service and excellent coverage in many of the de­veloped countries. Technology developments are now increasing the potential network capacity, reducing the size of mobiles, and bringing advanced features and services to the mobile user. The decade ahead with the opportunity to introduce new digital systems and create a world-wide land mobile standard looks particularly exciting.

Exercise 1 Write out of the text (47.6 – 47.8) all terms referring to cellular

• ⇐ Назад
• 123
• Далее ⇒