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GSM basics: an introduction.

The Global System for Mobile communication (GSM) is a huge, rapidly expanding and successful technology. Less than five years ago, there were a small number of companies working on GSM. Each of these companies had a few GSM experts who brought knowledge back from the European Telecommunications Standards Institute committees designing the GSM specification. Currently, there are hundreds of companies working on GSM and thousands of GSM experts.

In the US, bands have been allocated at approximately 2 GHz for a personal communications system (PCS). Unlike Europe and the Far East, the PCS license holders will not be forced to use any particular radio technology. The three main system contenders are GSM, code-division multiple access and 15-136 time-division multiple access (TDMA), all likely to have nationwide coverage. The ready availability of GSM equipment and expertise has made GSM at 1.9 GHz attractive for many operators. PCS1900 operators have banded together to form the North American Interest Group and help advance the development of GSM. The seven member companies include American Personal Communications (APC), American Portable Telecom, Bell South Personal Communications, Intercel, Omnipoint, Pacific Bell Mobile Services and Western Wireless Co. Many of the large GSM manufacturers are also backing PCS1900, including Nokia, Ericsson, Matra, AEG and Northern Telecom. The first commercial PCS system based on PCS1900 was launched by APC under the Sprint Spectrum name on November 15, 1995. The majority of US PCS licenses will became operational over the next two years.

GSM NETWORKS

The GSM system, shown in Figure 1, is made up of mobile stations (MS), both hand-held (or portables) and traditional mobiles, in a car that talk to the base station system (BSS) over an RF air interface. The base station system consists of a base transceiver station (BTS) and a base station controller (BSC). Typically, several BTS are located at the same site, producing two to four sectored cells around a common antenna tower. BSCs are often connected to BTS via microwave links. The BSC-to-BTS link is called the Abis interface. Usually, 20 to 30 BTS will be controlled by one BSC. A number of base station systems would then report back to the mobile switching center (MSC), which controls the traffic among a number of different cells. The Advanced Mobile Phone Service (AMPS) system does not have the base station system level of control. AMPS base stations, equivalent to the BTS in GSM, talk directly to the MSC.

Each MSC will have a visitors location register (VLR) in which mobiles that are out of their home cell will be listed so that the network will know where to find them. The MSC also will be connected to the home location register (HLR), the authentication center (AUC) and the equipment identity register (EIR) so the system can verify that users and equipment are legal subscribers. These connections help prevent the use of stolen or fraudulent mobiles. There are also facilities within the system for operations and maintenance center (OMC) and network management center (NMC) organizations. The MSC also has the interface to other networks such as private land mobile networks, public switched telephone networks and integrated services digital networks (ISDN). Addition of the equipment identity and home location registers to the MSC was unique to GSM.

[TABULAR DATA FOR TABLE I OMITTED]

A GSM CELL

In a typical GSM cell, shown in Figure 2, the individual cells can be located up to a 35 km radius for GSM900 and approximately 2 km for PCS1900 (because of lower power PCS1900 mobiles). The most obvious part of the GSM cell is the base station and its antenna tower. It is common for several cells to be sectored around a common antenna tower. The tower will have several directional antennas, each covering a particular area. This co-location of several BTS is sometimes called a cell site, or just a base station. The BTS are connected to their BSC by the Abis interface, which is a cable or an optical fiber interface.

Each BTS will be fitted with a number of Tx/Rx pairs or transceiver modules. The number will determine how many frequency channels can be used in the cell and depends on the expected number of users.

All BTS produce a broadcast channel (BCH). The BCH is like a lighthouse or beacon; it is on all the time and allows mobiles to find the GSM network. The BCH signal strength is also used by the network for a variety of user functions. This signal strength is a useful way of telling which is the closest BTS to the mobile. The BCH also has information coded onto it, such as the identify of the network (for example, APC or Bell South), paging messages for any mobiles needing to accept a phone call and a variety of other information. The BCH is received by all mobiles in the cell whether they are on a call or not.

The frequency channel used by the BCH is different in each cell. Channels can be reused by distant cells where the risk of interference is low.

Mobiles on a call use a traffic channel (TCH). The TCH is a two-way channel used to exchange speech information between the mobile and base station. Information is divided into the uplink and downlink, depending on its direction of flow. GSM separates out the uplink and downlink into different frequency bands. Within each band, the channel numbering scheme is the same. Effectively, a GSM channel consists of an uplink and a downlink frequency.

Note that while the TCH uses a frequency channel in both the uplink and downlink, the BCH occupies a channel in the downlink band only. The corresponding channel in the uplink is left clear. This channel can be used by the mobile for unscheduled or random-access channels (PACH). When the mobile wants to grab the attenuation of the base station (perhaps to make a call), it can ask for attention by using this clear frequency channel to send a PACH. Since more than one mobile may want to grab attention at the same time, colliding RACHs are possible and mobiles may need to make repeated attempts to be heard. Table 1 lists additional specifications about the air interface.

TDMA AND FDMA

GSM uses TDMA and frequency-division multiple access FDMA). The frequencies available are divided into two bands. The uplink is for mobile transmission, while the downlink is for base station transmission. Figure 3 shows part of one of these bands. Each band is divided into 200 kHz channels called absolute RF channel numbers (ARFCN). In addition to slicing up frequency, time is also sliced. Each ARFCN is shared between eight mobiles, each using the ARFCN in turn. Each mobile uses the ARFCN for one timeslot and then waits for its turn to come around again. Mobiles are allowed the use of the ARFCN once per TDMA frame.

The example shows four TCHs; each one uses a particular ARFCN and timeslot. Three of the traffic channels are on the same ARFCN using different timeslots. The fourth traffic channel is on a different ARFCN.

The combination of a timeslot number and ARFCN is called a physical channel. Not much space exists between timeslots and ARFCNs. It is important for the mobile and base stations to transmit their TDMA bursts at exactly the right time, frequency and amplitude. The BTS must be able to vary the timing of the MS transmission. Without timing advance, the transmitted burst from a user at the edge of a cell would arrive late and overlap (and corrupt) the signal from a user located right next to the base station (unless a guard time, between timeslots, greater than the longest signal travel time was used). By advancing the timing of the mobiles, their transmissions arrive at the base station at the correct time. As a mobile moves, the BTS will signal the MS to reduce its timing advance as it gets closer to the center of the GSM cell, and increase its timing advance as it moves away from the center of the cell. Poorly controlled modulation spectrum or spurious will also cause interference with adjacent ARFCN.

THE DOWNLINK AND UPLINK

Figure 4 shows an example of how information is transmitted. The uplink and downlink use the same timeslot number and ARFCN. Timeslot 2 is assigned in a traffic mode, receiving and transmitting information to the base station. The downlink on which information is received will be in the frequency range of 935 to 960 MHz. The uplink, the frequency at which the mobile will transmit information to the base station, will be in the frequency range of 890 to 915 MHz. The uplink and downlink make up a frequency pair, which for GSM900 is always separated by 45 MHz. The timeslots are offset by three slots between the downlink and uplink. Information is received in timeslot 2 in the downlink, and two timeslots are allocated to switch to the uplink frequency to be ready to transmit information. The system is then ready to receive the timeslot of information in the next frame.

THE SPEECH CODER

Most modern digital communication systems use some sort of voice compression. GSM uses a voice coder to model the tone and noise generation in the human throat and the acoustic filtering of the mouth and tongue. These characteristics are used to produce coefficients sent via the TCH.

The speech coder is based on a residually excited linear predictive coder, enhanced by including a long term predictor (LTP). The LTP improves speech quality by removing the structure from vowel sounds prior to coding the residual data. The coder outputs 260 bits for each 20 ms block of speech. This process yields a 13 kbps rate. Output bits are ordered, depending on their importance, into groups of 182 and 78 bits. The most important 182 bits are subdivided further, with the 50 important bits separated out. Figure 5 shows the speech coder operation.

The nature of the GSM air interface means that some bit errors will be introduced. The bits are handled in such a way that errors are more likely where they matter least. The sound quality is affected more by the most important coefficient bits than the least important. The least important or type 2 bits have no error correction or detection. The most important 1a bits have error detection cyclic redundancy check bits added. Both type 1a and the important 1b bits have convolutional error correction bits added.

It is interesting to think of GSM bits as aircraft passengers. There are three classes: 1a, 1b and 2. The most important bits get first-class treatment and are surrounded by a lot of error correction. In the case of 1a bits, error detection is present as well. These extra bits take up space in the TCH bursts. The second-class, type 2 bits, take up the least space on the TCH, just like tourist-class passengers on an airplane.

CONCLUSION

The GSM system is unique in many ways. One of the primary benefits is the system's wide acceptance. Users can make or receive calls with their mobiles from Italy to Ireland and in most Asian countries. With the frequency extensions to PCS1900 and DCS1800, this technology is becoming a worldwide standard for digital communications. Other benefits of GSM are a common system and increased voice security, and data services available via ISDN interconnects.
COPYRIGHT 1996 Horizon House Publications, Inc.
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Title Annotation:Global System for Mobile communication
Author:Harris, John
Publication:Microwave Journal
Date:Oct 1, 1996
Words:1896
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