Access Methods in GSM

Transcription

1 TDMA Methods, page 1 Access Methods in GSM 1. Fundamentals of Multiple Access Frequency division multiple access FDMA Time division multiple access TDMA Code division multiple access CDMA 2. TDMA in GSM RF data TDMA structure in GSM, frames and multiframes TDMA timers 3. Burst Structures Information bits Training sequence Bit synchronization Compensation of multipath reception Guard time Delay correction Burst types Frequency-correction burst Synchronization burst Dummy burst Access burst 4. The Concept of "Channel" in GSM Physical/logical channel Physical channels and their definition Main logical channels and their functions

2 TDMA Methods, page 2 1. Fundamentals of Multiple Access The 200 khz channel bandwidth of GSM systems seems fairly wide in comparison with that of conventional systems. This bandwidth is "divided up" using timeslots which allow one channel to be used by several subscribers (multiple access). The multiple access methods available and their characteristic features are described in the following. Frequency division multiple access FDMA For analog radio systems, the trend has always been towards a more efficient utilization of the available frequency spectrum by reducing the channel spacing. The number of radio channels obtained at a channel spacing of 12.5 khz is, of course, twice that obtained with 25 khz. However, improvements usually have to be traded off against some drawbacks: the narrower the channel spacing, the higher the required frequency accuracy. Consequently, the maximum deviation has to be reduced, which leads to poorer transmission quality due to the lower S/N ratio. Moreover, the gaps between the channels, which must be several kilohertz wide to provide a "safety margin", also reduce the available transmission bandwidth. Fig. 1: Channels in wideband and narrowband systems (fdma.dsf) Dividing the available frequency spectrum into a number of frequency channels enables several users to simultaneously access the various frequencies. This form of multiple access is called frequency division multiple access (FDMA). Consequently, all radio systems using a spectrum divided into channels are FDMA systems. Today, the technically useful limit is reached with a channel spacing of 10 to 12.5 khz. If time is considered as a third dimension, the following diagram, frequently used in GSM environments, is obtained:

3 TDMA Methods, page 3 Amp l i t u d e F r e q u e n c y T i me Ch a n n e l s N - 1 N N + 1 N + 2 Advantages of FDMA: Fig. 2: Frequency division multiple access FDMA - Simultaneous use of a given system bandwidth by many subscribers - More channels are available thanks to reduced channel spacing Disadvantages of FDMA: - Higher frequency accuracy required - Transmission quality decreases as the channel bandwidth is reduced - Better selectivity filters required - One transmitter and also one receiver is required per channel Time division multiple access TDMA With TDMA (time division multiple access) systems, the available bandwidth is divided into considerably fewer and so wider channels than in FDMA systems. It appears that each channel is simultaneously available to several subscribers but in fact each subscriber can use the whole channel only for the period of a timeslot. For the rest of the time, he has no access. This serial access by several subscribers is repeated over time.

4 TDMA Methods, page 4 Amplitude Frequency Time n-1 n n+1 n+2 Advantages of TDMA: Fig. 3: Time division multiple access TDMA - Simultaneous use of a given system bandwidth by many subscribers - Depending on the number of available timeslots, several subscribers can be served by one transmitter/receiver unit - Transmitter and receiver are not permanently on (saves battery power) - The instrument can perform other tasks in the intervals between transmission and reception of call (e.g. monitoring the field strength of neighbouring channels) - Reduced susceptibility to frequency-selective fading with large channel bandwidths Disadvantages of TDMA: - Accurate time (and frequency) synchronization of intruments required - Higher processing capacity required - Broadband modulators required Code division multiple access CDMA The advent of powerful, cost-effective signal processors meant that a less conventional multiple access method could be employed for mass communication systems. With code division multiple access CDMA, the whole system bandwidth is available to all subscribers all the time, i.e. all subscribers transmit and receive simultaneously but each subscriber uses a different code. Logic 1 is represented by a certain bit sequence, logic 0 is the inverse of this sequence. The different signals are distinguished in the receiver by cross-correlating the received sum signal, which contains the different codes of all active subscribers, with the bit sequence of the subscriber whose transmission the receiver wants to detect. UMTS (Universal Mobile Telecommunications System), the 3rd generation mobile telephone system uses this access method.

5 TDMA Methods, page 5 Amplitude Frequency Time Advantages of CDMA: Fig. 4: Code division multiple access CDMA - Simultaneous use of a given system bandwidth by many subscribers - Several signals can be received simultaneously by a single RF section - Reduced susceptibility to frequency-selective fading in the case of large channel bandwidths - More subscribers can be served - Reduced costs for radio network planning Disadvantages of CDMA: - Accurate time synchronization of subscribers required - Fast transmitter power control over a wide dynamic range required - No mass-market experience available 2. TDMA in GSM RF data In spite of the competition from other mobile telephone systems, it was possible to define common frequency bands for GSM on a worldwide basis. All nations who signed the GSM-MoU (memorandum of understanding) committed themselves to use the standardized frequency ranges when they installed their GSM system. The competition for frequencies mainly affects countries using NMT 900 (Nordic Mobile Telephone), whose frequency range coincides with the GSM P band. The TACS (Total Access Communication System) band too has some overlap with the GSM P band, and the G1 band (extended GSM 900) is completely within the TACS range. Cordless telephones to the CT1 standard also use the upper end of the GSM P band. CT1+ telephones which had been assigned a frequency range below the P band years ago to protect them against GSM are now being ousted by the G1 band.

6 TDMA Methods, page 6 GSM 900 GSM 1800 Frequency range P band G1 band Uplink 890 to 915 MHz 880 to 890 MHz 1710 to 1785 MHz (MS transmitting) Downlink (BTS transmitting) 935 to 960 MHz 925 to 935 MHz 1805 to 1880 MHz Duplex spacing 45 MHz 95 MHz Spectrum 2 x 35 MHz 2 x 75 MHz Number of channels Channel No.s 1 to to to 885 Channel spacing 200 khz Modulation GMSK with B x T = 0.3 Data transmission rate kbit/s Bit duration 3.69 µs Fig. 5: RF data for GSM 900 and GSM 1800 Depending on the resources of the network operator and the technical facilities of the mobile phone, up to 124 channels of the GSM 900 network are available in the P band (two frequencies per channel at a spacing of 45 MHz for uplink and downlink), and perhaps another 49 channels in the G1 band (also GSM 900 frequencies), and probably 374 channels in the E network (GSM 1800, duplex spacing 95 MHz). Normally, the network operators can use the frequencies (channels) assigned to them for their base stations as they choose. Each base station requires at least one channel (C0, also referred to as BCCH carrier) on which it continually sends synchronization information at full power and - depending on the expected traffic volume - additional frequency channels (traffic channels which are only used for actual calls). Why is GSM referred to as a TDMA system when it uses different frequency channels? The key point is that each frequency channel is divided into 8 timeslots. In the first channel (C0) of every base station, synchronization information is sent in timeslot 0 while the remaining 7 timeslots are used for calls (traffic) or dummy bursts so that power is always being transmitted. This is also necessary for synchronization and ensures that a telephone which is switched on in an area for the first time can find its GSM base station. To keep the hardware of the telephone as simple as possible and to ensure optimum utilization, it was decided that transmission and reception should not be simultaneous (intermittent operation of transmit and receive section). Nevertheless, this seems like duplex mode to the subscriber because the large amount of speech data transmitted in compressed form in the timeslots fills all the eight timeslots when expanded. Operation seen from the mobile:

7 TDMA Methods, page 7 RX RX RX * Duplex spacing TX TX 3 time slots * GSM900: 45 MHz GSM1800: 95 MHz Fig. 6: Transmission and reception in "duplex" mode The receive frequency and transmit frequency are generated by a single synthesizer in the mobile. The synthesizer lets the receiver "listen" to the base station in a timeslot and then three timeslots later lets the transmitter transmit (now the base station should listen). This sequence is repeated after eight timeslots (a frame consists of eight timeslots). The free slots in between are used, say, for field-strength measurements on the C0 frequencies of neighbouring base stations. The measured field strength is the criterion for deciding whether an ongoing call is handed over to another base station or not. To simplify timeslot counting, the timeslots of the base station and of the mobile are counted in the same way. For instance, if the downlink (base station sends) is assigned to timeslot 0, the mobile station must receive at the same time. The uplink (mobile station sends) for the phone is also assigned to timeslot 0, but because of the offset of 3 timeslots described above, the uplink timeslot is delayed by 1.73 ms (for timing see next section). TDMA structure in GSM, frames and multiframes GSM timing is based on 48 periods of a 13 MHz signal (approx ms, which corresponds to the transmission time of one bit). A certain number of these bits is combined to form a burst and is transmitted in one timeslot. Eight timeslots form a frame. A certain number of these frames is combined to give a multiframe. Since there are several types of multiframe, certain numbers are grouped together to form standard hyperframes and superframes. Detailed description:

8 TDMA Methods, page 8 Time slot (0,577 msec) number of bits: T=Tailbit, F=Flag, TS=Training Sequence, Guard=guard priod T Information F TS F Information T Guard ,25 TDMA frame (4,62 msec) Fig. 7: Timeslot in TDMA frame Two 57-bit information blocks, i.e. 114 bits, are transmitted in every timeslot. A TDMA frame contains eight of these timeslots and since a call can only use one timeslot per frame, the raw data transmission rate (coded speech or data signal plus error correction code) is about 114 bits/4.62 ms = 24.7 kbit/s. A known bit sequence, referred to as the training sequence, is transmitted between the information blocks. It is used for synchronization to the bit stream and for assessing the current transmission characteristics of the radio channel. The training sequence makes it possible to set channel equalizers in the receiver to considerably improve decoding. Since the transmission conditions in the radio channel may change rapidly, the training sequence is sent between the information blocks and transmitted with each burst. The guard periode has been inserted to prevent consecutive bursts time overlapping if signals are not fully time compensated (see further down). It is also required for ramping up resp. ramping down the transmitter (power ramping). It is certainly unusual to specify a guard time as a fraction of a bit transmission period (8.25), but interpreting this information in terms of time (8.25 bit periods x 3.69 µs) has proved to be useful.

9 TDMA Methods, page 9 This "odd" number is obviously due to the maximum signal delays resulting from the cell size planned in the GSM definition phase. The flag bit indicates whether the transmitted bits are normal information bits or whether some of the transmitted bits are used for signalling, e.g. when there is an urgent need to perform a handover. Frame numbers are also used on the control channel but the frames of control channels and traffic channels are numbered separately. This is necessary so that certain measurements can be performed, e.g. while a call is in progress. 26 TDMA frames are combined to form a 26-frame multiframe for all timeslots containing a traffic channel (i.e. voice and data signals and a small amount of signalling data to keep the link up). All time slots that are exclusively for signalling are counted using 51-frame multiframes. This method of counting conceals the fact that physical channels (specified in terms of frequency channels and timeslots) and logical channels (e.g. traffic channels, TCHs) are handled in a different way. Thinking in terms of logical GSM channels is an approach that has turned out to be useful. Logical channels have an almost parallel existence and must, of course, be mapped serially onto the physical channels by the hardware. The procedure is a bit confusing for a user who is not familiar with GSM but the approach has proved to be very useful for network operation because some logical channels are only transmitted when required and can be moved from one physical channel to another according to the traffic volume. Only very few logical channels have always to be associated with the same physical channels to allow the mobile to synchronize to an unknown GSM base station. If the first synchronization attempt succeeds, the information in a few fixed signalling bursts will be sufficient to decode the whole data stream. The TDMA structure provides for hyperframes and superframes above the 26-frame and 51-frame multiframes. The hyperframes and superframes can be used for both types. These hyperframes and superframes are used, for instance, for encryption algorithms. The following structure is obtained: 26-frame multiframe = 26 frames for timeslots containing traffic channels "in parallel" with 51-frame multiframe = 51 frames for timeslots containing control channels are "combined" in super frames: = 51 x 26 frames (least common multiple, makes a "combination" possible) = 1326 frames = 6.12 s and hyper frames: = 2,715,648 frames = 3 hours 29 min. 3.5 s = 2048 super frames

10 TDMA Methods, page 10 TDMA timers The frames within the hyperframe are continuously counted so that the TDMA clock restarts after approx. 3.5 hours. The frame number, therefore, represents the basic time unit for the GSM system. As it would be too cumbersome to use just seconds to measure time, larger units like minutes and hours are defined; the same thing happens with frame numbers which are expressed in larger units the various types of timer. These timers are defined as follows: T1: = FN div (26 x 51) value range: 0 to 2047 T2: = FN mod 26 value range: 0 to 25 T3: = FN mod 51 value range: 0 to 50 FN (Frame Number) value range: 0 to FN max = 51 x 26 x Fig. 9: TDMA timers The original frame number can be calculated from the three counter readings. For certain signalling tasks, not all the counters are needed. In other words, this means that some signalling procedures can be performed without all the counter readings being known. The division without remainder function "div" is used to calculate T1; div gives the whole number obtained when FN is divided by 1326 = 26 x 51. T2 and T3 are equal to the FN mod 26 and FN mod 51 respectively, the timers are repeatedly counting from 0 to 25 and 0 to Burst Structures Information is exchanged between the base station and mobile station in the timeslots. In each slot a certain amount of information, i.e. a burst, can be transmitted. Depending on the task to be performed, different types of burst can be used, although the most frequently used type is the "normal burst" shown in Fig. 10. It is used for signalling as well as for speech and data transmission.

11 1 time slot(0,577 msec) TDMA Methods, page 11 number of bits: T Information F TS F Information T Guard ,25 T=Tailbit, F=Flag, TS=Training Sequence, Guard=guard period Fig. 10: Normal burst Each part of the burst serves a specific purpose which will be described below: Information bits The normal burst is able to transmit a total of 114 bits so that a maximum data rate of approx kbit/s is obtained by 2nd generation GSM. The transmission rate within the system can only be increased when more than one timeslot is used (General Packet Radio Service GPRS, High-Speed Circuit-Switched Data HSCSD) or another modulation method (8PSK modulation with EDGE, Enhanced Data Rate for GSM Evolution). The bit rate in the control channels is much lower, i.e. the above transmission rate is only attained by the mobile if a traffic channel has been established. In this case the mobile and the base station use a control channel in addition to the speech and data channel, which uses up capacity and carries information on reception quality and power ramping. Fig. 11 shows the bit rates for the various channels: Useful data: Error protection: Total: Traffic channel: 22.8 kbit/s - Voice (full-rate): 13.0 kbit/s 9.8 kbit/s - Data: 2.4 kbit/s 20.4 kbit/s 4.8 kbit/s 18.0 kbit/s 9.6 kbit/s 13.2 kbit/s 14.4 kbit 8.4 kbit/s Control channel: 0.95 kbit/s Idle frame: 0.95 kbit/s Total: 24.7 kbit/s Fig. 11: Transmission bit rates

12 TDMA Methods, page 12 Training sequence In the middle of the normal burst, a 26-bit training sequence is sent, the bit sequence being known to the receiver. There are eight different sequences which are referred to as the training sequence code (TSC). The eight sequences must be stored in all receivers and at the beginning of a transmission the base station decides on the TSC to be used. The training sequence performs two main tasks: bit synchronization and estimating the channel impulse response (instantaneous response of the radio channel). Using this estimate, the channel equalizers in the receivers can be set for optimum data stream decoding. Bit synchronization Data are transmitted via the air interface in asynchronous mode. The receiver must be able to regenerate the bit clock from the data stream and needs features in the data stream to enable it to identify information units (block synchronization). Conventional data radio therefore uses data telegrams that start with a sequence so that the receiver can regenerate the bit clock. A predefined bit word tells the receiver when the actual information (block synchronization) starts. A receiver synchronized in this way is able to decode the data stream online. The GSM training sequence is used for fine bit synchronization and for block synchronization. Since the training sequence is not sent at the beginning of a burst, the received data stream must be buffered in the receiver and decoded later on. Synchronization itself makes use of cross-correlation, i.e. the stored data stream is compared bit-by-bit with the expected training sequence. When the position of the training sequence is known, the timing of the information bits is also known and fine tuning of the bit-clock is performed. A burst that does not contain the expected training sequence cannot be synchronized and decoded. Compensation of multipath reception The signal from the transmitter (in the Fig. below the base station is transmitting to the mobile, but the same explanation still applies, if the mobile is transmitting) arrives at the receiver not only along the direct path but also on various other paths as a result of reflections and diffraction caused by obstacles in the signal path. The propagation conditions on these additional paths are different to those on the direct path, for instance: - longer travel time because of longer path - various receive levels (depending on reflections) - different Doppler shifts (possibly due to different relative velocities)

13 TDMA Methods, page 13 BTS MS Fig. 12: Multipath reception due to reflections and diffraction Because of the different travel times, the signals have different phases at the receiving antenna. Depending on their phase, components may compensate - i.e. the total level goes to zero - or reinforce so that a strong signal is received for a short period of time. RF level variations are random, fading may be up to 40 db. In addition to RF level fading, there is another annoying effect which, without compensation, would make correct signal decoding rather difficult. Because of the extra length of the indirect paths, the signals at the receiving antenna not only have different phases but the modulated information arrives at different times. The sum of all the channel responses to a single pulse is called the CIR (Channel Impulse Response). If the indirect path is just one kilometer longer, the GSM echo bit reaches the receiver later than the directly received bit and so interferes with the next bit to be received. This intersymbol interference may affect several consecutive bits. With delays greater than 15 µs, identification of the received signal components and echoes becomes more and more difficult. This problem can also be solved with training sequences. The echoes on the indirect paths also contain the echoes of the training sequence. The cross-correlation method used to find the original training sequence may also be used to find the training sequence echoes as well as their delay and attenuation. With the aid of this information, the received signal can be corrected by a channel equalizer.

14 TDMA Methods, page 14 Sent pulse Received signal t T = propagation time D T Bit = bit duration Fig. 13: Channel impulse response Guard time Transmission in each time slot is terminated with a guard time of 8.25 bit periods (8.25 x 3.69 µs 30 µs) during which no information bits can be sent. During this time, the burst level must be reduced by up to 70 db to avoid the next timeslot being affected. The "owner" of the subsequent timeslot uses this time to increase his transmitter power to nominal. This means that the guard time is used twice for power ramping (the transmitter power must be increased and reduced within narrow tolerances).

15 TDMA Methods, page 15 Fig. 14: Guard time at the end of each timeslot Delay correction The integrity of a timeslot depends on subscribers transmitting only during the time assigned to them and otherwise keeping their transmitters off the air. This is only possible when all subscribers are in strict sync. For practical reasons, the clock signal is generated by the base station and all the mobiles synchronize to it. There will be no problems on the downlink, i.e. when (one) base station sends to (several) mobiles. Mutual interference may, however, occur in the uplink, where up to eight subscribers must share one radio channel, if the mobiles are not accurately synchronized to the timeslots. Where can problems with synchronization occur? Under the given conditions, radio signals propagate at almost the speed of light. Even at a speed of km/s they still take about 33 µs to cover a distance of 10 km. A mobile station 10 km away from the base station synchronizes to the received signal which has already travelled for about 33 µs before it arrives at the mobile. If the mobile station now transmits back to the base station (without any delay correction), this signal will require the same travel time. The base station, therefore, receives a signal which is delayed by about 66 µs in its own time frame. Considering that a timeslot is 577 µs wide (including the guard time of about 30 µs) and cells have a max. radius of. 35 km (limited by the delay correction factor described in the following) it is obvious that the neighbouring timeslot will be compromised if there is no compensation.

16 TDMA Methods, page 16 Effect of signal propagation time Propagation time ~33 µsec to phone Propagation time ~33 µsec to base station Mobile Station Base Station The above example with the numbers: Fig. 15: Signal delay and its effect Signal travel time over 10 km is 33 µs (distance / speed of light) The (time) sync signals from the base station require this time to arrive at the antenna of the mobile station, i.e. mobile synchronization is delayed by 33 µs. From the point of view of the mobile station, the mobile sends a burst to the base station with correct timing (without delay correction) but from the point of view of the base station the signal is sent 33 µs too late. The signal covers the same distance on its way to the base station antenna and so is delayed by another 33 µs. Relative to the base station s time frame, the received signal is delayed by 66 µs. The signal is not sent in the assigned timeslot and compromises neighbouring timeslots. The guard time at the end of each burst is only about 30 µs and is certainly not long enough in the above example (apart from it being required for power ramping of the transmitters). The greater the distance between mobile station and base station, the greater the effect of the signal delay. The only way to solve this problem is to make the mobile send the burst earlier. To do so, the base station has to measure the signal delays and send the appropriate correction factor to the mobile. A special burst type (access burst, see further down) is used for this purpose. This burst has a much wider guard time and is used by the mobile to attach to the base station when it wants to establish a connection.

17 TDMA Methods, page 17 Because of the longer guard time of this burst, interference with neighbouring timeslots is prevented. The base station can determine a correction factor (referred to as TA, Timing Advance in GSM, it represents the number of bit periods) and send it to the mobile station. The mobile station advances the sending time of its burst accordingly and the signal from the mobile station arrives at the base station in sync. If the mobile station is moving while a call is in progress, the distance between the mobile station and the base station generally changes and so also the signal delay. For this reason, the timing advance is checked about twice every second while a call is going on. A few technical limits of the GSM system (GSM 900 and GSM 1800) can be derived from the timing advance specifications: The timing advance is transmitted as a 6-bit word. With 6 bits, the numbers 0 to 63 can be represented. Increasing this number by one means that the mobile station has to advance transmission by one bit duration, i.e. by 3.69 µs. This means a delay of 63 x 3.69 µs = approx µs can be corrected, which corresponds to a distance of almost 70 km. Therefore, the mobile station cannot be more then about 35 km away from the base station. This also means that the distance between the base stations in the network cannot be more than 70 km. The timing advance also explains why, for instance, a mobile on a ship near to shore can find a GSM network (propagation conditions and coverage on water are optimal) but cannot register to the network because the base station is more than 35 km away. These extreme ambient conditions make it clear that the timing advance may also be a criterion for call handover or disconnection. On shore, this scenario is only of theoretical interest because, due to the traffic volume encountered, none of the base stations has to cover a cell radius of 35 km. The timing advance can be used to determine the distance between the mobile and the base station because each increment in the TA (1 bit duration = 3.69 µs) corresponds to a signal travel time of 1.1 km. If the base station uses an omnidirectional antenna, a valid timing advance indicates that the subscriber is on a 550 m wide circle centered on the base station. On the other hand, a valid timing advance is only available when a link has been set up, i.e. only when a call is in progress or during a location update. At any other time, the timing advance may be invalid because the mobile is not transmitting. Burst types In addition to the normal burst described previously, which is used in most of the cases, other burst types are available for special purposes (see Fig. below). All these burst are exactly 1 timeslot (577 µs) in duration.

18 TDMA Methods, page 18 Normal Burst T 3 F 26 Bits F 57 data bits 1 57 data bits Training Sequence 1 T 3 G 8,25 Frequency Correction Burst T fixed Bits T 3 G 8,25 Synchronisation Burst T 3 64 Bits 39 data bits 39 data bits Extended Training Sequence T 3 G 8,25 Dummy Burst T fixed Bits T 3 G 8,25 Access Burst T 8 41 Bits Training Sequence 36 data bits T 3 G 68,25 Abbreviations : T=Tail Bit (3 Bits/ 8 Bits in leading part of Access Burst F=Flag (1 Bit) G=Guard Period (8,25 Bits/ 68,25 Bits in Access Burst) Fig. 16: Burst types (burst_ty.dsf) Frequency correction burst The 142 "fixed bits" of the frequency correction burst are all set to logic 0. GMSK (Gaussian minimum shift keying), the type of modulation used for GSM, produces a stationary carrier-frequency deviation of approx khz with this burst. This burst is sent by the base station only and used by the mobiles for initial synchronization to the carrier frequency and for compensating any Doppler shift caused by a mobile moving at speed. It is sent by the base station every 10 frames (i.e. approx. every 46 ms) but only in timeslot 0 and only on carrier C0 (sometimes referred to as the BCCH carrier).

19 TDMA Methods, page 19 Synchronization burst The synchronization burst is also transmitted in timeslot 0 in the frame after the frequency correction burst. This burst, too, is only sent by the base station on the C0 carrier. The considerably longer training sequence is one significant difference between the synchronization burst and the normal burst. Like the 26-bit type, this training sequence is also used for bit synchronization but, because it is longer, synchronization is more accurate. The two 39-bit data blocks contain the timers T1, T2 and T3 in coded form and also a base station identification code (incl. the training sequence No.). When this "GSM time" is received, the mobile station is in sync with the base station. Dummy burst A base station must continuously transmit at nominal power on its C0 carrier in all time slots as this carrier is used by the mobile stations to find the nearest base station and to evaluate reception quality. If normal bursts are not available for transmission in a timeslot, dummy bursts are sent by the base station instead because an unmodulated carrier cannot be transmitted. These bursts, too, are used only by the base station on the C0 carrier but may be sent in any of the timeslots. Access burst As already pointed out, the access burst is sent when the mobile station calls the base station for the first time. The base station uses this burst for a delay measurement, determines the associated timing advance and informs the mobile station accordingly. This means that delay correction is performed for the next call from the mobile which can now use a normal burst with a much shorter guard time.

20 TDMA Methods, page The Concept of Channel in GSM Every base station sends on at least one frequency in 8 timeslots. It has become common practice to refer to physical channels that are defined by frequency and timeslot. Several "types" of data are sent on these physical channels, e.g. speech, test reports, instructions, etc. For these data types the term "logical channel" is used. Logical channels are considered to be "parallel" channels which are serially mapped by the hardware onto the physical channel (which must not always be the same; frequency and/or timeslot may be changed as required). For instance, the FCCH (Frequency Correction Channel), which is used for correcting the frequency of the mobile station, the SCH (Synchronization Channel) with the initial information on the base station, the BCCH (Broadcast Control Channel) acting as a kind of "notice board" with further information, and many other logical channels are transmitted in timeslot 0 (first time slot) of carrier C0. Some of these logical channels are only transmitted in specific contexts and their position in the physical data stream is not always the same. Physical channels and their definition: - ARFCN + TN The number of the carrier frequency channel (absolute radio frequency channel number) together with the timeslot number defines the simplest version of a physical channel. - several ARFCNs + TN + HSN + MAIO When frequency hopping is activated, the hopping sequence number (HSN) and the mobile allocation index offset value MAIO must also be specified. - ARFCN + TN + SSN If the half-rate speech codec is used for communication, two calls can share a fullrate channel. The subsequence number SSN is used to distinguish the calls. It indicates whether the half-rate link uses even or odd frame numbers. - several ARFCNs + TN + HSN + MAIO + SSN same as above, but the frequency of a half-rate channel is also assigned and frequency hopping is activated at the same time.

21 TDMA Methods, page 21 Main logical channels and their functions Traffic channels Traffic channels are used to carry digitized speech or other user data. They are normally classified according to transmission speed. For voice transmission, the following is defined: - Traffic channel using full-rate data transmission, a full-rate channel operating at 22.8 kbit/s. 13 kbit/s are used for speech transmission, the rest is basically used for error protection. - Traffic channel using half-rate transmission, a half-rate channel operating at 11.4 kbit/s. 6.5 kbit/s are available for speech transmission. The subscriber can also choose between half-rate and full-rate transmission for data. Available bit rates: Designation Explanation TCH/FS Full-rate speech traffic channel TCH/HS Half-rate speech traffic channel TCH/F kbit/s full-rate data traffic channel TCH/F kbit/s full-rate data traffic channel TCH/F kbit/s full-rate data traffic channel TCH/H kbit/s half-rate data traffic channel TCH/F kbit/s full-rate data traffic channel TCH/H kbit/s half-rate data traffic channel Fig. 17: GSM traffic channels and their bit rates Control channels Even if no call is in progress (traffic channel), the resources required for signalling are considerable. Information has to be continuously exchanged via the air interface (e.g. location update). The control channels allow the mobile station to receive information from the base station any time or to send information to the base station. There are three main groups of control channels:

22 TDMA Methods, page 22 Broadcast Channels This channel group is used by the base station to send relevant information to all active mobile stations (unidirectional transmission to mobile) - Frequency Correction Channel FCCH for the frequency synchronization already described. It is transmitted in frames 0, 10, 20, 30, 40 and 50 within the 51-frame multiframe - Synchronization CHannel SCH with "GSM time" and a code for base station identification. This channel is sent in the frame directly after the FCCH. - Broadcast Control Channel BCCH with information on the radio channel configuration of the home cell and of neighbouring cells, on the location area code for a location update and on the organization of the common control channels CCCH (described below). This channel also contains other important signalling information. The BCCH comprises four normal bursts which are sent in frames 2 to 5 of the 51-frame multiframe. - Cell Broadcast CHannel CBCH This is a kind of open information channel and comparable to teletext in TV broadcasting. Common Control Channels CCCH This group is used for information exchange between base station and mobile station (bidirectional) - mainly for access management - Paging CHannel PCH, used by the base station for paging mobile stations, e.g. for a mobileterminated call (a call is made to the mobile station). - Random Access CHannel RACH used by the mobile for a first call to the base station to request an exclusive control channel. In the case of a mobile-originated call (mobile station calls a subscriber), the mobile sends an access burst on this channel. - Access Grant CHannel AGCH is practically the response to the RACH. After having received the access burst, the base station tells the mobile the traffic channel. - Notification CHannel NCH enables the base station to notify incoming group calls. Dedicated Control Channels A bidirectional dedicated control channel performs signalling tasks independently or assigned to a traffic channel. - Slow Associated Control CHannel SACCH, a slow, dedicated control channel which is coupled to a traffic channel and used, for instance, for power control, setting the timing advance and for test reports (receive field strength and quality). The SACCH uses frame 12 of the 26-frame multiframe and is 4 bursts long. This means that it is sent in 4 consecutive 26-frame multiframes.

23 TDMA Methods, page 23 - Fast Associated Control CHannel FACCH This fast, dedicated control channel is coupled to a traffic channel and performs signalling tasks that cannot be postponed. Example: preparing a handover. This channel has to notify the "stealing" of bits for the FACCH via the two flags of the normal burst (traffic transmission). For this reason, the two flags for the normal burst immediately before and after the training sequence are called "stealing flags". The FACCH therefore "steals" transmission capacity from the traffic channel. - Stand-alone Dedicated Control CHannel SDCCH This independent control channel is used for exchanging information between the base station and the mobile station when no call is in progress. Example: location update, authentication and link setup up to the point when a call goes through.

24 TDMA Methods, page 24 Mapping of logical channels a) Traffic Channels Multiframe Structure (26 MF) TT TTTTTTTTTTATTTTTTTTTTTT I Mapping for Traffic Channels T = Traffic A = SACCH I = Idle b) Control Channels Multiframe Structure (51 MF) TS0 TS7TS0 TS7 ~ 4.62 ms F S B B B t Channel C0 (down link) FSBBBBCCCCFSCCCCCCCCFSCC CC I FSBBBBCCCCFSC 51 - Multiframe ~ 235 ms Channel C0 Time Slot 0 (down link) F = FCCH S = SCH B = BCCH C = CCCH I = Idle

25 TDMA Methods, page 25 CHANNEL MAPPING (1) FCH +SCH + BCCH + CCCH DOWNLINK F S B C F S C C F S C C F S C C F S C C I RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR UPLINK SDCCH/8(0..7) + SACCH/C8(0..7) DOWNLINK D0 D1 D2 D3 D4 D5 D6 D7 A0 A1 A2 A D0 D1 D2 D3 D4 D5 D6 D7 A4 A5 A6 A A5 A6 A7 D0 D1 D2 D3 D4 D5 D6 D7 A A1 A2 A D0 D1 D2 D3 D4 D5 D6 D7 A4 UPLINK CHANNEL MAPPING (2) FCCH +SCH + BCCH + CCCH + SDCCH/4(0..3) + SACCH/4(0..3) DOWNLINK F S B C F S C C F S D0 D1 F S D2 D3 F S A0 A1 I F S B C F S C C F S D0 D1 F S D2 D3 F S A2 A3 I D3 R R A2 A3 R R R R R R R R R R R R R R R R R R R R R R R D0 D1 R R D2 D3 R R A0 A1 R R R R R R R R R R R R R R R R R R R R R R R D0 D1 R R D2 UPLINK F = FCCH S = SCH B = BCCH C = CCCH D = SDCCH A = SACCH I = Idle = Frequency Correction Channel = Synchronization Channel = Broadcast Control Channel = Common Control Channel (= PCH + RACH + AGCH) = Stand-alone Dedicated Control Channel = Slow Associated Control Channel PCH = Paging Channel RACH = Random Access Channel AGCH = Access Grant Channel

26 TDMA Methods, page 26 List of Abbreviations Used: BSIC BTS BxT CDMA CIR DCS1800 FCB FDMA FN GMSK GSM GSM900 GSM1800 MS NMT RX SB S/N TA TACS TDMA TSC TX Base station identification code Base transceiver station Bandwidth/bit duration product Code division multiple access Channel impulse response Digital Communication System 1800 (new: GSM1800) Frequency correction burst Frequency division multiple access Frame number Gaussian minimum shift keying Global system for mobile communications GSM at 900 Mhz GSM at 1800 MHz Mobile station Nordic mobile telephone Receiver Synchronization burst Signal/noise ratio Timing advance Total access communication system Time division multiple access Training sequence code Transmitter