5.2 Time slots and bursts

3GPP45.001General descriptionGSM/EDGE Physical layer on the radio pathRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

The time slot is a time interval of  576,9 µs (15/26 ms), that is 156,25 symbol^[1] duration when using the normal symbol period (see 3GPP TS 45.010) or 187,5 symbol duration when using the reduced symbol period (see 3GPP TS 45.010), and its physical content is called a burst. Five different types of bursts exist in the system. A diagram of these bursts appears in figure 1.

– normal burst (NB): this burst is used to carry information on traffic and control channels, except for RACH, EC-RACH, PRACH, and CPRACH. It contains 116 encrypted symbols and includes a guard time of 8,25 symbol duration ( 30,46 µs);

– frequency correction burst (FB): this burst is used for frequency synchronization of the mobile. It is equivalent to an unmodulated carrier, shifted in frequency, with the same guard time as the normal burst. It is broadcasted together with the BCCH. The repetition of FBs is also named frequency correction channel (FCCH). For Compact, FB is broadcast together with the CPBCCH and the repetition of FBs is also named Compact frequency correction channel (CFCCH). In CTS, the frequency correction burst is broadcast in the CTSBCH-FB channel;

– synchronization burst (SB): this burst is used for time synchronization of the mobile. It contains a long training sequence and carries the information of the TDMA frame number (FN) and base station identity code (BSIC, see 3GPP TR 23.003). An exception applies to a PEO capable MS where the synchronization burst contains the 6 most significant bits of the 9 bit base station identity field (BSIC, see 3GPP TR 23.003). The 3 least significant bits of the 9 bit BSIC are obtained from messages sent over common control channels (see 3GPP TS 44.018). It is broadcast together with the frequency correction burst. The repetition of synchronization bursts is also named synchronization channel (SCH). For Compact, the repetition of synchronization bursts is also named Compact synchronization channel (CSCH). For EC-GSM-IoT the repetition of synchronization bursts is also referred to as Extended Coverage synchronization channel (EC-SCH). In this case the synchronization burst carries additional information apart from the reduced TDMA frame number per quarter hyperframe and BSIC (see 3GPP TS 45.002). In CTS, the synchronization burst is used for the CTSBCH-SB and the CTSARCH, and it carries different information depending on the channel using it;

– access burst (AB): this burst is used for random access and is characterized by a longer guard period (68,25 bit duration or 252 µs) to cater for burst transmission from a mobile which does not know the timing advance at the first access (or after handover).This allows for a distance of 35 km. In exceptional cases of cell radii larger than 35 km, some possible measures are described in 3GPP TR 43.030. The access burst is used in the (P)RACH, CPRACH, MPRACH and EC-RACH CC1 to CC4, after handover, on the uplink of a channel used for a voice group call in order to request the use of that uplink, as well as on the uplink of the PTCCH to allow estimation of the timing advance for MS in packet transfer mode;

– extended access burst (Extended AB): this burst is used for random access when executing the Multilateration Timing Advance using the Extended Access Burst method. This allows a mobile station which has made a successful random access attempt by using an access burst on (EC-)RACH containing 11 information bits, and has received a timing advance value on the access grant channel, i.e. on (EC-)AGCH, to make a subsequent access attempt by using the Extended AB on (EC-)RACH, except EC-RACH for CC2 to CC5, containing 30 information bits, as soon as possible after receiving the access grant while still using the principles for (EC-)RACH transmission opportunity selection described in 3GPP TS 44.018. The Extended AB has a burst size of 148 symbols, contains 30 information bits wherein 11 bits are sent using 36 encrypted bits and 19 bits are sent using 57 encrypted bits as per Figure 1a and using the commanded timing advance. Information sent in the access burst and Extended AB and the determined refined timing advance from both access attempts is used by the network within the Multilateration Timing Advance procedure, see 3GPP TS 43.059. The structure of the Extended AB is depicted in figure 1a. It is common with the access burst in figure 1 for the first 88 symbols, which are followed by another block of 57 encrypted symbols and another 3 tail symbols (TB), see 3GPP TS 45.002. Channel encoding for data transmitted in the Extended AB is specified in 3GPP TS 45.003 for (EC-)RACH carrying 30 information bits;

– extended synchronization access burst (ESAB): this burst is used for random access of CC5 users and is, as depicted in Figure 1b, characterized by a longer synchronization sequence (140 bits) and more encrypted data bits (102 bits) compared to access burst to cater for the required coverage extension;

– extended dual slot access burst (EDAB): this sequence of two bursts sent in consecutive timeslots in the same TDMA frame is used for random access channel request for uplink coverage class CC5 in EC operation applying the 2 TS EC-RACH format. The first burst is transmitted with equal size to the active part of the normal burst, with a timeslot length of 157 normal symbol periods on timeslots with TN = 0 and 4, and 156 normal symbol periods on timeslots with TN = 2 and 6, and the second burst transmitted in the consecutive TN in the same TDMA frame with equal size to the active part of the access burst with a timeslot length of 156 normal symbol periods. The structure of the extended dual slot AB is depicted in figure 1c and specified in 3GPP TS 45.002. Channel encoding for data transmitted in the Segmented AB is specified in 3GPP TS 45.003 for EC-RACH/132 carrying 11 information bits;

– higher symbol rate burst (HB): this burst is used to carry information on full rate packet data traffic channels using higher symbol rate (see 3GPP TS 45.004). It contains 138 encrypted symbols and includes a guard time of 10,5 reduced symbol periods (see 3GPP TS 45.010).

Figure 1: Time frames time slots and bursts

Figure 1a: Burst structure of Extended AB sent in 1 timeslot

Figure 1b: Burst structure of ESAB in 2 time slots

Figure 1c: Burst structure of EDAB sent in 2 consecutive timeslots