5 Multiple access and timeslot structure

05.013GPPGeneral descriptionPhysical layer on the radio pathTS

The access scheme is Time Division Multiple Access (TDMA) with eight basic physical channels per carrier. The carrier separation is 200 kHz. A physical channel is therefore defined as a sequence of TDMA frames, a time slot number (modulo 8) and a frequency hopping sequence.

The basic radio resource is a time slot lasting  576,9 µs (15/26 ms) and transmitting information at a modulation rate of  270.833 kbit/s (1 625/6 kbit/s). This means that the time slot duration, including guard time, is 156,25 bit duration.

We shall describe successively the time frame structures, the time slot structures and the channel organization. The appropriate specifications will be found in 3GPP TS 05.02 (multiplexing and multiple access).

5.1 Hyperframes, superframes and multiframes

A diagrammatic representation of all the time frame structures is in figure 1. The longest recurrent time period of the structure is called hyperframe and has a duration of 3 h 28 mn 53 s 760 ms (or 12 533,76 s). The TDMA frames are numbered modulo this hyperframe (TDMA frame number, or FN, from 0 to 2 715 647). This long period is needed to support cryptographic mechanisms defined in 3GPP TS 03.20.

One hyperframe is subdivided in 2 048 superframes which have a duration of 6,12 seconds. The superframe is the least common multiple of the time frame structures. The superframe is itself subdivided in multiframes; four types of multiframes exist in the system:

‑ a 26‑ multiframe (51 per superframe) with a duration of 120 ms, comprising 26 TDMA frames. This multiframe is used to carry TCH (and SACCH/T) and FACCH;

‑ a 51‑ multiframe (26 per superframe) with a duration of 235,4 ms (3 060/13 ms), comprising 51 TDMA frames. This multiframe is used to carry BCCH, CCCH (NCH, AGCH, PCH and RACH) and SDCCH (and SACCH/C), or PBCCH and PCCCH.

‑ a 52-multiframe (25,5 per superframe) with a duration of 240 ms, comprising 52 TDMA frames. This multiframe is used to carry PBCCH, PCCCH (PNCH, PAGCH, PPCH and PRACH), PACCH, PDTCH, and PTCCH. The 52-multiframe is not shown in Figure 1, but can be seen as two 26-multiframes, with TDMA frames numbered from 0 to 51. For Compact, this 52-multiframe (51 per superframe) is used to carry CFCCH, CSCH, CPBCCH, CPCCCH (CPNCH, CPAGCH, CPPCH, and CPRACH), PACCH, PDTCH, and PTCCH.

‑ a 52-multiframe (25.5 per superframe) for CTS, with a duration of 240 ms, comprising 52 TDMA frames. This multiframe is used to carry CTSCCH (CTSBCH, CTSPCH, CTSARCH and CTSAGCH). The 52-multiframe for CTS is shown in Figure 2b.

A TDMA frame, comprising eight time slots has a duration of  4,62 (60/13) ms.

5.2 Time slots and bursts

The time slot is a time interval of  576,9 µs (15/26 ms), that is 156,25 symbol[1] duration, and its physical content is called a burst. Four 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, PRACH, and CPRACH. It contains 116 encrypted symbol 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 broadcast 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 TS 03.03). 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). 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 TS 03.30. The access burst is used in the (P)RACH and CPRACH, 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.

Figure 1: Time frames time slots and bursts

5.3 Channel organization

The channel organization for the traffic channels (TCH), FACCHs and SACCH/T uses the 26‑frame multiframe. It is organized as described in figure 2, where only one time slot per TDMA frame is considered.

Figure 2: Traffic channel organization

The FACCH is transmitted by pre-empting half or all of the information bits of the bursts of the TCH to which it is associated (see 3GPP TS 05.03).

The channel organization for the control channels (except FACCHs and SACCH/T) uses the 51‑frame multiframe. It is organized in the downlink and uplink as described in figure 3.

The channel organization for packet data channels uses the 52- multiframe. Full rate packet data channels are organized as described in figure 2a1. Half rate packet data channels can be organized as described in figure 2a2.

52 TDMA Frames

X = Idle frame
B0 – B11 = Radio blocks

Figure 2a1: 52- multiframe for PDCH/Fs

Figure 2a2: 52- multiframe for PDCH/Hs

The channel organization for CTS control channels uses the 52-multiframe. It is organized as described in figure 2b.

52 TDMA Frames

A : TDMA frame for CTSARCH

B : TDMA frame for CTSBCH

P : TDMA frame for CTSPCH

G : TDMA frame for CTSAGCH

X : Idle frame

Figure 2b: 52-multiframe for CTS