Radio Physical Layer [Under GSM]
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Radio interface - 5.01, 5.02 «
Before we take a look at Abis interface, let us have a brief look at Radio Physical Layer - the modulation and access techniques GSM use, how does it organise in terms of channels etc.
For modulation GSM use GMSK (refer GMSK article).
GSM use TDMA with FDMA as an access method. Multiple frequencies are used with each frequency divided in (time) slots. Group of these slots is labeled as one "frame". These frames are further grouped to form multi-frame, super-frame, and hyper-frame. This frame arrangement is base physical structure. Frames are given numbers. Frame numbers repeat (1 to N frames and then start back from one). Frame numbers along with time slot number form physical identification. To make this work, MS first need to know the frame and time slot number (for at least one frame). This is done by transmitting all zero bits (or may be pre-known bit sequence) on (at least) one of the time slots on (at least) of the frames. This is sort of a marker. Zero bits mean constant modulated pattern and so help MS identify the frequency correctly. In GSM terminology, this is called "frequency correction channel" (e.g. 5 frames at regular intervals in 51 multi-frame). We will also need another channel to know the frame number. This is done having "synchronisation channel" at the same time slot as "frequency correction channel" at the following frame.
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One TS - 577 µs, 148 bits + 8.25 guard bits One Frame - 8 time slots (8 physical channels), 4.615 ms 26 Multiframe (TCH) - 26 Frames, 120 ms 51 Mutliframe (Signaling) - 51 Frames, 235.38 ms One Superframe - 26 x 51 or 51 x 26, 6 s 120 ms Hyperframe - 2048 Superframes, 3 hr 28 min 53 sec 760 ms |
In addition to above channels - as we have seen in article 1 - channels are needed for paging, broadcast, signaling, carrying transmission data etc. These are known as logical channels. The way logical channels are mapped to physical arrangement is known as channel configuration. The channel configuration is dynamic; it is broadcast in SYS_INFO 1- 4 messages over BCCH channel (physical location of which is pre-known). During active call, SYS_INFO 6 is used for the same via SACCH so that MS do not have listen to BCCH while on a call. One example channel configuration (based on DL - BCCH + CCCH + 4 SDCCH/4, 8 SDCCH/8 and UL - BCCH + CCCH + 4 SDCCH/4, 8 SDCCH/8) is shown below.
Note: SACCH 4-7 will fall in next multiframe at same location as SACCH 0-3 respectively. CCCH would mean AGCH or PCH - which blocks (4 bursts/timeslots) belong to which channel is noted in channel configuration.
Note: SACCH 1-3,4 will fall in next multiframe at same location as SACCH 5-7,0 respectively.
Below is a list of logical channels:
FCCH - Frequency correction channel (DL)
SCH - Synchronization
channel (DL)
BCCH - Broadcast common control channel (DL)
AGCH
- Access grant channel (DL)
PCH - Paging channel (DL)
CBCH -
Cell broadcast channel (DL)
SDCCH - Standalone dedicated
control channel
SACCH - Slow associated control channel
FACCH - Fast
associated control channel
RACH - Random access channel (UL)
CCCH
- Common Control channel (AGCH or PCH)
Channel configuration is required for both uplink (MS to BTS) and downlink (BTS to MS). The uplink (UL) and downlink (DL) reside on different frequencies. Both UL and DL time slots start and end at the same time,but UL is behind DL by 3 timeslots.
A word about SACCH and FACCH
SACCH consists of a "known" slot(s) that are "associated" to SDCCH or TCH. SACCH is used for signaling when SDCCH or TCH is already present (e.g. signal measurement related messaging). FACCH is not a separate physical channel but rather slots are stolen from TCH for FACCH (with the help of "stealing flag" bit of L1 header)
There is another concept specific to mobiles is "timing advance" (TA). The distance of mobile from BTS differ among mobiles. When MS is transmitting in UL, it should a little time before to take care of distance between MS and BTS. So the name timing "advance". TA is calculated by BTS and informed to MS. Obvious question would be: how BTS knows the distance to MS ? This is done intelligently: when MS want to request an access (say for location update), it would send a "small request" in common paging channel. The request is so small in terms of bits that even if MS is at longest distance from BTS, the request will be received at BTS within the same time slot. Now based on difference between time at which request received and start of time slot, BTS can calculate the delay that distance to MS caused. Refer the diagram below. This time should be twice the delay (as MS will receive after TA delay and response takes TA delay). One more thing that can be deduced here is area that BTS can serve is limited by width of time slot. For GSM, this is 35km.
Timing advance is informed to MS via Layer 1 header over SACCH (signaling messages).
GSM Frequecy bands
Popular frequency bands used in GSM are 850/1900 (most of North/South American market), 900/1800 (Europe/Asia/Australia, Brazil) MHz etc.
Note about Channel Coding and Interleaving
Channel coding refer to process by which bits are modified and added with, before transmission, so that by during reverse process, transmission error can be detected and corrected (till certain extent). GSM applies Fire code and then Convolution code for signaling data, Cyclic code and then Convolution code for transmission (i.e. voice) data. Firecode and Cyclic code add few bits to original data whereas Convolution code modifies input data. Channel decoding is reverse process of Channel coding.
Idea of Interleaving is to split the data blocks (received after channel coding) into parts and then interleave the parts in certain order for transmission. For example, the data is split in 8 parts per block like B1{ 1, 2, 3, 4, 5, 6, 7, 8 }, B2{1', 2', 3', 4', 5', 6', 7', 8' }then it may be interleaved like { ...., 5, 1', 6, 2', 7, 3', 8, 4', .... }. The intent is to avoid losing complete block if there is transmission issue with paritcular time slot so that lost bits of a block can be recovered by Channel decoding.
We will continue with A-bis interface in next article.
References: GSM book by Mouly and Pautet, GSM Networks book by Heine, Channel coding related article by Sandro Grech.
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