In Part 2 of our series on Global System for Mobile Communication (GSM) networks, I provide a brief introduction to the physical and logical channels used in GSM radio. I also describe how the base station (BTS) in the GSM network allocates downlink and uplink frequencies to a mobile station (MS).

Frequency allocation

A BTS in a GSM network usually has multiple transceivers. Each transceiver is assigned certain frequency bands for the uplink and downlink. Downlink is the communication link from the BTS to the MS, while uplink is in the reverse direction. For example, in GSM 850 systems, the frequencies ranges of 869.2–893.8 MHz and 824.2–848.8 MHz are respectively used for downlink (DL) and uplink (UL)

[1]. Each frequency range is divided into 124 pairs of DL/UL carriers spaced apart by 200 kHz.

Time slots, bursts, frame, multi-frames, super-frames, and hyper-frames

In each carrier pair, one frame is divided into eight time slots (0 to 7), with each slot having a length of 0.577 ms. This results in a time-division multiple access (TDMA) scheme. One time slot makes up one physical channel that conveys user and control signaling in bursts. The actual burst length is 0.546 ms, which is slightly shorter than the time slot duration in order for the system to tolerate timing alignment errors.

The logical GSM radio channels are based on the eight TDMA time slots (physical channels), resulting in a multi-frame structure. One logical channel can allocate more than one physical channel. In the GSM standard, 26 frames (120 ms each) are used for the multi-frame traffic channel (TCH), slow associated control channel (SACCH), and fast associated control channel (FACCH). A 51-frame multi-frame with a duration of 235 ms is used for signaling logical channels, including the stand-alone dedicated control channel (SDCCH), broadcast control channel (BCCH), synchronization channel (SCH), frequency correction channel (FCCH), and common control channels (CCCH).

Fifty-one 26-frame multi-frames or twenty-six 51-frame multi-frames form one super-frame with a duration of 6120 ms. 2048 super-frames construct a hyper-frame with a duration of 3 hours 28 minutes 53 seconds and 760 milliseconds [2].

Figure 1 shows an example of a 26-frame multi-frame consisting of full-rate TCH (TCH/F) + SACCH logical channels and a normal burst (NB) structure. It uses the 7th TDMA time slot in the 4th frame in the TCH channel. The last unused frame 25 is reserved for an additional SACCH channel in case a half-rate TCH channel is used.

Figure 2 shows an example of a 51-frame multi-frame consists of FCCH + SCH + BCCH + CCCH logical channels in the downlink direction. The FCCH and SCH channels are repetition of frequency correction bursts (FB) and synchronization bursts (SB) and have the same guard time period as NB. Refer to the GSM 05.01 document for more information about the organization of logical channels.

Figure 1: 26-frame multi-frame TCH/F + SACCH logical channels

Figure 1: 26-frame multi-frame TCH/F + SACCH logical channels

Figure 2: 51-frame multi-frame of downlink FCCH + SCH + BCCH + CCCH logical channels

Figure 2: 51-frame multi-frame of downlink FCCH + SCH + BCCH + CCCH logical channels

Conclusion

This blog post provided an introduction to how a GSM system allocates frequency in the downlink and uplink directions, as well as an example of frequency allocation for a GSM 850 MHz system. The construction of the logical channels in bursts nature from TDMA slots were also shown. In the next blog post in this series, I will discuss the function of each logical channel in a GSM system.

References

[1] ETSI. (2005) 3GPP TS 05.05 version 8.20.0 Release 1999. [Online]. http://www.etsi.org/deliver/etsi_ts/100900_100999/100910/08.20.00_60/ts_100910v082000p.pdf

[2] ETSI. (2004) 3GPP TS 05.01 V8.9.0 Release 1999. [Online]. http://www.etsi.org/deliver/etsi_ts/100500_100599/100573/08.09.00_60/ts_100573v080900p.pdf