GSM Radio Frequency Bands
GSM 900
Up-link 890 - 915 MHz (25 MHz band)
Down-link 935 - 960 MHz (25 MHz band)
GSM 1800
Up-link 1710 - 1785 MHz (75 MHz band)
Down-link 1805 - 1880 MHz (75 MHz band)
GSM 1900
Up-link 1850 - 1910 MHz (60 MHz band)
Down-link 1930 - 1990 MHz (60 MHz band)
Bands were divided into number of channels (bandwidth of each channel is 200 kHz).
For Example:
In GSM 900 band,
Bandwidth is 25 MHz
Channel Bandwidth is 200 kHz
Guard band is 200 kHz (100 kHz + 100 kHz)
Hence,
25 Mhz = 125 x 200 kHz = 124 Channels + 1 Guard band
So,
We get 124 pairs of carriers in the GSM 900 band.
By the same calculation we can get,
· 374 pairs of carriers in the GSM 1800 band
· 299 pairs of carriers in the GSM 1900 band
GSM Channel Concepts
HYPERFRAMES, SUPERFRAMES AND MULTIFRAMES: The longest recurrent time period of the structure is called hyperframe and has the duration of 3 h 28 min 53 sec 760 ms. Such a long period is needed for the support of cryptographic mechanisms.One hyperframe is divided into 2048 superframes, which have duration of 6.12 seconds. The superframe is itself subdivided into multiframes.
There are two types of multiframes in the system: · 26 frame multiframe (51 per superframe) with a duration of 120 ms, comprising 26 TDMA frames. This multiframe is used to carry the logical channels TCH, SACCH and FACCH, which are described in this chapter. · 51 frame multiframe (26 per superframe) with a duration of 235.4 ms, comprising 51 TDMA frames. This multiframe is used to carry the logical channels FCCH, SCH, BCCH, CCCH, SDCCH, SACCH, and CBCH, which are described in this chapter.
GSM Channel Concepts
GSM Channel Concepts
Using Time Division Multiple Access (TDMA) each carriers is divided into eight Time Slots (TS). One TS on a TDMA frame is called a physical channel, i.e. on each duplex pair of carriers there are eight physical channels.
A variety of information is transmitted between the BTS and the MS. The information is grouped into different logical channels. Each logical channel is used for a specific purpose such as paging, call set-up and speech. For example, speech is sent on the logical channel Traffic Channel (TCH). The logical channels are mapped onto the physical channels.
Logical Channels:
The logical channels can be separated into two categories. They are,
· Traffic channels and
· Signaling channels.
There are two forms of TCHs:
· Bm or full rate TCH (TCH/F) - this channel carries information at a gross rate of 22.8 kbit/s.
· Lm or half rate TCH (TCH/H) - this channel carries information at a gross rate of 11.4 kbit/s.
Signaling channels are subdivided into three categories:
· Broadcast CHannels (BCH)
· Common Control CHannels (CCCH)
· Dedicated Control CHannels (DCCH)
The following sections describe specific channels within these categories.
BROADCAST CHANNELS (BCH):
Frequency Correction CHannel (FCCH): On FCCH, bursts only containing zeroes are transmitted. This serves two purposes. First to make sure that this is the BCCH carrier, and second to allow the MS to synchronize to the frequency. FCCH is transmitted downlink only.
Synchronization CHannel (SCH): The MS needs to synchronize to the time-structure within this particular cell, and also ensure that the chosen BTS is a GSM
base station. By listening to the SCH, the MS receives information about the frame number in this cell and about BSIC (see Appendix) of the chosen BTS. BSIC can only be decoded if the base station belongs to the GSM network. SCH is transmitted downlink only.
Broadcast Control CHannel (BCCH): The MS must receive some general information concerning the cell in order to start roaming, waiting for calls to arrive or making calls. The needed information is broadcast on the Broadcast Control CHannel (BCCH) and includes the Location Area Identity (LAI), maximum output power allowed in the cell and the BCCH carriers for the neighboring cells on which the MS performs measurements. BCCH is transmitted on the downlink only.
Using FCCH, SCH, and BCCH the MS tunes to a BTS and synchronized with the frame structure in that cell. The BTSs are not synchronized to each other. Therefore, every time the MS camps on another cell, it must listen to FCCH, SCH and BCCH in the new cell.
COMMON CONTROL CHANNELS (CCCH):
Paging CHannel (PCH): At certain time intervals the MS listens to the PCH to check if the network wants to make contact with the MS. The reason why the network may want to contact the MS could be an incoming call or an incoming short message. The information
on PCH is a paging message, including the MS’s identity number (IMSI) or a temporary number (TMSI). PCH is transmitted downlink only.
Random Access CHannel (RACH): The MS listens to the PCH to determine when it is being paged. When the MS is paged, it replies on the RACH requesting a signaling channel. RACH can also be used if the MS wants to contact the network. For example, when setting up a mobile originating call. RACH is transmitted uplink only.
Access Grant CHannel (AGCH): The networks assigns a signaling channel (Stand-alone Dedicated Control Channel (SDCCH)) to the MS. This assignment is performed on the AGCH. AGCH is transmitted downlink only.
DEDICATED CONTROL CHANNELS (DCCH):
Stand alone Dedicated Control CHannel (SDCCH): The MS as well as the BTS switches over to the assigned SDCCH. The call set-up procedure is performed on the SDCCH, as well as the textual message transmission (short message and
cell broadcast) in idle mode. SDCCH is transmitted both uplink and downlink.
When call set-up is performed, the MS is told to switch to a TCH.
Slow Associated Control CHannel (SACCH): The SACCH is associated with SDCCH or TCH (i.e. sent on the same physical channel). On the uplink, the MS sends averaged measurements on its own BTS (signal strength and quality) and
neighboring BTSs (signal strength). On the downlink, the MS receives information concerning the transmitting power to use and instructions on the timing advance. SACCH is transmitted both uplink and downlink.
Fast Associated Control CHannel (FACCH): If a handover is required the FACCH is used. FACCH works in stealing mode meaning that one 20 ms segment of speech is exchanged for signaling information necessary for the handover.
Under normal conditions the subscriber does not notice the speech interruption because the speech coder repeats the previous speech block.
Cell Broadcast CHannel (CBCH): CBCH is only used downlink to carry Short Message Service Cell Broadcast (SMSCB) and uses the same physical channel as
the SDCCH.
CHANNEL COMBINATIONS:
Only certain combinations of logical channels are permitted according to the GSM recommendations. The figure below shows the way in which logical channels can be combined on to Basic Physical Channels (BPC). Numbers appearing in brackets after channel designations indicate sub-channel numbers.
Taking a part of a BPC within a multiframe structure and treating it as a separate resource forms a sub-channel.
(i) TCH/F + FACCH/F + SACCH/TF
(ii) TCH/H(0,1) + FACCH/H(0,1) + SACCH/TH(0,1)
(iii) TCH/H(0) + FACCH/H(0) + SACCH/TH(0) + TCH/H(1)
(iv) FCCH + SCH + BCCH + CCCH
(v) FCCH + SCH + BCCH + CCCH + SDCCH/4(0...3) +
SACCH/C4(0...3)
(vi) BCCH + CCCH
(vii) SDCCH/8(0...7) + SACCH/C8(0...7)
(viii) TCH/F + FACCH/F + SACCH/M
(ix) TCH/F + SACCH/M
(x) TCH/FD + SACCH/MD
Where CCCH = PCH + AGCH + RACH
SACCH/T means that SACCH is associated with a TCH while SACCH/C is associated with a control channel. Where the SMSCB is supported, the CBCH replaces SDCCH sub-channel 2 in cases (v) and (vii) of the above.
A combined CCCH/SDCCH allocation (case v) above may only be used when no other CCCH channel is allocated. The difference between channel combinations (ii) and (iii) is that combination (ii) addresses two different MSs, while combination (iii) addresses one single MS using both half rate traffic channels, for example, one for speech and the other for data.
TIME SLOTS AND TDMA FRAMES:
A TS has duration of 3/5200 seconds (577 ms). Eight TSs form a TDMA frame, with approximately 4.62 ms duration. At the BTS the TDMA frames on all of the radio frequency channels in the downlink direction are aligned. The same applies to the uplink. The start of a TDMA frame on uplink is, however, delayed by a fixed time corresponding to three TS periods. The reason for this delay is to allow the same TS number to be used in both uplink and downlink directions without requiring the MS to receive and transmit simultaneously.
