35 bps/Hz
MS
BTS
BTS
BTS
BSC
Mobile
Switching
Center
MS
MS
FIGURE 1.8 Simplified GSM architecture block diagram.
The demodulated user signals communicate to a Base Station Controller (BSC). The BSC is responsible
for frequency channel assignment, time slots, and controls frequency hopping [12]. Notice a BSC can
support multiple BTS to allow for handoffs to occur.
Next we aim to provide an overview of the logical and physical channels. The logical channels are
grouped into either traffic channels (TCH) or control channels (CCH). The TCH will carry either
speech or data information samples in the uplink and downlink. The CCH will carry control information,
either signaling or synchronization samples. We provide a pictorial presentation in Fig. 1.9.
The broadcast CCH (BCCH) are used for the following purposes:
??? Downlink only
??? Broadcast system information to the MS (such as frequency hopping sequence and RF channels)
??? Configurations of the CCH
??? Always transmitted (not frequency hopped)
??? Sends FCCH and SCH for frequency correction and frame synchronization info.
The common CCH (CCCH) are used for the following purposes:
??? Uplink and downlink
??? MS access management information
??? Sends PCH (paging channel) used to page the MS
??? Sends AGCH (access grant channel) used to allocate SDCCH (stand-alone dedicated control channel)
or TCH (traffic channel) to MS
??? Receives RACH (random access channel) used for access request; can either be a response to a page
or a call origination request.
Pages:
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37