- CDMA allows multiple users to communicate over the same frequency channel by using unique code sequences. Each user is assigned a code and any data transmitted is mixed together on transmission. The receiving device can separate the signals based on the individual codes. - GSM uses frequency division duplexing with different frequency bands for the uplink and downlink. The uplink band ranges from 890-915 MHz and the downlink from 935-960 MHz. Frequency carriers are spaced at 200 kHz intervals. - The main components of GSM architecture are the mobile station, base station subsystem (BTS and BSC), and mobile switching center. Logical channels in GSM include traffic channels, broadcast channels, paging channels

1. East Africa university
Lecturer :Eng Abdulkadir M.Daar

2. GSM Channels & Air
Interface

6. • CDMA accomplishes the
communication in different
code sequences.
• Special coding is adopted
before transmission, then
different information will lose
nothing after being mixed and
transmitted together on the
same frequency and at the same
time.
Time
Frequency
CDMA

7. Page 7
GSM 900
Uplink
890 915 935 960MHz
Downlink
 GSM systems use radio frequencies between 890-915 MHz for receive and between 935-960
MHz for transmit.
 RF carriers are spaced every 200 kHz, allowing a total of 124 carriers for use.
 An RF carrier is a pair of radio frequencies, one used in each direction.
 Transmit and receive frequencies are always separated by 45 MHz.
Frequency Spectrum

9. Frequency Spectrum

10. GSM Architecture Overview
• Main components of GSM
• The mobile station (MS)
• BSS Base Station Subsystem (BTS+BSC)
• MSS Mobile Station Subsystem (MSC/HLR, ..)
BSC
BTS MSC

12. Network Overview
BTS
BSC
MSC
VLR HLR AuC EIR
PSTN,
ISDN...
OMC
OMC
NMC
X.25 links
GSM interfaces
Voicemail Server
SM-SC
MS (Mobile Station)
BSS (Base Station System)
NSS
(Network Switching
Subsystem)
OMM (Operations & Maintenance
Management)
MS
RAN department
RF department
NSS department
Transmission department

13. GSMChannels
1. Logical Channel Type of information to be transmitted e.g.,
traffic or control logical channels.
2. Physical Channel Unit of radio resource of a radio system
e.g., frequency band, time slot, code, etc..

14. 1- Logical channels

15. • There are two major categories of GSM Logical
channels
A. Traffic channels
B. Control channels.

16. A- Traffic channels (TCH)

17. Traffic channels (TCH)
A. Full-rate channels (TCH/F) The channel bit rate of a full-rate
GSM channel is 22.7 kbit/s, although the actual payload data rate is 9.6-
14 kbit/s, depending on the channel coding.
B. Half-rate channels (TCH/H) The channel bit rate of a half-rate
GSM channel is 11.4 kbit/s, although the actual data capacity is 4.8-7
kbit/s, depending on the channel coding.

18. B- Control Channels

19. GSM Control Channels
• There are three main control channels in the GSM system
1. Broadcast Channel (BCH)
2. Common Control Channel (CCCH)
3. Dedicated Control Channel (DCCH)

20. TRX_900MHZ

21. TRX-1800MHZ

22. 1- Broadcast Channel (BCH)

23. • The broadcast channel operates on the forward link of a specific ARFCNs
• It transmits data only in (TS 0) of certain ARFCNs. Other TSs are available for
TCHs.
• The BCH provides synchronization for all mobiles within the cell.
• It is also monitored by mobiles in neighboring cells so that the received power
and MAHO(Mobile Assisted Handoff) decisions can be made by out-of-cell
users.

24. 1- BROADCAST CONTROL CHANNEL (BCCH)
• BCCH is a forward control channel.
• information such as cell and network identity.
• The BCCH is transmitted by the BTS at all times.
• The RF carrier used to transmit the BCCH is referred to as
the BCCH carrier.
• Information on BCCH is monitored by the MS periodically
(at least every 30 secs), when switched on & not in a call.
• The BCCH is transmitted at constant power at all times,
and its signal strength is measured by all MS which may
seek to use it.
24

25. BCCH Carries the following information
1. Location Area Identity (LAI).
2. List of neighboring cells which should be monitored by the MS.
3. List of frequencies used in the cell.
4. Cell identity.
5. Power control indicator.
6. DTX permitted.
7. Access control (for example, emergency calls, call barring).
8. CBCH description.
9. List of Channels currently in use within a cell.

26. 2- FREQUENCY CORRECTION CHANNEL (FCCH)
• FCCH is a forward control channel.
• It is transmitted on same ARFCN i.e. of BCCH
• The FCCH allows each subscriber unit to synchronize
its internal frequency standard (local oscillator) to
exact frequency of the base station

27. 3- SYNCHRONIZATION CHANNEL (SCH)
• It’s also a Downlink Channel.
• SCH allows each mobile to frame synchronize with the base station.
• It transmits two Important Information
1. Frame number.
2. Base Site Identity Code (BSIC).
• The frame number (FN) ranges from 0 to 2715647.
• The BSIC is uniquely assigned to each BTS in a GSM system.
• The BS issues course timing advancement command to the mobile
station over the SCH
27

28. 2- COMMON CONTROL CHANNEL
(CCCH)

29. • The common control channels occupy TS 0 of every GSM frame that is not
otherwise used by the BCH.
• CCCH consist of three different channels.
29

30. 1- PAGING CHANNEL (PCH)
• It exists only on downlink (Forward channel).
• The paging channel (PCH) provides paging signals to all mobiles.
• notifies a specific mobile of an incoming call which originates from
the PSTN.
• The PCH transmit the IMSI of the target subscriber, along with a
request for acknowledgment from the mobile unit.
• the PCH is also used to provide cell broadcast ASCII text messages to
all subscribers, as part of the SMS feature of GSM
30

31. 2- RANDOM ACCESS CHANNEL (RACH)
• The RACH is the only reverse link (uplink) channel.
• MS acknowledges a page from the PCH on RACH.
• RACH is also used by mobiles to originate a call.
• The RACH uses slotted ALOHA access scheme.
• At the BTS, every frame (even the idle frame) will accept RACH
transmissions from mobiles during TS 0
31

32. 3- ACCESS GRANT CHANNEL (AGCH).
• AGCH is used by the BS to provide forward link communication.
• The AGCH is used by the BS to respond to a RACH sent by a mobile
station.
• It carries data for MS to operate in a particular physical channel
(time slot and ARFCN).
• The AGCH is the final CCCH message sent by the base station
before a subscriber is moved off the control channel.
32

33. 4- CELL BROADCAST CHANNEL (CBCH)
• CBCH is used to transmit messages to be broadcasted to all
MSs within a cell.
• it is considered a common channel because the messages can
be received by all mobiles in the cell.
33

34. 3-DEDICATED CONTROL CHANNELS
(DCCH)

35. DEDICATED CONTROL CHANNELS (DCCH)
• There are three (03) types of Dedicated Control Channels in GSM.
• like Traffic Channels they are bi-directional.
• Same format and function in both the forward and reverse links.
• DCCHs may exist in any time slot and any ARFCN except TS 0 of
the BCH ARFCN.
35

36. 1- STAND-ALONE DEDICATED COTROL CHANNEL (SDCCH)
• The SDCCH carries signaling data following the connection of the MS with the
BTS just before a TCH assignment.
• The SDCCH ensures the Connection b/w MS and BS during the verification of
subscriber unit & allocation of resources for the MS.
• It is a dedicated point-to-point signaling channel which is not tied to the existence
of a TCH (stand-alone),
36

37. • The SDCCH is requested from the MS via the RACH and assigned via
the AGCH.
• The SDCCH can be thought of as an intermediate and temporary
channel.
• A SDCCH may also be used for
1. call setup,
2. Authentication
3. location updating
4. SMS point to point
5. e-Fax
6. USSD
37
1- STAND-ALONE DEDICATED COTROL CHANNEL (SDCCH)……

38. 2- SLOW-ASSOCITED CONTROL CHANNEL (SACCH)
• The SACCH is always associated with a TCH or a SDCCH and maps
onto the same physical channel.
• Each ARFCN systematically carries SACCH data for all of its current
users.
• On the downlink, SACCH sends slow but regularly changing control
information to the mobile station.
1. transmit power level instruction.
2. and specific timing advance instruction.
38

39. •On reverse channel SACCH sends
1. received signal strength (form Serving BTS).
2. quality of the TCH.
3. BCH measurement results (from neighboring
cells)
39
2- SLOW-ASSOCITED CONTROL CHANNEL (SACCH)…..

40. 3- FAST-ASSOCIATED CONTROL CHANNEL (FACCH)
• FACCH carries urgent messages, and contains essentially the
same type of information as the SDCCH.
• A FACCH is assigned whenever a SDCCH has not been
dedicated for a particular user and there is an urgent message
(e.g. handoff request).
• The FACCH gains access to a time slot by “stealing” frames
from the traffic channel to which it is assigned
40

41. 2- Physical Channel

42. GSM Physical Channel
A single GSM Carrier is divided into eight timeslots.
Therefore can support up to eight MS subscribers simultaneously.
The timeslots are arranged in sequence and are conventionally
numbered 0 to 7.
 Each repetition of this sequence is called a “TDMA frame”.