Broadcast Channel:
The Broadcast Control Channel
is transmitted by the BTS at all times. The RF carrier used to transmit the
BCCH is referred to as the BCCH carrier. The MS monitors the information
carried on the BCCH periodically (at least every 30 secs), when it is switched on
and not in a call.
The BCCH Consists of:
a. Broadcast Control Channel (BCCH):
Carries the following information:
-Location Area Identity (LAI)
-List of neighboring cells that should be monitored by the MS
-List of frequencies used in the cell
-Cell identity
-Power control indicator
-DTX permitted
-Access control (i.e., emergency calls, call barring etc.)
-CBCH description
The BCCH is
transmitted at constant power at all times, and all MS that may seek to use it
to measure its signal strength. “Dummy” bursts are transmitted to ensure
continuity when there is no BCCH carrier traffic.
b. Frequency Correction Channel (FCCH):
This is transmitted frequently on the BCCH timeslot and allows the mobile to
synchronize its own frequency to that of the transmitting base site. The FCCH
may only be sent during timeslot 0 on the BCCH carrier frequency and therefore
it acts as a flag to the mobile to identify Timeslot 0.
c. Synchronization Channel (SCH): The SCH
carries the information to enable the MS to synchronize to the TDMA frame
structure and know the timing of the individual timeslots. The following
parameters are sent:
-Frame number
-Base Site Identity Code (BSIC)
-The MS will
monitor BCCH information from surrounding cells and store the information from
the best six cells. The SCH information on these cells is also stored so that
the MS may quickly resynchronize when it enters a new cell
Common Control Channel:
The Common Control
Channel (CCCH) is responsible for transferring control information between
all mobiles and the BTS. This is necessary for the implementation of “call
origination” and “call paging” functions. It consists of the following:
a. Random
Access Channel (RACH) Used by the mobile when it requires gaining access to the
system. This occurs when the mobile initiates a call or responds to a page.
b. Paging
Channel (PCH) Used by the BTS to page MS, (paging can be performed by an IMSI,
TMSI or IMEI).
c. Access
Grant Control Channel (AGCH) Used by the BTS to assign a dedicated control
channel to a MS in response to an access message received on the Random Access
Channel. The MS will move to the dedicated channel in order to proceed with
either a call setup, response to a paging message, Location Area Update or
Short Message Service.
d. Cell
Broadcast Channel (CBCH) This channel is used to transmit messages to be
broadcast to all MS’s within a cell. The CBCH uses a dedicated control channel
to send its messages, however it is considered a common channel because all
mobiles in the cell can receive the messages.
Active MS’s must frequently monitor both BCCH and CCCH.
The CCCH will be transmitted on the RF carrier with the BCCH.
RACH:
Dedicated Control Channel:
The DCCH is a single timeslot
on an RF carrier that is used to convey eight Stand-alone Dedicated Control
Channels (SDCCH). A single MS for call setup, authentication, location updating
and SMS point to point use a SDCCH. As we will see later, SDCCH can also be
found on a BCCH/CCCH timeslot, this configuration only allows four SDCCHs.
a. Associated Control Channels (ACCH) These
channels can be associated with either an SDCCH or a TCH. They are used for
carrying information associated with the process being carried out on either
the SDCCH or the TCH.
b. Slow Associated Control Channel (SACCH)
Conveys power control and timing information in the downlink direction (towards
the MS) and Receive Signal Strength Indicator (RSSI), and link quality reports
in the uplink direction.
c. Fast Associated Control Channel (FACCH)
The FACCH is transmitted instead of a TCH. The FACCH ‘‘steals” the TCH burst
and inserts its own information. The FACCH is used to carry out user
authentication, handovers and immediate assignment.
All of the control channels
are required for system operation, however, in the same way that we allow different
users to share the radio channel by using different timeslots to carry the
conversation data, the control channels share timeslots on the radio channel at
different times. This allows efficient passing of control information without
wasting capacity that could be used for call traffic. To do this we must
organize the timeslots between those, which will be used for traffic, and
those, which will carry control signaling.
Common Channel Downlink:
F – FCH S
– SCH B – BCCH A – AGCH P – PCH D – SDCCH
H – SACCH
Common Channel Uplink:
R – RACH D-
SDCCH A- SACCH
Traffic Channel:
a. Traffic
Channel carries the voice data.
b. Two
blocks of 57 bits contain voice data.
c. One
TCH is allocated for every active call. While call is in progress if there is
degradation in quality of current channel, BTS may shift the communication to
another TCH on a different Carrier and/or Timeslot.
d. A
full rate TCH carries 13 kbps voice data, and half rate TCH carries a 6.5 kbps
voice data. Traffic Multi-frame:
-26 Frames – 120 ms
-24 carry speech, 1 idle, 1
SACCH
Framing Structure:
·
1 Frame = 8 bursts = 8*0.57692=4.616 ms
·
1 Control Ch. Multiframe = 51 TF = 234.6 ms
·
1 TCH Multiframe = 26 TF = 120 ms
·
1 Superframe = 51 * 26 CCH MF = 1326 TF = 6.12
sec
·
1 Superframe = 26 * 51 TCH MF = 1326 TF = 6.12
sec
·
1 Hyperframe = 2048 superframes = 2715648 TDMA
frames = 3 hrs. 28 min. 760 ms.
·
Multiframe is used for distribution of logical
channels
·
Superframe is used for Mobile synchronization
·
Hyperframe is used for signaling procedures and
Ciphering
Mobile Maximum Range:
·
Range = (Timing advance * bit period * velocity)
/ 2
·
Range = (63 * (3.693 * 10e-6) * (3 * 10e5)) /2
·
So range is 34.9 Kms.
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