IT 802DMobile Communications
Part 4Debasis Das
04/08/2023 Debasis Das Mallabhum Institute of Technology
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Radio Interface-1890 MHz to 915 MHz, 200 KHz each
935 to 960 MHz, 200 KHz each
MS to BTS , reverse links
BTS to MS, forward links
45 MHz gap125Possible channels,124 are used
Channels assignedIn pairs, 45 MHz gap
Absolute Radio Frequency Channel number (ARFCN) – 0 to 124 and 975 to 1023
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Radio Interface-2Physical Channel
Debasis Das Mallabhum Institute of Technology
124 channels, 935-960 MHz
124 channels, 890-915 MHz
TS0
Guard
space
TS1
TS2
. . . .TS7
tUserdata
Userdata
SS Training t
Guard
space
Time
3 57 57 31 126546.5 micro S
577 micro S
4.615 ms
GSM TDMA Frame
GSM time slot. Normal burst
04/08/2023 Debasis Das Mallabhum Institute of Technology
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Up/Down linksChannels 1 & 124 not used in GSM 90032channels for organizational data90 channels for usersOne BTS handles 1 organizational channel
and up to 10 user data channels.Each of 248 (124 x 2 Up and down),
separated by GSM time frameEach GSM time frame slot has a fixed pattern
04/08/2023 Debasis Das Mallabhum Institute of Technology
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GSM Time SlotData transmitted in small portions, burstsBurst is 546.5 micro sec long and contains 148 bitsRemaining 30.5 micro sec slot is used as guard spaces
Avoids multipath overlaps, gives transmitter time to switch on and off
156.25 bits in 577 micro sec leads to a data rate of 33.8 Kbps, 270 kbps over a radio carrier
All 3 bits of tail is set to 0Training sequence adapts receiver to current propagation
conditions and select the strongest signal in multipath situation
S- flag indicating user data or network control data4 more busts are defined
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Types of burstsFrequency correction bursts, helps MS to
correct the LO frequency
Synchronization burst wit extended training
sequence, help sync MS with BTS
Access burst set up initial connection with
BTS
Dummy burst, when no data is available for a
slot
04/08/2023 Debasis Das Mallabhum Institute of Technology
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Simplification of Transmitter Hardware
Up and downlinks are separated in frequency by 45
MHz in GSM 900 and 95 MHz in GSM 1800
TDM frames are shifted in time by three slots. If BTS
send data on t0 in slot 1 on the downlink then the
MS will access slot 1 of uplink in t0+3.577 micro sec
Thus MS does not need a full duplex transmitter
04/08/2023 Debasis Das Mallabhum Institute of Technology
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Point to NoteAs solution to frequency selective fading,
sometimes a slow frequency hopping
sequence is sometimes prescribed
MS and BTS may change carrier frequency
after each frame based on a common hopping
sequence
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Logical Channels & Frame HierarchyPhysical & logical Channels
1
2
7
3
4
8
5
6
4.615 ms8*124 Duplex channels
C1C2xC2C1C2xC2C1
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Logical Channels & Frame HierarchyTraffic & Control Channels-1Traffic Channels (TCH)
Available Full rate TCH (TCH/F), 22.8 kbps data rate Half rate TCH (TCH/H), 11.4 kbps data rate
Beginning of GSM TCH/F- 13 kbps, TCH/FS (22.8-13=9.8 kbps) TCH/H + TCH/HS may not give the right voice quality
Full rate codecs FR-13 kbps, HR-5,6 kbps
Enhanced full rate (EFR), better voice quality at 12.2 kbps, given low transmission errors
Adaptive multi rate (AMR) is used in 3GOther TCH/F4.8 and TCH/F14,4,coding, error correction differ
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Logical Channels & Frame HierarchyTraffic & Control Channels-2
Control Channel Broadcast control channel (BCCH) ( info carried are cell id, options
available, frequencies in the cell and neighboring cells) Frequency control channel (FCCH) , Synchronization channel
(SCCH) Common control channels ( CCCH)(info regarding MS and BS; BTS to
MS uses paging channel (PCH), MS to BTS uses random access channel (RACH). MS uses access grant channel (AGCH) to tell MS that it can use a TCH or a SCCH for further set up.
Dedicated control channel (DCCH) (bi directional) Stand alone dedicated control channel ( SDCCH), low rate, 782 bps
for signaling, authentication, registration, etc. data Slow associated dedicated control channel (SACCH), associated
with each TCH and SDCCH Fast associated dedicated control channel ( FACCS), uses TCH time
channel for fast data transfer, such as handover time
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Logical Channels & Frame HierarchyTraffic & Control Channels-3
No arbitrary transmission of channels, fixed patternsSpeech multi-frame, 26 TDMA frames
Slow signaling slot
Blank slot/S
Pattern repeats
Normal burst-114 bits every 4.615 ms=24.7 kbps24 of 26 TCH is used, effective rate then= 22.8 kbps
TCH frames
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GSM Logical Channel TypesTraffic channels (TCH)
Carries digital user speech or data, identical
on both forward and reverse links
Control channels (CCH)
Carries signaling and synchronizing commands
between base station and mobile station, some
CCH may defined for only the forward or the
reverse links
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Traffic Channels (TCH)Full rate TCH (22.8 kbps)
Full rate speech channel (TCH/FS)Full rate data channel ,9600 bps (TCH/F9.6)Full rate data channel, 4800 bps (TCH/F4.8)Full rate data channel, 2400 bps (TCH/F2.4)
Half rate TCH (11.4 kbps)Half rate speech channel (TCH/HS)Half rate data channel, 4800 bps (TCH/H4.8)Half rate data channel, 2400 bps (TCH/H2.4)
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Control Channels(CCH)Each control channel consists of several logical
channels, distributed in timeControl channel types
Broadcast channel (BCH)Common control channel (CCCH)Dedicated control channel (DCCH)
BCH and CCCH are allocated in TS0 slots only, broadcast only during certain frames in the 51 frame control Superframe and designated ARFCNs (34).
DCCH can be sent any time slot, frame
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Control Channel Types- BCHBroadcast channels (BCH)
Broadcast control channel (BCCH) Works as a TDMA beacon, helps synchronize MSs in a cell,
broadcasts on TS0 only Broadcast control channel (BCCH): cell & network id,
operating characteristics of cell (current control channel structure, channel availability, congestion, etc.), list of channels in use during frames 2 to 5 of control multi frame
Frequency control channel (FCCH): special pattern every 10th frame for synchronizing LO frequency in MS
Synchronization channel (SCH): serving base station is identified by base station identity code(BSIC) and frame number (FN), following FCCH and repeated every 10th frame
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Control Multi-frame
F BS B B B C C C C F S C C C F S C C F S C C I
Control channel multi-frame, forward link, TS0
0R
2R
1R
49R
50R
Control channel multi-frame, reverse link, TS0R: Reverse RACH burst (CCCH)
F: FCCH burst, S: SCH burst, B: BCCH burst, all three BCHC: PCH/AGCH burst (CCCH), I: idle
C
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Control Channel Types- CCCHUses TS0 if BCH is not using it or the idle frame
Paging channel (PCH), forward link only Broadcasts IMSI number of incoming call, request to
acknowledge on RACH, used to broadcast text messages as part of SMS feature of the GSM
Random access channel (RACH), a slotted ALOHA channel, reverse link only, Acknowledge a call, initiate a call by MS, during TS0,
base assigns a SDCCH for call establishment, connection confirmed over AGCH
Access grant channel (AGCH), forward link only Instructs a MS to operate in a physical link and ARFCN
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Control Channel Types- DCCHBi directional, any TDM slot & ARFCN except
TS0Standalone dedicated control channel (SDCCH)
Carries signaling data after a connection is made with base station and before a TCH assignment is made, lets MS and base remain connected as MSC verifies subscriber and allocates resources
Slow associated control channel (SACCH) Sends & receives regular control information such as
transmit power level instruction and specific timingFast associated control channel (FACCH)
Fast control requirements such as handover, steal frames from TCH
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Call From Within GSM-1Subscriber should be synchronized to the local baseAfter receiving FCCH, SCH and BCCH, subscriber
will get on to appropriate BCHSubscriber dials the number and hits sendSends a RACH using the same ARFCN on which it
was lockedBase sends AGCH on CCCH and assigns a SDCCHMobile unit gets time slot and ARFCN assignment
through AGCH and tune to it, wait for SACCH for time advance and transmitter power commands
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Call From Within GSM-2While MS and base finish authentication and
user validation, MSC locates and establishes a connection with th4 called party in the PSTN, MSC switches the speech path to the serving base station.
Base station command via SDCCH about TS and ARFCN assignments for a specific TCH
Transferred to TCH to and fro transfers start and SDCCH is vacated
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Call from PSTNPage is responded by the MS, replies with
RACH
Base uses AGCH and CCCH to assign a new
SDCCH and a SACCH while network and
base are connected
MS does timing advance and authentication
on SDCCH, base issues a new physical
channel assignment and the TCH assignment
is made
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Traffic Multi-frame
Burst
Slot
0 1 2 …….. 7
577 micro sec4.615 ms
0 1 2 …….. 25traffic multi-frame, 120 ms
0 1 2 …….. 51
6.12 secSuperframe
0 1 2 …….. 2045 2046 2047
Hyperframe3 h, 28 m, 53.76 s2,715,647 frames