GSM CHANNEL CONCEPT

Back to basics

CONTENTS• INTRODUCTION• HYPER,SUPER & MULTIFRAMES• Frame Counters IN GSM• GSM BURSTS• Logical Channels

Broadcast channelsCommon Control ChannelsDedicated Control Channels• Mapping OF Logical channels on Frame• Mobile Power ON scenario

INTRODUCTION(1/4)• FDMA divides the frequency spectrum into small slices

Carrier Separation is 200 kHz, which provides:• 124 pairs of carriers in the GSM 900 band (carrier also called ARFCN)• 374 pairs of carriers in the GSM 1800 band• 299 pairs of carriers in the GSM 1900 band, etc.)• TDMA divides each channel into 8 timeslots such that each carrier is shared by 8 users• The basic radio resource is a time slot with a duration of 577 µs• 8 Timeslots of 577 µs constitutes a 4.615 ms TDMA Frame• Up link and Down link use the same Time slot number• Up link and down link use the same channel number (ARFCN)

INTRODUCTION(2/4)• CA /ARFCN: Cell Allocation (CA) is the subset of the total frequency band that is

available for one BTS. We can say total transport resources available for communication between BTS and MS.

• Radio Frequency Carriers:

GSM 900 GSM 1800 GSM 1900

UPLINK 890 - 915 MHz 1710 - 1785 MHz 1850 - 1910 MHz

DOWNLINK 935 - 960 MHz 1805 - 1880 MHz 1930 - 1990 MHz

INTRODUCTION(3/4)

Basic Physical Channel (BPC): Using TDMA each of this carriers is divided into 8 timeslots.Each of this timeslot is called Basic Physical Channel.

C0BPC (577µs)

T0 T1 T2 T3 T4 T5 T6 T7

INTRODUCTION(4/4)• The start of a TDMA frame on uplink is delayed by a fixed time (3 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

Hyper, Super & Multiframes(1/3)

• In a GSM system, every TDMA frame is assigned a fixed number, which repeats itself in a time period of 3 hours, 28 minutes, 53 seconds, and 760 milliseconds. This time period is referred to as hyper frame

• There are two types of multiframes in the system:1. 26 frame multiframe with a duration of 120 ms

Consists 26 TDMA frames. Used to carry the logical channels TCH, SACCH and FACCH.

1. 51 frame multiframe with a duration of 235.4 msConsists 51 TDMA framesUsed to carry the logical channels FCCH, SCH, BCCH, CCCH, SDCCH, SACCH etc.

Hyper, Super & Multiframes(2/3)

• Super frame consists of 51 (26 frame multi frames) or 26 (51 frame multi frames) with a duration of 6.12 seconds.

• A Hyper frame consists of 2048 super frames.• Frame number is generated by BTS. BTS broadcast current frame number

5 times in 51 frame multiframe.• The frame hierarchy is used for synchronization between BTS and MS.

Hyper, Super & Multiframes(3/3)

Frame Counters in GSM(1/2)• There are three counters in GSM: T1, T2 & T3 • T1 counter counts the super-frames.

o Whenever a super-frame is completed, T1 is incremented by 1 o T1 = FN DIV (51*26)o 0 T1 2047

• T2 counter counts the speech frames, which only occur in 26 multi-frame structureo T2 = FN MOD 26o 0 T2 25

• T3 counter counts the signaling frames, which are 51-multi-frame structure o T3 = FN MOD 51o 0 T3 50

• FN = 26 * 51 * T1 + ((( T3 - T2 ) MOD 26 ) * 51 ) + T3

Frame Counters in GSM (2/2)

FN = 26 * 51 * T1 + ((( T3 - T2 ) MOD 26 ) * 51 ) + T3

After maximum values, start again with zeroes.That happens about every three and half hours.

T1 gets a new value always when T3 and T2 get zero values together.

T1 maximum value is 2047.

GSM Bursts(1/7)• Burst define a format in which a particular information is transmitted on TS (The physical

content of TS is called a burst)• The time interval of a TS corresponds to 156.25 bits• These bits are divided into different parts depending on the type of burst• There are 5 types of bursts in GSM

o Normal Burst GMSK: 114 bits of useful information 8-PSK: 342 bits of useful information (three times content)

o Frequency Correction Bursto Synchronization Bursto Access Bursto Dummy Burst

Normal Burst for GMSK(2/7)

Source: Unknown

Normal Burst for GMSK (3/7)• Used to carry Traffic or control information• Tail bits are sequence of zero bits to indicate the start and the end of burst• Data bits carries Traffic or control information• Training bits carries a training sequence used by adaptive equalizer to

estimate the channel• Flag bit indicates type of information being transmitted (User info/signaling

info (TCH or FACCH)• Guard bits are used to avoid overlapping between adjacent timeslots

Frequency Correction Burst(4/7)• This burst format is used in downlink direction on FCCH for frequency synchronization of MS• Contains a sequence of 142 zeros• After modulation the resulting signal is pure sine wave• Tail bits are sequence of zero bits to indicate the start and the end of burst

Tail3

All zeros142

Tail3

Guard8.25

Frequency Correction Burst

Synchronization Burst(5/7)• This burst format is used in downlink direction on SCH for time synchronization of MS• Carries BSIC and FN in data field• Contains a long Training sequence (synchronization field)

Tail3

Data39

Synchronization64

Data39

Tail3

Guard8.25

Synchronization Burst

Access Burst(6/7)• This burst format is used in Uplink direction on RACH to access the network• Long guard period ensures that the mobile transmissions from all parts of the cell arrive at the

BTS within the duration of the burst

Tail8

Synchronization41

Tail3

Guard68.25

Access Burst

Data36

Dummy Burst(7/7)• BTS always transmit on C0 (BCCH frequency in the cell) to enable MS to perform power

measurements continuously• The power measurements are useful to select the BTS for initial access or for Handover• In order to achieve this dummy burst are transmitted on C0 when no other burst is transmitted

on C0• Dummy Bursts are transmitted with a pre-defined sequence of fixed bits

Tail3

Mixed58

Training26

Mixed58

Tail3

Guard8.25

Dummy Burst

Information bits - receive

AirInterface

(6)

(5)

Rec. 05.02

(4)

Info bits transmit

Rec. 05.03

(2)(1)Code 1 (block)

Code 2 (conv)

Reordering and partitioning

(3)Inter-

leaving

Encryption unit

Burst building

Burst multiplex

Rec. 05.04

Differential encoding

GMSK Modulation

Rec. 05.05

Antenna

Transmitter

Transmitter

Logical Channels• Tasks performed in GSM are supported by number of functional channels known as

logical channels• Different logical channels are mapped in either direction on Physical Channels• The logical channels carrying control information are generally mapped on to one or two

timeslots of one carrier (Known as BCCH Frequency) in a cell• The remaining "logical channels" are used to carry traffic

LOGICAL CHANNELS

SIGNALLING

FULL RATE22.8 Kb/S

HALF RATE11.4 Kb/S

BROADCAST COMMON CONTROL DEDICATED CONTROL

FCCH SCH BCCH

PCHRACH

AGCH

SDCCH SACCH FACCH

FCCH -- FREQUENCY CORRECTION CHANNELSCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNELPCH -- PAGING CHANNELRACH -- RANDOM ACCESS CHANNELAGCH -- ACCESS GRANTED CHANNELSDCCH -- STAND ALONE DEDICATED CONTROL CHANNELSACCH -- SLOW ASSOCIATED CONTROL CHANNELFACCH -- FAST ASSOCIATED CONTROL CHANNEL

DOWN LINK ONLY

UPLINK ONLY

BOTH UP &DOWNLINKS

TRAFFIC

Broadcast Channels: FCCH(1/3)• Frequency Correction Channel: FCCH• One way channel operating in forward direction and using frequency correction burst

format• Bears information for Frequency Synchronization• 142 all 0 bits in this burst causes GMSK modulator to deliver an unmodulated carrier for

the entire duration of the timeslot• Upon detecting this sine wave the MS can adjust its frequency reference appropriately

Broadcast Channels: SCH(2/3)• Synchronization Channel (SCH)• Uses the synchronization burst format• Raw Data information for SCH is of 89 bits• 64 bits are same for each cell and helps them to achieve timing synchronization. • 6 bits are for the identification of BTS and mapped on Base Station Identity Code (BSIC)

= NCC (3 bit) + BCC (3 bit) • BSIC avoids ambiguity or interference which can arise when a MS can receive SCH from

two cells using the same BCCH frequency. • Network Colour Code (NCC)

Used to identity the BTS for which measurement is made.• Base-Station Colour Code (BCC)

Each 8 BCC value maps to a different Training Sequence.Different training sequences allow for a better transmission in case of interference

• 19 bits represent the TDMA frame number (reduced frame number)

Broadcast Channels: BCCH(3/3)• Broadcast control channel (BCCH)• One way channel operating in the forward direction and using the normal burst format• BCCH Occur in timeslot 0 of some specific carriers known as BCCH carriers• After locking on to the frequency and frame structure in the cell, MS needs some more

general information broadcast on the BCCH for call setup purposes• Cell Identity (CI)• Network Identity (LAI) • Control Channel structure• BCCH Frequencies of neighboring cells• GPRS Supported or not.

Common Control Channels: PCH(1/3)• Paging Channel: PCH• One way channel operating in the forward direction and using the

normal burst format• Mobile subscribers are paged this channel for incoming calls or short

messages• Every MS in a cell periodically listen to this channel• Uses same coding scheme as used for BCCH

Common Control Channels: RACH(2/3)• Random Access Channel: RACH• One way channel operating in the reverse direction and using the access burst format• When MS wants to initiate dialogue with network, this channel is used to send request to

network for a dedicated resource• The actual communication between the MS and the network will takes place later on the

dedicated channel. • If the request is not granted within a specific time period, the MS repeats the request on RACH.

Common Control Channels: AGCH(3/3)• Access Grant Channel: AGCH• AGCH is a one way channel operating in forward direction and using the

normal burst format• In response to requests from different MS on RACH, the network allocates

a specific dedicated signaling channel (SDCCH) against each request for further communication.

• The response to the request is sent on AGCH.• Uses same coding scheme as used for BCCH

Dedicated Control Channels: SDCCH(1/4)• Stand-alone Dedicated Control Channel: SDCCH • Two way channel using normal burst format• As per the allocation conveyed over the AGCH, both the MS & the BTS

switch over to the assigned SDCCH for a further communication• The Following tasks require the use of SDCCH• Location Updates• Call Setup• SMS• Uses the same coding scheme as used by BCCH

Dedicated Control Channels: SACCH(2/4)• Slow Associated Control Channel: SACCH• Two way channel using normal burst format• SACCH is always associated with TCH or SDCCH• When associated with a TCH, the SACCH occurs in 12 or 25 frame of each

26-frame multi frame• Each message comprises of 456 bits so 4 multi frames are required to

transmit a message• Since a 26-frame multiframe requires 120ms, a SAACH message over 4

multi-frame requires 480ms. • Thus, power control that is linked to SAACH exchanges is hindered by the

low rates of SAACH exchange. (But then, SAACH was meant to be slow!)

Dedicated Control Channels: SACCH(3/4)• Used to convey the periodic carrier-signal strength measurements to the

network• While an MS is busy on a call over a traffic channel (TCH) or in

communication with MSC on the SDCCH, MS takes periodic carrier-signal strength measurements on own base station & neighboring base stations.

• Based on the analysis of measurements taken by BTS & the MS, the BSC conveys information on timing advance & MS transmitter power control

• Uses the same coding scheme as used by BCCH

Dedicated Control Channels: FACCH(4/4)• Fast Associated Control Channel: FACCH• FACCH is a two way channel using normal burst format• FACCH can be associated with SDCCH or TCH• FACCH works on the principle of stealing i.e 20 milliseconds of speech

burst is replaced by FACCH signaling• FACCH is used to convey Handover information• Uses the same coding scheme as used by BCCH

TRAFFIC CHANNELS(TCH)• Full Rate Traffic Channel • This channel carries information at rate of 22.8 Kbps• Half Rate Traffic Channel• This channel carries information at rate of 11.4 Kbps• Enhanced Full Rate Speech• GMSK Adaptive Multi Rate (Half Rate and Full Rate)• Circuit Switched Data (Transparent): 600/1200, 2400, 4800, 9600, 14400.• Circuit Switched Data (Non-transparent): 9600, 14400.• Group 3 Fax: 2400, 4800, 9600, 14400.• CS 1 to 4 • MCS 1 to 9• High-Speed Circuit Switched Data (HSCSD) for 9.6/14.4 kbps• Enhanced Circuit Switched Data 28.8/32.0/43.2 Kbps per TS

Mapping OF logical Channels on Frame(1/2)

Generally two configurations are mainly used• Separate SDCCH: FCCH + SCH + BCCH + CCCH

Addresses a channel configuration in which no SDCCH are available on TS 0.In this case SDCCH sub channels are defined on TS 1Rest of the TS are used by Traffic channels• Combined SDCCH: FCCH + SCH + BCCH + CCCH + SDCCH/4

Addresses a channel configuration in which all control channels are assigned to TS 0In this case TS1 is also available for Traffic channels

Mapping OF logical Channels(2/2)• The downlink direction of TS 0 of the BCCH-TRX is used by various

channels.• FCCH• SCH• BCCH• Four SDCCH sub channels (optional);• CCCH• This use is possible because the logical channels can time-share TS 0 in

different TDMA frames of 51 frame Multi frame

Mapping of BCCH and CCCH• Multiplexing of FCCH + SCH + BCCH + CCCH on TS 0 of radio frequency

C0 (51 Frame Multiframe)• Cycle of 51 TDMA frame (0-50), The structure is repeated after IDLE frame• It contains one block of 4 frames for BCCH and 9 Blocks of 4 frames for

CCCH (AGCH/PCH)

Mapping of BCCH and CCCH(Source: GSM Networks by Gunnar

Heine)

Mapping of RACH on Uplink• In the Uplink direction TS0 on carrier frequency C0 is used to access the

network• Only RACH is sent on this timeslot in uplink direction

0 1 2 3 4 5 6 7

TDMA frame - 4.615 ms

0 1 2 3 4 5 6 7

TDMA frame - 4.615 ms

0 1

Timeslot 0, C0, uplink

Each burst on the uplink is a RACH

Mapping of Dedicated Control Channels

0 1 2 3 4 5 6 7TDMA frame - 4.615 ms

0 10 1 2 3 4 5 6 7TDMA frame - 4.615 ms

D0 D7 IA0 A3

D0 D7 IA4 A7

Multiplexing of Dedicated Control Channel on TS 1 of radio frequency C0

• (SDCCH/8 + SACCH/8) The Structure is cyclic over 102 frames• 102 frames = Two 51 multi-frames• In first multi-frame, there are 4TS for each of the 8 SDCCH

channel and 4TS for 4 of the 8 SAACH.• In second multi-frame, there are again 4TS for each of the 8

SDCCH channel and 4TS for remaining 4 of the 8 SAACH.• 8 MS can share this timeslot simultaneously

• (TCH/F + SACCH/TF + FACCH/F) The Structure is cyclic over 26 frames• TCH are mapped together with SACCH• FACCH is also used together with TCH/F.• FACCH works in stealing mode (The speech burst are replaced by FACCH

signaling)

Mapping OF TCH on Time Slots

TDMA frame - 4.615 ms

0 1 2 3 4 5 6 7

TDMA frame - 4.615 ms

0 1 2 3 4 5 6 7

TDMA fra

0 1 2 3

Multiplexing of Dedicated Control Channel on TS 2 of radio frequency C0

AT T T IT T T

MS Power-on Scenario (1/3)• Search of the strongest (BCCH) carrier

o Power measurement based on list of preferred BCCH carriers (if available in SIM card memory)

o Altenatively: Power measurements on all carriers• Search of FCCH (Frequency Burst)

o Identification of the strongest BCCH carrier found in step 1o Mono-frequent characteristic of FCCH facilitates robust detection

by narrowband bandpass filter even if SNR is lowo Successful FCCH detection allows

Coarse time synchronization (adjustment of search window for subsequent SCH detection)

Frequency synchronization of MS i.e. fine tuning of oscillators in MS based on FCCH frequency estimation (nominal frequency: 67.7 kHz)

MS Power-on Scenario (2/3)• Search of Synchronisation Burst SB

o SCH follows immediately on FCCH burst within TDMA frameo Training sequence was defined to be sufficiently long (64 bit) in

order to take into account that the time sync in the preceding step is only coarse (need for long search window)

o Successful SB detection allows: Fine tuning of MS frequency oscillator Fine tuning of TDMA time SCH information reading (BSIC & RFN)

• Reading of logical BCCH channelo Channel configuration of the current cello Frequencies of BCCH carriers of neighbouring cells

MS Power-on Scenario (3/3)• Periodic Reading of PCH (Paging Channels) as well as receive signal

power measurements of BCCH carriers in adjacent cellso The 6 strongest BCCH carrier are kept in mind

• Periodic synchronization on the 6 strongest BCCH carrierso i.e. reading the SCH/BCCH information (see steps 1-4)

• Cell change o If the BCCH carrier currently in use does not fulfill the quality

requirements but at least one of the BCCH carriers of a neighboring cell fulfills the selection criteria (Cell Reselection)

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