GSM CHANNEL CONCEPTBack to basics
CONTENTSINTRODUCTIONHYPER,SUPER & MULTIFRAMESFrame Counters IN GSMGSM BURSTSLogical ChannelsBroadcast channelsCommon Control ChannelsDedicated Control ChannelsMapping OF Logical channels on FrameMobile Power ON scenario
INTRODUCTION(1/4)FDMA divides the frequency spectrum into small slicesCarrier 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 band299 pairs of carriers in the GSM 1900 band, etc.)TDMA divides each channel into 8 timeslots such that each carrier is shared by 8 usersThe basic radio resource is a time slot with a duration of 577s8 Timeslots of 577 s constitutes a 4.615 ms TDMA FrameUp link and Down link use the same Time slot numberUp 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:
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.
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 frameThere are two types of multiframes in the system:26 frame multiframe with a duration of 120 msConsists 26 TDMA frames. Used to carry the logical channels TCH, SACCH and FACCH.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. Whenever a super-frame is completed, T1 is incremented by 1 T1 = FN DIV (51*26)0 T1 2047T2 counter counts the speech frames, which only occur in 26 multi-frame structureT2 = FN MOD 260 T2 25T3 counter counts the signaling frames, which are 51-multi-frame structure T3 = FN MOD 510 T3 50FN = 26 * 51 * T1 + ((( T3 - T2 ) MOD 26 ) * 51 ) + T3
Frame Counters in GSM (2/2)FN = 26 * 51 * T1 + ((( T3 - T2 ) MOD 26 ) * 51 ) + T3After 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 bitsThese bits are divided into different parts depending on the type of burstThere are 5 types of bursts in GSMNormal BurstGMSK: 114 bits of useful information8-PSK: 342 bits of useful information (three times content)Frequency Correction BurstSynchronization BurstAccess BurstDummy Burst
Normal Burst for GMSK(2/7)Source: Unknown
Normal Burst for GMSK (3/7)Used to carry Traffic or control informationTail bits are sequence of zero bits to indicate the start and the end of burstData bits carries Traffic or control informationTraining bits carries a training sequence used by adaptive equalizer to estimate the channelFlag 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 MSContains a sequence of 142 zerosAfter modulation the resulting signal is pure sine waveTail bits are sequence of zero bits to indicate the start and the end of burst
Tail3All zeros142Tail3Guard8.25Frequency Correction Burst
Synchronization Burst(5/7)This burst format is used in downlink direction on SCH for time synchronization of MSCarries BSIC and FN in data fieldContains a long Training sequence (synchronization field)
Access Burst(6/7)This burst format is used in Uplink direction on RACH to access the networkLong guard period ensures that the mobile transmissions from all parts of the cell arrive at the BTS within the duration of the burstTail8Synchronization41Tail3Guard68.25Access BurstData36
Dummy Burst(7/7)BTS always transmit on C0 (BCCH frequency in the cell) to enable MS to perform power measurements continuouslyThe power measurements are useful to select the BTS for initial access or for HandoverIn order to achieve this dummy burst are transmitted on C0 when no other burst is transmitted on C0Dummy Bursts are transmitted with a pre-defined sequence of fixed bits
Information bits - receiveAirInterface(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 buildingBurst multiplex
Rec. 05.04Differential encodingGMSK Modulation
Logical ChannelsTasks performed in GSM are supported by number of functional channels known as logical channelsDifferent logical channels are mapped in either direction on Physical ChannelsThe logical channels carrying control information are generally mapped on to one or two timeslots of one carrier (Known as BCCH Frequency) in a cellThe remaining "logical channels" are used to carry traffic
LOGICAL CHANNELSSIGNALLINGFULL RATE22.8 Kb/SHALF RATE11.4 Kb/SBROADCASTCOMMON CONTROL DEDICATED CONTROLFCCHSCHBCCHPCHRACHAGCHSDCCHSACCHFACCHFCCH -- 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 CHANNELDOWN LINK ONLYUPLINK ONLYBOTH UP &DOWNLINKSTRAFFIC
Broadcast Channels: FCCH(1/3)Frequency Correction Channel: FCCHOne way channel operating in forward direction and using frequency correction burst formatBears information for Frequency Synchronization142 all 0 bits in this burst causes GMSK modulator to deliver an unmodulated carrier for the entire duration of the timeslotUpon detecting this sine wave the MS can adjust its frequency reference appropriately
Broadcast Channels: SCH(2/3)Synchronization Channel (SCH)Uses the synchronization burst formatRaw Data information for SCH is of 89 bits64 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 formatBCCH Occur in timeslot 0 of some specific carriers known as BCCH carriersAfter locking on to the frequency and frame structure in the cell, MS needs some more general information broadcast on the BCCH for call setup purposesCell Identity (CI)Network Identity (LAI) Control Channel structureBCCH Frequencies of neighboring cellsGPRS Supported or not.
Common Control Channels: PCH(1/3)Paging Channel: PCHOne way channel operating in the forward direction and using the normal burst formatMobile subscribers are paged this channel for incoming calls or short messagesEvery MS in a cell periodically listen to this channelUses same coding scheme as used for BCCH
Common Control Channels: RACH(2/3)Random Access Channel: RACHOne way channel operating in the reverse direction and using the access burst formatWhen MS wants to initiate dialogue with network, this channel is used to send request to network for a dedicated resourceThe 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: AGCHAGCH is a one way channel operating in forward direction and using the normal burst formatIn 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 formatAs per the allocation conveyed over the AGCH, both the MS & the BTS switch over to the assigned SDCCH for a further communicationThe Following tasks require the use of SDCCHLocation UpdatesCall SetupSMSUses the same coding scheme as used by BCCH
Dedicated Control Channels: SACCH(2/4)Slow Associated Control Channel: SACCHTwo way channel using normal burst formatSACCH is always associated with TCH or SDCCHWhen associated with a TCH, the SACCH occurs in 12 or 25 frame of each 26-frame multi frameEach message comprises of 456 bits so 4 multi frames are required to transmit a messageSince 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 networkWhile 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 controlUses the same coding scheme as used by BCCH
Dedicated Control Channels: FACCH(4/4)Fast Associated Control Channel: FACCHFACCH is a two way channel using normal burst formatFACCH can be associated with SDCCH or TCHFACCH works on the principle of stealing i.e 20 milliseconds of speech burst is replaced by FACCH signalingFACCH is used to convey Handover informationUses the same coding scheme as used by BCCH
TRAFFIC CHANNELS(TCH)Full Rate Traffic Channel This channel carries information at rate of 22.8 KbpsHalf Rate Traffic ChannelThis channel carries information at rate of 11.4 KbpsEnhanced Full Rate SpeechGMSK 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 9High-Speed Circuit Switched Data (HSCSD) for 9.6/14.4 kbpsEnhanced 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 usedSeparate SDCCH: FCCH + SCH + BCCH + CCCHAddresses 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 channelsCombined SDCCH: FCCH + SCH + BCCH + CCCH + SDCCH/4Addresses 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.FCCHSCHBCCHFour SDCCH sub channels (optional);CCCHThis 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 CCCHMultiplexing 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 frameIt 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 UplinkIn the Uplink direction TS0 on carrier frequency C0 is used to access the networkOnly RACH is sent on this timeslot in uplink direction01234567TDMA frame - 4.615 ms01234567TDMA frame - 4.615 ms01Timeslot 0, C0, uplink Each burst on the uplink is a RACH
Mapping of Dedicated Control Channels01234567TDMA frame - 4.615 ms0101234567TDMA frame - 4.615 msD0D7IA0A3D0D7IA4A7Multiplexing of Dedicated Control Channel on TS 1 of radio frequency C0
(SDCCH/8 + SACCH/8) The Structure is cyclic over 102 frames102 frames = Two 51 multi-framesIn 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 framesTCH are mapped together with SACCHFACCH 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 SlotsTDMA frame - 4.615 ms01234567TDMA frame - 4.615 ms01234567TDMA fra 0123Multiplexing of Dedicated Control Channel on TS 2 of radio frequency C0
MS Power-on Scenario (1/3)Search of the strongest (BCCH) carrierPower measurement based on list of preferred BCCH carriers (if available in SIM card memory)Altenatively: Power measurements on all carriersSearch of FCCH (Frequency Burst)Identification of the strongest BCCH carrier found in step 1Mono-frequent characteristic of FCCH facilitates robust detection by narrowband bandpass filter even if SNR is lowSuccessful 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 SBSCH follows immediately on FCCH burst within TDMA frameT...