GSM system

Global System for Mobile Communications Introduced in 1991. Settings of standards

under ETSI (European Telecommunication Standards Institute)

Services Telephone services Data services Short message paging

GSM systemSystem Architecture : three major subsystems

Network and Switching Subsystem (NSS) MSCs, Visitor Location Register (VLR), Home Location Register

(HLR), Authentication Center (AUC) and Equipment Identity Register (EIR).

Switching of GSM calls between external networks and the BSCs. Managing and providing external access to several customer

databases, such as HLR : contains subscriber information (International Mobile

Subscriber Identity -IMSI) and location information for each user who resides in the same city as the MSC.

VLR : temporarily stores the IMSI and customer information for each roaming subscriber who is visiting the coverage area of a particular MSC. Once a roaming mobile is logged in the VLR, the MSC sends the necessary information to the visiting subscriber’s HLR so that calls to the roaming mobile can be appropriately routed over the PSTN by the roaming user’s HLR.

AUC : Strongly protected database which handles the authentication and encryption keys for every single subscriber in the HLR and VLR.

Radio Subsystem (Base Station Subsystem BSS): Mobile stations (MS), Base Transceiver

Station (BTS) and the Base Station Controller (BSC)

Provides and manages radio transmission paths between the MS and MSC.

One BSC controls up to several hundred BTSs.

BSC performs handover for MS under the control of same BSC.

Operation Support Subsystem (OSS) Support the operation and maintenance of

GSM and allows system engineers to monitor, diagnose and troubleshoot all aspects of the GSM system.

Interacts with the other GSM subsystems. Charging and billing.

Interface

TDMA/FDMA/FDD

Uses both TDMA and FDMA to transmit and recover information.

Systems are FDD.

Location Management

In mobile networks, location management is an important mechanism that enables the network to discover the current attachment point of a MS so as to facilitate successful information delivery.

For GSM networks, location information about MSs is maintained by registration, whereby MSs update their registration area(s) information with HLR and VLR, in the event of changes in the registration areas(s).

Radio bandwidth normally needs to be reserved for carrying location management traffic, which comprises of two major components: paging and location update. Paging is the process of searching for a MS in the vicinity of its last-known location, typically in an area of several cell sizes which is also known as a location area (LA), so as to connect an incoming call.

Location update is the process undertaken by a MS to inform the network of any changes in its current location. Location updates typically occur each time a MS crosses an LA boundary.

Location Management

Current GSM Location Management Scheme. A service area comprises of 9 cells are partitioned into 3 static LA, each having 4, 2 and 3 cells, respectively.

Location Management

The location management scheme of GSM Mobile is given in IS-41. It is based on partitioning the cells into static registration areas known as location areas (LAs).

Location updates are to be performed when MSs cross the partitioning of LA so that up-to-date location information for each MS on is stored in the respective HLR and VLR.

Since the location information of each MS is tracked on a resolution that is dependent on the size of the LA, to facilitate a successful data delivery, all the cells within the LA have to be paged concurrently.

Frequency domain

The frequency band for uplink (reverse) is 890-915MHz, downlink (forward) is 935-960MHz.

The bandwidth for the GSM system is 25MHz, which provides 125 carriers uplink/downlink each having a bandwidth of 200 kHz. The ARFCN (Absolute radio frequency channel numbers) denotes a forward and reverse channel pair which is separated in frequency by 45 MHz. Base-to-mobile : Fu (n) = 890.2 + 0.2(n −1) MHz Mobile-to-base : Fd (n) = Fu (n) + 45 MHz

In practical implementations, a guard band of 100kHz is provided at the upper and lower end of the GSM spectrum, and only 124 (duplex) channels are implemented.

Frequency domain

There are a total of eight channels per carrier. Every eighth timeslot on a TDMA channel, the user transmits or receives his information.

A second frequency band from 1710-1785 MHz and 1805-1880 MHz (three times as much as primary 900MHz) are also specified in 1990, a total of 374 duplex channels – DCS 1800

GSM has chosen the four-cell repeat pattern for the frequency reuse cell sets. In most cases, each cell is divided into 120-deg sectors, with three base transceiver subsystems in each cell. Each base transceiver has a 120-deg antenna.

These 12 sectors (called cells in GSM system) share the 124 channels.

Time domain

RF carrier channel is time division multiple accessed by users at different locations within a cell site.

Frame duration is 4.615ms, and each frame consists of 8 time-slots.

Each of the time-slot is a traffic channel having time duration 0.577ms (3/5200s).

The start of an uplink TDMA frame is delayed with respect to downlink by three timeslots. Staggering TDMA frames allows the same timeslot number (TN) to be used in both directions.

Time domain

Multiframe

26 frames (traffic or speech ~): Traffic CHannel (TCH), Slow Associated Control CHannel (SACCH), Fast Associated Control CHannel (FACCH).

51 frames (control ~) : Broadcast Common Control (BCC), Stand Alone Dedicated Control Channels,..

Superframe : 51 traffic multiframes or 26 control multiframes.

Hyperframe : 2048 superframes (3 hrs 28 min 52.76 s), to support encryption with high security and frequency hopping.

Timeslot and Frame structure

0 2045 2046 2047321 2044

3210 4947 48 50

0 24 251

1 super high frame = 2048 super frame = 2715648 TDMA frame

1 super frame = 1326 TDMA frame （ 6.12s）

0 1 2524 504910

1 multiplex frame = 26 TDMA frames （ 120ms）1 multiplex frame = 51 TDMA frame

0 1 765432

1 TDMA frame = 8 timeslot （ 120/26 = 4.615ms）

BCCHCCCHSDCHSACCH/TCH

FACCH

Physical Channel & Logical Channel Physical Channel : specified by ARFCN and

TN. Logical Channel : is mapped onto the

physical channel. E.g, TCHs and control channels.

Channel Type

channel

TCH

CCH

Voice CH

Data CH

FR Voice Traffic Channel (TCH/FS)

HR Traffic Channel (TCH/HS)

4.8Kbit/s HR TCH (TCH/H4.8)

9.6Kbit/s FR TCH(TCH/F9.6)

4.8Kbit/s FR TCH (TCH/F4.8)

BCH

FCCH (down)SCH (down)BCCH (down)

CCCHRACH (up)AGCH (down)

PCH (down)

DCCHSDCCHFACCHSACCH

14.4Kbit/s FR TCH (TCH/F14.4)

Enhanced FR Traffic Channel (TCH/EFR)

Traffic Channel (TCH)

Traffic channels carry digitally encoded user speech or user data. Have identical functions and formats on both the forward and reverse link.

Full rate : user data is contained within one TS per frame. Half rate : user data is mapped onto the same time slot, but is sent in alternate frames. TCH/FS : Full rate speech, raw data sent at 13kbps, with

GSM channel coding 22.8kbps. TCH/HS : Half rate speech, raw data sent at 6.5kbps, with

GSM channel coding 11.4kbps. TCH/F9.6 : Full raw data rate 9.6kbps, send at 22.8kbps. TCH/F4.8, TCH/F2.4 TCH/H4.8, TCH/H2.4

Traffic or speech multiframe

Control multiframe

Control (Signaling) Channels

Broadcast CHannel (BCH) Common Control CHannel (CCCH) Dedicated Control CHannel (DCCH)

BCH

Broadcast channel (BCH) -- Only on downlink Broadcast control channel (BCCH)

Broadcast cell and network information Cell and network identity List of channels in use

Frequency correction channel (FCCH) Occupies TS0 of first frame Repeated every 10th frame within a control channel multiframe Synchronization of local oscillator (radio frequency) to base station

oscillator. Synchronization channel (SCH)

Broadcast in TS0 immediately following a FCCH frame. Allows for frame synchronization Base station issues timing advancement commands.

Common control channel (CCCH) Occupies TS0 of every control frame not used by BCH or the Idle

Frame. Paging channel (PCH)

Provides paging signals from base to mobiles. Notifies specific mobile of incoming call

Random access channel (RACH) Uplink channel Used by mobiles to acknowledge page from PCH Used by mobiles to originate a call

Access grant channel (AGCH) Provides forward link communication to mobile. Specifies time slot, radio channel and dedicated control channel. AGCH is final CCCH message before mobile is moved off the

control channel.

Dedicated control channel (DCCH) Bi-directional channels with same format and

function on uplink and downlink. May exist in any time slot and on any radio channel

except TS0 of the control radio channel. Stand-alone dedicated control channel (SDCCH)

Carries signaling data following the connection of the mobile with BS.

Intermediate and temporary channel for mobiles while waiting for the BS to allocate a TCH channel.

Ensures that mobile and base remains connected during authentication and resource allocation.

Maybe assigned their own physical channel or may occupy TS0 of the BCH if there is low demand for BCH or CCCH traffic.

Dedicated control channel (DCCH) Slow associated control channel (SACCH)

Always associated with a traffic channel On downlink the SACCH carries power control and timing

advance instruction On uplink the SACCH carriers signal strength and quality

information SACCH is allocated every 13th frame of a traffic channel.

Fast associated control channel (FACCH) 1. Carries urgent messages to the mobile, for example

handover. 2. FACCH gains access by stealing frames from TCH (e.g.

data transmission slot are stolen).

Time Slot Bursts

Time slot data bursts take on one of the 5 formats according to the logical channel.

A normal burst consists of 148 bits Guard time of 8.25 bits to avoid frame

overlap. Two batched of 57 bits are information bits 26 training bits for equalization. Two stealing bits for FACCH

Time Slot Bursts

Mobile-Assisted handover

Four purposes for handover Rescue low-quality channel Recovering cochannel interference Traffic balancing among cells (avoid congestion or load

balancing) – directed handover. Recovering in the event of failure of a control channel

Three cases of handover, From one radio channel to another of the same BSC. Between channel of different BSCs under the control of the

same MSC. Between different MSCs in the same licensed operator.Between different licensed operators there are no procedure.

Two modes of handover

Synchronous: the old and new cells are synchronized so that their TDMA timeslots start at exactly the same time.

Asynchronous: this may lead to a longer interruption of communication during handover since MS has to re-initialize timing advance at the new cell.

Speech coding

The GSM speech coder is based on the Residually Excited Linear Predictor (RELP)

Enhanced by a long term predictor (LTP) The coder provides 260 speech codec bits for each

20ms, ie. The speech codec bit rate is 13 kbps. 40% average voice activity exploited by a

discontinuous transmission mode. A voice activity detector (VAD) is used in the speech coder – off the transmitter for power saving.

Half rate codec works at 6.5 kbps.

Speech coding

Channel coding – data channels Full rate 22.8kbps :

The output bits of the speech coder are ordered into groups for error protection, based upon their significance in contributing to speech quality. Out of the total 260 bits in a frame,

The most important 50 bits, called type Ia bits, have 3 parity check (CRC) bits added to them.

The next 132 bits along with the first 53 are reordered and appended by 4 trailing zero bits, and then encoded for error protection using a rate ½ convolution encoder with constraint length K=5.

The least important 78 bits do not have any error protection. For TCH/F.9.6,

Modulation & Interleaving

Modulation 0.3 GMSK. The channel data rate of GSM is 270.833333 kbps

Interleaving To minimize the effect of sudden fades on the received

data, the total of 456 encoded bits within each 20 ms speech frame or control message frame are broken into eight 57 bit sub-blocks. These 8 sub-blocks which make up a single speech frame are spread over eight consecutive TCH time slots.

If a burst is lost due to interference or fading, interleaved data will help to spread the effect over a few error-correction-frames. Hopefully channel coding ensures that enough bits will still be received correctly.

Frequency hopping

Under normal conditions, each data burst belonging to a particular physical channel is transmitted using the same carrier frequency.

If users in a particular cell have severe multipath problems, the cell may be defined as a hopping cell by the network operator.

Frequency hopping is carried out on a frame-by-frame basis, thus hopping occurs at a maximum rate of 216.7 hops per second (1/0.004615 – frame rate). As many as 64 different channels may be used before a hopping sequence is repeated.

Others – Apparent bandwidth efficiency

GSM bit rate = 270.83 kbps, bandwidth = 200kHz Bandwidth efficiency = 1.354 bs/Hz

– The speech codec rate for each time slot = 456b/20ms (=22.8kbps).

– Each voice channel actually allocated for 270.83/8 =33.854kbps. – Number of bits/slot = 33.854x4.615 = 156.25 bits (148+8.25 guard

time) – For each TDMA slot in each frame, 114 bits are transmitted, and

only 24 data frames per 26 frames are transmitting, therefore the vocoder output rate = 114/0.004615 x 24/26 = 22.8kbps

– However, of the 114 data bits in a slot, only 65 are raw speech codec bits Raw data rate = 22.8*65/114=13 kbps 65 bits in 20ms = 13 kbps