GSM System

EE5401 Cellular Mobile Communications

Institute for Infocomm Research National University of Singapore 188

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

System Architecture : three major subsystems.



– Network and Switching Subsystem (NSS) : 1. MSCs, Visitor Location Register (VLR), Home Location

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

2. Switching of GSM calls between external networks and the BSCs.

3. 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): 1. Mobile stations (MS), Base Transceiver Station (BTS)

and the Base Station Controller (BSC) 2. Provides and manages radio transmission paths

between the MS and MSC. 3. One BSC controls up to several hundred BTSs.



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

– Operation Support Subsystem (OSS) 1. Support the operation and maintenance of GSM and

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

2. Interacts with the other GSM subsystems. 3. Charging and billing.

Interface

TDMA/FDMA/FDD

– Uses both TDMA and FDMA to transmit and recover information.

– Systems are FDD.



Location Management

In cellular wireless 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.



LA 1

LA 2Legends

:

Public Switched Telephone Network;Mobile-services Switching Center/Visitor Location Register;

Base Station Controller;Base Transceiver Station;

Moving/Still MSs;

Home Location Register;

LA 3

Loca

tion

Upd

ate

Pagi

ng

Paging

BSCHLR

/

BSC1BSC2 BSC3

HLR

MSC/VLR

MSC/VLR

PSTN

2

1

Paging

PSTNMSC/VLR

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.

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.



It is also to be noted that both the location update traffic and paging traffic are inversely proportional to one another with respect to the size of the LA, i.e. location update load decreases whilst paging load increases for increasing sizes of LA, and vice versa.

Paging has to be made at every cell of a LA.

20

20

4)10101010( ×+++160=

10

10 1010

2)2020( ×+80=

Note: the number indicates the nomalizedpaging traffic of the cell or cells cluster.

Avoid setting the LA boundary at places when cell crossing rate is high.

20=Aλ

A

B C

D E

10=Bλ10=Cλ

20=Dλ10=Fλ

30=ABµ

30=BAµ 20=BDµ

20=DBµ

20=BCµ20=CBµ

20=ADµ

20=DAµ

10=CDµ

10=DCµ

20=DEµ20=EDµ

15=CEµ

15=ECµ

LA1 LA2



Frequency domain

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

- 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 : )1(2.02.890)( −+= nnFu MHz

Mobile-to-base : 45)()( += nFnF ud 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.



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 900 MHz) 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.

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.



Physical Channel : specified by ARFCN and TN.

Logical Channel : is mapped onto the physical channel. E.g, TCHs and control channels.



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

- TCH data may not be sent in TS0 within a TDMA frame on certain ARFCNs which serve as the broadcast station (control burst) for every frame. Furthermore, frames of TCH data are broken up every thirteenth frame by SACCH or idle frame.



Traffic or speech multiframe

Control multiframe

T11



Control (Signaling) Channels : Three types :

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

Broadcast channel (BCH)

– Only on downlink – Three types of BCH – Broadcast control channel (BCCH) 1. Broadcast cell and network information 2. Cell and network identity 3. List of channels in use – Frequency correction channel (FCCH) 1. Occupies TS0 of first frame 2. Repeated every 10th frame within a control

channel multiframe 3. Synchronization of local oscillator (radio

frequency) to base station oscillator. – Synchronization channel (SCH) 1. Broadcast in TS0 immediately following a FCCH

frame. 2. Allows for frame synchronization 3. Base station issues timing advancement commands. Assume that the mobile 2 was allocated TS2 and he is close to the BS and that mobile 1 has been allocated TS1 and is at the boundary of the cell site. Due to higher distance resulting in large propagation delay, the transmitted



signal of mobile 1 during TS1 reaches the BS later. If no action is taken, this will overlap with other transmission slots. The only solution is that the MS advances its emission relative to its reception by a time corresponding to and from propagation delay. This value is call the timing advance. The timing advance value can be computed by the BTS and is then provided to the MS through signaling. The maximum timing advance of 233us (6 bits data) is adequate for MS to be up to 35 km from the BS, which is the maximum allowable cell radius of the GSM systems.

Common control channel (CCCH)

– Occupies TS0 of every control frame not used by BCH or the Idle Frame.

– 3 different CCCH types – Paging channel (PCH) 1. Provides paging signals from base to mobiles. 2. Notifies specific mobile of incoming call – Random access channel (RACH) 1. Uplink channel 2. Used by mobiles to acknowledge page from PCH 3. Used by mobiles to originate a call – Access grant channel (AGCH) 1. Provides forward link communication to mobile. 2. Specifies time slot, radio channel and dedicated

control channel. 3. AGCH is final CCCH message before mobile is

moved off the control channel.



Dedicated control channel (DCCH)

– Bidirectional 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.

– 3 different DCCH are specified. – Stand-alone dedicated control channel (SDCCH) 1. Carries signaling data following the connection of

the mobile with BS. 2. Intermediate and temporary channel for mobiles

while waiting for the BS to allocate a TCH channel. 3. Ensures that mobile and base remains connected

during authentication and resource allocation. 4. Maybe assigned their own physical channel or may

occupy TS0 of the BCH if there is low demand for BCH or CCCH traffic.

– Slow associated control channel (SACCH) 1. Always associated with a traffic channel 2. On downlink the SACCH carries power control and

timing advance instruction 3. On uplink the SACCH carriers signal strength and

quality information 4. 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).

Example : Mobile to PSTN GSM Set-up - Synchronize to nearest BTS by monitoring BCH. - Dial intended number - Burst of RACH data - BTS responds with AGCH message on CCCH and

assigns the mobile a new channel for SDCCH connection.

- The MS listens to the SACCH frame to get timing advance and power control info.

- MS sends authentication and validation requests. - MS is instructed by the BTS over the SDCCH to retune

to new radio channel and TS for TCH assignment.

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

– A normal burst consists of 148 bits – Guardtime 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



Mobile-Assisted handover

– There are four purposes for handover (1) rescue low-quality channel (2) recovering cochannel interference (3) traffic balancing among cells (avoid congestion or

load balancing) – directed handover. (4) recovering in the event of failure of a control channel



– There are three cases of handover, (1) From one radio channel to another of the same BSC. (2) Between channel of different BSCs under the control

of the same MSC. (3) Between different MSCs in the same licensed

operator. Between different licensed operators there are no procedure. – There are two modes of handover :

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

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

By repetitively monitoring the SCHs of all the surrounding cells between transmission and reception of traffic bursts, the MS can compute beforehand the timing advances required for all surrounding cells. The MS is then presynchronized with any such cell to which it may be handed. This speeds up the handover process.



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.



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 convolutional encoder with constraint length K=5.

• The least important 78 bits do not have any error protection.



– For TCH/F.9.6,



Channel coding – control channel

- GSM Control channel messages are defined to be 184 bits long, and are encoded using a shortened binary cyclic fire code, followed by a half-rate convolutional coder.

- The fire codes uses the generator polynomial

1)1)(1()( 317232640317235 +++++=+++= xxxxxxxxxG w

hich produces 184 message bits, followed by 40 parity bits.

- Four tail bits are added to clear the convolutional coder which follows, yielding a 228 bits per block.

- This block is applied to a half-rate K=5 convolutional code using the generator polynomials 43

0 1)( xxxG ++= and 43

1 1)( xxxxG +++= (same as type Ia TCH). The resulting 456 encoded bits are interleaved onto eight consecutive frames in the same manner as TCH speech data.



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 subblocks 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.



Verify the following:

– 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 Or 65 bits in 20ms = 13 kbps

Business

GSM System