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GSM AIR

INTERFACE

KIPYEGON ODUNGA KABUGA OKEROSI BITOK

(Group V) {

14.08.2012

mtd-8101

}


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IntroductionThis presentation will cover in summary the following:

1. The air interface or radio interface refers to the manner in which communication

is achieved between the mobile handset and the base station.

2. Modulation techniques allow us to put information onto the radio wave.

3. Multiple Access techniques allow us to share the limited resources of the radio

spectrum among a number of users.

4. In GSM, there are many signaling and communication activities that must be

carried out. These are specified as logical channels, which must be mapped onto

the physical channels provided by the radio interface.

5. This mapping is achieved by using Multiframes.


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Introduction to GSM Air InterfaceA GSM interface is used to strictly define the actions, procedures and functions that

must

be implemented before any two or more GSM network elements can communicate.

The air interface also known as Um refers to the manner in which communication is

achieved between the mobile station and the base station. Its also referred to as the

radio interface.


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ModulationDefinition The process of putting a baseband signal (voice or data) onto a carrier wave

Modulation techniques

Analogue:

- Amplitude modulation (AM)- Frequency modulation (FM)

Digital:

- Pulse code Modulation (PCM)

- Frequency Shift Key modulation(FSK)

- Phase Shift Key modulation (PSK)

- Gaussian minimum shift keying (GMSK)


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Multiple Access Techniques in GSM

GSM applies two forms of multiple access

FDMA- Frequency division multiple access - provides carriers

TDMA- Time division multiple access - share access to the carriers

The third method of Multiple Access is Code division multiple access (CDMA )but not

applied in GSM networks.

These methods of access can be used together in one network to reduce congestion.

Access technique allow several users to share the limited resources of the radiospectrum among a number of users.


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F D M AFrequency Division Multiple Access

Available frequency spectrum is divided into channels each

of the same bandwidth

Channel separation achieved by filters:

Good selectivity

Guard bands between channels

Signaling channel required to allocate a traffic channel to

a user

Only one user per frequency channel at any time


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T D M A

Time Division Multiple Access

Access to available spectrum is limited to timeslots

User is allocated the spectrum for the duration of one timeslot

Timeslots are repeated in frames


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Frequency Allocation

GSM Uses both FDMA and TDMA multiple access techniques for both the 900 and

1800 bands.

Both uplink and downlink have each 25mhz of total spectrum available

The spectrum is divided into 124 carrier frequencies /channels

Space between two carrier frequencies is 200khz

Each carrier has 8 time slots


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Frequency Plan 900MHz band


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Protocols on the GSM air Interface


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Speech over the radio interface


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GSM ChannelsWe have two types of channels in GSM

Physical Channels - The physical resource available for use Logical Channels - The various ways that we use the resource

A physical channels is shared as carriers through methods such as FDMA and TDMA.A logical channel is however divided into Traffic channels and Control channels. See below:


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Traffic Channel - TimeslotsA full rate traffic channel is allocated to one timeslot. Normally, bits 1-7 of ti meslot 0

are used for control signaling.

TCH/F = 13 Kb/s voice or 9.6 Kb/s data

TCH/H = 6.5 Kb/s voice or 4.8 Kb/s data

One Physical channel (equivalent of 1 Timeslot )can support 1 TCH/F or 2 TCH/H

The half rate is normally implemented if there is congestion in the network.


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Logical Channels

TCH - Traffic ChannelsTCH/F Traffic Channel (full rate) (U/D)TCH/H Traffic Channel (half rate) (U/D)

BCH Broadcast ChannelsFCCH Frequency Correction Channel (D)SCH Synchronization Channel (D)BCCH Broadcast Control Channel (D)

CCCH Common Control ChannelsPCH Paging Channel (D)RACH Random Access Channel (U)AGCH Access Grant Channel (D)CBCH Cell Broadcast Channel (D)NCH Notification Channel (D)

DCCH Dedicated Control ChannelsSDCCH Stand alone Dedicated Control Channel (U/D)

FACCH Fast Associated Control Channel (U/D)U-Uplink D- Downlink


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GSM Frame structure

The basic element in the GSM frame structure is the frame itself.

It comprises the eight slots, each used for different users within the TDMAsystem

It defines the structure upon which all the timing and structure of the GSM

messaging and signaling is based.The fundamental unit of time is called a burst period and it lasts forapproximately 0.577 ms (15/26 ms).

Eight of these burst periods are grouped into what is known as a TDMAframe.


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Frame Hierarchy


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TDMA Frames- Time Division means that the frequency is divided up into blocks of time and only

certain logical channels are transmitted at certain times.

- The time divisions in TDMA are known as Time Slots .

- Each TDMA frame is divided into 8 time slots

- GSM uses Gaussian Minimum-Shift Keying (GMSK) as its modulation method.

- -GMSK provides a modulation rate of 270.833 kilobits per second (kb/s).At that rate, a maximum of 156.25 bits can be transmitted in each time slot (576.9 s).


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TDMA MultiframesTDMA frames are grouped into two types of Multiframes

26-frame Multiframes for traffic channels (Traffic channel Multiframes) which is

composed of 26 TDMA frames

51-frame Multiframes for control channels (Control channel Multiframes) which is

composed of 51 TDMA frames


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Traffic Channel (TCH) Multiframes

The diagram shows a Traffic Channel (TCH) Multiframes with TS2 (green) being

allocated to a Mobile Station (MS).

The red arrow indicates the sequence of transmission. The sequence starts in TDMA

frame 0 at TS0, proceeds through all eight time slots, then starts again with TDMA

frame 1. The MS has been allocated a Traffic Channel in TS2. Therefore the MS will

only transmit/receive during TS2 of each TDMA frame.


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Bursts

The information content carried in is called a burst. Each burst allows 8.25 bits for

guard time . This is to prevent bursts from overlapping and interfering with

transmissions in other time slots.

There are four main types of bursts

1.NORMAL BURST This GSM burst is used for the standard communications between the base-station and the mobile, and typically transfers the digitized voice data.

2.FREQUENCY CORRECTION BURST This burst is used for frequency synchronization of themobile station. The broadcast of the FB usually occurs on the logical channel FCCH.

3.SYNCRONIZATION BURST It is used for time synchronization of the mobile. The data payloadcarries the TDMA Frame Number (FN) and the Base Station Identity Code (BSIC). It is broadcastwith the frequency correction burst. The Synchronization Burst is broadcast on theSynchronization Channel (SCH) .

4.RANDOM ACCESS BURST This burst is used the by mobile station for random access. It isdesigned to compensate for the unknown distance of the mobile station from the tower, whenthe MS wants access to a new BTS, it will not know the correct Timing Advance.


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Interleaving IntroductionInterleaving is a technique where sequential data words or packets are spread across several

transmitted data bursts.

In this way, if one transmitted burst or group is lost as a result of noise or some other drop-out, then

only a small proportion of the data in each original word or packet is lost and it can be reconstructed

using the error detection and correction techniques employed.

This method rearranges a group of bits in a particular way. It is combined with FEC codes in order to

improve the performance of the error correction mechanisms. Interleaving decreases the possibility of

losing whole bursts during the transmission, by dispersing the errors. Since the errors become less

concentrated , it is then easier to correct them.

At the physical layer a burst in GSM transmits 2 blocks of 57 data bits each. Thus, the 456-bit block

output of the channel coder fit into 4 bursts (4*114 = 456). The 456 bits are, thus, divided into 8*57-bit

blocks. As interleaving is applied during the forming of the blocks, the 1st block of 57 bits contains the

bit numbers (0, 8, 16, .....448), the second one the bit numbers (1, 9, 17, .....449), etc. The last block of

57 bits will then contain the bit numbers (7, 15, .....455).


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Interleaving . .Interleaving for the GSM speech channels

The 456-bit block, obtained after the channel coding, is divided in 8*57-bit blocks in the same way as itis explained in the previous paragraph. But these 8 blocks are distributed differently.

The first 4 blocks of 57 bits are placed in the even-numbered bits of 4 consecutive bursts. The otherfour blocks are placed in the odd-numbered bits of the next four bursts. The interleaving depth of theGSM interleaving for speech channels is then 8. A new data block also starts every 4 bursts. Theinterleaver for speech channels is called a block-diagonal interleaver.

Interleaving for the GSM data TCH channels

A particular interleaving scheme, with an interleaving depth equal to 22, is applied to the block of 456 bitsobtained after the channel coding. The block is divided into 2 blocks of 6 bits each , 2 blocks of 12 bitseach , 2 blocks of 18 bits each and 16 blocks of 24 bits each . It is spread over 22 bursts in the followingway :

the 1st and 22nd bursts carry one block of 6 bits each (2)

the 2nd and 21st bursts carry one block of 12 bits each (2)

the 3rd and 20th bursts carry one block of 18 bits each (2)

from 4th to 19th burst, a block of 24 bits is placed in each burst (16)


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Channel CodingChannel coding adds redundancy bits to the original information to detect and correct, if

possible, transmission errors.

Channel coding for the GSM data TCH channels

Channel coding is performed using two codes: a block and a convolutional code.

o The block code is defined in the GSM Recommendations 05.03. It receives an input block of 240 bits and adds 4-zero tail bits at the end of the input block; this results a block output of 244

bits.

o A convolutional code adds redundancy bits to protect the data. A convolutional encoder

contains memory. This property differentiates the two types of code. A convolutional codecan be defined by three variables : n, k and K. The value n corresponds to the number of

output bits from the encoder, k to the number of input bits and K to the memory of the

encoder. The ratio (R) of the code is defined as R = k/n.


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Channel CodingFor example, a convolutional code with k=1, n=2 and K=5, uses a ratio of R = 1/2 and delay of K=5, which means that it will add one redundant bit for each input bit (1 in 2 output bits is

an input bit) . The code uses 5 consecutive bits to compute the redundancy bit. As theconvolutional code is a 1/2 rate for an input block of 244 bits an output block of 488 bits is generated. These 488 bits are punctured to produce a block of 456 bits . 32 bits, obtained as follows, are not transmitted :

C (11 + 15 j) for j = 0, 1, ..., 31 The output block of 456 bits is then passed to the interleaver .

Channel coding for the GSM speech channels

Before applying channel coding, the 260 bits of a GSM speech frame are divided in 3

different classes according to function and importance. The most important class is theclass Ia containing 50 bits. Next in importance is the class Ib, which contains 132 bits . Theleast important is the class II, which contains the remaining 78 bits. The different classesare coded differently:


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Power ControlThe power levels and power control of GSM mobiles is of great

importance because of the effect of power on the battery life.

The power of the GSM mobiles is closely controlled so that the

battery of the mobile is conserved, the levels of interference are

reduced and performance of the base-station is not

compromised by high power local mobiles.

BTS controls the power output of the mobile, keeping the GSM

power level sufficient to maintain a good signal to noise ratio,

while not too high to reduce interference, overloading, and also

to preserve the battery life.

The BTS controls the power of the mobile by sending a GSM

"power level" number. The mobile then adjusts its power

accordingly.

GSM power level table ( GSM 1900)


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GSM Power Classes

This is achieved by allocating a GSM power

class number to a mobile. This GSM power

class number indicates to the base station the

maximum power it can transmit and hence the

maximum power level number the base station

can instruct it to use.

The GSM power classes vary according to theband in use.

GSM power class designations have been

allocated to indicate the power capability of

various mobiles.

Not all mobiles have the same maximum power output level. In order that the base station

knows the maximum power level number that it can send to the mobile, it is necessary for

the base station to know the maximum power it can transmit.


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Brief Summary


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References

AIRCOM International (2002), Document number: P/TR/005/G102/3.0a, GSM

system overview, England, AIRCOM International

www.aircom.co.uk

http://www.radio-electronics.com

http://www.wikipedia.org/

http://gsmfordummies.com/tdma/tdma.shtml

http://www0.cs.ucl.ac.uk/staff/t.pagtzis/wireless/gsm/radio.html
http://www.aircom.co.uk/http://www.radio-electronics.com/http://www.wikipedia.org/http://gsmfordummies.com/tdma/tdma.shtmlhttp://www0.cs.ucl.ac.uk/staff/t.pagtzis/wireless/gsm/radio.htmlhttp://www0.cs.ucl.ac.uk/staff/t.pagtzis/wireless/gsm/radio.htmlhttp://gsmfordummies.com/tdma/tdma.shtmlhttp://www.wikipedia.org/http://www.radio-electronics.com/http://www.radio-electronics.com/http://www.radio-electronics.com/http://www.aircom.co.uk/


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thanks KIPYEGON ODUNGA KABUGA OKEROSI BITOK

}

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