GSM Air Interface

(𝑈𝑚)PRESENTED BY:

NAVEEN JAKHAR, ITS

ABHISHEK SINGH, ITS

Introduction

The Air-interface is the central interface of every

mobile system and typically the only one to which a

customer is exposed.

The physical characteristics of the Air-interface are

particularly important for the quality and success of

a new mobile standard.

RF Spectrum

GSM 900

Mobile to BS (UP-LINK) -890 to 915 MHz

BS to Mobile (DOWN -LINK) -935 to 960 MHz

Bandwidth - 25 MHz

GSM 1800 or PCS 1800

Mobile to Cell (UP-LINK) -1710 to 1785 MHz

Cell to Mobile (DOWN -LINK) -1805 to 1880 MHz

Bandwidth - 75 MHz

GSM SPECIFICATIONS

Carrier Separation - 200 kHz

Duplex Distance - 45 MHz

No. of RF Carriers - 124

Access Method - TDMA/FDMA

Modulation Method - GMSK

Transmission Rate - 270.833 Kbps

Speech Coding - Full rate 13 Kbps

Half rate 6.5 Kbps

GSM Air Interface structure

(FDMA/TDMA)

GSM utilizes a combination of frequency division

multiple access (FDMA) and time division multiple

access (TDMA) on the Air-interface. That results in a

two-dimensional channel structure,

WHY FDMA and TDMA?

In a pure FDMA system, one specific frequency is

allocated for every user during a call. That quickly

leads to overload situations in cases of high

demand.

In fullrate configuration, eight time slots (TSs) are

mapped on every frequency; in a halfrate

configuration there are 16 TSs per frequency.

in a TDMA system, each user sends an impulse like

signal only periodically, while a user in a FDMA

system sends the signal permanently.

The difference between the two is illustrated in

Figure

DIGITAL VOICE TRANSMISSION

Speech Coding

In GSM speech coding a block of 20 ms is

encoded in one set of 260 bits.

This calculates as 50X 260 = 13 kbps. Thus GSM

speech coder produces a bit rate of 13 kbps per

subscriber.

This provides speech quality which is acceptable for

mobile telephony and comparable with wire-line

PSTN phones.

CHANNEL CODING

Interleaving

The process of interleaving smaller packages of 456 bits over a larger time period, that is, distributing them in separate TSs.

How the packets are spread depends on the type of application the bits represent.

Signalling traffic and packets of data traffic are spread more than voice traffic.

The whole process is referred to as interleaving.

The goal of interleaving is to minimize the impact of the peculiarities of the Air-interface that account for rapid, short-term changes of the quality of the transmission channel. It is possible that a particular channel is corrupted for a very short period of time and all the data sent during that time are lost.

Interleaving does not prevent loss of bits, and if there is a loss, the same number of bits are lost.

However, because of interleaving, the lost bits are

BURST FORMATTING

Adds training sequence that helps in the working

equalizer to assist in countering the effects of radio

channel on the signal.

Total of 136 bits added, bringing overall total to 592

bits.

Each TS of TDMA frame is 0.577 ms long and during

this time 156.25 bits are transmitted.

One burst contains only 148 bits. Rest of the space,

8.25 bits time, is empty and is called Guard Period (

GP ).

GP enables MS/BTS to “ramp up” and “ ramp

down”.

GMSK

The modulation method in GSM is GMSK which

facilitates the use of narrow bandwidth and

coherent detection capability.

Rectangular pulses are passed through a Gaussian

filter prior to their passing through a modulator.

The modulation scheme almost satisfies the

adjacent channel power spectrum density

requirements of -60dB specified by CCIR.

FRAME HIERARCHY

The frame hierarchy is used for

synchronization between BTS and MS,

channel mapping, and ciphering.

Every BTS permanently broadcasts the

current frame number over the

synchronization channel (SCH) and

thereby forms an internal clock of the

BTS. There is no coordination between

BTSs; all have an independent clock,

except for synchronized BTSs.

That information is very important,

particularly during the initial access to a

Physical Versus Logical

Channels

Physical channels are all the available TSs of a BTS,

whereas every TS corresponds to a physical

channel. Two types of channels need to be

distinguished, the halfrate channel and the fullrate

channel.

Logical channels are piggybacked on the physical

channels. Logical channels are, so to speak, laid

over the grid of physical channels. Each logical

channel performs a specific task.

LOGICAL CHANNELS

Frequency Correction Burst

This burst format is used by FCCH channel only.

The whole data space (142 bits) is used for

unmodulated carrier (pure sinusoid) or carrier

modulated with all zero bits.

This pure carrier is the ‘identity’ of a beacon

frequency (also called BCCH-frequency or base-

frequency) and FCCH slot

Synchronization Burst

This burst format is used by SCH channel only.

This channel makes a mobile station time-

synchronized with the base station clock. That is why

the synchronization training sequence is very large

for this burst comparing to other burst types.

Only one training sequence is defined for this burst.

Access Burst

This burst format is used by RACH and AGCH channels.

When a mobile station sends an RACH message and receivesan AGCH reply, neither MS nor the BTS does have the timing-advance information.

For that reason, the actual message is relatively short andhave a long guard band (GB) in order to make sure that therewill be no overlap with the next burst.

The length of the guard band in the access burst (68.25 bits x3.69 = 251.16 ms) is equivalent to 37.5 km propagation delay.

The GSM allows a cell radius up to of 35 km.

That is, an RACH message from an MS at a distance of up to 35km from the base station can reach to the base stationantenna without overlapping the next burst.

The FACCH channel uses this burst during handover operation

Normal Burst

This burst format is used by all other channels

(except FCCH, SCH, RACH and AGCH).

This normal burst is used by TCH, SDCCH, SACCH,

FACCH, BCCH and PCH.

A few important features of the burst is stated

below.

Maximum 57 x 2 = 114 bits of voice/data per burst

Flag bit is to indicate if the channel is carrying user

traffic (Flag = 0) or control message bits (Flag = 1).

That is the flag is 0 for TCH and 1 for others.

Dummy Burst

This is like normal burst but has no meaning of its

payload bits

CONTROL MULTIFRAME

TRAFFIC MULTIFRAME (FULL

RATE)

This example shows 2 users using full rate voice traffic channels. (One user uses the slot 2 at every frame and the other user use the slot 4 at every frame).

At the center of the 26-frame traffic channel multiframe (i.e, Frame 12) is the Slow Associated Control Channel (SACCH) which carries link control information to and from the MS–BTS. At the last frame is 1 idle frame. All the remaining frame are allocated for Traffic. There is no dedicated FACCH frame or slots. FACCH steals TCH whenever it needs.

TRAFFIC MULTIFRAME (HALF

RATE)

This example shows 26-multiframe structure for TCH/HR, showing 2 users using HR voice traffic channel. In this example, the two users shares the slot 2 of every frame in alternating fashion. You would notice that there are two Frames for SACCH. The Frame 12 is for SACCH of user 1 and Frame 25 is for SACCH of user 2.

CONTROL MULTIFRAME

(BROADCAST CHANNELS) An example of control channel

multiframe structure for Broadcast Channel (Base Control Channel) which is made up of FCCH, SCH, BCCH, CCCH. It is for Downlink multiframe structure.

In Uplink, every frame is for single channel - RACH. Some key facts about DL Base Control Channel are

There are five FCCH equally spaced within the 51 multiframe.

Each FCCH is followed by a SCH, meaning that there are five SCH as well.

Four frames (Frame 2-5) are

allocated for BCCH.

The last frame (Frame 50) is allocated for Idle.

All the remaining Frames are allocated for CCCH(e.g, PCH or AGCH).

Mapping of Logical Channels Onto

Physical Channels

OPERATIONAL ASPECTS

Subscribers are not allocated dedicated channels

TCH allocated to users only when needed

Hence IDLE MODE & DEDICATED MODE

IDLE MODE

When MS is powered on (active) without being in

dedicated mode

MS stays continuously in touch with BS

Listens to transmissions from BS to intercept Paging

Messages ( for incoming calls)

• Monitors Radio Environment in order to evaluate

Channel Quality & choose the most suitable BS

• Listens to BS to avail short message broadcast

service

ACCESS PROCEDURE

Access to system ( switch over from IDLE to

DEDICATED Mode)

MS indicates to BS that it needs a connection

BS accepts the request & indicates which traffic

channel it may use

For above purpose specific transmission is done over

“ Common Channels”

OTHER FEATURES

Discontinuous Transmission

Synchronization between uplink and downlink

Diversity

Discontinuous Transmission

Speech activity only 40% of time.

Needs Voice activity detection.

Annoying clicks/inefficient DTX.

Generation of Comfort Noise at receiver to avoidthe feeling of the set being dead.

SYNCHRONIZATION BETWEEN

UPLINK AND DOWNLINK

For technical reasons, it is necessary that the MS and

the BTS do not transmit simultaneously. Therefore,

the MS Is transmitting three timeslots after the BTS.

The time between sending and receiving data is

used by the MS to perform various measurements on

the signal quality of the receivable neighbour cells.

Depending on the distance between the two, a considerable propagation delay needs to be taken into account.

That propagation delay, known as timing advance (TA), requires the MS to transmit its data a little earlier as determined by the “three timeslots delay

DIVERSITY

Space Diversity

Mounting two receiver antenna physically

separated a distance.

- Probability of both of them being affected by a

deep fading dip at same time is low.

THANK YOU!