Properties of radio path

1 © NOKIA BSS.PPT/EG

Properties of Radio Path


4. Radio Path

Module Objectives

This module describes the GSM Radio Path. After studying this material you

should be able to:

Explain at least one of the problems in the air interface and its solution in the GSM

Name at least three logical channels in the air interface and explain their functions without using any reference.



• Fast Fading (Rayleigh Fading)-Caused by multipath propogation. Signal received is the vector sum of original transmitted signal.

-Typical in PLMN, in city areas where many reflectors exist, tall buildings and mountains.

• Fast Fading-Selective Fading (time dispersion)-Flat Fading


Properties of Radio Path• Selective Fading

-Reflected signal coming from objects that are far away (1-5km)

-Bit Rate in GSM is 270kbit/s.Time corresponds to 1.1km bit length.This distance&longer distances causes problems.-Typical in areas of mountainous terrain or areas with large expansions of water or both.-Result is a problem called ISI (Inter Symbol Interference)

BTSBTS


Properties of Radio Path• Selective Fading-Solution

-We model the air interface and treat it as a filter.

-26 bits are added into the middle of every burst called the training sequence code.

-After receiving a burst, we pass the burst through a filter in MS/BTS.

-When we find the best possible filter we use this filter to interpret the data/speech bits.

-Not all possible filters are tried but only the ones that are the most likely to reduce time used in filtering.

-We use Viterbi Equalizers.

Air-Interface FilterSignal awaited to be received


Properties of Radio Path• Flat Fading-Caused by the Vector Summation of signals from near objects.

-Summation can be beneficial but can also be zero and cause fading dips.

-In GSM 900/DCS 1800 dips occur approximately 17/8.5cm.

RX sensitivity

Approx. 17cm

Fading dips

-Fading dips are most likely to occur at every half wave length and therefore are frequency dependent.

-Faster the MS moves, less information is lost because of fading dips.



• Flat Fading-Solutions

-Frequency Hopping

-Channel Coding/Speech Coding

-Interleaving

-Antenna/Receiver Diversity


Properties of Radio Path• Frequency Hopping

F2F1

F3F4

Time

-Dips are frequency dependent so by changing the transmitted frequency we can move the dips to different locations.

-Beneficial especially to slow moving MSs

-Optional in the BTS, but obligatory in MS

-For each cell(sector)the hopping sequence is the same.

-Information of the sequence is broadcast on the control channels.

-Hopping sequence must be different on each BTS site.

-Frequency that sends the control channels must remain on the same frequency and power level.


Properties of Radio Path• Speech Coding

-Performed in TCSM and MS.-In GSM we use LPC-LTP-RPE codingLPC – Linear predictive codingLTP – Long term predictionRPE – Regular pulse excitationHybrid Speech Coder,Waveform Coder&Vocoder

-In order to reduce synthetic sound caused by vocoder coder we use some additional information from waveform coder.

-20ms samples of speech, Parameters of filters are transmitted.Not actual speech.



• Channel Coding(Block Coding and Convolutional Coding)• Block Coding

-Used to detect errors-Adds additional bits into the information sent (check-sum bits, convolutional-coder reset bits)

• Convolutional Coding-Used to detect and correct errors detected-Doubles the amount of bits coded


SPEECHSEGMEN-TATION

SPEECHCODING

BLOCKCODING

CONVOL-TUTIONALCODING

INTER-LEAVING

CIPHERINGBURST

FORMAT-TING

TRXMODULA-

TION

33.8 Kbits

900/1800 MHz

20 ms 13 Kbits/sec= 260 bits

22.8 Kbits= 456 bits

Channel coding




Received signal

RXRX

Signal Processing

Antennas

•Antenna/Receiver Diversity

-Two receiver aerials

-Signal received seperately and strongest is selected by DSPU.

-Aerials six/three meters apart, thus giving 6db gain in signal strength.

-GSM 900 approximately 6 meters,GSM 1800 approximately 3 meters.



Solution-Adaptive power control, transmission power of BTS/MS changes-Adaptive power control is active on all speech and common channels but not on broadcast channels

BTSBTS

•Slow Fading

-Caused by hills and other objects on the signal way


Channel Organization

• There are 2 different types of channels in GSM/DCS• Physical Channel

-Single TSL on a single frequency-8 physical channels per frequency (TDMA frame)-Information sent on 1 channel is termed a burst

• Logical Channel-Within a burst

-Information sent is of particular type (speech/signalling/data/sms)-There are 11 logical channels-Logical channels are mapped so that they take minimum space in order to maximize amount of traffic channels.


Broadcast channels

Frequency Correction Channel (FCCH)• Pure sine wave.• The MS searches for this channels to switch on.• Downlink.

Synchronisation Channel (SCH)• After locking to the frequency the MS synchronises with

the SCH.• The SCH contains the BSIC of the BTS and the TDMA frame

number (used in encryption).

Broadcast Control Channel (BCCH)• Common information about the BTS:

– Used frequencies– Frequency hopping sequence– Channel combination– Paging groups– Surrounding cell information


Common control channels

Paging Channel (PCH)• Used by BTS to page a mobile.• A downlink channel only.

Random Access Channel (RACH)• Used by the MS to request a dedicated control

channel.• Used for e.g. mobile originated calls.• An uplink channel only.

Access Grant Channel (AGCH)• Used by the BTS to assign a dedicated control

channel.• A downlink channel only.


Dedicated channelsStand Alone Dedicated Control Channel (SDCCH)

• Bi-directional channel.• Used for call set-up procedures, e.g. authentication.• The traffic channel (TCH) is assigned by using SDCCH.

Slow Associated Control Channel (SACCH)• Associated with SDCCH and TCH.• Measurement reports.• MS power control.• Timing alignment.

Fast Associated Control Channel (FACCH)• Associated with TCH.• For quick control communication, e.g. handover.• Physically replaces 20 ms of speech, “stealing mode”


Traffic channelsFull Rate

• Bi-directional channel.• Used for speech or data

transmission.• User data bit rate 13 kbit/s.

Half Rate• Bi-directional channel.• Used for speech or data

transmission.• User data bit rate 6-7 kbit/s.

Enhanced Full Rate (EFR)• Bi-directional channel.• Used for high quality speech

transmission.• User data bit rate 13 kbit/s.


4. Radio PathTraining MaterialTraining MaterialTDMA Frames and Multiframes (Downlink)

TSL0 TSL1 TSL2 TSL3 TSL4 TSL5 TSL6 TSL70 FCCH TCH1 SCH TCH2 BCCH TCH3 BCCH TCH4 BCCH TCH5 BCCH TCH6 CCCH TCH7 CCCH TCH8 CCCH AGCH TCH9 CCCH TCH

10 FCCH TCH11 SCH TCH12 CCCH SACCH13 CCCH TCH14 CCCH PCH TCH15 CCCH TCH16 CCCH TCH17 CCCH TCH18 CCCH PCH TCH19 CCCH TCH20 FCCH TCH21 SCH TCH22 SDCCH TCH23 SDCCH TCH24 SDCCH TCH25 SDCCH IDLE26 SDCCH27 SDCCH28 SDCCH29 SDCCH30 FCCH31 SCH32 SDCCH33 SDCCH34 SDCCH35 SDCCH36 SDCCH37 SDCCH38 SDCCH39 SDCCH40 FCCH41 SCH42 SACCH43 SACCH44 SACCH45 SACCH46 SACCH47 SACCH48 SACCH49 SACCH50 IDLE51 FCCH52 SCH

MU

LTIF

RA

ME

, 51

TD

MA

FR

AM

ES

MU

LTIF

RA

ME

, 26

TD

MA

FR

AM

ES


GSM burst types (1)

tailbits

3

fixed bits ("0")142

tailbits

3

guardperiod

8,25 bits

FREQUENCY CORRECTION BURST

tailbits

3

encrypted bits57

SB1

training sequence

26

SB1

encrypted bits57

tailbits

3

guardperiod

8,25 bits

NORMAL BURST

TDMA FRAME ~ 4.615 ms

576.9 µs


GSM burst types (2)

ext. tailbits

8

synchronisationsequence

41

encrypted bits36

tailbits

3

extended guard period68,25 bits

ACCESS BURST

tailbits

3

encrypted bits39

extended training sequence

64

encrypted bits39

tailbits

3

guardperiod8,25 bits

SYNCHRONISATION BURST

tailbits

3

mixed bits142

tailbits

3

guardperiod

8,25 bits

DUMMY BURST


Burst types• Frequency correction burst

Used to transmit the FCCH channel. No information.

• Synchronisation burstUsed to transmit synchronisation information.

• Access burstUsed to send RACH information.

RACH contains the first message from the MS to the BTS.

It has a long guard period to allow the BTS to calculate the MS distance from the BTS and to provide timing advance information to the MS.

• Normal burstUsed to send all other logical channel information.

• Dummy burstUsed to fill up unused timeslots in the TRX, which transmits the BCCH channel.No real information.

Properties of radio path

Engineering

gsm radio path

control channels

radio path module objectives

bits convolutional coding

cause fading dips

transmitted frequency

physical channels

common channels