Properties of radio path
-
Upload
deepak-joshi -
Category
Engineering
-
view
249 -
download
0
description
Transcript of Properties of radio path
1 © NOKIA BSS.PPT/EG
Properties of Radio Path
2 © NOKIA BSS.PPT/EG
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.
3 © NOKIA BSS.PPT/EG
Properties of Radio Path
• 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
4 © NOKIA BSS.PPT/EG
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
5 © NOKIA BSS.PPT/EG
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
6 © NOKIA BSS.PPT/EG
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.
7 © NOKIA BSS.PPT/EG
Properties of Radio Path
• Flat Fading-Solutions
-Frequency Hopping
-Channel Coding/Speech Coding
-Interleaving
-Antenna/Receiver Diversity
8 © NOKIA BSS.PPT/EG
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.
9 © NOKIA BSS.PPT/EG
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.
10 © NOKIA BSS.PPT/EG
Properties of Radio Path
• 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
11 © NOKIA BSS.PPT/EG
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
Properties of Radio Path
12 © NOKIA BSS.PPT/EG
Properties of Radio Path
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.
13 © NOKIA BSS.PPT/EG
Properties of Radio Path
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
14 © NOKIA BSS.PPT/EG
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.
16 © NOKIA BSS.PPT/EG
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
17 © NOKIA BSS.PPT/EG
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.
18 © NOKIA BSS.PPT/EG
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”
19 © NOKIA BSS.PPT/EG
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.
21 © NOKIA BSS.PPT/EG
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
22 © NOKIA BSS.PPT/EG
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
23 © NOKIA BSS.PPT/EG
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
24 © NOKIA BSS.PPT/EG
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.