2. 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. 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. 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. 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 Filter
Signal awaited
to be received
6. 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. Properties of Radio Path
• Flat Fading-Solutions
-Frequency Hopping
-Channel Coding/Speech Coding
-Interleaving
-Antenna/Receiver Diversity
8. Properties of Radio Path
• Frequency Hopping
F2
F1
F3
F4
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. Properties of Radio Path
• Speech Coding
-Performed in TCSM and MS.
-In GSM we use LPC-LTP-RPE coding
LPC – Linear predictive coding
LTP – Long term prediction
RPE – Regular pulse excitation
Hybrid 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. 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
12. 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. 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. 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.
15. 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
16. 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.
17. Dedicated channels
Stand 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”
18. Traffic channels
Full 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.
22. Burst types
• Frequency correction burst
Used to transmit the FCCH channel. No information.
• Synchronisation burst
Used to transmit synchronisation information.
• Access burst
Used 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 burst
Used to send all other logical channel information.
• Dummy burst
Used to fill up unused timeslots in the TRX, which transmits the BCCH channel.
No real information.
