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1. 1. Properties of Radio Path
2. 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. 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. 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. 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. 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. 7. Properties of Radio Path • Flat Fading-Solutions -Frequency Hopping -Channel Coding/Speech Coding -Interleaving -Antenna/Receiver Diversity
8. 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. 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. 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
11. 11. SPEECH SEGMEN- TATION SPEECH CODING BLOCK CODING CONVOL- TUTIONAL CODING INTER- LEAVING CIPHERING BURST FORMAT- TING TRX MODULA- 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. 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. 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. 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. 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. 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. 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. 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.
19. 19. 4. Radio PathTraining MaterialTraining Material TDMA Frames and Multiframes (Downlink) TSL0 TSL1 TSL2 TSL3 TSL4 TSL5 TSL6 TSL7 0 FCCH TCH 1 SCH TCH 2 BCCH TCH 3 BCCH TCH 4 BCCH TCH 5 BCCH TCH 6 CCCH TCH 7 CCCH TCH 8 CCCH AGCH TCH 9 CCCH TCH 10 FCCH TCH 11 SCH TCH 12 CCCH SACCH 13 CCCH TCH 14 CCCH PCH TCH 15 CCCH TCH 16 CCCH TCH 17 CCCH TCH 18 CCCH PCH TCH 19 CCCH TCH 20 FCCH TCH 21 SCH TCH 22 SDCCH TCH 23 SDCCH TCH 24 SDCCH TCH 25 SDCCH IDLE 26 SDCCH 27 SDCCH 28 SDCCH 29 SDCCH 30 FCCH 31 SCH 32 SDCCH 33 SDCCH 34 SDCCH 35 SDCCH 36 SDCCH 37 SDCCH 38 SDCCH 39 SDCCH 40 FCCH 41 SCH 42 SACCH 43 SACCH 44 SACCH 45 SACCH 46 SACCH 47 SACCH 48 SACCH 49 SACCH 50 IDLE 51 FCCH 52 SCH MULTIFRAME,51TDMAFRAMES MULTIFRAME,26TDMAFRAMES
20. 20. GSM burst types (1) tail bits 3 fixed bits ("0") 142 tail bits 3 guard period 8,25 bits FREQUENCY CORRECTION BURST tail bits 3 encrypted bits 57 S B 1 training sequence 26 S B 1 encrypted bits 57 tail bits 3 guard period 8,25 bits NORMAL BURST TDMA FRAME ~ 4.615 ms 576.9 µs
21. 21. GSM burst types (2) ext. tail bits 8 synchronisation sequence 41 encrypted bits 36 tail bits 3 extended guard period 68,25 bits ACCESS BURST tail bits 3 encrypted bits 39 extended training sequence 64 encrypted bits 39 tail bits 3 guard period 8,25 bits SYNCHRONISATION BURST tail bits 3 mixed bits 142 tail bits 3 guard period 8,25 bits DUMMY BURST
22. 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.