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GSM Radio Link Management

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GSM Radio Link Management

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    2. GSM Network Overview

    3. GSM Radio Link Management Um The Air Interface Mobile Radio Propagation Types of signal strength variations Transmitting and receiving Process Speech Coding Bit Interleaving Time slot staggering Timing Advance & Time alignment Power control Discontinuous Transmission (DTX) FREQUENCY HOPPING

    4. Um The Air Interface Between BTS and MS Utilizes a radio wave which is subject to attenuation,reflection,and interference from other transmitter. These effects will causes loss of signal strength and distortion which will impact on the quality on radio link and voice channel. To cope with the harsh conditions,GSM make use of an efficient and protective signal processing.

    5. Mobile Radio Propagation

    6. Types of signal strength variations The signal strength variation for mobile are due to different types of signal strength fading There are two types of signal strength variations 1. Macroscopic Variations Due to the terrain contour between BTS and MS The fading effect is caused by shadowing and diffraction of radio waves. 2.Microscopic variations. Due to multipath, Short-term or Rayleigh fading. As the MS moves, radio waves from many different path will be received.

    7. Rayleigh Fading Occur as the mobile moves over short distances. Caused by signal scattering in the vicinity of the mobile unit e.g. by hill,building or traffic. The result is that not one but many different paths are followed between transmitter and receiver(Multipath Propagation) The reflected wave will be altered in both phase and amplitude.The signal may effectively disappear if the reflected wave is 1800 out of phase with the direct path signal. The partial out of phase relationships among multiple received signal produce smaller reduction in received signal strength.

    8. Effects caused by Rayleigh fading Reflection and multipath propagation can cause positive and negative effects. Coverage Extension Multipath propagation allow radio signal to reach behind hills and buildings and into tunnels. Constructive and destructive interference

    9. Transmitting and receiving Process The two major process involved are coding and modulation

    10. Coding Preparing the basic data signal so that they are protected and put it in a form that the radio link can handle Coding is included in Speech coding Channel coding Bit Interleaving Encryption. Modulation Processing that involves the physical preparation of the signal so that the information can be transported on a RF carrier

    11. Speech Coding There are 3 different ways of speech audio coding used in telephony 1.Analog FM Speech signal is filtered until it occupies 3 KHz B.W In FM modulator,the signal modulates the frequency deviation of the carrier ,resulting in a B.W of 25KHz.

    12. Speech Coding 2.Telephone PCM PCM used in PSTN consists of a 64 kbits/s stream consisting of 8000 samples taken per second,each sample being coded in 8 bits.

    13. Speech Coding 3.Linear Predictive Coding Digital cellular cannot handle the high bit rate used for PCM Processing have been developed for reduction of bit rate. Full rate channel operate with a protected bit rate of 22.8kbits/s where in the speech itself only uses 13kbits/s. The GSM speech coding scheme at 13kbits/s is called RPE-LTP(Regular pulse Excitation –Long Term Prediction)

    14. Speech coding techniques There are 3 classes of speech coding Techniques 1.Waveform Coding Speech is transmitted as good as possible P.C.M is an example of waveform coding Bit rate ranges from 24-64kbps 2.Hybrid Coding A mix of waveform coding and parameter coding. This technique is applied in GSM(13kbps) 3.Parameter Coding Only a very limited quantity of information is sent. Only 1-3kbps is required for the speech transmission. The regenerated speech is intelligible but it suffers from noise.

    15. Speech coding in the MS There are 3 steps 1. The 64kbits/s PCM transcoded from A law quantized 8bits per sample into a linearly quantised 13bits per sample bit stream ,that correspond to a 104kbits/s bit rate. 2. The 104kbits/s stream is fed into the RPE-LTP speech encoder which takes the 13 bits samples in a block of 160 samples(every 20ms) 3. RPE-LTP encoder encoder produces 260bits in every 20ms (The resulting bit rate of 13kbits/s)

    16. Channel Coding The data to be transmitted over the interface must be specially protected against the corruption due to the effects of fading and co-channel interference. Additional bits are generated for this purpose and permits the detection of transmission error and to a certain extent the reconstruction of the original data. The channel coding process increases the bit rate from 13kbis/s to 22.8kbits/s result in 456bits coming out during the 20ms.

    17. Bit Interleaving To distribute sub blocks of data obtained by channel coding in such a way that one data block is distributed over several TDMA frame. The sub blocks are rearranged and transmitted. 1. The 456-bit block from the channel coder are fed to the bit interleaver where they are split into 8 sub blocks of 57 bits. 2. The 57 bits at a time will be interleaved with 57 bits of from an adjacent block to form what is called a burst of 114 data bits.

    18. Burst building for TCH data

    19. FDMA/TDMA Scheme

    20. Benefits of bit Interleaving The bit interleaving reduces the adverse effects of Rayleigh fading by preventing entire data blocks from being destroyed by a signal fade.

    21. GSM Speech to Radio waves

    23. Time slot staggering. It is the principle of deriving the time slot organization of uplink from the timeslot organization of downlink. A particular time slot of uplink is derived from the downlink by shifting the downlink time slot number by 3. Reason:- By shifting 3 time slot , the mobile avoids to transmit and receive simultaneously. This allows an easier implementation of the mobile station. Typically , a mobile station will receive during one time slot,and then shift the frequency by 45MHz to transmit some time later.

    24. Time alignment BTS receives signals from different mobile stations . When a mobile station is far from the BTS,the BTS must deal with the propagation delay. It is essential that the burst received at the BTS fits correctly into the time slot. Otherwise,the burst from the mobile stations using adjacent time slot could overlap,resulting in poor transmission.

    25. Timing Advance

    26. Timing Advance In order to solve the problem of propagation delay,a compensation mechanism is necessary in the mobile station. The mobile station is able to advance its transmission time by a time known as the timing advance.

    27. Time alignment definition Time alignment is the process of transmitting early the burst to the BTS(the timing advance) to compensate for the propagation delay. BTS continuously measures the time offset between its own burst schedule and the reception schedule of the mobile station burst. Based on these ,measurement, the BTS is able to provided the mobile station with the required timing advance via the SACCH. Timing advance is derived from the distance measurement. The BTS sends to each mobile station a timing advance parameter Each mobile station advances its timing,with result that signals from different mobile station arriving at BTS are compensated for propagation delay.

    28. Time alignment process. The MS at 10Km away from BTS must start transmitting 66micro sec earlier to compensate for the round trip delay.

    29. Time alignment process A 6 bit number indicates how many bits the MS must advance its transmission.This time advance is TA The 68.25 bit long GP(guard period) of the access burst provides the required flexibility to advance the transmission time. The time advance TA can have a value between 0 and 63 bits lengths. 1 TS=156.25 bits in 0.577ms (i.e Each burst consists of 148 usable bits of 3.69 m sec each & between the bust there is a guard period of 30.5 m sec (=8.25 bits) to distinguish the consecutive bursts in normal burst) The maximum mobile range of 35Km is rather determined by the timing advance value than by the signal strength.

    30. Power control Enables Mobile station and BTS to increase or decrease the transmission power Separately performed for the uplink and down link. BSC is responsible for initiating the power control Power Control - Why? To save Mobile Station battery power To reduce interference on other channel

    31. Power Control - How? MS Measures (Down link Measurement) Strength of the signal from BTS Quality of the signal from BTS BTS Measures (Up link Measurement) Strength of the signal from MS Quality of the signal from MS MS and BTS send measurement report periodically to BSC through SACCH every 480 ms If the BSC notices that the signal strength is below the lower pre defined threshold, it sends a power control command to the MS to increase its transmitter power by a pre-defined step (2dB)

    32. Discontinuous Transmission (DTX) A mechanism that which allows the radio transmitter to be switched off during speech pauses Discontinuous Transmission – How? Voice activity detector detects weather there is speech or no speech If no speech, transmission will cease after a defined period of time. The transmitter will periodically send a signal called silence indicator block. This signal provides comfort noise level information to the MS or BTS.

    33. Discontinuous Transmission – Why? To decrease the co-channel interference level Saves the battery power for the MS. Disadvantage Slightly deteriorates the quality of transmission. Note : Does not save time slots on the air interface

    34. FREQUENCY HOPPING Change of frequency after every frame in a pre-determined manner Since multipath fading is dependent on carrier frequency, slow frequency hopping helps alleviate the problem

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