In order for several links to be in progress simultaneously in the same geographical area without mutual interference , multiple access techniques are deploy
The commonly used multiple access techniques
– Frequency Division Multiple Access (FDMA )
– Time Division Multiple Access (TDMA )
--Code Division Multiple Access (CDMA )
Terresterial interface
The terrestrial interfaces comprises all the connections between the GSM system entities ,apart from the Um or air interface.The terrestrial interfaces transport the traffic across the system and allows the passage of thousands of data messages to make the system function.
The standard interfaces used are
– 2 Mb/s
– Signalling System (C7 or SS7
– Packet Switched Data
– A bis using the LAPD protocol
INTERFACE NAMES
UM,ABIS,A,B,C,D,E,F,G,
Physical channel - Each timeslot on a carrier is referred to as a physical channel. Per carrier there are 8 physical channels.
Logical channel - Variety of information is transmitted between the MS and BTS. There are different logical channels depending on the information sent. The logical channels are of two types
• Traffic channel
• Control channel
*Channels ( Down/Up Link)*
Two types of Logical Channel
Traffic Channel(TCH)
Transmits traffic information, include data and speed.
Transmits all kinds of control information.
BCCH( Broadcast Control Channel )
• Downlink only
• Broadcasts general information of the serving cell called System Information
• BCCH is transmitted on timeslot zero of BCCH carrier
• Read only by idle mobile at least once every 30 secs.
SCH( Synchronisation Channel )
• Downlink only
• Carries information for frame synchronisation. Contains TDMA frame number and BSIC.
FCCH( Frequency Correction Channel )
• Downlink only.
• Enables MS to synchronise to the frequency.
• Also helps mobiles of the ncells to locate TS 0 of BCCH carrier.
CCCH Channels
RACH( Random Access Channel )
• Uplink only
• Used by the MS to access the Network.
AGCH( Access Grant Channel )
• Downlink only
• Used by the network to assign a signalling channel upon successfull decoding of access bursts.
PCH( Paging Channel )
• Downlink only.
• Used by the Network to contact the MS.
DCCH Channels
SDCCH( Standalone Dedicated Control Channel )
• Uplink and Downlink
• Used for call setup, location update and SMS.
SACCH( Slow Associated Control Channel )
• Used on Uplink and Downlink only in dedicated mode.
• Uplink SACCH messages - Measurement reports.
• Downlink SACCH messages - control info.
FACCH( Fast Associated Control Channel )
• Uplink and Downlink.
• Associated with TCH only.
• Is used to send fast messages like handover messages.
• Works by stealing traffic bursts.
Diversity
Receive diversity provides an effective technique for both overcoming the impact of fading across the radio channel and increasing the received signal to interference ratio.
The former is achieved by ensuring “uncorrelated” (i.e. low enough correlated) fading between antenna branches i.e. not all antennas experience fades at the same time.
SPEECH CODING
• The transmission of speech is one of the most important service of a mobile cellular system.
• The GSM speech codec, which will transform the analog signal(voice) into a digital representation, has to meet the following criterias
• A good speech quality, at least as good as the one obtained with previous cellular systems.
• To reduce the redundancy in the sounds of the voice. This reduction is essential due to the limited capacity of transmission of a radio channel.
• The speech codec must not be very complex because complexity is equivalent to high costs.
• The final choice for the GSM speech codec is a codec named RPE-LTP (Regular Pulse Excitation Long-Term Prediction).
CHANNEL CODING
• Channel coding adds redundancy bits to the original information in order to detect and correct, if possible, errors ocurred during the transmission.
• The channel coding is performed using two codes: a block code and a convolutional code.
• The block code receives an input block of 240 bits and adds four zero tail bits at the end of the input block. The output of the block code is consequently a block of 244 bits.
• A convolutional code adds redundancy bits in order to protect the information. A convolutional encoder contains memory. This property differentiates a convolutional code from a block code.
• A convolutional code can be defined by three variables : n, k and K.
• The value n corresponds to the number of bits at the output of the encoder, k to the number of bits at the input of the block and K to the memory of the encoder.
• The ratio, R, of the code is defined as R = k/n.
Example - Let's consider a convolutional code with the following values: k is equal to 1, n to 2 and K to 5. This convolutional code uses then a rate of R = 1/2 and a delay of K = 5, which means that it will add a redundant bit for each input bit. The convolutional code uses 5 consecutive bits in order to compute the redundancy bit. As the convolutional code is a 1/2 rate convolutional code, a block of 488 bits is generated. These 488 bits are punctured in order to produce a block of 456 bits. Thirty two bits, obtained as follows, are not transmitted :
C (11 + 15 j) for j = 0, 1, ..., 31
• The block of 456 bits produced by the convolutional code is then passed to the interleaver
• k=1
1 bit input Convolution code R = k/n = ½ n=2 2 bit input
CHANNEL CODING FOR GSM SPEECH CHANNELS
• . Before applying the channel coding, the 260 bits of a GSM speech frame are divided in three different classes according to their function and importance.
• The most important class is the class 1a containing 50 bits.Next important is the class 1b, which contains 132 bits.The least important is the class 2, which contains the remaining 78 bits.
• The different classes are coded differently.
• First of all, the class 1a bits are block-coded. Three parity bits, used for error detection, are added to the 50 class 1a bits.The resultant 53 bits are added to the class 1b bits.
• Four zero bits are added to this block of 185 bits (50+3+132). A convolutional code, with r = 1/2 and K = 5, is then applied, obtaining an output block of 378 bits.
• The class 2 bits are added, without any protection, to the output block of the convolutional coder. An output block of 456 bits is finally obtained
INTERLEAVING
• An interleaving rearranges a group of bits in a particular way.
• It is used in combination with FEC codes( Forward Error Correction Codes ) in order to improve the performance of the error correction mechanisms.
• The interleaving decreases the possibility of losing whole bursts during the transmission, by dispersing the errors.
• As the errors are less concentrated, it is then easier to correct them.
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