CDMA vs GSM-two main competing network technologies

-Global System for Mobile Communications

-Code Division Multiple Access

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• GSM started to be used in USA in limited scale.

• In GSM, Frequency Division Multiplexing is used.

• 55 MHz higher for GSM.

• the GSM channels are much wider than the AMPS channels.

• GSM follows GSM standards.

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• A GSM system has 124 pairs of simplex channels.

• Each frequency band is 200 khz. Having each separate connection to it.

• Currently active station is assigned one time slot on one channel pair.

• Not practically, 992 channels supported in every cell but not active as to reduce the frequency conflicts.

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• All 8 shaded time slot belongs to same connection.

• Transmission and receiving doesn’t happen in same time slot because GSM radio can’t perform both the task at the same time.

• If the mobile station assigned to 890.4/935.4 MHz and time slot 2 wanted to transmit to the base station, it would use the lower four shaded slots (and the ones following them in time), putting some data in each slot until all the data had been sent

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• Each TDM frame has a specific structure.

• The group of TDM frames also have the specific structure.

• These TDM frames has some specific hierarchy.

• TDM slot consists of a 148-bit data

• 577 µsec – time for one TDM slot.

• Each frame starts and ends with 000 for reaching to the destination.

• Consist of 57 bit information field

• Each has control bit which identify that whether it is voice or data

• 26 bit sync is for receiver to identify sender’s frame boundaries.

• Data frame is transmitted in 547 microsecond

- Data frame is transmitted in 547 µsec, but a transmitter is only allowed to send one data frame every 4.615 msec.

- Shares channel with seven other stations

- each channel is 270,833 bps

- Gross of seven is 33854 kbps

- Payload field is 24.7kbps. (270833 bytes of each channel) before error correction

- After error detection this becomes 13 kbps for voice.

• 8 data frame = 1 TDM frame

• 26 TDM frame = 120msec multi frame

• In multi frame slot 12 for control, slot 25 for future use.

• 24 slots are available for user traffic.

• Broad case control channel - continuously sends signal to output

• Dedicated control channel is used for locating update, registration and call setup.– The information’s which are used to maintain the

database will be sent to the dedicated control channel.

• There is common control channel, which is split into three logical subchannels

• 1. paging channel – use to announce incoming call• 2. random access channel – request a slot on

dedicated control channel.• 3. access grant channel – requested slot is

assigned here

CDMA

• It is a basis for third generation mobile systems.

• CDMA works differently compare to GSM

• Used in US in second generation mobile system

• CDMA is described in International Standard IS-95

• CDMA allows each station to transmit over the entire frequency spectrum. (not the slots)

• Multiple simultaneous transmission separated using code theory.

• In CDMA, each bit time is subdivided into intervals called chips

• 64 or 128 bits per chip.

• Each station is assigned a unique m-bit code called a chip sequence.

• To transmit 1 bit, station sends chip sequence

• To send 0 bit, station sends one’s complement of its chip sequence.

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• m = 8, if station A is assigned the chip sequence 00011011, it sends a 1 bit by sending 00011011 and a 0 bit by sending 11100100.

• each station uses the full 1 MHz

• effective bandwidth per station is higher for CDMA than FDM

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