April 9, Mobile Computing COE 446 Wireless Multiple Access Tarek Sheltami KFUPM CCSE COE Principles of Wireless Networks K. Pahlavan and P. Krishnamurth

Outline Fixed Frequency Assignment FDMA TDMA CDMA FDD TDD April 9, 20152

3 Fixed-Assignment Access for Voice- oriented Networks All existing voice networks use fixed assignment access methods Fixed allocation of channel resources are made available on a predetermined basis to a single user for the duration of the communication session Three basic fixed-assignment multiple access are FDMA, TDMA and CDMA The choice of an access method will have a great impact on the capacity and QoS provided by the network

April 9, Fixed-Assignment Access for Voice- oriented Networks.. If the downlink and uplink channel use the same frequency band for communication, but alternated time slots, the system is referred as TDD In TDD, Users are allocated one or more timeslots for uplink and downlink transmission TDD systems are often used in local area pico- or microcellular systems PCS networks Picocellular correspond to radio cells covering a building or parts of buildings any where between m Microcellular spans hundreds of meters to a kilometer. If the downlink and uplink use different carrier frequencies that are sufficient separated, the duplex scheme is referred to as FDD FDD is mostly used in macrocellular systems designed for coverage of several tens of kilometers

April 9, AP/ mini-BTS Radio Microcell (~0.5-1 Km radius) Picocellular (~30-100m radius) Radio macrocell (hundreds of Kilometers)

April 9, Fixed-Assignment Access for Voice- oriented Networks.. FDMA All users can transmit signals simultaneously All users separated from one another by their frequency of operation FDMA/FDD Downlink and uplink channel use different frequency carriers Fixed sub-channel pair is assigned to an MT during the communication session At the receiving end, the MT filters the designated channel out of the composite signal

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8  30 kHz of BW for downlink and uplink  There are 421 channels in 25 MHz of spectrum assigned to each direction, 395 for voice traffic and the rest for signaling

April 9, Fixed-Assignment Access for Voice- oriented Networks.. FDMA/TDD The downlink and uplink take turns via alternating time slots

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April 9,  The total allocated BW for CT-2 cordless phones = 4 MHz, supporting 40 carrier, each using 100 KHz of BW

April 9, Fixed-Assignment Access for Voice- oriented Networks.. TDMA TDMA users share the same frequency band by taking assigned turns in using the channel The major advantage of TDMA over FDMA is the format flexibility Because of the fully digital format flexibility of buffering and multiplexing functions, time slot assignment among multiple users are readily adjustable to provide different access rates for different users

April 9, Fixed-Assignment Access for Voice- oriented Networks.. TDMA A transmit controller assigns time slots to users An assigned time slot is held by a user until the user releases it At the receiving end, a receiver station synchronizes to TDMA signal frame and extracts the time slot designated for that user The synchronization is the heart of TDMA TDMA/FDD TDMA/TDD

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April 9,  8 slots TDMA scheme  Each carrier supports 8 simultaneous users, each with 13 kbps encoded digital speech within 200 kHz carrier BW  Total of 124 frequency carriers (FDMA) are available in the 25 MHz allocated band in each direction  100 kHz of band is allocated as guard band at each edge of the overall allocated band Global System of Mobile Communications

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April 9,  BW per carrier = MHz, which can support up to 12 ADPCM  Total BW = 10 MHz  Frame duration is 10 ms, with 5 ms for portable-to- fixed station and 5 ms fix- to-portable station  64 bit guard time  The transmitter transfers information in signal bursts, which it transmits in slots of duration 10/24 = ms  With 480 bit per slot, the total bit rate is  Mbps Digital Enhanced Cordless Telephone

April 9, Fixed-Assignment Access for Voice- oriented Networks CDMA With growing interest in the integration of voice, data and video traffic in telecommunications, CDMA becomes increasingly attractive Integration of various types of traffic is readily accomplished in CDMA Accommodate various wireless users with different BW requirements switching methods and technical characteristics without any need for coordination Power control techniques are essential in efficient CDMA

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April 9, CDMA..  Let the data rate of the i th user by R i (t)  Let the code assigned to the i th user by C i (t)  i.e. all the codes are orthogonal  Each user uses its code to spread its signal – the signal transmitted by the i th user is S i (t) = R i (t)C i (t)  The signal received BS is the sum of all transmitted signals (ignore multi-path copies for the time being), ∑ S i (t) = ∑ R i (t)C i (t)

April 9, CDMA..  Demodulation (De-spreading):  Receiver dedicates a path structure per user – multiplies the received signal with the k th user code  C k (t) X ∑ S i (t) = C k (t) X ∑ R i (t)C i (t)  = R k (t)  i.e. only the k th signal is retrieved from the k th receiver path  Codes are only orthogonal if:  Perfect synchronization is achieved  No multipath exists

April 9, CDMA.. Multiple users use the same band at the same time User is differentiated by a code that acts as the key to identify that user Code are selected so that when they are used at the same time in the same band a receiver knowing the code of a particular user can detect that user among all received signals CDMA/FDD and CDMA/TDD In CDMA, each user is a source of noise to the receiver of other users If we increase the number of users beyond a certain value, the entire system collapses

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