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Mobile Computing COE 446

Wireless Multiple AccessTarek Sheltami

KFUPMCCSECOE

http://faculty.kfupm.edu.sa/coe/tarek/coe446.htm

Principles of Wireless Networks

K. Pahlavan and P. Krishnamurth

Outline Fixed Frequency Assignment

FDMA TDMA CDMA

FDD TDD

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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

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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 30-100m

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

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AP/mini-BTS

Radio Microcell(~0.5-1 Km radius)

Picocellular(~30-100m radius)

Radio macrocell(hundreds of Kilometers)

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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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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

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Fixed-Assignment Access for Voice-oriented Networks..

FDMA/TDD The downlink and uplink take turns via

alternating time slots

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

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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

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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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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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BW per carrier = 1.728 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 = 0.417 ms

With 480 bit per slot, the total bit rate is

1.152 Mbps

Digital Enhanced Cordless Telephone

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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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CDMA.. Let the data rate of the ith user by Ri(t) Let the code assigned to the ith user by Ci(t)

i.e. all the codes are orthogonal Each user uses its code to spread its signal – the signal

transmitted by the ith user is Si(t) = Ri(t)Ci(t) The signal received BS is the sum of all transmitted

signals (ignore multi-path copies for the time being),

∑ Si(t) = ∑ Ri(t)Ci(t)

i j

0, i jC (t) X C (t)

1, i j

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CDMA..

Demodulation (De-spreading): Receiver dedicates a path structure per user –

multiplies the received signal with the kth user code Ck(t) X ∑ Si(t) = Ck(t) X ∑ Ri(t)Ci(t) = Rk(t) i.e. only the kth signal is retrieved from the kth

receiver path Codes are only orthogonal if:

Perfect synchronization is achieved No multipath exists

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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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