ma duplexing hand-off

8/12/2019 MA Duplexing Hand-Off

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Lecture 6Multiple Access, Duplexing and Hand-off

Dr. Mohammed Usman


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GSM-TDMASpeech codec produces 260 bits for each 20 ms speech

segmentSpeech rate = 13 kbpsECC applied to the 260 bits 456 bitsEach GSM time slot can carry 114 data bits (57 x 2)

456 bits carried over 4 time slots over different framesOne time slot to one user13 kbps speech carried over a single time slot13 x 8 = 104 kbps is the speech data rate over a single TDMAframeEach TDMA frame is a channel of 200 kHz BWSpectral efficiency = 104 kbps/200 kHz = 0.52 bits/sec/HzNote: Gross GSM air interface data rate = 270.833 kbps


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Multiple Access - CDMAUsers identified by a unique pseudo random code

called spreading sequence or PN codeMultiple users use the same frequency channelsImportant properties required for PN codes

High auto-correlationLow cross-correlation

No. of users that can be served = as many unique PNcodes with above properties

In practice, interference limits the no. of users (CDMAis interference limited) More on this later


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Multiple Access - OFDMAOFDMA similar to FDMA but frequency channels

are orthogonal to each otherOFDMA uses narrowband channels Advantage of orthogonality

Channels can overlap each other i.e. no need for guardbands

Users are allocated multiple narrowband channels inparallel

Low rate data transmitted in parallel What are the advantages of doing this?


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FDD vs TDDFDD paired spectrum wasteful of BW

TDD unpaired spectrum higher spectral efficiencyTDD more flexible for asymmetric data trafficFDD different frequency bands for transmission andreception separate h/w for Tx and Rx partTDD can dynamically adapt to changing trafficpatternsIn FDD, a user will require 2 interference-free channels

whereas in TDD only 1 interference-free channel isrequired


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Hand-offHand-off: transfer of a mobile station from one BTS toanother BTSProper hand-off management is required

To reduce call dropping probabilityTo prevent ping-pong effect (users moving around the

cell boundary)Need for hand-off

Rx signal strength falling below receiver sensitivity levelCIR falling below the threshold of 18 dBNo free channels and user located in overlappedcoverage region of two BTSs


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Hand-offTypes of hand-offs

Horizontal hand-off within the same system (same airinterface) Vertical hand-off from one system to another (different airinterfaces)

Horizontal hand off is of two types (based on where thehand-off takes place)

Intercell hand-off from one BTS to anotherIntra-cell handoff from one sector to another sector of thesame BTS

Types of hand-offs (based on how the hand-off procedureis implemented)

Hard hand-off: Break before makeSoft hand-off: Make before breakSofter hand-off: Signals received via multiple channels duringsoft hand-off are combined to produce a clearer signal(applies only for intra-cell hand off )


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Hand-off initiationContinuous monitoring of several parameters

RSSI, SNR, BER etc.On what basis (when) is hand-off initiated?BTS - 1

BTS - 2

A B C

hT

Point-A: Hand-off based on RelativeRSSI (ping-pong effect can happen)

Point-B: Hand-off based on RelativeRSSI with threshold

Point-C: Hand-off based on RelativeRSSI with hsyteresis h

RSSIBTS-1RSSIBTS-2


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Illustration: issues with multimedia

3G BTS typical capacity = 2.5 MbpsIn one hour, BTS can handle 2.5 Mbps x 3600 = 9 GigabitsTypical voice data rate = 12.2. kbps Assuming average call duration = 2 minutes (120 sec)

Each voice call will consume 1.4 MegabitsNo. of voice calls supported = 9 gigabits/1.4megabits = 6250Low resolution video data rate = 128 kbpsIf 5% of 6250 users access video of 2 min duration, they willconsume (0.05 * 6250) * (128 kbps * 120s) = 4.8 Gigabits overthe hour4.2 Gigabits remaining for voice calls which is less than halfthe voice capacity without video.


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Illustration: issues with multimedia

If 5% subscribers access video of six minutes duration,the consumption will be (0.05 * 6250) * (128 kbps * 360s) = 14.4 Gigabits which is way over the 9 Gigabitscapacity of the network

Multimedia distribution poses stringent challenges


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Frame efficiencyCalculate the frame efficiency of GSM TDMA frame

Calculate the spectral efficiency of GSM air interface

10 2 3 4 5 6 7

10 2 3 4 .. .. .. .... .. .. 122 123 124

TB 3 Data 57 F1Training 26 Data 57F1 Guard 8.25TB 3


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Interpret and explain


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Fundamental but very important

For a fixed SNR, if the data transmission rate is increased,the transmission BW also has to increaseMost of the noise is external to the communication systemSystem can control either the transmit power ortransmission BWIn a power limited system, transmission BW is traded-offfor efficient power utilizationIn a BW limited system, transmit power is traded off withfor efficient BW utilizationThe trade-off is made to achieve a certain acceptable errorprobability

ma duplexing hand-off

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