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Mobile and Personal Communications

Dr Mike Fitton,mike.fitton@toshiba-trel.com

Telecommunications Research LabToshiba Research Europe Limited

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“Mobile and Personal Communications” Outline of Lectures

l Personal communication system requirementsl Multiple Access Techniques

– Frequency Division Multiple Access– Time Division Multiple Access– Code Division Multiple Access

l Techniques to improve performance– Equalisation– Diversity and Diversity Combining

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Evolution of personal cellular communicationsEvolution of personal cellular communications

• Cellular systems are expanding in capacity and services• Increasing integration between wireless systems

– Wireless LAN, wireless PAN (Bluetooth), etc

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Multiple Access

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Multiple Access Requirements

A cellular system employs a multiple access technique to control the allocation of the network resources. The purposes of a multiple access technique are:

l To provide each user with unique access to the shared resource: the spectrum.

l To minimise the impact of other users acting as interferers.l To provide efficient use of the spectrum available.l To support flexible allocation of resources (for a variety of services).

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Frequency Division Multiple Access (FDMA)

l Each user is assigned a unique frequency for the duration of their call.

l Severe fading and interference can cause errors.

l Complex frequency planning required. Not flexible.

l Used in analogue systems, such as TACS (Europe), and AMPS (USA).

2 users shown

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Time Division Multiple Access (TDMA)

l Each user can use all available frequencies, for a limited period. The user must not transmit until its next turn.

l High bit rates required, therefore possible problems withintersymbol-interference.

l Flexible allocation of resources (multiple time slots).

l Used in second generation digital networks, such as GSM (Europe), and D-AMPS (USA).

2 users shown

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Frequency Hopping Code Division Multiple Access (FH-CDMA)

l Each user regularly hopsfrequency over the available spectrum.

l Users are distinguished from each other by a unique hopping pattern (or code).

l Interference is randomised.l Used in BluetoothTM

only 1 user shown

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Direct SequenceCode Division Multiple Access (DS-CDMA)

l All users occupy the same spectrum at the same time.

l The modulated signal is spreadto a much larger bandwidth than that required by multiplying with a spreading code. Users are distinguished from each other by a unique spreading code.

l Very flexible, but complex.l Currently used in 3G and 2nd

generation IS-95

only 1 user shown

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Summary of Multiple Access Techniques:The Cocktail Party

To illustrate the nature of the multiple access techniques, consider a number of guests at a cocktail party. The aim is for all the guests to hold an intelligible conversation. In this case the resource available is the house itself.

l FDMA: each guest has a seperate room to talk to their partner.l TDMA: everyone is in the same room, and has a limited time to hold

their conversation (so they must talk very quickly).l FH-CDMA: the guests run from room to room to talk.l DS-CDMA: everyone is in the same room, talking at the same time, but

each pair talks in a different language.

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Duplex Communication

l Two way communication is called duplex (eg. for cellular radio). One way is called simplex (eg. for paging).

l The link from the base-station to mobile is the down-link. The link from the mobile to base-station is the up-link.

l The up-link and down-link can exist simultaneously on different frequencies: Frequency Division Duplex (FDD).

l The up-link and down-link can exist on the same frequency at different times: Time Division Duplex (TDD).

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Picocells up to 2Mbpsup to 384kbpsMicrocells

Hierarchical cell structureHierarchical cell structure

Macrocells, up to 144kbps

Possible Application:MPEG4 video

File transferNews download

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Performance enhancements

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The effects of Equalisation

l Frequency-selective fading arises due to time-dispersion in the multipath channel. This type of wideband fading causes irreducible errors, unless its effects are mitigated.

l Equalisation is employed to remove the harmful frequency-selective fading. It acts as an adaptive filter, to produce an output signal with a flat frequency response. Consequently, error-free transmission at high data rates is possible.

1.8101.800 1.802 1.804 1.806 1.808 1.810-40

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Frequency (GHz)

(i) Channel (Frequency Domain) (ii) Forward Filter (Frequency Domain)

Trms = 2.67µs

Noise EnhancingAmplification

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Linear Transversal Equaliser

l The linear transversal equalisation (LTE) is one of the simplest forms of equaliser.

l The tap coefficients (C1 to Cn) are adapt to suit the current channel conditions. Normally this adaptation is done on a training sequence.

l In the presence of severe amplitude and phase distortion, the required inverse filter tends to result in an unacceptable degree of noise amplification.

T T T

Zk

r(t-T) r(t-2T) r(t-nT)INPUT

r(t)

C0

DECISIONDEVICE

e kTRAININGSEQUENCE

+

+

++

+

-

OUTPUT

r(t)

ERROR

Forward Filter

C1 C2 Cn

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Decision Feedback Equaliserl The equaliser output signal is the

sum of the outputs of the feedforward and feedbacksections of the equaliser.

l The forward section similar tothe LTE

l Decisions made from the output of the equaliser are now feed back through a second filter.

l If these decisions are correct, the ISI caused by these symbols can be cancelled without noise enhancement

l However, errors made in hard decisions are fedback through the equaliser and can cause error propagation

T T T

C C Cn-1 n-2 0

Zk

r(t+[n-1]T) r(t+[n-2]T) r(t)INPUT

r(t+nT)

Cn

DECISIONDEVICE

ekTRAININGSEQUENCE

+

+

++

+

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OUTPUT

ERROR

Forward Filter

TTT

Xk^Xk-1

^

b1b2bm

Xk-2^

-

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+

Feedback Filter

Xk-m^

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Diversity

l Diversity: the provision of two or more uncorrelated (or independent) fading paths between transmitter and receiver.

l The uncorrelated fading statistics are combined or selected in some form.

l Performance improvement results as it is unlikely that all the diversity paths will be poor at the same time. Consequently, the probability of outage is reduced.

l Methods for generating uncorrelated paths for diversity combining include time, frequency, polarisation, angle, and space diversity.

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Space Diversity

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(i) Space Diversity (ii) Power Variation with Distance

B

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Polarisation and Angle Diversity

(i) Polarisation Diversity (ii) Angle (Pattern) Diversity

time

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Branch 1 Branch 2

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Branch 4Branch 5

Branch 6

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Time and Frequency Diversity

A B C

Tc

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(i) Time Diversity (ii) Frequency Diversity

f2TX1

•Less desirable: extra signal bandwidth is required

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Diversity Combining:Switched (or Scanning) Combining

RFSwitch

fc + f

IF

IFDemod

ToDetector

ComparatorFixed

Sync

RSSI

Threshold

A

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RFSwitch

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(i) Switch Diversity with Fixed Threshold

fc + f

IF

IFDemod

ToDetector

Comparator

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RSSI

Average

(ii) Switch Diversity with Adaptive Threshold

l The current branch remains selected until a metric fails a certain threshold, usually the Received Signal Strength Indicator (RSSI). The next branch is then blindly selected.

l An adaptive threshold removes unnecessary switching. When the signal fades relative to the mean, switching occurs.

l This system is cheap and simple, but not ideal.

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Diversity Combining:Selection Combining

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Detect the Best Signalfrom the M Receivers

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RSSI

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RSSI

l The most appropriate branch is always selected. Slight performance advantage over switch diversity.

l The system is expensive, as all branchs have to beanalysed.

l Using RSSI as a indication of quality is non-ideal, since it is unduly affected by interference.

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Diversity Combining:Equal Gain Combining (EGC)

l Post-detection combining.l All branchs are merely

cophased and summed.

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hasi

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min

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M

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Output

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Diversity Combining:Maximal Ratio Combining (MRC)

1

Cop

hasi

ng &

Sum

min

g

M

a1

2

a2

aM

Detector

Output

ia = 1 for Equal Gain Combining

NOTE:

l Each branch is weighted before summation in proportion to its own signal-to-noise ratio.

l Slightly better performance than EGC, but requires the complexity of estimating signal-to-noise ratio.

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The Effect of Diversity onFading Statistics

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l The fading statistics are improved with the applications of diversity.

l It is much less likely for deep fades to occur.

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The Effect of Diversity onPerformance

l The BER in a Rayleigh fading channel can be significantly reduced with the use of diversity.

l Diversity can offer an 8-12 dB gain in Rayleigh channels.

l It can also increase the maximum bit rate in a dispersion limited environment by a factor of two.

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