coding and modulation for a hostile channel

Coding and Modulation for a Hostile Channel Seminar Report ‘03

INTRODUCTION

The use of electric medium as a potential medium has been

suggested over the past generations. Actually for some decades power

wires have been used for low speed data communication. However

power line has largely been unpredictable as practical high speed

communication channel.

Power lines form a potentially convenient and inexpensive

communication medium for control signaling and data transmission, as

houses and industrial buildings are coupled to the power grid, with

power outlets available in virtually all rooms where a communication

terminal is to be used, and with a simple standard interface in the form

of a wall socket plug. But they present a difficult challenge, because

reliable high-speed communication is sought over a medium designed

for electrical energy delivery rather than for data transmission, and

because strongly competitive wireless solutions are available. There are

two main applications for power line communication (PLC): the

existing line in a building can be used as a local area network

backbone, or integrated into a wide area network. This paper briefly

summarize what is known about channel modeling for PLC and

describe some of the solutions advocated for transmission.

Dept. of ECE MESCE Kuttippuram-1-


PROPERTIES

PLC channels suffer from a number of technical problems they

are

Frequency varying and time varying attenuation of the medium

Dependence of the channel on the connected load.

Suffer from reflection caused by the impendence mismatches.

High interference due to noisy load

Electromagnetic compatibility issues, that limit the accurate

transmitted power

Block diagram of PLC Channel.


Time varying system

Colored background

noise

Narrowband noise

From transmitter

Impulse noise

50/60 Hz power wave

To receiver


The total signal attenuation consists of mainly two parts:

Coupling losses – Depend on the design and can be made as small as

possible.

Line losses – very high and ranges from 40 dB/km to 100 dB/km. This

large attenuation produces very low SNR at receiver.



CHANNEL MODELING

PLC channels are considered as a multi path environment with

frequency selective attenuation. Most widely known model for the

transfer function H(f) of the PLC channel is multi path model proposed

by Philips Zimmermann and Dostert which takes the following term

for H(f) in the frequency range from 500KHz to 20 MHz.

Here N is the number of relevant propagation paths, and are

link attenuation parameters; K is a exponent (with typical value from

0.5 to 1), gi is the weighting factor for path i, di its length and i is the

delay. Clearly longer the path itself the more attenuation each path

experience. Therefore the number N tends to be small for out door

channel. If the indoor PLC is considered the multi path model is still

valid, the main difference being that N is no much larger than in out

door case.



The above parameters must be determined through

measurements. It is essentially focuses the phenomenonological aspects

of the channel; not explaining the physics of the signal over power

cables.

So another modeling was proposed, which is known as bottom

up model. This approach is based on the multi conductor transmission

line there in (MTL). It reveals that frequency response of the PLC has

highly predicable features. It also proves the property that PLC is

isotropic (same transfer function from both sides).



MODULATION SCHEMES

Selection of modulation schemes must account for three major

factors, they are:

Presence of low signal to noise ratio (SNR) by noise and

impedance mismatches.

Time varying & frequency varying nature of channel.

Electromagnetic compatibility that limit the transmitted power.

The different modulation schemes possible for PLC channel are

given below.

SINGLE CARRIER MODULATION

It is an attractive proposition from the complexity point of view.

However it need,

Powerful detection and realization techniques because PLC

introduces strong intersymbol interference.

When equalization network used it enhances the noise, it is main

disadvantage.

It also needs high frequency which increases the signal attenuation.



High intersymbol interference.

Multi accessing is not possible.

SPREAD SPECTRUM MODULATION

Primary advantage is its ability to reject interference, whether it

is unintentional or intentional. Definition may be stated in two words:

1. Spread spectrum is a means of transmission in which the data

sequence occupies a bandwidth in excess of minimum bandwidth

necessary to sent it.

2. It is accomplished before the transmission through the use of a

code that is independent of the data sequence. The same code is

used at receiver to dispread it.

All standard modulation technique (PCM, FM, etc.) do satisfy

part 1 of the definition but they are not spread spectrum techniques

because they do not satisfy part 2 of the definition.



Idealized model of baseband spread-spectrum system.

(a) Transmitter. (b) Channel. (c) Receiver.

One method of widening the bandwidth of information bearing

(data) sequence involves the use of modulation. In the above diagram

let b(t) be the binary data sequence and c(t) denote pseudo noise (PN)

sequence. Value of b(t) and c(t) are ±1. The desired modulation is

achieved by applying the data signal b(t) and PN signal c(t) to a

product modulator (Multiplier) as in diagram. We know from the

Fourier transform theory that multiplication of 2 signals produces a

signal in whose spectrum equals the convolution of the spectra of the

two component signals. Thus if the message b(t) is narrowband and PN



signal c(t) is wideband, the product (modulated) signal m(t) will have a

spectrum that is nearly the same as the wide band PN signal. In other

words in context of our present application, the PN sequence perform

the role of a spreading code.

To recover the original signal the received signal r(t) is applied to a

demodulator that consists of a multiplier followed by an integrator &

decision device.

ie; z (t) = c(t) r (t)

= c (t) [c (t) b(t) + i (t)]

= c (t)² b (t) + c(t) b(t)

= b (t) + c (t) b (t). (because c(t) = )

b(t) can be extracted by using a base band (low pass) filter with a

bandwidth just large enough to accommodate the recovery of data

signal b(t), most of the power in the spurious component is filtered out.

However spread spectrum requiring low SNR has comparatively bad

modulation efficiency. Also multiplexing is not easy.



Illustrating the waveforms in the transmitter

MULTI CARRIER MODULATION

Multiplexing is a signal processing operation where by a

number of independent signal can be combined into a composite signal

suitable for transmission over common channel.

To transmit a lot of signal over the same channel signal must be

kept apart so that they do not interfere with each other and thus they

can be separated at receiving end. This is accomplished by separating

signals in frequency or in time. The different techniques collectively



known as multi carrier modulation techniques. To overcome the

hostile PLC environment OFDM (Orthogonal Frequency Division

Multiplexing) was proposed. OFDM splits available bandwidth into

small frequency bands called sub carriers. Unusable sub carriers may be

then masked out and best modulation and coding techniques can be

applied to usable sub carriers.

OFDM can adapt to bandwidth/data rates according to channel

conditions. The main properties of OFDM from perspective of utility

are:

Very stable & reliable approach.

High modulation efficiency (5bits/Hz).

High SNR required.

Minimum Inter Symbol interference effects.

However it needs insertion of a cyclic prefix (CP) which

depends on the channel memory which is quite a long in PLC channels.

CP acts as a guard level. It reduce inter carrier interference, and is

removed at receiver.



OFDM Transceiver



CODING

Reliable data communication over hostile channel requires

forward error correction (FEC) coding, inter leaving, automatic repeat

request (ARQ). ARQ- it requires a feedback channel for requesting

message transmission. The encoded bits are applied to modulator for

transmission and received signals are demodulated and decoded. It also

includes transmit and receive controllers that exchange information via

feedback channels.

Block diagram of ARQ system

Dept. of ECE MESCE Kuttippuram

Encoder Modulator , channel , demodulator

Receiver controller

Feedback channel

Source

Destination

Discrete channel

Transmit controller

Decoder

-13-


The simplest ARQ system employs in a start and stop strategy

which proceeds as follows:

1. A block of data is encoded into a code word for transmission

over the communication channel

2. Transmitter stops and waits until it receives acknowledgment of

the correct reception of the code word or a request for

retransmission.

Clearly this strategy requires only error detection (rather than

both correction and detection) in the receiver. Virtually error free

transmission can be attained by proper choice of the code for error

detection.

Interleaving – Inter leaver scrambles encoded data stream in a

deterministic manner. Specifically successive bits are separated as

widely as possible. In the receiver deinterleaver is used to perform

reverse operation. Error burst, that occur after inter leaving are spread

out in data sequence to be decoded thereby spanning many code words

Interleaving method for burst error channel

Dept. of ECE MESCE Kuttippuram

Encoder Burst error channel

Deinter- leaver

DecoderInterleaver

-14-

Inpu

t dat

a

Out

put d

ata


OFDM can be fruitfully combined with coding by using bit inter

leaved coded modulation. It allows flexibility (eg: choice of code can

be made independent of modulation scheme) and is specially used for

fading channel.

Block diagram of OFDM with BICM


Encoder

DFT

Equalizer

Bit interleaver

Remove cyclic prefix

Compute bit metric

Modulator

PLC Channel

Bit deinterleaver

IDFT

Add cyclic prefix

Decoder


CONCLUSION

Electric power lines are presently being considered for high bit

rate data services, because they offer convenient and inexpensive

communication medium for data transmission. Since the power line

channel is used in the frequency range it was not originally designed

for, designing a simple modem for reliable transmission on the hostile

channel is a challenging task. We have reviewed some of the main

solution that have been proposed to solve this problem.

With the recent development in the modulation and coding

schemes PLC home networkings has overcome almost all hurdles in its

way and have become the strongest contender technology in the home

networking with added advantages over its competitors such as ease of

installation, cheaper cost and high data rates. These advances in the

PLC home networking appliances has made the ‘smart home’ a reality



REFERENCES

IEEE communication magazine April 2003, May 2003

Communication systems- Simon Haykin.

www.cdmatech.com

www.google.com



ABSTRACT

Designing a system able to cope with a hostile channel is a

typical challenge for data communication Engineers. High- data- rate

communication over power lines is an exemplar case: while power lines

are potentially convenient and in expensive “no new wire” medium for

data transmission their features make it very difficult to design a simple

modem. This paper illustrates main features of hostile channel, and

some of the solution advocated for modem design.



CONTENTS

INTRODUCTION

CHANNEL MODELING

MODULATION SCHEMES

CODING

CONCLUSION

REFERENCES



ACKNOWLEDGEMENT

I extend my sincere thanks to Prof. P.V.Abdul Hameed, Head of

the Department for providing me with the guidance and facilities for

the Seminar.

I express my sincere gratitude to Seminar coordinator

Mr. Manoj K, Staff in charge, for his cooperation and guidance for

preparing and presenting this seminar.

I also extend my sincere thanks to all other faculty members of

Electronics and Communication Department and my friends for their

support and encouragement.

Roy Varghese. E


coding and modulation for a hostile channel

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