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coding and modulation for a horrible channel
Post: #1

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.

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.
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.

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.

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).

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.
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.
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. © 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
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

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

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
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

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 Ëœsmart homeâ„¢ a reality

¢ IEEE communication magazine April 2003, May 2003
¢ Communication systems- Simon Haykin.


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 seminars.
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

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