coding and modulation for horrible channel

8/7/2019 Coding and Modulation for Horrible Channel

1/20

Presented by:

Balachandra Hyati

1DA07EC014


2/20

Introduction

Channel modeling

Channel frequency respense

NoiseModulation techniques

Adaptive transmission techniques

Coding

Advantages and disadvantagesApplications

Conclusion

References


3/20

Power lines form a potentially convenient

and inexpensive communication medium for

control signaling and data transmission. Electric power lines, which have been used

for some time for remote metering and control

and are presently being considered for high bit

rate data services.


4/20

The modeling problem for PLC(power line communation),which consists of investigating the characteristics of thepower network as a communication channel is difficult task.

Adequate models have not been standardized yet.

PLC channels as shown in figure(1) suffer from a number oftechnical problems: Frequency-varying and time-varying attenuation of the

medium Dependence of the channel model on location, network

topology, and connected loads High interference due to noisy loads High, nonwhite background noise Various forms of impulse noise


5/20

Impulsenoise

50/60 Hzpowerwave

Time varyingsystem

Coloredbackground

noise

Narrowbandnoise

+T

o receiver

Fromtransmitter

Figure 1.Block diagram of a PLC channel


6/20

The channel, assumed to be linear, exhibits notches due to

reflections and cancellations causing narrowband fades.

PLC channel can be regarded as a multipath environment,

since the frequency response changes when some electricaldevice is switched, the PLC channel is time-varying.

The most widely known model for the transfer functionH(f)

of the PLC channel is the multipath model proposed by

Philipps, and Zimmermann and Dostert , which takes the

following form forH(f) in the frequency range from 500 kHz

to 20 MHz:


7/20

The dominant channel disturbances occurring in

the frequency range are between several 100 kHz

and 20 MHz are:

Colored background noise

Narrowband noise

Impulse noise


8/20

Single Carrier Modulation

Spread Spectrum

Multicarrier Modulation


9/20

Single-carrier modulation is an attractive proposition from thecomplexity point of view.

Since the power line channel introduces strong intersymbolinterference (ISI), powerful detection and equalization techniques

are called for. The deep frequency notches present in the channel transfer

function prevent the use of linear equalizers, as the noiseenhancement they cause is a serious drawback on a very noisy

channel.

On the other hand, nonlinear symbol or sequence detectorsexhibit a computational complexity that increases exponentially

with the length of the impulse response, which is considerable inthe power line channel.


10/20


11/20

A scheme suitable for frequency-selective channels is orthogonalfrequency-division multiplexing (OFDM).

Here quadrature amplitude modulation (QAM) or PSK symbols aredivided in blocks, and the data within blocks are transmitted

in parallel on many orthogonal subcarriers occupying a subband which is

so narrow that the associated subchannel has a flat frequency response. Hence, a frequency-selective channel becomes equivalent to a set of

multiple flat-fading subchannels.

ISI is cancelled, and equalization of the data with coherent detectionsimply amounts to a normalization by a complex scalar, while incoherentdetection does not require any further equalization.

Each symbol can be detected separately, an approach that can beextremely successful in preventing errors caused by strong channelattenuation or noise in specific subbands.

Hence, OFDM is robust against narrowband

interference and high noise levels.


12/20


13/20

Adapting Power Level Through power control, all

transmission parameters are unchanged as the channel varies,

but the transmission level is varied according to the channel

fluctuations.

Adapting Constellation Size Adaptive modulation

increases the data transmission efficiency without increasing

the multi-access interference power, instead using a

hierarchy of different constellations of increasing size. By

continuously monitoring the channel state, modulation indexand symbol rate can be varied to adapt the modulation

scheme to the channel SNR.

Adapting Code Rate The coding scheme can respond to

channel variations by selecting the code rate.


14/20

OFDM can be combined with coding by using bit-interleaved coded modulation

(BICM) as shown in figure(2). This links binary coding and nonbinary

modulations in a simple way.

Figure 2.Block diagram of OFDM with BICM.

EncoderBit

interleaver Modulator IDFT

DFT Removecyclic prefix

PLCchannel

Add cyclicprefix

EqualizerComputebit metric

Bitdeinterleaver

Decoder


15/20

The performance of BICM with Gray mapping of bits onto signalscomes very close to the optimum for the AWGN channel, whileits structure is simpler than with coded modulation or multilevelcoding, allows a large degree of flexibility and is especially wellsuited for fading channels.

Since OFDM generates a number of parallel independent AWGNchannels, the synergy of OFDM with BICM appears promising.

In general, it can be expected that for low SNR bit loading will benecessary, while for higher SNR a higher-order modulationscheme is appropriate to exploit the capabilities of the system.

Adaptivity may also be introduced in the coding scheme.For example, a code may be used to detect transmissionerrors; if the transmitter does not receive from the decoderan acknowledgment that reception of a frame wassuccessful, retransmission occurs.


16/20

Advantages Power lines form a potentially convenient and inexpensive

communication medium for control signaling and datatransmission, as houses and industrial buildings are coupled to the

power grid, with power outlets available in virtually all roomswhere a communication terminal is to be used.

It is possible to get high bit rate data services.

Disadvantages PLC channel suffer from frequency-varying and time varying

attenuation of the medium.

Dependence of channel


17/20

Home networking(LAN):

PLCs can be used in a home to interconnect homecomputers,as well as any home entertainment devices

that have an Ethernet port. Internet access(broadband over powerlines):

BPLs is the use PLC technology to provide broadbandinternet access through ordinary power lines.

International Broadband ElictricCommunications(IBEC) and other companiescurrently offer BPL service to several electriccooperatives.


18/20

Electric power lines are presently being

considered for high-bit-rate data services

because they offer a convenient and

inexpensive communication medium for data

transmission.

Since the PLC channel is used in a frequency

range it was not originally designed for,designing a simple modem for reliable

transmission on this hostile channel is a

challenging task.


19/20

[1] Y.-F. Chen and T.-D. Chiueh, Baseband TransceiverDesign of a 128-Kbps Power-line Modem for HouseholdApplications, IEEE Trans. Power Delivery, vol. 17, no.2, Apr. 2002, pp. 33844.

[2] H. Philipps, Modelling of Powerline Communication

Channels, Proc. Intl. Symp. Power-Line Commun. andIts Apps., Lancaster, U.K., Mar. 30Apr. 1, 1999.

[3] M. Zimmermann and K. Dostert, A Multipath Modelfor the Powerline Channel, IEEE Trans. Commun., vol.50, no. 4, Apr. 2002, pp. 55359.

[4] T. C. Banwell and S. Galli, A Novel Approach to Accurate

Modeling of the Indoor Power Line Channel. Part I:Fundamental Analysis and Circuit Models, and Part II:Transfer Function and Channel Properties, submittedIEEE Trans. Commun., Jan. 2003.

[5] Proc. 2001 Intl. Symp. Power-Line Commun. and ItsApps., Malm, Sweden, April 46, 2001.


20/20

THANK YOU

coding and modulation for horrible channel

Documents