ADAPTIVE MODULATION AND CODING.pptx

ADAPTIVE MODULATION AND CODING

ADAPTIVE MODULATION AND CODING

PRESENTED BY:

ANAMIKA YADAVMANASA ANN THOMAS

10.Juli.2015Communication Systems (SS2015)12 CONTENTS

INTRODUCTIONWHY DO WE NEED AMC?CONSTRAINTSDEFINITIONS USEDSYSTEM MODELADAPTIVE TECHNIQUESVARIABLE RATE AND VARIABLE POWER MQAMADAPTIVE CODED MODULATIONCURRENT SYSTEMS USING AMC

3INTRODUCTION

Adaptive modulation and coding enables robust and spectrally efficient transmission over time-varying channels. AMC was introduced in the late sixties and early seventies.

The basic premise is to estimate the channel at the receiver and feed this estimate back to the transmitter, so that the transmission scheme can be adapted relative to the channel characteristics.

4WHY DO WE NEED AMC?

Without AMC, there is inefficient utilization of channel and wastage of power.

With AMC, data rate and signal power are varied as per the channel conditions for efficient utilization of resources.

AMC guarantees 99% of data transfer even at worst channel conditions without any wastage of resources.5CONSTRAINTS

Adaptive modulation requires a feedback path between the transmitter and the receiver, which may not be feasible for some systems.

If the channel changes faster than it can be estimated and fed back to the transmitter, adaptive modulation will perform very poorly.

How often the transmitter can change its rate and power.67SYSTEM MODEL

8Adaptive Transmission SystemAssume linear modulation.Modulation uses ideal Nyquist data pulses.Flat fading channel as a discrete time channel.Estimate the power gain or Rx SNR at time i and adapt the modulation and coding parameters.Most common parameters to adapt are the data rate, transmit power and coding parameters.9ADAPTIVE TECHNIQUES

Variable-Rate TechniquesVariable-Power TechniquesVariable Error ProbabilityVariable-Coding TechniquesHybrid Techniques

1011Variable Error ProbabilityAdapting the instantaneous BER subject to an average BER constraint Pb.

Variable-Coding TechniquesUse of different channel codes to provide different amounts of coding gain to the transmitted bits.Implemented by multiplexing together codes with different error correction capabilities.We can also use rate-compatible Punctured convolution codes (RCPC).12Hybrid Techniques

These can adapt multiple parameters of the transmission scheme, including rate, power, coding and instantaneous error probability.Joint optimization of the different techniques is used to meet a given performance requirement.Rate adaptation is often combined with power adaptation to maximize spectral efficiency.131415Average Spectral Efficiency in Log-Normal Shadowing

16Average Spectral Efficiency in Rayleigh Fading

17Channel Inversion with Fixed Rate

We transmit fixed rate MQAM to attain a target Pb and fixed SNR at receiver.Power adaptations at Tx cause channel inversion.Truncated channel inversion will block the transmission over channel if received SNR is below some threshold value.

18Discrete Rate AdaptationRestrict the adaptive MQAM to a limited set of constellations.Assume a set of square constellations of size M0=0, M1=2 and Mj=22(j-1), j=2,.,N-1 for some N.

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Figure : Discrete-Rate Efficiency in Rayleigh FadingDiscrete-rate Discrete-power Adaptation21

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23Average Fade Region DurationChoice of the number of regions depends upon how fast channel changes, number of constellations available in channel and how fast channel can adapt.Average time Rx SNR remains in a fading region determines tradeoff between number of regions and power-constellation adaptation.

24Channel Estimation Error and Delay

25ADAPTIVE CODED MODULATION

Additional coding gain can be achieved with adaptive modulation by superimposing trellis code or more general coset codes on top of the adaptive modulation. To exploit the separability of code and constellation design inherent to coset codes.It is a natural coding scheme to use with variable-rate variable-power MQAM, since the channel coding gain is essentially independent of the modulation.26Adaptive Coded Modulation Scheme:

CURRENT SYSTEMS USING AMC

EGPRS for data transmission in GSM cellular system uses 8PSK with 5 different code rates and GMSK with 4 different code rates. Total 9 different coding and modulation schemes.GPRS for data transmission in IS-136 TDMA cellular systems can use 4, 8, and 16 level PSK modulation.2728Reference

Wireless Communications by Andrea Goldsmith (Edition 2005)

29THANK YOU