31-08-2012, 04:13 PM
Energy-Efficient Transmission of DWT Image over OFDM fading Channel
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Abstract
In many applications retransmission of lost packets
are not permitted. In an OFDM system, due to channel
fading, only a subset of carriers are usable for successful data
transmission. If the channel state information is available at the
transmitter, it is possible to take a proactive decision of mapping
the descriptions optimally onto the good subcarriers and discard
at the transmitter itself the remaining descriptions, which would
have been otherwise dropped at the receiver due to unacceptably
high channel errors.
In this paper we present a energy saving approach to transmission
of discrete wavelet transformation based compressed
image frames over the OFDM channels. Based on one-bit
channel state information at the transmitter, the descriptions in
order of descending priority are assigned to the currently good
channels. In order to reduce the system power consumption, the
mapped descriptions onto the bad subchannels are dropped at the
transmitter. Via analysis, supported by MATLAB simulations, we
demonstrate the usefulness of our proposed scheme in terms of
system energy saving without compromising the received quality
in terms of peak signal-noise ratio.
INTRODUCTION AND MOTIVATION
It is always desired to increase the data rate over wireless
channels. But high rate data communication is significantly
limited by Inter Symbol Interference (ISI) and frequency
selective fading nature of the channel. Rayleigh fading channel
is an example of frequency selective and time varying channel.
Multi-carrier modulation is used for such channels to mitigate
the effect of ISI. OFDM is a multi-carrier modulation scheme
having excellent performance which allows overlapping in
frequency domain. In OFDM, individual subchannels are
affected by flat fading, so for a period of time, condition
of the subchannels may be good, or they might be deeply
faded. The packets which are transmitted through these faded
subchannels are highly prone to be lost at the receiver due to
non-acceptable errors. OFDM system provides an opportunity
to exploit the diversity in frequency domain by providing a
number of subcarriers, which can work as multiple channels
for applications having multiple bit streams.
SYSTEM MODEL
In our system model, an image frame is compressed using
DWT, and the compressed data is arranged in data vectors,
each with equal number of coefficients. These vectors are
quantized and binary coded to get the bit steams, which
are then packetized and intelligently mapped to the OFDM
system, such that poorer subchannels can only affect the lesser
important data vectors. We consider only one-bit channel
state information available at the transmitter, informing only
about the subchannels to be ‘good’ or ‘bad’. For a good
subchannel, instantaneous received power should be greater
than a threshold Pth. Otherwise, the subchannel is in fading
state and considered ‘bad’ for that batch of coefficients. Note
that the data transmitted through deeply faded subchannels
are highly prone to error and are likely to be discarded at the
receiver.∗ Thus, the binary channel state information gives an
opportunity to map the bit streams intelligently and to save a
reasonable amount of power. Below, we described the DWTOFDM
system model in details.
RESULTS AND DISCUSSION
The analytical results from the formulation in Section III
as well as the simulation results to validate the analysis are
presented here. For simulations we transmitted standard ‘Lena’
image of size 256 × 256 pixels. We simulated the OFDM
system with N × M = 128 subcarriers. In this way, 32
packets can be transmitted simultaneously through the OFDM
system. Packets are distributed in time and frequency domain
as described before, but the packets which are transmitted back
to back through same group of 4 subchannel are corrupted
due to slowly time varying nature of fading. We maintain
coherence time to be more than that of the packet transmission
time through a subchannel, and the channel condition is fed
back for each packet. We simulated block fading channel with
number of sub-bands N = 4 and the coherence bandwidth
equivalent to 32 subcarriers (M = 32). QPSK is used as
modulation scheme. Thus, 128 × 2 bits per OFDM symbol
are transmitted through a subchannel.
CONCLUSIONS
To conclude, we present a case of DWT compressed image
transmission over OFDM channels where binary channel state
information is available at the transmitter, but retransmission
is not allowed. We propose a energy saving approach, where
the compressed coefficients are arranged in descending order
of priority and mapped over the channels starting with the
good ones. The coefficients with lower importance level, which
are likely mapped over the bad channels are discarded at the
transmitter to save power without significant loss of reception
quality. Our analytic observations on reception quality and energy
saving performance are validated by extensive MATLAB
simulations.