Energy efficient error-correcting coding for wireless systems


Shao, Xiaoying (2010) Energy efficient error-correcting coding for wireless systems. thesis.

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Abstract:The wireless channel is a hostile environment. The transmitted signal does not only suffers multi-path fading but also noise and interference from other users of the wireless channel. That causes unreliable communications. To achieve high-quality communications, error correcting coding is required to mitigate the noise and interference encountered during the signal transmission. However, the current design of error correcting codes does not take the power consumption in ADCs into account. ADCs consume about 50% of the total base-band power. The power-efficiency of ADCs does not increase in the same speed as the baseband signal processing. Digital signal processing follows Moore’s law. Given the same specification, the power consumed in ADCs halves every 2.7 years but the power consumption in the baseband signal processing decreases a factor of 10 every 5 years. In the case of RF signal processing, the power efficiency is limited by the semi-conductor technology. Therefore, ADCs are the main bottleneck for an energy-efficient wireless receiver. Quantized channels arise in practical communication systems where ADCs are used to sample the analog transmitted signals. Conventional narrow-band systems usually do not take quantization into account, since a large number of quantization levels is used. In this case the difference between the quantized and unquantized channel can
be neglected. To lower the resolution of ADCs in the narrow-band wireless system, the design of the error correcting coding should consider the quantization effect. This
thesis describes the design of a coding scheme for the quantized channel. The approach
is based on multi-level coding and binary block codes to achieve the theoretical limits
in the narrow-band wireless channel.
Wide-band wireless systems often employ OFDM to ease the equalizer in the receiver.
OFDM has a high Peak-to-Average Power Ratio (PAPR) which is the main disadvantage
of OFDM. When signal peaks in the OFDM signal are clipped, all sub-carriers
are affected. Because the wide-band wireless channel is often modeled as a frequency
selective fading channel, some part of the channel may suffer from deep fading and
can not afford any distortion. Consequently, the communication is unreliable. That
urges the usage of high-resolution ADCs in OFDM systems. Current OFDM systems employ fixed high-resolution ADCs which are designed for the worst-case scenario.
However, the worst-case scenario does not happen often. This thesis proposes to apply
resolution adaptive ADCs in the OFDM system. In such a case, the ADC can
be designed for each channel condition instead of fixing for the worst-case scenario.
Correspondingly, the power consumption in ADCs reduces.
A further resolution reduction in ADCs can be achieved by designing an energy efficient
error correcting coding for the OFDM system. The error correction coding
in the current OFDM system is based on the joint coding scheme, which encodes
the source data over all the sub-carriers in parallel. The joint coding scheme works
better than separate coding, as it employs the fact that sub-carriers with high energy
can compensate for sub-carriers with low-energy. Its drawback is each sub-carrier
must be decoded, also the ones in deep fading. Hence, the maximum level of the
noise floor endured by the joint coding scheme is limited to the dynamic range of
the channel. Correspondingly, the minimum resolution of the ADC required by the
joint coding scheme in a certain channel condition is dependent on the dynamic range
of the channel. A way to reduce the dynamic range of the channel is to neglect
the deep-fading part of the channel and to exploit the high-energy part only. Obviously,
the joint-coding scheme can not apply this. Therefore, we propose in this
thesis an energy-efficient error correction scheme based on fountain codes for OFDM
systems. Fountain codes can reconstruct the original source file by only collecting
enough fountain-encoded packets. It does not matter which packet is received as we
only need to receive enough packets. In other words, fountain-encoded packets are
independent with respect to each other. Since fountain codes are designed for erasure
channels, error correction codes are required to transfer the noisy wireless channel into
an erasure channel. That inspires us to exchange the code rate of error correction
codes with the number of sub-carriers to be discarded. In this case, the resolution of
ADCs and thus the power consumption can be reduced even more.
Item Type:Thesis
Electrical Engineering, Mathematics and Computer Science (EEMCS)
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