Xiaoying Shao

An opportunistic error correction layer for OFDM systems

We propose a novel cross layer scheme to reduce the power consumption of ADCs in OFDM systems. The ADCs in a receiver can consume up to 50% of the total baseband energy. Our scheme is based on resolution-adaptive ADCs and Fountain codes. In a wireless frequency-selective channel some subcarriers have good channel conditions and others are attenuated. The key part of the proposed system is that the dynamic range of ADCs can be reduced by discarding subcarriers that are attenuated by the channel. Correspondingly, the power consumption in ADCs can be decreased. In our approach, each subcarrier carries a Fountain-encoded packet. To protect Fountain-encoded packets against bit errors, an LDPC code has been used. The receiver only decodes subcarriers (i.e., Fountain-encoded packets) with the highest SNR. Others are discarded. For that reason a LDPC code with a relatively high code rate can be used. The new error correction layer does not require perfect channel knowledge, so it can be used in a realistic system where the channel is estimated. With our approach, more than 70% of the energy consumption in the ADCs can be saved compared with the conventional IEEE 802.11a WLAN system under the same channel conditions and throughput. In addition, it requires 7.5 dB less SNR than the 802.11a system. To reduce the overhead of Fountain codes, we apply message passing and Gaussian elimination in the decoder. In this way, the overhead is 3% for a small block size (i.e., 500 packets). Using both methods results in an efficient system with low delay.

A Simple Two-Dimensional Coding Scheme for Bit Patterned Media

This paper presents a simple code to combat the two-dimensional inter-symbol interference (2D-ISI) effect that is present in data storage on magnetic bit patterned media. Whether the ISI effect is constructive or destructive depends on the surrounding bits. Therefore, we propose a simple 2D coding scheme to mitigate the ISI effect. With this 2D coding scheme in square patterned media, every 2-by-3 array has one redundant bit which has the opposite or same value of one of its adjacent bits. Compared to the 2D coding scheme in under the condition of the same areal density, the proposed 2D coding scheme increases the allowable bit-position jitter in square patterned media by 1% at a BER of 10-4; while it allows the effective storage capacity to be increased by around 5.5%.

Energy Efficient Error Correction in Mobile TV

The current error correction layer of digital mobile TV is designed for worst case scenarios, which often do not apply. In this paper, we propose a new opportunistic error correction layer based on fountain codes and a resolution adaptive ADC, which has been integrated into the OFDM-based physical layer. The key element in the new proposed system is that only packets are processed by the receiver which have encountered high-energy channels. Others are discarded. With this new approach, around 84% of the energy consumption in ADCs can be saved compared with the conventional mobile TV system under the same channel conditions.

Practical Evaluation of Opportunistic Error Correction

In [1], we have proposed a novel cross-layer scheme based on resolution adaptive ADCs and fountain codes for the OFDM systems to lower the power consumption in ADCs. The simulation results show that it saves more than 70% power consumption in ADCs comparing to the current IEEE 802.11a system. In this paper, we investigate its performance in the real-world. Measurement results show that the FEC layer used in the IEEE 802.11a system consumes around 59 times of the amount of power in ADCs comparing to the LDPC codes from the IEEE 802.11n standard, whose power consumption in ADCs is around 26 times of the proposed cross-layer method. In addition, this new cross-layer approach only needs to process the well-received packets to save the processing power. The latter can not be applied in the current FEC schemes.

Opportunistic error correction for MIMO

In this paper, we propose an energy-efficient scheme to reduce the power consumption of ADCs in MIMO-OFDM systems. The proposed opportunistic error correction scheme is based on resolution adaptive ADCs and fountain codes. The key idea is to transmit a fountain-encoded packet over one single sub-carrier per antenna and to transmit more packets than needed for decoding. Comparing to the IEEE 802.11n standard, this approach allows around 83% of power saving in ADCs for a 2×2 system and 90% for a 4×4 system. With the current FEC scheme defined in the IEEE 802.11n standard, a 4×4 system requires twice amount of power in ADCs as a 2×2 system when receiving the same amount of information with the same power. However, using opportunistic error correction in a 4×4 system costs only around 1.4 times amount of energy in ADCs comparing to a 2×2 system.

Opportunistic error correction for mimo-ofdm: from theory to practice

Opportunistic error correction based on fountain codes is especially designed for the MIMOOFDM system. The key point of this new method is the tradeoff between the code rate of error correcting codes and the number of sub-carriers in the channel vector to be discarded. By transmitting one fountain-encoded packet over a single sub-carrier per antenna, the ADC is allowed to only take care of the sub-carriers with high energy in the channel vector. In such a case, the power in the ADC is reduced by quantizing the received signal coarsely. Correspondingly, this approach can afford higher level of noise floor than the joint coding scheme adopted by the current MIMO-OFDM system. In this paper, we evaluate its performance in the aspect of mitigating the noise and interference. At the same code rate, simulation results show that opportunistic error correction works better (i.e.requires lower SNR) than the FEC layers defined in the IEEE 802.11n standard. With respect to RCPC with interleaving, the SNR gained by opportunistic error correction decreases as the multiplexing gain increases. Furthermore, we evaluate their performance in the real world. This novel approach does not have the same SNR gain in practice as in the simulation, compared to the FEC layers in the IEEE 802.11n standard. Measurement results show that this new scheme survives in most of the channel conditions (i.e. 92%) with respect to RCPC with interleaving (i.e. 86%) and the LDPC code from the IEEE 802.11n standard (i.e. around 80%).

A concatenated coding scheme for biometric template protection

Cryptography may mitigate the privacy problem in biometric recognition systems. However, cryptography technologies lack error-tolerance and biometric samples cannot be reproduced exactly, rising the robustness problem. The biometric template protection system needs a good feature extraction algorithm to be a good classifier. But, an even effective feature extractor can give a very low-quality biometric channel (i.e. high Bit Error Rate (BER)). Using the Spectral Minutiae method to identify fingerprints is one of the examples, which gives a BER of 40 ~ 50% to most of the matching channels. Therefore, we propose a concatenated coding scheme based on erasure codes to achieve a robust and secure biometric recognition system. The key idea is to transmit more packets than needed for decoding and allow the erasure-encoded packet suffering high BER to be discarded. The erasure decoder can reconstruct the secret key by collecting enough surviving packets. By applying the spectral minutiae method in the FVC2000-DB2 fingerprint database, the unprotected system achieves an EER of 3.7% and our proposed coding scheme reaches an EER of 4.6% with a 798-bit secret key.

A robust cross coding scheme for OFDM systems

In wireless OFDM-based systems, coding jointly over all the sub-carriers simultaneously performs better than coding separately per sub-carrier. However, the joint coding is not always optimal because its achievable channel capacity (i.e. the maximum data rate) is inversely proportional to the dynamic range of the channel. In this paper, we propose a novel cross coding scheme to increase the maximum data rate (i.e. the noise floor) achieved in frequency selective channels. The proposed OEC-II is based on fountain codes. The key element in OEC-II is to exchange the modulation order with the discarded subcarriers. In such case, the system can increase the noise floor by only taking care of the sub-carriers with high energy. By transmitting a fountain-encoded packet over a single sub-carrier, the packets can be discarded if they have encountered a low-energy channel. Fountain codes can recover the source data by only using the surviving packets. With the same effective transmission data rate (i.e. 10.8 Mbits/s), OEC-II with QAM-16 has a SNR gain of around 7 dB over the FEC layer from the IEEE 802.11a standard, around 8 dB over the FEC layer from the IEEE 802.11n standard, around 7 dB over OEC-II with QPSK and around 3 dB over OEC-II with QAM-64.

Another approach to save energy in OFDM systems

In this paper, we propose an energy-efficient error correction scheme to lower the power consumption of the ADCs in the OFDM system. The proposed opportunistic error correction scheme is based on resolution adaptive ADCs and fountain codes. The key idea is to reduce the dynamic range of the channel by discarding part of the channel in deep fading. Correspondingly, the power consumption in ADCs can be decreased. In our approach, each sub-carrier transports a fountain-encoded packet. The receiver only decodes fountain-encoded packets with high SNR. Others are discarded. To compensate for the discarded packets, a high order modulation is used. The new error correction layer does not require perfect channel knowledge, so it can be applied in a real system. With our approach and 16-QAM, the energy consumption in ADCs is reduced by around 73% with non-perfect channel estimation comparing to the traditional IEEE 802.11a system under the same channel conditions and throughput.