Liangping Ma

Matched filtering assisted energy detection for sensing weak primary user signals

Energy detection is widely used by cognitive radios for spectrum sensing. During a silent period, secondary users (SUs) are kept silent so that the energy detector does not confuse SU signals for primary user (PU) signals. Due to imperfect coordination, an SU may transmit during a silent period and cause possible false alarms. We propose to leverage matched filters that already exist in many SUs to alleviate the impact of such SU interference by combining the matched filtering result and the energy detection result. The analysis shows that for practical purposes, our algorithm virtually eliminates all of the negative impact of SU interference with only negligible penalty in delay and energy consumption.

Computational Complexity of Interference Alignment for Symmetric MIMO Networks

It is known that the problem of maximizing the sum degrees of freedom (DoF) for an arbitrary MIMO network without symbol extension is NP-hard in the number of transmitter-receiver pairs. The DoF achievability problem is also NP-hard if each transmitter/receiver has at least three antennas, and is polynomial-time solvable if each transmitter/receiver has no more than two antennas. It was conjectured that for the special case of the symmetric network (where all the transmitters have the same number of antennas and all the receivers have the same number of antennas), polynomial-time algorithms might exist for these two problems. In this paper, we show that the conjecture holds only in a very limited sense. We also show that for two important special cases of the symmetric network, both problems are polynomial-time solvable if the number of antennas at each transmitter/receiver is no more than two, and generally are NP-hard otherwise.

Making overlay cognitive radios practical

The overlay approach to dynamic spectrum access recently proposed in information theory allows both primary users (PUs) and secondary users (SUs) to simultaneously access the same spectrum with comparable power levels while ensuring no degradation to the performance of PUs. However, this approach is based on a number of idealized assumptions that are difficult to satisfy in practice, and existing efforts to address this issue fall outside physical layer processing. In this paper, we propose a number of physical layer mechanisms to make the overlay approach practical. In particular, we leverage the broadcast nature of the wireless medium and the latest breakthrough in full-duplex radios to resolve the synchronization problem and to get around the non-causal assumption while naturally offering delay diversity.

Cyclic Prefix Adaptation with Constant Overall Symbol Time for DFT-Spread-OFDM and OFDM

For DFT-spread-OFDM or OFDM, if the delay spread varies in a wide range and the symbol duration is relatively short, adapting the cyclic prefix (CP) duration rather than using a fixed one may significantly improve the spectral efficiency while preventing inter-symbol interference (ISI). In practice, it may be beneficial to have a constant overall DFT-spread-OFDM/OFDM symbol time, which is the sum of the duration of a CP and the duration of a data portion. We propose to adapt the CP duration to the delay spread without changing the overall symbol time for DFT-spread-OFDM or OFDM, and address implementation challenges. In particular, we propose changing the clocking rate of ADC and DAC or using a Farrow filter to reduce the computational complexity of arbitrary-size DFT/IDFT resulting from the adaptation.