Lei Ke

Monobit digital receivers: design, performance, and application to impulse radio

Digital receivers for future high-rate high-bandwidth communication systems will require large sampling rate. This is especially true for ultra-wideband (UWB) communications with impulse radio (IR) modulation. Due to high complexity and large power consumption, multibit high-rate analog-to-digital converter (ADC) is difficult to implement. Monobit receiver has been previously proposed to relax the need for high-rate ADC. In this paper, we derive optimal digital processing architecture for receivers based on monobit ADC with a certain over-sampling rate and the corresponding theoretically achievable performance. A practically appealing suboptimal iterative receiver is also proposed. Iterative decision-directed weight estimation, and small sample removal are distinctive features of the proposed detector. Numerical simulations show that compared with full resolution matched filter based receiver, the proposed low complexity monobit receiver incurs only 2 dB signal to noise ratio (SNR) loss in additive white Gaussian noise (AWGN) and 3.5dB SNR loss in standard UWB fading channels.

Degrees of Freedom Region for an Interference Network With General Message Demands

We consider a single-hop interference network with K transmitters and J receivers, all having M antennas. Each transmitter emits an independent message and each receiver requests an arbitrary subset of the messages. This generalizes the well-known K -user M-antenna interference channel, where each message is requested by a unique receiver. For our setup, we derive the degrees of freedom (DoF) region. The achievability scheme generalizes the interference alignment schemes proposed by Cadambe and Jafar. In particular, we achieve general points in the DoF region by using multiple base vectors and aligning all interferers at a given receiver to the interferer with the largest DoF. As a byproduct, we obtain the DoF region for the original interference channel. We also discuss extensions of our approach where the same region can be achieved by considering a reduced set of interference alignment constraints, thus reducing the time-expansion duration needed. The DoF region for the considered system depends only on a subset of receivers whose demands meet certain characteristics. The geometric shape of the DoF region is also discussed.

Finite-resolution digital receiver design for impulse radio ultra-wideband communication

Receiver design for impulse radio based ultrawideband (UWB) communication is a challenge. High sampling rate high resolution digital receiver is usually difficult to implement. Some tradeoffs can be made on the digital receiver, such as limiting amplitude resolution to only one bit, which results in a previously considered monobit receiver. In this paper, we consider the design of finite-resolution digital UWB receivers. We derive the optimal post-quantization processing, and analyze the achievable bit-error rate performance using an approximation of the log-likelihood ratio. Optimal thresholds for 4- and 3-level quantization are obtained. Training-based receiver template estimation is presented. Our work discloses the incremental gain that additional quantization levels offer and the results provide useful guidelines for designing impulse radio UWB digital receivers.

Degrees of freedom region for an interference network with general message demands

We consider a single-hop interference network with K transmitters and J receivers, all having M antennas. Each transmitter emits an independent message and each receiver requests an arbitrary subset of the messages. This generalizes the well-known K -user M-antenna interference channel, where each message is requested by a unique receiver. For our setup, we derive the degrees of freedom (DoF) region. The achievability scheme generalizes the interference alignment schemes proposed by Cadambe and Jafar. In particular, we achieve general points in the DoF region by using multiple base vectors and aligning all interferers at a given receiver to the interferer with the largest DoF. As a byproduct, we obtain the DoF region for the original interference channel. We also discuss extensions of our approach where the same region can be achieved by considering a reduced set of interference alignment constraints, thus reducing the time-expansion duration needed. The DoF region for the considered system depends only on a subset of receivers whose demands meet certain characteristics. The geometric shape of the DoF region is also discussed.

On the Degrees of Freedom Regions of Two-User MIMO Z and Full Interference Channels with Reconfigurable Antennas

We study the degrees of freedom (DoF) regions of two-user multiple-input multiple-output (MIMO) Z and full interference channels in this paper. We assume that the receivers always have perfect channel state information. We derive the DoF region of Z interference channel with channel state information at transmitter (CSIT). For full interference channel without CSIT, the DoF region has been obtained in previous work except for a special case M1< N1<min(M2,N2), where M_i and N_i are the number of transmit and receive antennas of user i, respectively. We show that for this case the DoF regions of the Z and full interference channels are the same. We establish the achievability based on the assumption of transmitter antenna mode switching. A systematic way of constructing the DoF-achieving nulling and beamforming matrices is presented in this paper.

Finite-Resolution Digital Receiver Design for Impulse Radio Ultra-Wideband Communication

Receiver design for impulse radio (IR) based ultra-wideband (UWB) communication is a challenge, because full-resolution digital receiver is difficult to implement under todays technology due to high sampling rate required. Some trade-offs can be made to the digital receiver, such as limiting amplitude resolution to only 1 bit, which results in a previously considered so-termed mono-bit receiver. In this paper, we consider the design of finite-resolution digital UWB receiver. We derive the optimal post-quantization processing, and analyze the achievable bit-error rate (BER) performance using an approximation of the log-likelihood ratio. We evaluate the effect of quantization threshold. The optimal threshold for two-bit quantization is obtained. Our work discloses the incremental gain that each sampling bit could bring and the results provide guidelines for designing IR UWB digital receivers.receivers.

Interference alignment and Degrees of Freedom region of cellular sigma channel

We investigate the Degrees of Freedom (DoF) Region of a cellular network, where the cells can have overlapping areas. Within an overlapping area, the mobile users can access multiple base stations. We consider a case where there are two base stations both equipped with multiple antennas. The mobile stations are all equipped with single antenna and each mobile station can belong to either a single cell or both cells. We completely characterize the DoF region for the uplink channel assuming that global channel state information is available at the transmitters. The achievability scheme is based on interference alignment at the base stations.

Performance analysis of IR-UWB 1-bit digital receivers

This paper considers the practical implementation of low-resolution digital receivers in impulse radio ultra-wideband (IR-UWB) communication systems for low complexity and low power consumption. The performance of 1-bit digital receivers is investigated. The maximum mutual information per sample is derived and the structure of the maximum likelihood receiver is analyzed. The bit error rate performances under different modulation schemes are also studied. The analysis demonstrates that the signal to noise ratio degradation for 1-bit receivers in multi-path UWB channel is only 1.96 dB. The simulation is in good agreement with our analytical derivations.

Capacity analysis of mimo ad hoc network with stream number selection

We study the problem of stream number selection in an ad hoc multiple-input multiple-output (MIMO) network, where there are a number of co-existent MIMO transmitter-receiver pairs. We assume that there is no channel state information (CSI) at the transmitters, and the receivers use single-user detection. It is shown that the link capacities at high signal-to-noise ratio (SNR) is mainly limited by the number of degrees of freedom available at the receiver side. In general the number of streams per transmitter should not be more than Ntau/L when the interference level is high, where Ntau is the number of receive antennas and L is the number of simultaneous links. When the interference level is low, the number of streams should be set equal to the number of transmit antennas.