Chengkang Pan

Adaptive subcarrier and power allocation for multiuser MIMO-OFDM systems

This paper addresses the optimal resource allocation problem for multiuser MIMO-OFDM systems. We apply an optimization algorithm to obtain a joint subcarrier and power allocation scheme based on orthogonal frequency division multiple access (OFDMA) combined with dirty paper coding (DPC) assuming instantaneous channel state information (CSI), which is called as DPC-OFDMA. The ultimate objective is to minimize the total transmit power subject to individual required data rates constraints. To reduce the complexity of the optimal solution, the analysis is considered in two stages. The first stage addresses subcarriers allocation, in which users are allowable to share subcarriers. The second stage employs DPC technology to deal with simultaneous transmissions of the users sharing the same subcarriers. An efficient algorithm to choose the best possible ordering for DPC and the optimal precoding design of each user are also involved. Simulation results show that DPC-OFDMA scheme has high spectral and power efficiency than conventional fixed schemes, where fixed power and subcarriers are allocated to each user.

Relay MAC Channels: Capacity and Resource Allocation

We investigate the relay multiple access channels (RMAC) with two users and a destination. We determine the lower bound and upper bound on the capacity of RMAC under time-division (TD) decode-and-forward (DF) mode by using superposition modulation. A relay scheme with resource allocation is proposed to achieve the lower bound. Analytical results and simulation results show that the proposed scheme can achieve larger capacity region than that of direct transmission (DT) for the fixed channel gain case. But they provide the same maximum sum capacity. Whereas, the proposed relay scheme can provide higher outage capacity than DT scheme due to the fact that the two sources can share the resources from each other.

Capacity, power allocation and partners selection for SIMO relay channels

In this paper, we investigate the cooperative channels with one source-destination pair and multiple partners (relays), where only the destination is equipped with multiple antennas. The achievable capacity with the optimal power allocation and partner selection is analyzed with the total transmit power constraints under the different cooperation modes, including amplify-and-forward (AF) and decode-and-forward (DF). With the partial channel state information (CSI) at the destination, we develop three algorithms to choose the possible best partner(s) for AF, repetition-coded DF and Gaussian-coded DF, which are called Selective AF (SAF), Selective repetition-coded DF (SRDF), and Selective Gaussian-coded DF (SGDF), respectively. In these algorithms, only one partner will be employed as relay for SAF and SRDF to maximize the capacity while multiple partners are selected for SGDF. An efficient quasi-distributed protocol to support SAF, SRDF, and SGDF is also involved. We study the impact of the number of receive antennas and SNR on SAF, SRDF, and SGDF. Numerical results show that SAF and SRDF have higher spectral efficiency than direct transmission (DT), especially in low SNR regime and for the small number of receive antennas and large number of users cases, while SGDF outperforms DT evidently in every scenario. Copyright

Channel-aware multi-user uplink transmission scheme for SIMO-OFDM systems

The problem of medium access control (MAC) in wireless single-input multiple-output-orthogonal frequency division multiplexing (SIMO-OFDM) systems is addressed. Traditional random access protocols have low overheads and inferior performance. Centralized methods have superior performance and high overheads. To achieve the tradeoff between overhead and performance, we propose a channel-aware uplink transmission (CaUT) scheme for SIMO-OFDM systems. In CaUT, users transmit request-to-send (RTS) at some subcarriers whose channel gains are above a predetermined threshold. Using the channel state information provided by RTS, access point performs user selection with receive beamforming to decide which users can access and then broadcasts the selection results via clear-to-send (CTS) to users. We present a distributed power control scheme by using a simple fixed modulation mode. We optimize the modulation order and channel gain thresholds to maximize the separable packets subject to the bit-error-rate (BER) and temporal fairness requirements and the individual average transmit power constraints. The performance of CaUT scheme is analyzed analytically and evaluated by simulations. Simulation results show that CaUT can achieve more significant throughput performance than traditional random access protocols.

Space sense random access with collision mitigation in uplink WLANs

Space sense random access (SSRA) with collision mitigation is proposed for TDD wireless local area networks (WLANs) with multiple antennas. Before transmitting a packet, the user performs space sense to detect the status of the channel. If the channel is idle, it then transmits a common training sequence for channel estimation. Multiple subslots are available for channel estimation and the user randomly selects one subslot to achieve the possible collision-free channel estimation. It is shown that SSRA removes the requirement of orthogonal training sequences while avoiding possible packet collision and further providing multipacket reception capability. Compared to the conventional slotted ALOHA scheme, SSRA can achieve much higher throughput, as verified by extensive simulation results.

Space sensing based random access in SIMO-OFDM systems

The problem of cross-layer medium access control (MAC) in a SIMO-OFDM system is addressed and a space sensing based random access (SSRA) scheme is proposed. In SSRA scheme, the transmit slot is divided into three parts: space sensing period, channel estimation period and packets transmission period. Without orthogonal training sequences, users can sense the possible spatial collisions in the following way. Each user sends a short random sequence to the access point (AP) and the AP forwards the receive signal to the users with an amplify-and-forward form. Each user detects the receive signal to determine whether a collision in packets transmission period will appear. After space sensing, the user sensing no collision sends a common training sequence to AP for channel estimation. Multiple nonoverlapping pilot subslots are set and the user randomly selects one for training sequence transmission. Failing channel estimation means failing packet reception. Thus, SSRA gives a more practical multipacket reception (MPR) model. The throughput performance of the proposed SSRA scheme is analyzed analytically and evaluated through simulations.

Multipacket reception in MIMO-OFDM systems

The medium access control (MAC) layer in many wireless systems is designed to serve one user at a slot when conventional slotted ALOHA protocol is applied. However, the problem of packet collisions can be solved at the physical (PHY) layer by using signal processing technique. By exploiting orthogonal frequency division multiple access (OFDMA) and spatial division multiple access (SDMA), a decentralized MAC strategy with multi-packet reception (MPR) is proposed for MIMO-OFDM systems. First, employing receive beamforming, we study the possible optimal number of users which are likely to transmit at the same subcarrier and slot to maximize the separable packets subject to their BER (SINR) requirements. Then the transmission probability is designed to achieve the desired number of users. The performance and benefits of the proposed strategy is analyzed analytically and evaluated through simulations.

A Multipacket Reception Protocol Based on Cooperative Communication for WMNs

Cooperative communication enables nodes to share their antennas to form a virtual multiple-input multiple-output system and achieve spatial diversity gain. A multipacket reception protocol, based on cooperative communication and network-assisted diversity multiple access, is proposed to employ multiple receive antennas to separate the collided packets quickly and provide the multi-packet reception capability in wireless mesh networks. The theoretical analysis and simulation results show that the proposed protocol is capable of fully utilizing the spatial resources and outperforms the conventional NDMA and TDMA in terms of system throughput.

Cooperative multiple access channels: Achievable rates and optimal resource allocation

We investigate the multiple access channels (MAC) where sources can cooperate via half-duplex relaying and refer to it as cooperative MAC channels (CMAC). Assuming perfect channel state information (CSI) at the transmitters and the receivers, we determine the bounds on the achievable rate region of a Gaussian CMAC channel and an inner bound on the outage capacity region of a fading CMAC channel. Based on superposition modulation, a half-duplex cooperative relay scheme with optimal resource allocation is proposed to achieve the bounds of capacity region. Analytical results and simulation results show that the achievable rate region of a Gaussian CMAC channel is larger than that of a Gaussian MAC channel with direct transmission (DT) schemes. But they have the same achievable sum rate. Moreover, the proposed scheme can provide higher outage capacity region than DT schemes in a fading MAC channel due to the fact that sources can share the resources with each other to reduce outages.

Multipacket Reception in SIMO-OFDM Systems

The problem of random access (RA) in a wireless SIMO-OFDM system is addressed and a decentralized medium access control (MAC) strategy is proposed based on the so called random orthogonal frequency division multiple access (ROFDMA). By employing receive beamforming, the problem of packet collisions at the same subcarrier are solved by using random spatial division multiple access (RSDMA), which therefore provides a multipacket reception (MPR) capability. First, we study the optimal number of users allowed to transmit at the same subcarrier and the same slot to maximize the separable packets subject to their BER requirements. Then the transmission probability based on users channel gain is designed to achieve the desired number of users with the highest probability. Finally, a MPR scheme under average transmit power constraint is proposed with modulation selection. The performance of the proposed scheme is analyzed analytically and evaluated through simulations