There are two key components in the design of future high-speed, robust wireless networks, namely powerful transmission technologies and efficient resource management. With the emergence of new transmission technologies, i.e. multiple-input multiple-output (MIMO), orthogonal frequency division multiplexing (OFDM), cognitive radio (CR) and etc., the efficient resource management becomes one major challenge in the design of future wireless networks. In this thesis, we would like to shed some lights on this challenge. Specifically, we investigate the resource allocation problem in the following systems:...[ Read more ]

There are two key components in the design of future high-speed, robust wireless networks, namely powerful transmission technologies and efficient resource management. With the emergence of new transmission technologies, i.e. multiple-input multiple-output (MIMO), orthogonal frequency division multiplexing (OFDM), cognitive radio (CR) and etc., the efficient resource management becomes one major challenge in the design of future wireless networks. In this thesis, we would like to shed some lights on this challenge. Specifically, we investigate the resource allocation problem in the following systems:

Cellular systems with multi-antenna and/or multi-carrier: In such systems, the channel state information at the transmitter (CSIT) is usually imperfect (contain errors). It’s shown that the naive scheduler designed for perfect CSIT suffers a significant performance degradation from CSIT error in such systems. Therefore, we first investigate the optimal cross-layer design in the downlink multi-antenna OFDMA systems with imperfect CSIT. We propose a novel open-loop cross-layer scheduler taking the CSIT error into consideration. It’s demonstrated by simulations that this novel scheduler obtains a significant goodput gain over the naive scheduler.

The above open-loop design relies heavily on the knowledge of CSIT error statistics. However, this knowledge is usually hard to obtain in practice. Therefore, we continue to consider the closed-loop cross-layer design which should be more robust to the open-loop design. Specifically, we consider two different systems: (1) multiuser multi-antenna systems and (2) OFMDA systems. We propose novel closed-loop approaches for both systems with imperfect CSIT, where the CSIT error statistics is not required for the scheduler design and we adapt the transmission parameters (such as power, rate, user selection) with the ACK/NAK feedbacks from mobiles. Simulation results illustrate that based on the ACK/NAK feedbacks, the performance of the proposed closed-loop cross-layer designs is very robust with respect to the CSIT error, unknown CSIT error statistics as well as channel variations due to Doppler.

Cellular systems with HARQ: Most of the existing works handle HARQ and cross-layer scheduling in a decoupled manner based on heuristic approaches. We introduce an integrated design framework for cross-layer scheduling and HARQ design in multiuser systems with slow fading channel and outdated knowledge of CSIT. We consider both the chase combining and incremental redundancy in HARQ. Based on information theoretical approach, we derive the asymptotically optimal cross-layer policy (power allocation, rate allocation and user selection policies) to optimize the average system goodput with HARQ. In addition, we derive analytically the closed-form expression of the average system goodput, from which we can obtain useful design insights such as the role of HARQ in the overall cross-layer gain, the tradeoff between the system goodput and the HARQ delay as well as the sensitivity of the average system goodput with respect to the CSIT quality.

Cognitive cellular systems: In most of the existing works on CR systems, the spectrum sensing and the cross-layer scheduling are designed separately. Specifically, the sensing module first determines whether or not a channel resource is available for the CR system based on the sensing information. The cross-layer scheduling module then schedules the data transmission of different users on the available channels based on the hard-decision sensing information (HSI). This isolated design is not efficient when the spectrum sensing is imperfect. We propose a joint spectrum sensing and cross-layer scheduling design framework for downlink cognitive cellular system with imperfect sensing. In our joint design framework, the mobiles’ sensing feedbacks (named as RSI, raw sensing information) is used directly in the cross-layer scheduler, and it’s shown that such joint design can exploit the spectrum hole much more efficiently, leading to much more transmission opportunities for the CR systems and therefore, much higher system goodput than the conventional isolated design.

Relay networks: Utilizing the relay buffers, an opportunistic decode-wait-and-forward relay scheme is proposed for a point-to-point communication system with a half-duplexing relay network to better exploit the time diversity and relay mobility. For instance, we analyze the asymptotic throughput-delay tradeoffs in a dense relay network for both two types of channel fading: (1) fixed relays with microscopic fading channels (multipath channels), and (2) mobile relays with macroscopic fading channels (path loss). In the first scenario, the proposed scheme can better exploit the multi-relay diversity in the sense that with K fixed relays and a cost of O(K) average end-to-end packet delay (compared with the O(1) average end-to-end packet delay of the existing designs), it could achieve the same optimal asymptotic average throughput as the existing designs (such as regular decode-and-forward relay schemes) with K² fixed relays. In the second scenario, the proposed scheme achieves the maximum throughput of Θ(log K). This system throughput is unattainable for the existing designs with low relay mobility, the proposed relay scheme can exploit the relays’ mobility more efficiently.