On the Study of Cross-layer Optimization for Wireless Cooperative Networks

In this paper, we study the effect of cooperative transmission on the routing decision for wireless ad-hoc networks. The influence of cooperative transmission to the wireless link cost is first studied, with or without node selection, which shows that the quality of wireless links could be improved significantly. To reduce system overhead, a distributed strategy of node selectio n is proposed by carefully designing the carrier sensing protocol in the medium access control layer. Then routing optimization is investigated to understand the effects of improved link cost to th e routing decision, where the optimal solution of the optimization problem is developed and later used as a quantitative criterion of the routing decision. Our developed analytical and simulation results show that the criteria using cooperative transmission typically yield more efficient routes than the non-cooperative schemes. I. I NTRODUCTION Future wireless communication networks are expected to support the mixture of real-time applications, such as voic e and multimedia teleconferencing, and non-real-time data a pplications, such as web browsing, messaging and file transfers. Compared with wired environments, the associated com munication channels and traffic patterns in mobile wireless networks are more unpredictable. Hence all of these applications impose stringent and diversified Quality of Service (QoS) requirements, which cannot be satisfactorily addres sed through the traditional layering network-protocol archit ecture. Correspondingly, there has recently been increased intere st in protocols for wireless networks to exploit the significan t interactions between various layers of the protocol stack f or performance enhancements. And it has been shown that these cross-layer designs and protocols could be essential for wi reless ad-hoc and sensor networks where unpredictable variab les such as node mobility, node density and network dimensions make the diverse and stringent wireless QoS requirements difficult to satisfy. Due to the unreliability of wireless links, it has been of interest to study the impacts of physical-layer techniques on the design of upper-layers, including medium access contro l (MAC), packet scheduling, power control, routing, transpo rt protocol, and ultimately the QoS at the application level in wireless networks. Opportunistic scheduling could be seen as one of successful examples of cross-layer design, where Research was sponsored by US Army Research laboratory and the UK Ministry of Defence and was accomplished under Agreement Number W91 1NF06-3-0001. The views and conclusions contained in this docu ment are those of the authors and should not be interpreted as representing th e official policies, either expressed or implied, of the US Army Research Laborator y, he U.S. Government, the UK Ministry of Defense, or the UK Government. T he US and UK Governments are authorized to reproduce and distribut e reprints for Government purposes notwithstanding any copyright notatio n hereon. scheduling protocols are designed by taking advantage of the knowledge of wireless link conditions [1], [2]. Among many candidates of physical-layer techniques, multiple-i nput multiple-output (MIMO) has received significant attention , which can provide spatial diversity and hence represents a powerful technique for interference mitigation and reduct ion [3], [4]. Cooperative communication provides an alternati ve way to achieve spatial diversity, where single-antenna ter minals in a multiple-user environment share their antennas an d form a cluster to assist each other with their data transmiss ion [5], [6]. In [7] and [8] the authors study the cross-layer rou ting protocol design for energy-constrained networks, where th e cooperative transmission technique is used to form a virtua l antenna array. It is shown in [7] and [8] that the proposed pro tocol could yield a different route compared with tradition ally non-cross-layer protocols if the circuit processing energ y is c nsidered, and the two protocols will choose the same route otherwise. Recently the routing optimization is analyzed by using a probabilistic link model [9], which pints out that the broad casting nature of wireless communications should be utiliz ed by routing protocols to achieve robustness at the networkin g level. Recall that cooperative transmission has been recog nized as an effective technology to utilize such feature of wirele ss communications [5], [10], [11]. Inspired by this observati on, we focus on studying the effects of cooperative transmissio n on the routing decision in this paper. The effects of coopera tive transmission to the link quality are first studied with or wit hout node selection, and it is shown that cooperative transmissi on can improve the quality of a wireless link, such as lower powe r consumption or higher reliability. Although incorporatio n f a strategy of an appropriate node for cooperative transmissi on could yield more performance gain, but a centralized strate gy could cause too much overhead. To reduce the system overhead, we propose a distributed strategy of node selection by using a carrier sensing protocol at the medium access contro l (MAC) layer. The problem of routing optimization is then investigated in order to study the effects of improved link cost to the routin g decision. The objective function of interest is to minimize th total power consumption with a given end-to-end reliabilit y constraint. The optimal solution of this optimization prob lem indicates the minimum total power consumption of a route in order to satisfy the required error performance, which is then used as a criterion to compare and select the best among different routes. Our analytical results show that the rout e chosen by the cooperative criterion can consume much less transmission power compared with the route using only direc t transmission. For certain path loss factors and provided th at