The IEEE 802.16e world interoperability for microwave access (WiMax) system plays an important role in the future wireless metropolitan area network (WMAN). Orthogonal frequency division multiple access (OFDMA), adopted in the IEEE 802.16 e WiMax system, has many advantages in the physical layer, but also poses many challenges for radio resource allocation. One of interesting radio resource allocation issue in the OFDMA system is to partition the overall radio resource (bandwidth and time duration) into two portions: one for random access and the other for connection-oriented access. In the IEEE 802.16 e WiMax system, a truncated binary backoff algorithm is adopted to resolve the contention in random access, while the time-division OFDMA is used for the connection-oriented access. The main contribution of this paper is to design an analytical approach to determine the optimal amount of reserved radio resource in both time and frequency domains for random access, with the objective of maximizing the overall efficiency of radio resource while satisfying the delay requirements for supporting real-time services. Furthermore, an analytical model for calculating the access latency and the efficiency of the reserved radio resources is developed.

Through wide-band spectrum sensing, cognitive radio (CR) can identify the opportunity of reusing the frequency spectrum of other wireless systems. However, wide-band spectrum sensing requires energy consumption processes. In this paper, we aim to relieve the burden of spectrum scanning in a CR system by means of location awareness. We investigate to what extent a CR system with location awareness capability can establish a scanning-free region where a peer-to-peer connection of the secondary CR users can coexist with an infrastructure-based connection of the primary user. We compute the concurrent transmission probability of a peer-to-peer connection and an infrastructure-based connection in a system based on the carrier sense multiple access with collision avoidance (CSMA/CA) medium access control (MAC) protocol. It has been shown that the frequency band of the legacy system can be reused up to 45% by the overlaying cognitive ad hoc network if certain location techniques help CR users locate primary and other secondary users. In summary, a CR system equipped with location awareness techniques can dramatically reduce the need of spectrum sensing thanks to the capability of identifying the concurrent transmission region in a hybrid infrastructure-based and ad hoc overlaying systems. Hence, from another aspect, the issue of wide-band spectrum sensing in CR systems is resolved fundamentally.

Orthogonal frequency division multiple access (OFDMA), adopted in the IEEE 802.16e WiMax system, possesses many advantages in the physical layer, but also poses many challenges from the aspect of radio resource allocation. In the IEEE 802.16e WiMax system, a truncated binary backoff algorithm is adopted for resolving the contention in random access, while the time-division OFDMA is used for the connection-oriented access. One interesting radio resource allocation issue in such an OFDMA system is to partition the overall radio resource (bandwidth and time duration) into two portions: one for random access and the other for connection-oriented access. The main contribution of this paper is to design an analytical approach to determine the optimal amount of reserved radio resource in both time and frequency domains for random access with the objective of maximizing the overall efficiency of radio resource while satisfying the delay requirements for supporting real-time services. Furthermore, an analytical model for calculating the access latency and the efficiency of the reserved radio resources is developed.

Relay stations (RSs) are usually used to enhance the signal strength for the users close to the cell boundary. However, transmission through a relay station needs two transmission phases, i.e., one is from the base station to the relay station and the other is from relay station to mobile stations. Thus, relay may also decrease system capacity if two-phase transmission time is considered. As a result, whether or not data are transmitted by one-hop or two-hop transmission should be determined based on both signal strength and throughput. In this paper, we investigate the optimal relay location aiming to maximize system capacity. We consider two relay selection rules for determining whether a two-hop transmission is necessary: signal strength-oriented and throughput-oriented. We find that the signal strength-oriented two-hop transmission may yield even lower system capacity than the one-hop transmission. Based on the throughput-oriented rule, we find that the throughput in the two-hop transmission can be higher than that in the one-hop transmission at some locations. We also identify the optimal relay location that can achieve the highest system capacity.

As the IEEE 802.11 wireless local area networks (WLANs) have become very popular and the emerging IEEE 802.16e wireless metropolitan area networks (WMANs) also attract a great deal of attentions, in the future we can expect the appearance of the hybrid IEEE 802.11 WLANs and IEEE 802.16e WMANs systems. Clearly, seamless handoffs in this hybrid network will become an increasingly important issue. However, the system which has shortest handoff latency may not always have the highest throughput. In this paper, we develop a dynamic network selection scheme to determine the connecting system in a new "WLAN to hybrid WLAN/WiMax" handoff scenario, in which we consider both the vertical and horizontal handoffs. The proposed scheme requests the station to wait an additional network selection time "tw" before the selection to maximize the amount of delivered bits during a dwelling time. We also formulate an optimization problem to find the optimal network selection time t*w, and show that the proposed scheme can improve the delivered information bits during handoff.

Adaptively adjusting transmission rate and power to concurrently enhance goodput and save energy is an important issue in the wireless local area network (WLAN). However, goodput enhancement and energy saving are two contradictory goals, since high data rate usually requires high transmission power to meet the bit-error-rate (BER) requirement. To achieve the tradeoff between energy efficiency and goodput, therefore, we suggest a fast channel-driven rate and power adaptation (CDRPA) algorithm. Furthermore, we develop a physical (PHY)/medium access control (MAC) cross-layer analytical method to evaluate goodput and energy efficiency, incorporating the impacts of Nakagami fading channel and the carrier sense multiple access (CSMA) MAC protocol. This paper compares two adaptation

In recent years, the IEEE 802.11 wireless local area networks (WLANs) have become a very popular access network technique. However, due to the large handoff latency, most application scenarios are limited to hot spot areas without handoff across different access points (APs). As the advance of many real-time or highly interactive applications such as voice over IP (VoIP), one of burning issues for the WLAN is to reduce the handoff latency. Current studies in the literature have indicated that the latency in searching available APs dominates the handoff latency. Thus, we propose a simple analytical model to calculate the search latency and success probability for the handoff in the IEEE 802.11 WLAN. A new performance metric, namely the "effective search time (EST)", is introduced to evaluate the latency that is required for a station to successfully search a channel from neighboring APs. Based on the developed model, the optimum numbers of the probe request and response transmission for minimizing the EST can be obtained. Our results demonstrate that, in the environment of 100 contending stations, the EST with optimum number of transmissions can be reduced by 30% compared to that in the legacy IEEE 802.11 WLAN.

In this paper, we propose a fast channel-driven rate and power adaptation (CDRPA) algorithm for wireless local area networks (WLAN). The proposed algorithm jointly adjusts the rate and power for data frame transmission in a generalized Nakagami fading channel. Different from the previous works, the CDRPA algorithm bases on the channel quality observed from the previous ACK frames and makes the selection only from the limited combinations of rate and power, instead of exhausted searching. Thus, the CDRPA algorithm greatly reduces the complexity compared to the previous approaches. Furthermore, to meat the different performance requirements, we propose several alternatives based on the CDRPA algorithm. Prom the viewpoint of energy saving, the CDRPA algorithm determines the transmit power first to maximize energy efficiency; while achieving the maximum goodput performance, the CDRPA first selects the appropriate transmit rate to ensure the successful transmissions. The numerical results show that the CDRPA algorithm can approach to the optimum performance in terms of energy efficiency and goodput while keeping the low complexity

In this paper we develop a cross-layer analytical approach from both the physical layer and the medium access control (MAC) layer to evaluate the performance of the IEEE 802.11 wireless local area network (WLAN). From the physical layer, this analytical approach incorporates the effects of both capture and directional antenna, while from the MAC layer, our model takes account of the carrier sense multiple access with collision avoidance (CSMA/CA) protocol. Through a cross-layer modeling technique, this analytical framework can provide valuable insights of the physical layer impact on the throughput performance of the CSMA/CA MAC protocol. These insights can be helpful in developing a MAC protocol to fully take advantage of directional antennas for enhancing the performance of the WLAN.

The desired properties of a medium access control (MAC) protocol in mobile ad hoc network (MANET) include: (1) meet quality of service (QoS) requirements for real-time nodes, (2) be decentralized, (3) achieve fairness from viewpoint of throughput or energy consumption, and (4) be immune to the hidden node problem. Though there have been numerous proposed MAC protocols for the IEEE 802.11 WLAN, few of them possess all of the four properties mentioned above. Our protocol can support real-time traffic and satisfy QoS requirements, and can achieve fairness among non-real-time nodes. Also, without using any centralized control, it can be easily deployed in MANET. An analytic model of the protocol's throughput has also been developed. We compare the protocol's throughput obtained from its analytic model and simulation to validate each other.