无线局域网(Wireless Local Area Network，WLAN)已经成为无线智能终端接入互联网的重要方式。目前主流的WLAN是基于IEEE 802.11标准进行实现。为了应对不断增长的WLAN性能要求，一些已有的研究工作通过在IEEE 802.11标准中引入额外的控制信息，设计新的WLAN机制，从而提高WLAN在信道接入控制和能量消耗方面的性能。携带机制被广泛用于实现节点之间新引入的控制信息的传输。但是，传统携带机制不仅消耗额外的信道资源而且无法可靠地将具有广播特性的控制信息从一个节点传输到多个节点。因此，在WLAN中设计新的控制信息传输机制是一个重要的研究问题。新设计的控制信息传输机制可以被用于解决WLAN中存在的各种问题，从而提高WLAN的性能。由于WLAN具有部署简单、设备成本低和传输速率高等优点，移动运营商通过把蜂窝网的数据流量卸载到WLAN，减轻蜂窝网的负载压力。为了提高卸载到WLAN的数据流量的容量，设计高效的流量卸载策略是另外一个重要的研究问题。本文具体的研究内容和贡献如下： 1.基于物理层的正交频分多址(Orthogonal Frequency Division Multiplexing， OFDM)技术，设计了一种新的通信机制CoS(Communicationthroughdata symbol Silence)，传递轻量的控制信息，并且针对具有频率选择性衰落特性的信道场景，提出改进的通信机制CoS+。CoS在物理层主动擦除数据包传输中的一些数据符号，并利用被擦除的数据符号的间隔长度编码需要传输的额外的控制信息。CoS把额外的控制信息嵌入到物理信号中，并没有修改IEEE 802.11标准中规范的数据包格式。实际的WLAN通信系统存在信噪比间隔，使得信道编码的纠错能力没有被充分利用。因此，只要合理地设计被擦除的数据符号的数量，CoS不会影响原始数据包的正确解码。本文通过实验测量了在不同信道条件下CoS能够擦除的数据符号的数量。在频率选择性衰落信道中，由于无线传输导致的错误数据比特主要分布在信道条件比较差的数据子载波上。基于这个观察，改进的通信机制CoS+在信道条件差的数据子载波上擦除数据符号，从而减少由于擦除数据符号引入的错误数据比特。实验测量表明，在频率选择性衰落信道中CoS+比CoS能够擦除更多的数据符号，从而能够传输更多的控制信息。 2.基于控制信息传输机制CoS，提出了一种信道接入控制机制。IEEE 802.11标准在介质接入控制(Medium Access Control，MAC)层采用分布式协调功能(Distributed Coordination Function，DCF)。为了减少分布式竞争接入机制DCF中的传输碰撞，本文提出信道接入控制机制CoS-MAC，利用CoS在当前数据包传输中嵌入为下一次数据包传输选择的退避计数器的数值。在CoS-MAC中，基于CoS的分布式信息交换，网络中的节点知道彼此的退避过程何时结束，从而减少传输碰撞。CoS-MAC将DCF中的随机退避转换成确定性退避。节点之间的退避计数器信息的传输的可靠性对于网络性能有着显著的影响。仿真结果表明，相对于DCF机制和已有的采用传统携带机制实现退避计数器信息交换的机制，CoS-MAC显著减少了WLAN中的传输碰撞，提高了WLAN的吞吐量。 3.基于控制信息传输机制CoS，提出了一种节能机制。在WLAN中，节点在基于DCF竞争信道的过程中，会由于网络中的其它节点的数据包传输而冻结自己的退避过程，并接收目的地址不是自己的数据包。在IEEE 802.11标准中，节点需要在接收整个数据包之后才能获得MAC帧头中的目的地址信息。因此，即使节点在获得目的地址信息之后会丢弃无用的数据包，但是在物理层接收和处理无用数据包的过程中已经浪费了能量。本文提出的节能机制利用CoS在物理层将数据包的目的地址和传输持续时间嵌入到数据包传输的前部。因此，节点只需要在物理层接收部分数据包就能提取嵌入的信息，从而在物理层立即停止无用的数据包的接收和处理，并可以在当前数据包传输结束之前维持在低功耗状态，从而节省能量。本文提出的节能机制不需要对现有的WLAN协议进行修改，可以直接应用到现有的WLAN系统。仿真结果表明，提出的节能机制显著减少了节点在DCF机制中执行退避过程消耗的能量。 4.针对蜂窝-WLAN融合网络中的流量卸载，提出了D2D(Device to Device)通信辅助的流量卸载机制。由于WLAN中的DCF是竞争接入机制，如果过多的蜂窝用户被卸载到WLAN，将导致WLAN的网络吞吐量由于激烈的竞争接入而下降，从而减少了能够从蜂窝网卸载到WLAN的数据流量的容量。为了减少直接被卸载到WLAN的蜂窝用户的数目，本文提出的流量卸载机制利用蜂窝网中的D2D通信汇聚被卸载的蜂窝用户的数据流量，只允许少量汇聚节点直接接入WLAN，从而减少WLAN中的信道接入竞争，确保WLAN的吞吐量比较高。本文将提出的流量卸载机制建模成一个联合优化问题，最大化卸载到WLAN的数据流量的容量，同时保证卸载用户的数据速率，并提出启发式算法获得一个近似最优解。数值仿真结果表明，相对于将被卸载的蜂窝用户直接接入WLAN的传统卸载机制，本文提出的流量卸载机制显著提高了卸载到WLAN的数据流量的容量。 关键词：无线局域网；控制信息传输；跨层设计；信道接入控制；节能；D2D通信；流量卸载

 WLAN (Wireless Local Area Network) has become an important way for wireless intelligent terminals to access the Internet. Currently, most WLANs are achieved based on IEEE 802.11 standards. To handle the increasing requirements in WLAN performance, some existing research work introduces extra control messages into IEEE 802.11 standards to design new WLAN schemes to enhance WLANs’ performance in channel access control and energy saving. The piggybacking scheme is widely used to achieve the transmissions of newly introduced control messages among nodes. However, the traditional piggybacking scheme not only consumes extra channel resources but also lacks reliability to transmit control messages with broadcast nature from one node to many nodes. Therefore, it is an important research issue in WLANs to design a new control message transmission scheme. The newly designed control message transmission scheme can be used to solve the problems existed in WLANs to improve WLANs’ performance. Since WLANs have many advantages, such as simple deployment, low equipment cost, and high transmission rate, mobile operators reduce the press on cellular networks by offloading data traffic from cellular networks to WLANs. To enhance the capacity of data traffic offloaded into WLANs, designing an efficient traffic offloading strategy is another important research issue. The detailed research contents and contributions of this dissertation are as follows. 1.This dissertation designs a novel communication scheme CoS (Communication through data symbol Silence) based on the physical layer’s OFDM (Orthogonal Frequency Division Multiplexing) technique to convey lightweight control messages, and proposes an improved communication scheme CoS+ for wireless channels with frequency selective fading. CoS erases some data symbols in a data packet transmission at the physical layer and exploits the intervals between the erased data symbols to encode extra control messages. CoS embeds extra control messages into physical signals, so it does not modify the data packet format defined in IEEE 802.11 standards. Since practical WLAN communication systems exist the SNR gap, the correcting capability of the channel code is under-utilized. CoS does not affect the correct decoding of the original data packet as long as the total number of erased data symbols is designed carefully. This dissertation conducts experiments to measure the number of data symbols that CoS can erase in various channel conditions. In a frequency selective fading channel, the data bit errors induced by wireless transmissions mainly stem from weak data subcarriers. Based on this observation, the improved communication scheme CoS+ erases data symbols on the selected weak data subcarriers, which reduces the number of data bit errors induced by erased data symbols. Experimental results show that compared with CoS, CoS+ can erase more data symbols and convey more control messages in frequency selective fading channels. 2.This dissertation proposes a channel access control scheme based on the communication scheme CoS. IEEE 802.11 standards adopt DCF (Distributed Coordination Function) at the MAC (Medium Access Control) layer. To reduce transmission collisions among nodes in the distributed contention access scheme DCF, the proposed channel access control scheme CoS-MAC exploits CoS to embed the backoff counter value selected for a node’s next data packet transmission into the current data packet transmission. In CoS-MAC, nodes are aware of when others will finish their backoff procedures by exploiting the distributed message exchanges provided by CoS, which reduces transmission collisions. CoS-MAC turns random backoff in DCF into deterministic backoff. The reliability of control message transmissions among nodes has a significant impact on network performance. Simulation results show that compared with DCF and the existing scheme adopting the traditional piggybacking to exchange backoff counter values among nodes, CoS-MAC significantly reduces nodes’ transmission collisions and enhances network throughput in practical WLANs. 3.This dissertation proposes an energy saving scheme based on the communication scheme CoS. In WLANs, when a node performs channel contention based on DCF, it freezes its random backoff procedure due to other nodes’ data packet transmissions and receives data packets of which the destination address is not itself. In IEEE 802.11 standards, a node needs to receive the whole data packet before it can obtain the destination address information in the MAC header. Thus, even if the node drops useless data packets after obtaining the destination address information, the energy consumed in receiving and processing useless data packets has been wasted. The proposed energy saving scheme exploits CoS to embed a data packet’s destination address and transmission duration into the front part of a data packet transmission. Thus, nodes only need to receive the front part of a data packet at the physical layer to obtain the embedded information. They can immediately stop useless data packets’ receiving and processing and switch to a low power state before the current data packet transmission ends, which saves energy. The proposed energy saving scheme does not modify the protocol of WLANs, so it can be applied directly to existing WLAN systems. Simulation results show that the proposed energy saving scheme significantly reduces a node’s energy consumption during the backoff procedure performed in DCF. 4. This dissertation proposes a D2D (Device to Device) communication assisted traffic offloading scheme in integrated cellular and WLAN networks. In WLANs, DCF is a contention-based access scheme. If too many cellular users are offloaded to a WLAN, the WLAN’s throughput reduces due to the increased access contention, which reduces the capacity of data traffic that can be offloaded to the WLAN. To reduce the number of cellular users offloaded directly to a WLAN, The proposed traffic offloading scheme exploits cellular networks’ D2D communications to aggregate traffic from cellular users who are offloaded to a WLAN and only allows a small number of aggregation users to directly connect to the WLAN, which reduces access contention in the WLAN and ensures the WLAN’s throughput is high. This dissertation models the proposed traffic offloading scheme as a joint optimization problem that maximizes the capacity of offloaded traffic while guaranteeing offloading users’ data rates. A heuristic algorithm is proposed to obtain a near-optimal solution to the joint optimization problem. Numerical simulation results show that compared with the traditional traffic offloading scheme where offloading users directly connect to a WLAN, the proposed traffic offloading scheme significantly improves the capacity of traffic offloaded into a WLAN. Key Words: WLAN; Control Message Transmission; Cross-layer Design; Channel Access Control; Energy Saving; D2D Communication; Traffic Offloading