Research On Control Message Transmission And Traffic Offloading In Wireless Local Area Networks

WLAN(Wireless Local Area Network)has become an important way for wire-less intelligent terminals to access the Internet.Currently,most WLANs are achieved based on IEEE 802.11 standards.To handle the increasing requirements in WLAN per-formance,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 savin g.The piggybacking scheme is widely used to achieve the transmissions of newly introduced control messages among nodes.How-ever,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 de-sign 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 deploy-ment,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(Communica-tion 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 fre-quency 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 practi-cal 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 sub-carriers.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 com_munication scheme CoS.IEEE 802.11 standards adopt DCF(Distributed Coordination Function)at the MAC(Medium Access Control)layer.To reduce transmission col-lisions 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 deter?ministic 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' transmis-sion collisions and enhances network throughput in practical WLANs.3.This dissertation proposes an energy saving scheme based on the communica-tion 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 des-tination 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 transmis-sion 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 process-ing 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 offload-ing 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 ag-gregation 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 heuris-tic 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.