Study Of The Convergence Of Cellular And Ad Hoc Networks

In the late eighties of last century, the setup of cellular networks made the large scale wireless communication possible for the first time, and by then the industry of wireless communication entered into its vigorous development. Today, in the 21^st century, there are a lot of new concepts and new technologies related to wireless communication. Wireless communication system also undergoes its evolution from the first generation to the third generation. All this shows a truth that the demands of services motivate the technologies to develop continuously. Now voice is no longer a real profit center or competitive edge for the network operators, and instead are the high speed wireless data service and the broadband multimedia service. The customers need to get a rich experience from a combined service with voice, data and images. In such a background, the capacity limitation of cellular networks is a problem we can hardly obviate. And hence, to meet the demands of future mobile communications there is a new hotspot rising in the research field, which is the convergence of cellular and Ad Hoc networks.This thesis focuses on the issues of implementing a hybrid cellular and Ad Hoc network over the TD-SCDMA air interface by means of software update. The co-existence of conventional uplink, downlink and the new peer-to-peer links will introduce considerable mutual interference. On account of this, a multi-carrier solution is applied in our discussion to get a reasonable frequency allocation scheme for the hybrid network. The conventional cellular links and the signaling links use the main carrier, while the direct-connected links are on the secondary carrier. Thus the interference between the two kinds of links is restricted. In this solution, the terminal should access the system first on the main carrier with a cellular mode, and hence all the terminals have the initial synchronization with the common reference of the base station. To make the terminals be able to set up reception synchronization without the administration of the base station, a frame synchronization solution with training-sequence correlation detection scheme is proposed. By making use of the initial synchronization with the base station, and also the limited distance supported by the peer-to-peer communication, the detection window can be restricted to a small range, and hence the detection accuracy is increased accordingly. Link-level simulation shows that in its application scenario this solution can meet almost the same performance as the conventional downlink cell searching sequence. Since the CDMA system is a self-interference system, the distributed topology in the new network plan brings challenges to the MAC-layer protocol design. Targeting to these problems, a new rehearsal-assisted distributed MAC protocol is proposed. By making use of the free timeslot of the original TD-SCDMA subframe on the secondary carrier, a separate access control timeslot is allocated to resolve confliction detection, interference control and channel allocation. The protocol can be divided into three steps: probing, rehearsal and feedback. The communication on the data channel with the new link adopted is simulated in the rehearsal step in the access control timeslot, and hence the iterative mutual interference between the new user and the accessed users are tested. As a result, this protocol greatly increases the system capacity without impairing service quality. Furthermore, when there are a lot of concurrent traffic, the per packet access delay performance is improved as well.Since the distributed peer-to-peer links are featured by short communication distances and small interference ranges, an intra-cell spatial resource reuse scheme is proposed in this thesis. With this scheme, multiple P2P links in the same cell can reuse the same radio resource as long as they are separated far away enough. On account of the new interference scenario in this scheme, a simplified SINR model is first deduced in the frequency domain with the assumption of a worst interference model. And then analysis is taken in the time domain to arrive at a quasi-realistic SINR model, which provides the basis for system simulations. The results of the system simulations show that the system capacity can get great improvement with this scheme. Making a comparison between this result and the hard capacity defined by the radio resource, the critical values of SINR targets that discriminate the system as resource-restricted or interference-restricted are deduced. By means of a hexagon padding scheme, the relation between the co-channel reuse distance and single-hop distance is disclosed to help to apply our scheme into a real system.