HWN* RN Site Planning

Plans have already been proposed to select RN sites for in other hybrid wireless networks. The solutions depend on the varied network performance objectives. Among these the packing based RN placement plan is simple, effective and straight-forward but is only suitable for ideal HWN* deployment scenarios where the BS sites are also packed with similar discipline plus the geographical site availability issues are not considered for both BS and RN. It is well known from planar geometry that to cover a two dimensional district with equal sized circles, the best possible packing solution can be obtained by surrounding each circle by six circles. But to have connections between the RNs to form a virtual RN backbone, an overlap is needed between relay cells. The framework therefore considers a situation where the locations of the RNs are centered with maximum coverage.

As the research addresses radio resource management, routing and node mobility problems, the RN site location planning should not be designed too idealistic such as in the packing based RN placement or not only include one or two parameters. We therefore propose a novel heuristic RN placement algorithm considers both physical distance through multi hop distance and Signal to Interference Ratio (SIR) through channel availability. The algorithm is devised in three steps described as:

1. Identify ideal RN locations based on radio resource management and MT mobility behaviour, then generate a set of RN position candidates.
2. Further formulate the RN site positioning as a constrained optimisation problem, of which the goal is to maximise the overall network throughput, the potential gain of MANET RN based services and minimise the hop distance and delay, so that more MTs can be served with guaranteed QoS.
3. Test RNs positioning sites combination recursively and update each RN's position based on performance result.

These three procedures are executed recursively until the algorithm converges. By imposing such apian, in practice, one can expect the system performance at disadvantaged locations should be improved with enlarged network dimensioning. In order to provide fixed RN assistance for both cellular and MANET interfaces radio resource management and routing, initial RN placement site candidates and possible topologies are considered in the following scenarios as presented in Figure 1. The scenario I, II and III cover the mobility management problems, scenario IV discusses the routing problem and scenario V is concerned with the relay structure.

 

Figure 1: The RN node test scenarios I, II and III for the cellular network, the test scenario IV for the MANET and virtual RN backbone scenario V for RN positioning evaluation

When RNs participate in cellular resource sharing and traffic handover along with the BSs and MTs (BSONRN mode), the situation is a little more complex than the BSON mode as more hops are involved during resource transfer, although RN does not have the traffic admission functionality. The heuristic algorithm first proposes to place RNs in positions within the coverage of several BSs, such as the shaded area presented in the Scenario I where the RN is located within the coverage of both BS1 and BS2. The RN can assist advanced communication mechanisms such as cell breathing traffic balancing and TDMA based soft handover. For example, suppose that in Scenario I the RN is associated with both BS1 and BS2. If BS1 reduces signal coverage radius to improve interference and capacity in considered area, the RN will lose its association to BS1, and it can transmit data to BS2. The BS2 may at this stage reduce or increase its coverage radius based on the information and traffic condition. The software handover support is similar to load balancing since the RN also acts as data relayer. Suppose a MT is moving from BS1 to BS2 and it currently receives data from the RN. If the received signal from BS2 becomes larger than it from BS1, the MT performs an intra cellular network handover from one cell to another cell without changing the serving RN. Scenario II places together two RNs at the coverage edge of BS1 and BS2 to facilitate the communication between two cells (It is assumed each cell has only one BS). From the SIR values combining with Shannon's formula, if the bandwidth allocation λB ratio to BS transmissions and the bandwidth allocation ratio to RN transmissions λR together is 1, Scenario II can have a slightly better system capacity performance compared to Scenario I because the average received signal power strength in Scenario I is lower than in Scenario II. However, the sites deployment of Scenario II can introduce much larger latencies, service interruption time and equipment cost, while most cellular coverage overlaps. To conclude, the shaded area in Scenario I is considered as a better site candidate plan other than Scenario I. Scenario III is also considered as an ideal location candidate. The RN at this position extends the cellular coverage to places without fixed infrastructure support.

As the attractor point mobility model is used to model MT movement, the MTs at some stage converge to the attraction points, dwell for a certain period of time and move within a short radius. In order to mitigate resource contention in the "hot spot", reduce service interruption time and latencies for ad hoc communications, a RN should be positioned where more MTs can associate to the RN within one or two hop distance as illustrated in Scenario IV. It is also important to place RN in places based on MT traffic density prediction. Scenario V presents the RN candidate sites combination topologies, it is preferred that RNs can compose a mesh like service layer with virtual backbone connections using ad hoc frequencies thus Scenario V topology right is recommended other than Scenario V topology left. The next step of the heuristic algorithm is to decide the number of RNs needed to assist traffic relaying with guaranteed QoS. The strategy first updates system traffic load information and initialises the HWN* system. MTs then continue moving using attraction point mobility model. As BSs and attraction points sites are pre-defined by the system so only MTs trajectories are needed to be used for Scenario IV analysis. A system performance result for a RN site combination is recorded and compared with theoretical analysis. The RN's location will be changed at disadvantaged locations if the result is not satisfactory. The algorithm runs recursively until an optimal solution is found. Meanwhile, in the final sites selection, a hard distant limit δ is introduced to solve BS RN overlapping problem to regulate the distance between one RN candidate site and any BS.

HWN* Routing Management

Efforts must be invested on HWN* to avoid congestion, compute the next route, discover medium resources and gather data. This section proposes a novel cascaded Adaptive Distributed Cross-layer Routing (ADCR) scheme for the HWN* framework, using a minimal number of hops and considers dynamic routing models aimed at reducing latency, preserving communications and delivering good overall and per node throughput. A cross-layer network design that seeks to enhance the system performance by jointly designing MAC and network layers has been adopted.

The cascaded ADCR includes three sub packet transmission modes labeled as One-Hop Ad-Hoc Transmission (OHAHT) for point-to-point direct communication, Multi-Hop Combined Transmission (MHCT) for radio resource relaying using fixed RNs or MTs, and Cellular Transmission (CT) for a traditional cellular service. The RNs of the core network compose a mesh structure with fixed routing tables using ad hoc frequencies, while BSs are connected to each other via wires and the link between the RN and BS are established using cellular channels and directional antennae. In areas without infrastructure support, two MTs may communicate directly, or through intermediate MTs. When a MT transmits packets to a BS through RNs, the RNs extend the signalling coverage of the BSON thus enhanced resource sharing performance is expected. The QoS flows can consume all the bandwidth on certain links, thus creating congestion for, or even starvation of best effort sessions. Statically partitioning the link resources can result in low network throughput if the traffic mix changes over time. Thus, a mechanism that dynamically distributes link resources across traffic classes based on the current load conditions in each traffic class is critical for performance. By proposing the cascaded ADCR for HWN*, the framework discourages applications from using any route that is heavily loaded with low priority traffic. Traditional routing strategies that use global state information are not considered. Problems associated with maintaining global state information and the staleness of such information is avoided by having individual MTs infer the network state based on route discovery statistics collected locally, and perform traffic routing using this localised view of the network QoS state. Each application, categorised by the service class with the choice of three possible transmission modes, maintains a set of candidate paths to each possible destination and route flows along these paths. The selection of the candidate paths is a key issue in localised routing and has a considerable impact on how the ADCR performs. The high priority traffic is given high priority in accessing comparatively expensive cellular resources, while low priority traffic tries to access lower cost ad hoc resources. Per MT bandwidth is used as the metric for local route statistics collection since it is one of the most important metrics in QoS routing. Furthermore, important metrics such as end-to-end delay can be improved by an increased bandwidth as long as the traffic load is not largely increased. As in QoS based investigation for inter system handover management. Traffic sessions are divided by HPUs, NPUs and LPUs. In case of network congestion, CT mode may temporarily become unavailable to NPUs when HPUs are not fully accommodated, while LPUs sessions may be only granted MHCT and OHAHT mode access to mitigate network congestion, reduce transmission delay and improve per MT throughput.

To avoid having higher traffic classes being influenced by lower traffic classes in terms of queueing delay, a waiting time limitation is placed on each traffic class using a forced starvation packet switch model. A traffic flow maintains two queues: a slot queue and a packet queue. The slot queue is decoupled for traffic class identification from the packet queue for transmission. The RN is specially designed so that it reserves QoS guaranteed free channels for signalling information exchange in the slot queue. Each MT and RN maintains a routing table. A RN's routing table contains the information of other fixed RNs thus the routing delay and multi-hop signalling overhead in MANET RN mode are largely reduced. On the other hand, a RN can also analyse the current system traffic load condition through feed-back from other fixed RNs. The purpose of bandwidth reservation is to let RNs that receive the relaying discovery command in the slot queue check if they can provide the bandwidth required for the connection. Start and finish tags are associated with slots but not with packets. When a packet arrives for a flow, it gets added to the packet queue, and a new slot is added to the slot queue. Corresponding start and finish tags are assigned to the new slot. The way to raise priority in slot queue is that the packets related to a high profile have shorter back-offtime to increase the probability of early medium access. As for the status table maintenance, information flooding is restricted to a limited scope. Once a positive acknowledgment message is confirmed by a requesting RN, the relay paths will not be changed unless resource contention happens. Given the fact that maintaining global RNs channel status in each RN slows down RN response time, each RN only updates neighbouring RNs' information, periodically.

In OHAHT, the requesting MT A first broadcasts SEARCH messages to every node in its transmission range including its associated RN and BS. If the destination MT B is within its transmission range and there is no ad hoc based media contention between MT A and MT B, MT B can respond to MT A with an ACK message. Once MT A confirms the acknowledgment, it starts a connection SETUP session immediately. The OHAHT transmission model can be extended to multi-hop ad hoc communication and it is only activated on demand.

The MHCT involves RNs acting as intermediate nodes for message relaying. BS provides assistance on cellular link establishment using cellular location registers. In the connection setup process for communication between MT A and MT B via the RN infrastructure, MT A first broadcasts SEARCH messages to every node to find MT B. After the SEARCH session, MT A may find that the cellular resources can be used through RNs by receiving three ACK messages from the serving BS of MT B, RNs and the MT B. It may also find that the ad hoc frequency based RN mesh is adequate for communications by receiving two ACK messages from the serving RN of MT B, and the MT B, respectively. These positive acknowledgments require MT B to send an ACK to its serving BS and serving RN, then the serving BS and the serving RN feedback the ACKs to MT A. Once the positive ACK is confirmed, MT A can either start a connection SETUP from MT A → RN, then RN → BS, and finally BS → MT B, or from MT A → RN then RN → M T B. The DATA transmission process follows the same packet delivery route, and further route discovery is prohibited to reduce the signalling overhead.

The label routing concept originated in Asynchronous Transfer Mode (ATM) network is introduced to MHCT mode since the position of RN is fixed and label based RN switching provides faster packet forwarding than routing because its operation is relatively simple.

The label is a fixed-length identifier. Multiple labels can identify a path or connection from source MT to destination MT. The structure of label has four parts. The first part CAST of the label is 3 bits and relates to cast options. Only uni-cast is considered in the current research. As practically the RN mesh can be very large, the second part of the Label is 20 bits which includes a node identifier and is unique in the network ensuring conflicts do not occur. The third part is 3 bits QoS, which means class of service. The last part of the label is TTL. All label information has a time-related restriction. After time out for a label, all corresponding entries will be deleted from the label routing table. Label routing uses a label to directly index into a connection table entry to determine the next hop, lending itself to a simpler lookup implementation than the complex IP routing and hop-by-hop IP address lookup. All intermediate nodes in the virtual connection can forward packets more efficiently. The path from a source MT to a destination MT is identified by multiple labels. The protocol distributes labels and set up new route after the path is computed by the routing protocol. The path finding process dynamically initialised by the label request packet carrying a unique label and flow information, where low path setup delay is guaranteed. The path between MT and destination MT is composed of multiple segments. The path is separated by segments and all data packets are relayed by these segments. Each segment is a real connection between two nodes and labeled by the sending-side node of label relay message in this segment. Figure 1 presents an example routing presentation and routing tables for three nodes MT A, RN B and MT C. The relay nodes only need to find the available entry indexed by a label in the packet, swap it with the respective Label out of this entry, and then send it out to the next relay node. Furthermore, the fixed RN placement reduces the frequency of label entry changes with reliable service.

 

Figure 1: Presentation of routing presentation and routing tables for three nodes MT A, RN B and MT C

In the CT mode, the source node MT A first broadcasts SEARCH messages to every node to find destination MT B. After the SEARCH session, the MT A finds that it is able to communicate with MT B directly via BSs, while the connection can be setup through a virtual wireless backbone. The positive acknowledgment of a connection requires MT B to send an ACK to its serving BS, then the serving BS informs the serving BS of MTA or the BS feedbacks the ACK to MT B when both MT A and MT B share the same serving BS. Once the positive ACK is confirmed, MT A starts connection SETUP from MT A → BS, then BS → BS, and finally BS → M T B. The DATA transmission process follows the same packet switched delivery route. The cellular transmission also includes cellular transmission with RN support. For cellular based RN connection, the packet tries to establish a link with a BS first then the traffic will be connected to BS RN mode. Dynamic channel allocation can be realised in a distributed manner given that the channel usage does not break the two channel interference constraint which are cosite constraint where there are minimum channel separations within a cell and non-cosite constraint where minimum channel separation between two adjacent BSs is kept.

HWN* MANAGEMENT FRAMEWORK AND ALGORITHMS

HWN* Handover Management

In the HWN* context, each communication mode exhibits different capacity characteristics therefore selecting the most appropriate mode for a particular service request is critical to guarantee QoS to the end user. A balanced traffic load distribution between the available modes is also important to avoid one mode becoming excessively loaded, which leads to an unstable state. The HWN* requires an algorithm that includes the service transfer function between the cellular communication modes and MANET communication modes. Another novelty of the HWN* handover mobility management is that user segregation is provided to guarantee QoS to particular user classes by the prioritisation of user requests when the HWN* system approaches a congested state. As the RN provides stable infrastructure support for a MANET, it will be beneficial to switch more service from the cellular service to the MANET service. The HWN* concentrates on the mode selection considering differentiated user negotiation, multiple handover triggers and congestion control. Here the handover mobility management procedure  refers to a process of transferring a MT from its serving BS to another BS or from one medium access interface to another interface. The RN using cellular frequencies relays the handover traffic but doe s not have traffic admission functionality. While for the MANET mode and MANET with RN mode, the procedures refer to a new path finding process similar to rerouting, which is described as part of the cascaded ADCR routing algorithm. To mutually mitigate the traffic burden between communication modes, the HWN* dynamically allocates the radio resources without a fixed plan.

Three service classes are devised to describe subscriber behaviour: High Profile Users (HPUs), Normal Profile Users (NPUs) and Low Profile Users (LPUs). Principally, HPUs get the guaranteed QoS service and the class is assured with any amount of bandwidth, firm end-to-end delay bounds and limited queue loss for data flow. Next are NPUs with less QoS guaranteed medium access opportunities compared to HPUs, but the users will get a close service quality as the one received by HPUs. LPUs are a best-effort class with absence of QoS specification using currently available medium resources. HPUs have the highest access priority in any of the communication modes of the HWN*, and traffic admission of NPUs and LPUs has to consider the impact of ongoing HPUs sessions. NPUs are also configured to have a higher probability than LPUs in terms of resource acquisition and this probability is decided by an Association Level (AL) set, which will be described later. Inter-system and intra-system mobility management are addressed separately to reduce system complexity.

MANET RN and MANET modes can potentially become the primary communications methods if distributed management, link reliability and security problems of the MANET mode can be solved. Specifically for the MANET mode, the issue is that data relaying via a third party MT is not safe as discussed in MCN review. Data relaying via provider owned fixed RN provides a permanent solution. The RN integration largely moderates the problem by providing reliable relays between communication parities. Upon communication request, HPUs are configured to search for cellular based service, NPUs sfor a MANET RN service while LPUs for MANET service by default. The signalling, path discovery and route establishment for MANET and MANET RN modes are completed before the BSON and BSON RN modes, if all four modes are available. Inter-system and intra-system traffic handovers are only triggered when essential to avoid unnecessary network management overhead.

The control entity for inter system mobility cooperation is called the HWN* Mobility Controller, which is responsible for managing the modification of a route in an attempt to maintain or enhance the QoS level. The unit is located in each BS and a BS periodically monitors receives, specifies, filters and analyses frequencies in use by nearby MTs, RNs and BSs, geographically locates MTs so that the terrain blocks interfering signals, and may use directional antennas to reduce unwanted signals. The BS also gathers the information on the MANET with RN mode channel availability from its associated RN. A MT, either requiring differentiated service or not, makes a distributed decision on inter-network and intra-network handovers based on information gathered from its associated BS mobility controller as well as information gathered after probing one hop point to point direct communication. To obtain an effective handover process, while reducing the unnecessary handover rate, it is proposed to setup a dedicated Status Check Point embedded in the HWN* Mobility Controller where the necessary measurements are taken and then fed back to the handover algorithms of the nodes involved. The check result indicates the likelihood of a handover, which depends on interference level and physical layer information such as Bit Error Rates (BER), velocity, buffer size, etc. Since the HPUs applications have higher priorities over NPUs and LPUs, subscribers from this profile are more likely to get an accept ticket, which is issued by the Negotiation Unit in continuing with traffic handover. If an accept ticket is not issued, the MT will not use the status check data to request continuing with the handover process unless the status check point data necessitates handover. Then the mobility controller will decide to accept, queue or reject the MT handover request. The negotiation process between HPUs, NPUs and LPUs is based on Association Level (AL), which makes decisions and feeds back to the HWN* Mobility Controller. The AL is a set of parameters monitoring channel availabilities, an AL that scores higher than the threshold means that the channels are already occupied by ongoing sessions. The AL set is further sub-classified as AL in the BSON, BSON RN, MANET and MANET.

The handover algorithms in the HWN* should allow subscribers to seamlessly move without dropping their communication session and considers differentiated QoS issues, for example, it must guarantee QoS for HPUs that agree to pay more than NPUs and LPUs. Two handover types are therefore used as described previously, Intra Network Handover occurs when a MT enters into another entity that belongs to the same network with a cellular TDMA MAC interface. An Inter Network Handover happens when a MT leaves the serving network and communicates with another entity that belongs to a different network. For all service classes, the intra network handover is selected before considering inter network handover as less traffic management overhead may be produced. In intra network handover, after obtaining pass tickets from the Status Check Pointand the Negotiation Unit, the Network Selector entity that embedded in the mobility controller informs the MT if the RN should participate in handovers or not, then the MT makes a local decision. The inter network handover is seen as a switching process between ad hoc and cellular services which has different properties and procedures compared to the intra network handover. The Status Check Point is activated first to avoid extra expense and the Negotiation Unit keeps monitoring the channel availability status and updating the AL in a short interval to grant or reject handovers. The Network Selector always tries to divert the traffic back to intra network handover before it is informed that the intra network handover is not possible. If the MT is currently communicating in ad hoc modes, the selector will search for available B Ss in neighbouring cells. If the MT is using cellular modes, it will look for either direct point-to-point communication or search for a fixed RNs involved multi-hop communications. The Network Selector also uses several short network search expire time s for both intra-network search and inter-network search to make sure a MT is not long isolated during the network selection process

HWN* Framework towards 4G Mobile Communication Networks

The evolving cellular communications industry was originally focused on providing voice services. However, ever increasing demand for multimedia mobile services required new architectures to extend indoor and outdoor coverage, increase system capacity, improve service quality and reduce transmission delay. For example, a large amount of bandwidth is needed to support mobile broadband applications. Current solutions require the installation of expensive BSs to increase the system capacity with reduction of cellular cell sizes, which result in large costs in equipment, wiring and complex system management. The alternative approach is to install wirelessly linked multi-functional Relay Nodes (RN), which not only creates smaller cell sizes but also provides packet relay functions, introduces networking flexibility, saves end terminal power and largely mitigates the pass loss. In this work, we propose to realise high quality mobile communication services in a novel system infrastructure that includes a cellular network, a Mobile Ad hoc Network (MANET) and an integrated dual radio access interface on each RN and MT

The proposal brings out many research challenges as the infrastructure lends itself to complex change in topology, medium access, resources sharing and routing path selection. The ad hoc interface has limited transmission range thus multi-hop communication and RN infrastructure support are required for data exchange. The cellular interface offers robust communication but the bottleneck is at the resource sharing between base stations and relay stations, which require the development of a traffic sharing approach between entities. In order to exploit the advantages of the infrastructure, algorithms and protocols have been proposed to tackle resource management problems e.g. inter-network traffic management and heterogeneous network route selection in a distributed manner.

Hybrid wireless networks with multiple active interfaces towards 4G wireless networks are still being developed and as yet no real world prototype has been deployed except the 3GPP Long Term Evolution (LTE) test operating by Motorola, Nortel and NTTDoCoMo, the IEEE 802.16j multimedia traffic relaying project and Alcatel-Lucent 3G Femto high speed mobile home access where a large number of Femto relay cells are required on per house basis. The HWN* represents another possible realisation of the hybrid wireless network concept. The HWN* and associated algorithms are proposed and evaluated by means of computer simulation (Rea, 2006) to analyse node mobility, scheduling algorithm, handovers, routing, resource sharing and topology design schemes. The complexity of the HWN* system is such that performance evaluation does not lend itself to pure mathematical treatment but more accurate evaluation is only possible by means of simulation, which also provides more practical options for parameter changes.

Briefly review the state of the art of the cellular network, MANET, relay concept, recent hybrid network system approaches, and hybrid wireless networks related algorithms to provide an understanding of the effectiveness of our HWN* infrastructure. The major achievement of our research is then discussed. Apart from the cost-effective HWN* architecture proposal with minimal change on existing cellular and MANET structures, other core contributions can be summarised as:

• Network Design: In order to maximise the spectrum usage and facilitate load balancing, RN positioning planning has been developed and investigated. Our approach focuses on heuristic relay placement to explore the node mobility pattern's impact, cellular system, MANET and relay characteristics.
• Solving complexity: For large scale hybrid networks, more components are added to the system. Therefore simple route management algorithms may produce larger end-to-end transmission delays, which results in a slower application response time. The design of cascaded routing algorithm has contributed a novel approach to deal with complex systems over several layers. The scheme includes several route change triggers and considers cross layer design in overlayed networks.
• Quality of Service: The HWN* is expected to provide services with different QoS requirements, which results in complex user management. The research provides a user classification strategy with differentiated service class profiles for the evaluation of traffic admission, mobility management, handover management and cascaded routing. For the inter-network handovers, congestion control between service classes has been introduced, which discourages applications from using heavily loaded cellular resources and encourages the MANET usage.

The algorithms and strategies developed as part of this research aim to benefit resource management and routing for the HWN*. These methodologies, meanwhile, have potential to be applied on cooperative vehicle networks including vehicle-to-vehicle communication and vehicle-to-infrastructure communication, and wireless sensor network packet relay infrastructure and related medium access & route optimisation.