Wireless Mesh Networks and Cooperative Relaying Technologies

     The Wireless Mesh Network (WMN) is a kind of multi-hop Ad hoc network integrated with mobile Ad hoc network technologies and cellular network technologies. Unlike mobile Ad hoc networks, which are designed for peer-to-peer communications between mobile terminals, the WMN is mainly designed to provide wireless access for user terminals[1-7].

     Having such features or advantages as self-networking, self-management, self-healing, self-balancing,
self-management of nodes, multi-hop, simple installation, and wide coverage, the WMN can dynamically achieve mobile broadband access and support multiple services and radiolocation technology.
Recently, WMN has become one of the hottest research issues in the research of Wireless Broadband Access (WBA) technologies. The Institute of Electrical and Electronics Engineers (IEEE) is now working with the telecom industry, both equipment manufacturers and research institutes, in the standardization of WMNs, and some related specifications have been written in such standards as IEEE 802.11s,
IEEE 802.16j, IEEE 802.15.5, IEEE 802.20 and IEEE 802.16m[8].

1 Characteristics of WMN

1.1 Classification of WMNs
In the Open System Interconnection (OSI) model, Layer 2 is Data Link Layer and Layer 3 is Network Layer. Accordingly, the WMNs are classified into two categories: Layer 2-based or switching technologies-based and Layer 3-based or routing technologies-based WMN[9].

     From the perspective of Transfer Control Protocol/Internet Protocol (TCP/IP), the Layer 2-based WMN is just like one IP subnet. The wireless Mesh technologies in IEEE 802 series standards and most wireless Mesh products in current market fall into this category. In Layer 2, such functions as access control, Mesh networking, routing, link congestion control, quick move, handover and security authentication are realized. Adopting switching technologies, the existing WMN products of most manufacturers complete data exchange between Access Points (APs) in Layer 2. But these products do not have routing function, so data exchange is actually performed by a root node. For example, in current Wireless Fidelity (Wi-Fi) Mesh solutions, if node A wants to transmit data to node B, in most cases, it cannot directly select the shortest transmission path, but has to transmit the data to the root node for data exchange before sending them to node B. Therefore, this switching technologies-based WMN performs well in the scenarios where small coverage is required.

     Unlike Layer 2-based WMNs, the Layer 3-based WMN divides the entire network into several IP subnets, which communicate with each other by means of IP routing. This category of WMNs is based on routing technologies, so data exchange at the root node becomes unnecessary. When node A transmits data to node B, it will select the shortest path. The WMNs with routing function are similar to the wired networks in architecture. Their routers can not only provide access to Internet for the users within the coverage range, but also act as network infrastructures to route the data to the destination. Therefore, the routing technologies-based WMN is quite flexible in networking and powerful in fault tolerance. In addition, it simplifies Line-of-Sight (LOS) communications and expand the network coverage with minimal infrastructures and costs.

1.2 Basics for WMN Implementation
The WMN, either Layer 2-based or Layer 3-based, is essentially a multi-hop network. Its performance depends on the quality, reliability and efficiency of communications between two neighboring nodes (which can be treated as a source node and a destination node). Taking advantage of the multi-hop path information between the source node and the destination node, the cooperative relaying technology can improve the performance of wireless links between nodes to meet the communication quality requirements of the WMN.

     Due to the mobility of WMN nodes, the WMN topology changes continuously. As a result, traditional Internet routing protocols are no longer applicable to the WMN environment, and  special routing protocols have to be developed for WMNs to ensure certain data rate and transmission reliability between two nodes in the Physical Layer.

In developing the protocols for WMNs, one should consider not only the improvement of protocol performance of one layer but also the effects of the protocols on related layers. In other words, one should not be bound by the traditional layers of Open Systems Interconnection/Reference Model (OSI/RM), but optimize the overall network performance based on the status and requirements of related modules/protocols in each layer as well as dependence relation and effects between layers. This means cross-layer design approaches should be adopted. Specifically, cross-layer design is to fully and reasonably use the existing network resources to achieve the ultimate objectives: maximize the system throughput, minimize the overall transmit power, and optimize Quality of Service (QoS).

2 Integration of WMNs and Cooperative Relaying Technologies

2.1 Wireless Mesh Architecture in Backbone Network
The WMN is based on IP protocols. In terms of network topology, the WMN can be regarded as wireless and compact Internet. With the development of broadband wireless technologies, people expect wireless networks with higher data rate, higher spectrum efficiency, wider coverage, and more extensive service support. For example, B3G/4G systems are required to have a transmission rate of at least 1 Gb/s with a bandwidth of no less than 100 MHz. Such a continuous bandwidth is not available in existing bands. Therefore, higher bands, such as 5 GHz and 6 GHz, have to be allocated to B3G/4G systems.

     Higher working frequency and broader bandwidth are helpful for increasing data rate, but the coverage shrinks with the increase of frequency. Consequently, a contradiction arises between data rate and coverage, for which a trade-off is needed to balance them. For example, in IEEE 802.11n, the Media Access Control (MAC) sublayer is expected a maximum data rate of 100 Mb/s within the coverage range of 15 m. In case of Ultra Wideband (UWB), the rate can reach 480 Mb/s, but the coverage is only 2 m or even shorter. As a result, to achieve a high data rate in a wide coverage, lots of APs have to be deployed in the wireless networks. However, in practice, deploying a large amount of APs is quite difficult for two reasons: the cost is too expensive; and it is impractical to connect all APs into the wired backbone, especially in the regions without wired services.

     The relay-based Mesh architecture is a feasible solution, and under this architecture, many existing wireless Mesh products in the market can be integrated with such systems as Wireless Personal Area Networks (WPANs), Wireless Metropolitan Area Networks (WMANs), Wireless Local Area Networks (WLANs) and WBAs. If the Mesh network is an ocean, these WPANs, WMANs, WLANs and WBAs are just islands. They cannot provide services for the users until their central control units are connected to the backbone network. Therefore, to enable the wireless Mesh architecture in the entire network, the first thing is to solve the wireless implementation problem of the
backbone[10].

2.2 WMN Architecture
The WMN is a full multi-hop wireless network, which data flow in and out of wired Internet gateways. Unlike the flat structure of Ad Hoc network, the WMN is hierarchical. By function, WMN nodes can be divided into two types, as shown in Figure 1: Mesh routers and Mesh clients. These Mesh nodes can make a full Mesh or a partial Mesh. In the full Mesh, any two nodes in the network can be directly connected, while in the partial Mesh, only some nodes can directly communicate with each other.

     A Mesh router functions as a router and a gateway/bridge. It can be configured with one or several wireless interfaces, which are based on either the same or different wireless access technologies. Compared with traditional routers, Mesh routers can achieve the same coverage with lower transmit powers by way of multi-hops. Moreover, by enhancing its MAC protocol, a Mesh router can obtain a better scalability in the multi-hop Mesh network environment. A Mesh client not only has a networking function, but also can act as a router, but it cannot play the role of a gateway or bridge. Besides, it can be configured with only one wireless interface and support one wireless access technology.

     Currently, there are two basic wireless Mesh architectures: infrastructure Mesh and client Mesh. But the hybrid Mesh architecture (illustrated in Figure 2), an integration of the above two, seems  promising. In the hybrid Mesh architecture, clients can be connected to the backbone via Mesh routers, or can directly construct a Mesh network (i.e. local Ad hoc network). The backbone Mesh network provides access to other networks, including Internet, Worldwide Interoperability for Microwave Access (WiMAX), Wi-Fi, and cellular networks. Meanwhile, the routing function of the client can internally enhance the connectivity and coverage of the Mesh network.

2.3 Characteristics of Cooperative Relaying Technologies
Cooperative relaying is also called cooperative diversity. Its principle comes from the idea of
 "virtual antenna array". Cooperative relay is actually an extension of a single path relay. In which, one or several nodes with a common coverage area, called relay node(s) R, are added between the source node S and the destination node D, and the destination node D can combine the data from both the source node and relay nodes. In this way, the pressure arisen from multiple antennas unable to be configured at the terminal is alleviated[11]; Figure 3 shows an example in downlink, where the Base Station (BS), Relay Station (RS) and User Terminal (UT) act as the source node, relay node and destination node respectively.

     By the transmission/receiving timeslots between nodes, cooperative relaying can be divided into three types, as shown in Table 1, where "X →Y " means the signal is transmitted from the node "X " to the node "Y ". In either type, the destination node combines the signals received in different timeslots to obtain spatial diversity gains. As signal information of multiple paths is used, spatial diversity gain is achieved at the destination node, thus the data rate and reliability of relay links are guaranteed.

2.4 Integration of WMNs and Cooperative Relaying Technologies
As we have discussed above, the Mesh architecture of WMN is mainly implemented in the Network layer or data link layer. But cooperative relaying technologies are implemented in Physical Layer to ensure data rate and reliability of relay links. Figure 4 illustrates the application of cooperative relaying technologies in a fixed wireless Mesh network[12]. In the fixed WMN, Mesh nodes include APs and fixed/roaming RSs. Included in the Mesh architecture or not, UTs must be connected to the Mesh nodes before they receive services from the WMN. The APs can be BSs or RSs, or they can be directly connected to the backbone (often the Internet). As shown in Figure 4, node C can combine the signals from node A and node B to achieve cooperative relaying, and to obtain spatial diversity gain as well.

     The cooperative relaying technologies can be used in any wireless relay link. Besides, to make full use of spectrum resources and Multiple-Input Multiple-Output (MIMO) technology, multiple antennas can be configured at source node, relay node, and destination node. With cooperation between different RSs as well as cooperative MIMO relaying at the destination node, the data rates of relay links are greatly improved. The basic principle of the cooperative MIMO relaying is illustrated in Figure 5. Simulation results[13] show that the transmission rate of cooperative MIMO relaying is about 2 times over common MIMO relaying. With the development of wireless technologies, the application of cooperative MIMO relaying technology in WMNs will eventually be a trend for development[13].

3 Main Problems Involved in Application of Cooperative Relaying Technologies in WMNs

3.1 Standardization
In the development of WMNs, market occupation goes ahead of standardization. To achieve a
carrier-class application, the WMN has three obstacles to overcome: First, the key technologies for wireless Mesh networking; second, the decrease of network performance resulting from quick move of nodes, especially at Physical Layer; third, the standards for integrating various WMN technologies.

     So far, many research institutes have made deep research on cooperative relaying, and some research results have been released in IEEE802.16j. But current researches are all made on cellular networks. To integrate the cooperative relaying technology into WMNs and standardize it, there is still a long way to go.

3.2 Research on Key Technologies
Like regular relay stations, the performance of cooperative relaying stations is mainly affected by several critical processes, including resource allocation, selection of cooperative relaying nodes (or discovery and maintenance of cooperative routing), transmission latency and handover. In terms of implementation, the following aspects of cooperative relaying technologies have to be studied:

• Selection of cooperative relaying stations and resource allocation: The two processes are closely related. Particularly in Orthogonal Frequency Division Multiple Access (OFDMA) systems, they should be jointly implemented, and the signaling load should be considered as the most important problem to be solved.
• Transmission latency: The increase in hop counts leads to a longer transmission latency of WMN links, which is a great challenge for wide application of the technologies.
• Relay link rate adjustment: In fixed wireless Mesh networks, transmission between many nodes is in LOS range. In case the transmission between the source node and the relay node is in LOS range, but the transmission between the relay node and the destination node is in Non-Line-of-Sight (NLOS) range, the common MIMO technology cannot be used directly. As a result, it is necessary to coordinate transmission rates of relay links to improve overall link performance.
• Discovery and maintenance protocols for relay routing: This problem is also present in the regular relaying system, especially the maintenance protocol for the relaying routing where UTs move.

3.3 Industrialization
Currently, the WMN products are mainly manufactured by some small companies. Except Nortel, which introduces complete WMN solutions, other powerful companies have not brought any WMN products into the market. To achieve a carrier-class application of WMNs, the telecom industry should make more efforts and give greater support to WMNs. Only when the WMN technologies are widely used, such technologies as cooperative relaying can be further developed.

4 Conclusion
The multi-hop based WMN is a new kind of network. Although there are still many critical problems to be solved, the WMN has a vast development space in the broadband wireless access systems due to its advantages: flexible networking, easy maintenance, wide coverage, low cost, small risk, and good reliability. Moreover, in the development of WMNs, such technologies as cooperative relaying will play an important role in improving the link performance.

References
[1] HOSSAIN E, LEUNG K K. Wireless mesh networks: architectures and protocols [M]. Berlin, Germany: Springer, 2007.
[2] SCHULTZ D C, WALKE B, HIERTZ G R, et al. Meshing for relay based deployment concepts [R].
WWRF-WG4 White Paper, 2006.
[3] AKYILDIZ I F, WANG X, WANG W. Wireless mesh networks: a survey [J]. Computer Networks, 2005, 47 (4): 445-487.
[4] JAIN R. Wireless mesh and multi-hop relay hop relay networks [R] Saint Louis, MO, USA: Washington University in Saint Louis, 2006.
[5] LAN K C, et al. Implementation of a wireless mesh network testbed for traffic control [C]//Proceedings of 16th IEEE International Conference on Computer, Communications and Networks (ICCCN’07), Aug 16, 2007, Honolulu, HI, USA. 2007: 1022-1027.
[6] De C PASCHOALINO R, MADEIRA E R M. A scalable link quality routing protocol for multi-radio wireless mesh networks [C]//Proceedings of 16th IEEE International Conference on Computer, Communications and Networks (ICCCN’07), Aug 16, 2007, Honolulu, HI, USA. 2007: 1053-1058.
[7] 张勇, 郭达. 无线网状网原理与技术[M]. 北京: 电子工业出版社. 2007. 7.
[8] IEEE C802.16m-07/076r2 [S]. Draft IEEE 802.16m requirement. TGm Requirements Drafting Group, 2007.
[9] 无线网状网 [EB/OR]. http://www.51cto.com/art/200605/26368.htm.
[10] HORAK R. Webster’s new world telecom dictionary [M]. New York, NY, USA: John Wiley & Sons, 2007.
[11] IST-4-027756 WINNER II D3.5.1. Relaying concepts and supporting actions in the context of CGs [S]. October 2006.
[12] ZHAO Zhipeng, BELFIORE J C. Application of cooperative diversity in 802.11a Ad-hoc networks [C]//Proceedings of 16th IEEE International Conference on Computer, Communications and Networks (ICCCN’07), Aug 16, 2007, Honolulu, HI, USA, 2007: 1016-1021.
[13] SAKAGUCHI K. FUMIE O. Multiple access interference cancellation and link multiplexing for MIMO Mesh network [C]//Proceedings of 16th IEEE International Conference on Computer, Communications and Networks (ICCCN’07), Aug 16, 2007, Honolulu, HI, USA. 2007: 1028-1033.

[Abstract] The Wireless Mesh Network (WMN) has become a focus in research of wireless broadband communications. In a switching technologies-based wireless Mesh network, the entire network is regarded as an IP subnet, so it cannot be applied in situations where large coverage is required. The use of cooperative relaying technologies can improve the transmission rate and reliability of wireless link; while the wireless Mesh network, once integrated with cooperative relaying technologies and routing technologies, can improve its spectrum efficiency and cover a wide area. However, there are many problems to be solved with respect to standardization, key technologies research and industrialization. Therefore, the application of cooperative relaying technologies in wireless Mesh networks is still a great challenge.