1、 Rec. ITU-R F.1704 1 RECOMMENDATION ITU-R F.1704 Characteristics of multipoint-to-multipoint fixed wireless systems with mesh network topology operating in frequency bands above about 17 GHz (Question ITU-R 107/9) (2005) Scope This Recommendation provides guidance for the system configuration and ch
2、aracteristics of Multipoint-to-Multipoint (MP-MP) fixed wireless systems (FWSs) with mesh network topology operating in frequency bands above about 17 GHz. The Annex analyses improvement of availability and reduction of transmit power as well as route diversity effect and the required function for M
3、P-MP systems. The ITU Radiocommunication Assembly, considering a) that FWSs operate in various frequency bands above 17 GHz; b) that various techniques for the use of these frequencies are being implemented by administrations; c) that the radio-wave propagation characteristics above about 17 GHz are
4、 predominantly governed by precipitation fading and absorption and only suited to short range radio system applications in countries affected by rain (see Recommendation ITU-R P.837); d) that the radio-wave propagation characteristics at these frequencies are known to differ in some respects from th
5、ose of lower frequencies and that some of these differences might be exploited to the advantage of certain types of systems; e) that the equipment designs might differ from those used in the lower frequency bands; f) that new applications and network configurations are being used in high-density dep
6、loyment of FWSs in bands above about 17 GHz; g) that the high concentrations of service users in urban, suburban and industrial areas require high-density deployment of user terminals in these areas; h) that MP-MP systems with mesh network topology would be effective because of their potential for r
7、oute diversity; j) that under certain conditions, a MP-MP system with mesh network topology may be considered as an effective technique for the improvement of availability and/or the reduction of the transmit power in deployment of FWSs operating in frequency bands above about 17 GHz, recommends tha
8、t Annex 1 should be used as guidance for the system configuration and characteristics of MP-MP systems with mesh network topology operating in frequency bands above about 17 GHz. 2 Rec. ITU-R F.1704 Annex 1 System configuration and characteristics of MP-MP systems with mesh network topology operatin
9、g in frequency bands above about 17 GHz 1 Introduction The use of MP-MP systems with mesh network topology are considered an effective means to mitigating the degradation of telecommunication quality in the FWSs operating in frequency bands above about 17 GHz. This Annex describes the overview of th
10、e system configuration for the MP-MP systems with mesh network topology and shows the results of the quantitative analysis on the improvement of availability and the reduction of transmit power due to the diversity gain utilizing mesh network topology. System functional requirements to maximize the
11、route diversity effect and practical examples of the required functions are also shown. In addition, field experimental results on diversity gain are introduced (see Appendix 1 to this Annex). 2 Overview of system configuration Figure 1 illustrates an example of MP-MP system with mesh network topolo
12、gy. The wireless mesh network consists of wireless nodes, which are either customer sites, relay nodes without originating/terminating traffic, or points of interface (POI) to other networks such as Internet service providers (ISP) networks. A wireless node is connected to others via wireless links.
13、 The end-to-end traffic is conveyed over the single-hop route and/or multi-hop routes. Whereas the single-hop route consists of one wireless link, multi-hop routes consist of multiple wireless links. The entire network can be regarded as a MP-MP system. When at least one diversity route is available
14、 in the network, the system is specifically referred to as “an MP-MP system with mesh network topology”. Rec. ITU-R F.1704 3 3 Improvement of availability and reduction of transmit power MP-MP systems with mesh network topology have inherent capability for route diversity between a pair of nodes. Th
15、e end-to-end telecommunication traffic is forwarded from a source node to a destination node via intermediate transient nodes, and there can be several routes between the pair of source and destination nodes. If one of the wireless links within a route between a pair of source and destination nodes
16、become unavailable due to rain attenuation, the telecommunication traffic on the wireless route can be rerouted to other routes. Due to the route diversity effect, the availability of end-to-end telecommunications of the proposed mesh wireless network can be improved in comparison with the conventio
17、nal P-P systems, P-MP systems, or MP-MP systems without mesh network topology. This section shows an analytical model and various numerical results of the analytical study on the improvement of availability and the diversity gain in MP-MP systems with mesh network topology. In the analytical model,
18、the probability of simultaneous degradation of multiple links due to rain attenuation is expressed by the multivariate gamma distribution function with correlation. 3.1 Analytical model Figure 2 depicts a mesh network to be evaluated. Multiple links connected to Node A within a mesh network (see Fig
19、. 2a) are simplified by a model (see Fig. 2b), where multiple links provide L-branch diversity to Node A. In the simplified model, it is assumed that the separation angle between adjacent links is identical and all links have the same length d. 3.1.1 Node unavailability In Fig. 2b), Node A becomes u
20、navailable when all links connected to the node are unavailable simultaneously. Hence, the probability that all the links providing the L-branch diversity are unavailable simultaneously is called node unavailability (i.e. outage probability), hereafter. Let Xidenote a stochastic variable for the rai
21、n attenuation of i-th link and f (Xi, Xj, ., Xk) be the joint probability density function of Xi, Xj, . and Xk. The node unavailability)(LNUp , that is the probability that all the diversity branches to the node becomes unavailable simultaneously, is derived from: LuLNUpp.12)(= (1) 4 Rec. ITU-R F.17
22、04 where, LLxxLudXdXdXXXXfp .),.,(.2121.12= (2) Note that x in this equation represents the threshold of rain attenuation to maintain a wireless link available. Here we assume that the probability of the rain attenuation for a wireless link obeys the gamma distribution and that there is a certain le
23、vel of correlation between the rain attenuation levels for wireless links. Along with the multivariate gamma distributions with arbitrary correlations, 12 LupLis derived from: LnnLunxnnnp+=)(/,()1(! )()(0.12, (3) where: (a): complete gamma function (a, x): incomplete gamma function of the second kin
24、d. The above equation is exact for L 2 and approximate for L 2. The shape parameter has a typical value of around 0.005-0.01 in the countries affected by rain at millimetre-wave bands. The correlation among L branches is represented by11)Rdet(=L, where: =111R121212112LMOMMLLLLL(4) ifis a correlation
25、 coefficient between i -th and j -th links and can be evaluated by averaging a spatial correlation of the rainfall intensity over the two links (through double integral). Regarding a spatial correlation coefficient of the rainfall intensity, a conventional formula of ( )rr3.0exp = is traditionally u
26、sed, where r is a distance between two locations (km). 3.1.2 Route unavailability For the investigation of route unavailability of mesh networks, a simple model with square lattice is given as shown in Fig. 3. With respect to the single link route between Nodes A and B, there are many alternative ro
27、utes available. One of the shortest routes except the single link route between Nodes A and B is the route, which consists of links No. 2, No. 3, and No. 4. The other one is the route consisting of links No. 5, No. 6, and No. 7. Rec. ITU-R F.1704 5 The unavailability of all possible routes between t
28、he two nodes is referred to as, Route unavailability. Considering only one alternative route between Nodes A and B consists of the links No. 2, No. 3 and No. 4, other than the single link route (i.e., there are two routes in total), the route unavailability between nodes A and B can be derived from
29、equation (5): ( )12342341)2(BA1aaaRUpppp +=(5) Note that the superscript “A-B(2)” of )2(BARUp means that there are two routes between Nodes A and B. The variable kijap.is the probability that all the links i, j, ., k are available and can be derived from equation (6): ()()()()()()Lnnkjixxkjikijanxnn
30、ndXdXdXXXXf+=/,1!.,.,.000.(6) where (a, x) is the incomplete gamma function of the first kind. L in the equation represents the number of links (i.e., i, j, ., k) being considered. In case that there are two alternative routes (i.e., one route consists of links No. 2, No. 3, and No. 4, and the other
31、 one consists of links No. 5, No. 6, and No. 7), the route unavailability between Nodes A and B can be derived from equation (7): ()A-B(3) 1 234 567 1234 1567 234567 1234567RU1aa a a a a apppppppp= + + + (7) 3.2 Availability improvement and diversity gain This section presents some numerical results
32、 of the analytical study in terms of the availability improvement and the diversity gain based on the analytical model given in the previous section. The frequency of wireless links is assumed to be at 26 GHz except for Fig. 6 that represents the dependency of the diversity gain on the frequency. Fi
33、gure 4 shows the node unavailability as a function of the number of diversity branches L (i.e., wireless links connected to the node). The node unavailability is plotted for the unavailability of 102-105for each diversity branch. The node unavailability without diversity branches are indicated at L
34、= 1. It is shown that the number of branches more than 4 gives only a marginal 6 Rec. ITU-R F.1704 improvement. Figure 5 shows the node unavailability as a function of the link length d for the case of L = 4. Four-branch diversity with 2 km radio links to the node can reduce the unavailability by a
35、factor of about 0.3-0.5 for the specified availability of 102-105. As the link length increases, the spatial correlation of rain attenuation between links decreases. Accordingly, the greater improvement of the node unavailability is achieved due to the larger diversity effect. Rec. ITU-R F.1704 7 Fi
36、gure 6 a) shows the diversity gain as a function of the link length d in case of L = 4. The diversity gain, which contributes to the increase of the margin in the link budget, can contribute to the reduction of transmit power, the employment of smaller antenna with less gain, or the ease of receiver
37、 system design by relaxing the noise figure. From the Figure, it is found that the diversity gain almost linearly increases as the link length extends. The smaller the node unavailability)(LNUp , the more diversity gains can be achieved. From Fig. 6 b), it is also found that the diversity gain becom
38、es larger as the frequency becomes higher. Figure 7 shows the route unavailability as a function of the link length d . It is taken for granted that more routes make the route unavailability lower. The lower bound of the route unavailability is also presented in the Figure. The lower bound is estima
39、ted from an assumption that an infinite number of routes are available between Nodes A and B in Fig. 3 and is derived from equation (8): 124578914791258)(BAuuuRUpppp +=(8) Equation (8) shows the probability that either one of the nodes (i.e., Node A or Node B) becomes unavailable. Finally, Fig. 8 sh
40、ows the diversity gain as a function of the link length d in the case that an infinite number of routes are assumed to be available between Nodes A and B in Fig. 3. This gives the upper bound of the diversity gain. Outcome of this result is the similar to that obtained from Fig. 6. 3.3 Summary of th
41、e analytical results From the above results, the following can be concluded; 1. A large amount of reduction in the transmit power can be expected due to diversity gain. For instance, around 10 dB is expected assuming: 4 km link length, 26 GHz frequency band, 1 x 105link unavailability, spatial corre
42、lation of ( )r3.0exp = , where r is distance (km). 2. Higher diversity gain can be expected as: the unavailability of each link is smaller, the link length is longer; and, the frequency band is higher. 3. In terms of the diversity effect, 4 branches (links) per node are sufficient. 4 System function
43、s required for exploiting route diversity 4.1 Required functions The following functions are required to MP-MP systems with mesh network topology in order to facilitate the route diversity effect such as the improvement of the route availability and the reduction of the transmit power. 4.1.1 Functio
44、n of establishing multiple routes between a pair of nodes In order to exploit route diversity, a networking function to establish multiple routes, which include multi-hop routes, is required over the physical mesh topology in MP-MP systems. 8 Rec. ITU-R F.1704 Rec. ITU-R F.1704 9 10 Rec. ITU-R F.170
45、4 4.1.2 Diversity route selection function If one of the configured routes becomes unavailable, telecommunication traffic conveyed over the route should be rerouted to other available routes. Thus, a function to select appropriate routes is indispensable to maintain the undisrupted telecommunication
46、s over the routes. It is noted that additional effectiveness of multiple routes would be expected using a traffic load balancing mechanism to enhance system capacity. 4.1.3 Link quality management function In order to make use of the diversity route selection function, it is necessary to collect inf
47、ormation on the quality of wireless links over the mesh network. For the smooth hand-over of telecommunication traffic between routes and the minimization of the unavailable period of telecommunication services, such information should be collected and reflected in a sufficiently frequent and prompt
48、 way. 4.2 Practical examples to realize the required functions Practical examples to realize the required functions mentioned above are introduced in this subsection. 4.2.1 Function of establishing multiple routes between a pair of nodes Routing protocols that have been used in the Internet Protocol
49、 (IP) layer so far, such as the open shortest path fast (OSPF) and the Routing Information Protocol (RIP), select only one route between a pair of source and destination. Thus, for each pair of source and destination nodes, routers establish the one route determined by these routing protocols. However, the router in the mesh wireless network architecture, which is equipped at each wireless node, must have a function for forwarding IP data packets to multiple routes.
