Properties of Mobile Tactical Radio Networks on VHF Bands

Transcription

1 Proerties of Mobile Tactical Radio Networks on VHF Bands Li Li, Phil Vigneron Communications Research Centre Canada Ottawa, Canada / ABSTRACT This work extends a network model that was develoed for the study of mobile tactical scenarios emloying VHF combat radios. The model uses VHF radio signal measurement data, integrating ractical network deloyment scenarios and node mobility models to evaluate network link robabilities, and to derive fundamental network roerties including the average network node degree, network connectivity strength, network ath ho count distribution and dynamics of network link udates. The results illustrate imortant network characteristics and rovide insights for assessing alicability and otimization of network rotocols for tactical radios oerating in the VHF band. 1.0 INTRODUCTION Mobile Tactical Networks (MTN) that use tactical radios oerating in the VHF military bands [4-5] offer the otential for imroved coverage and scalability through self-forming multi-ho caabilities. The MTN aims to deliver realtime integrated voice and data across command osts, vehicles and dismounted soldiers to rovide enhanced C2 and situational awareness in the military oerational theatre. To suort the multi-ho networking functions required by the MTN, many recent rotocols roosed for mobile ad hoc networks (MANET) [6-9] may be considered candidate solutions. However, most of these roosals assume WiFi (e.g., IEEE ) radios when conducting rotocol analysis and simulations. The communication roerties exhibited by VHF tactical radios are very different from those of generic WiFi radios, e.g., limited bandwidth and different signal ath loss roerties. To establish effective network rotocol design guidelines and to evaluate networking solution otions ragmatically, it is imortant to understand the fundamental roerties of MTN in tyical oerational scenarios. The urose of this work is thus to investigate the network characteristics of MTNs in a ractical deloyment scenario. The aroach taken combines an analytical network model [1], tactical VHF radio signal measurement data [2-3] and realistic mobility scenarios, taking into account different terrain tyes. In the model, numerical comutations are alied to calculate the changing distance matrix of the network nodes through the duration of network simulation to cature the toology udates. The model comutes the network link and ath robability matrices, the network node degree, the link udate ratio, the ath length and cost distribution, etc. The results obtained illustrate the fundamental network roerties of the MTN, which rovide significant insights for assessing alicability and otimization of the mobile ad hoc network rotocols and schemes in the MTN environment. The rest of the aer is organized as follows: section 2 briefly describes the network model; the network deloyment scenario is resented in section 3; section 4 illustrates the network roerties obtained and section 5 concludes the aer. RTO-MP-IST

2 Proerties of Mobile Tactical Radio Networks on VHF Bands 2.0 NETWORK MODEL FOR MOBILE VHF NETS The network model emloyed in this study is established for a mobile tactical network alying comrehensive radio signal measurement data [1-3]. As described in [1], the network model is based on an undirected geometric random grah where a tactical link is described using ( r, t), which is the robability of having a link between node i and j at distance metric r at time t [1, 10]. The mobility scenario generator BonnMotion [12] is emloyed to roduce MTN deloyment scenarios for the model. The outut file of BonnMotion traces the trajectory of each node by logging ositions of the node through time. Then given a time interval samled at instants nt, i.e., t = {T, 2T,, nt, }, the 2-dimensional osition coordinates of all nodes in the network at each samling instant t are collected in the N 2 osition matrix X t) {( x ( t), x ( )) i = 1,2,... N, where N is the total number of radio nodes. A distance matrix is N 2 ( = i1 i2 t } then formed for all nodes at each samling instant as d ( t) = { d ( t) i, = 1,2, d ( t) = ( xik ( t) x jk ( t)) k = 1,2 2 N N j }..., N, where. Matrix d ( t) N N is then alied to calculate ( r, t), using the radio signal measurement data taken in the terrain tye of the deloyment field [2, 3], as described in [1]. With T sufficiently small, the resulting ( r, t) characterizes the network link status through the entire modelling duration. Given ( r, t), the average node degree, the robability that a link changes its state at any given time t, and the exected number of links that change states at any given node i at time t can all be derived as resented in [1]. To gather the network connectivity information, ath robability matrices of the network for both voice and data ackets are comuted at each samling time t. Realtime voice traffic requires aths of high availability and low latency, without erforming retransmissions even if errors occur in the recetion. The ath matrix for voice VP [t], with (i,j)th element [t], is comuted assuming that the transmission from source to v destination refers all the intermediate hos to be available simultaneously, and that the link robabilities are indeendent of each other. A modified Floyd shortest ath algorithm is used selecting the maximum ath robability [1] as: VP[ t] = [ ( r, t)]; for k = 1 to N do for all i, j, do If v [ t] < ), v [ t] = max( v [ t], v [ t] v [ t]) ; where r is the error threshold. ( r ik kj Other auxiliary data structures are included in the ath comutation algorithm to cature the list of hos contained along each ath; these are not shown here to kee the descritions succinct. Denote the required delivery ratio of voice ackets as The error threshold is dr. L is the length of the acket and b is the BER. dr r = for a ath of m hos, assuming indeendent bit errors. Taking the ml ( 1 b ) NATO standard MELPe voice format of 7 bytes er frame and 4 frames acked into one voice acket to 15-2 RTO-MP-IST-092

3 Proerties of Mobile Tactical Radio Networks on VHF Bands enhance the transmission efficiency as in many common imlementations, an examle voice acket size is assumed containing 50 bytes, i.e., L =400, including headers in comressed format. The voice aths are comuted at each samling time t using dr =95% and b = The ath for data communications differs from that for voice. Data communications tolerate end to end latency but not errors. Retransmissions will be used, such as in the er-ho ARQ scheme to ensure reliable delivery. In this case, the ho cost is formulated as the exected number of transmissions taken for the acket to traverse a ho. This ho cost reresents the sectral cost in terms of number of sends, aroximating the occuancy of timeslots in a TDMA network. As earlier, the link robability between nodes i and j at time t is ( r, t), hence the robability that a acket is successfully received is b L = ( r, t)(1 ). The robability that node i receives the acknowledgement (ACK) from j after 2 sending a acket is ( 1 (1 )) =, assuming only one ACK is sent for each received acket. Then the 1 exected number of times that node i sends the acket is. At each time, the robability that a sent acket 2 is received successfully at j is. If received correctly, a single ACK is generated. The total exected number 1 1 of ACKs sent by node j for this acket is then 1 =. The total exected number of acket 2 transmission attemts and acknowledgements sent across the ho between nodes i and j, which is the sectral cost of the ho, is thus: w ( t) = + =. 2 2 The ath matrix for data, DP [t] with its (i,j)th element [t], is thus comuted as: DP[ t] = [ w ( t)]; for k = 1 to N do for all i, j, do d [ t] = min( d [ t], d [ t] d [ t]) ; ik + kj The above two algorithms generate the ath matrices for voice and data at each samling instant during the modelling time of the network. The results will be illustrated in Section 4. d 3.0 NETWORK SCENARIO The network mobility traces are generated using the mobility scenario generator BonnMotion [12]. The semi-rural area is selected for network deloyment, using the measurement data taken from this tye of terrain [2-3]. The semi-rural environment measured has a significant amount of rural and forested areas, RTO-MP-IST

4 Proerties of Mobile Tactical Radio Networks on VHF Bands some overgrown farm fields and a few two or three story brick buildings [3]. The scenario has 38 deloyed radio nodes in a field of 400 km 2 (20 km by 20 km), which is divided into four 10 km by 10 km non-overlaing quarters. Among all the nodes, 3 of them form a commander grou ositioned in the area of 5 km by 5 km at the center of the field. A grou of 8 nodes is deloyed in each of the four quarters. The commanders and the 4 grous are all running the reference oint grou mobility (RPGM) model [11]. The remaining three are individual nodes with assigned tasks such as reconnaissance or secial oerations. These three nodes move in the entire field of 20 km by 20 km according to a random wayoint model. Excet the commander grou, all the nodes travel in the seed range of m/s with an average ause time of zero to 10 minutes. The commander grou has an average ause time of 30 minutes and moves between the seed levels of 0-8 m/s. Within each grou, the maximum distance from any node to the grou center varies between 3 km and 5 km. A total of 10 networks are generated to average the comutation results. The modelling time for each network is 9000 sec and the samling interval is chosen to be 3 sec for semi-rural scenarios, which is sufficient to cature the udates caused by the defined mobility. Figure 1 below illustrates some sna shots of one of such networks generated in the model, at different samling times. km Network at Time=30 minutes ç ç à à à à à àà ô ç ôô ì ì ì æ æ æ æ æ æ km Network at Time=75 minutes ì ì ç ìì ìì km 10. æ grou 1 à grou 2 ì grou 3 grou 4 ô Commanders ç Secial Oerations 5. à à àà àà à ô ô ç ô æ æ æ æ æ æ æ 0 ç km Figure 1 Tactical network deloyment scenario examle 4.0 NETWORK PROPERTIES The radio signal frequency is at 57.0 MHz with channel bandwidth of 25 khz. The data rates range from 20kbs to 96 kbs. The transmission ower is 46 dbm for vehicle mounted unit. The recetion noise floor in the field is -126 to -95 dbm. The required SNR for achieving the data rate, deending on the radio design, may range from 6 dbm to 23 dbm. From these arameters, the ath loss margin is estimated to be around 140 db. Given the reference distance d 0 = 100 m, the ath loss arameters comuted from the measurement 15-4 RTO-MP-IST-092

5 data have the exonent η =3.18, the intercet α d ) = 68.8 dbm, and the standard deviation of the ( 0 shadowing δ = 4.11 dbm, for the selected terrain of semi-rural. Then alying the algorithms resented in the revious section and the formulas in [1], the link robability ( r, t) at each samling instant t is obtained, which leads to the node degree distribution, as shown in Figure 2. Proerties of Mobile Tactical Radio Networks on VHF Bands cdf ath loss margin=130db 133 db 135 db db ath loss margin =140dB node degree Figure 2 Distribution of network node degree The average node degree varies from about 17 nodes to 33 nodes when the ath loss margin changes from 130 db to 140 db, which is fairly high. To find out how the network connectivity is imacted under the different network conditions, the average network connectivity level (t) is defined as: C l C ( t) = l 2 N N i= 1 j= i+ 1 γ ( i, j, t) N ( N 1), where γ ( i, j, t) is the largest ath robability between node i and j comuted over all ossible routes at time t. Note that C l (t) is the ratio of the exected number of aths over the total number of routes (node airs) in the network. The matrix Γ( t) = [ γ ( i, j, t)] is obtained by alying the voice ath comuting algorithm with r =1. The network C l (t) s for ath loss margin of 130 db and 140 db are lotted in Figure 3 (a), showing the variation through the entire 9000 seconds of network modelling time. A network connectivity level of 1 indicates a fully connected network between all node airs. It can be seen that when the average node degree decreases from 33 to 17, the network connectivity level moves from close to 1 all the time, to ranging between 85% and 99%. Taking any of the three commander nodes in the network, its average ath robability to all the other nodes in the network, referred as the average overall ath robability, can be comuted at each samling time t. It is found that the average overall ath robability of any of the commander nodes is closely related to the network connectivity level (t). In Figure 3 (b), the average overall ath robability of one of the C l commander nodes is lotted together with the network connectivity level C l (t), for ath loss of 140 db, to RTO-MP-IST

6 Proerties of Mobile Tactical Radio Networks on VHF Bands illustrate this observation. The imlication of this relationshi is elaborated on later in the section. Connectivity Level ath loss margin 130dB ass loss margin 140dB Time HsL Figure 3 (a) Average network connectivity levels time HsL commander node network Conn. Level Figure 3 (b) Average overall ath robabilities of commander nodes vs. network connectivity level The voice ath matrix VP[t] is comuted with dr =95%, yielding the distribution of the voice ath length in the network, which is deicted in Figure 4, for networks with average node degree ranging from 17 to 32 nodes, i.e., the ath loss margin of 130 to 140 db. When the ath ho count is larger than three, the end-toend throughut delivered on the very low seed tactical links may be ractically too small and the latency too high. When the average node degree is below 25, i.e., the ath loss margin <135dB, certain voice ath will extend to more than 3 hos, causing QoS concerns. A ractical MTN is often connected with many nodes within 1 ho and others reachable through a single relay. A few nodes at certain times may temorarily require two relay nodes to connect when they are searated due to ath obstacles. Such strong connectivity maintains the high reliability and low latency required by tactical communications. At a ath loss margin of 140 db, the network is connected with nodes mostly 1 and 2 hos aart, and only a few aths 15-6 RTO-MP-IST-092

7 Proerties of Mobile Tactical Radio Networks on VHF Bands traversing 3 hos. The average smallest best ossible ath robability found in matrices (t) was then 0.985, reresenting a strongly connected tactical network. 1.0 cdf ath loss margin 130dB ath loss margin 135dB ath loss margin 138dB aths loss margin 140dB Ho Count Figure 4 Distributions of voice ath ho counts The data ath matrix DP[t] is comuted by alying the algorithm resented in the revious section. The distribution of data ath ho counts is illustrated in Figure 5(a) for different ath loss margins (denoted as lm in the figure). It can be seen that data aths have relatively shorter ho counts than voice aths. However, because of the required reliability and the ARQ transmission scheme, the real sending cost on a data ath is different from its ho count. On a single ho, two sends or even more will be needed to deliver a single data acket successfully. Each send consumes bandwidth and ower. Thus the real cost of data aths is calculated as the number of transmissions required for a successful acket delivery, as the ath sectral cost, described in the algorithm resented in the revious section. The distribution of the data ath sectral cost is illustrated in Figure 5 (b). The cost of data aths in the network is relatively higher than the voice ath, because the send cost of voice traffic is the same as the ho count of its ath. cdf HaL data ath ho count lm=130db lm=135db lm=140db cdf HbL data ath sectral cost lm=130db lm=135db lm=140db Figure 5 Distributions of data ath ho counts and costs The robability that a link changes its state from samling instant t-1 to t can be calculated as [1]: _ linkchange( i, j, t) = ( 1 ( r, t 1)) ( r, t) + ( r, t 1)(1 ( r, t)) RTO-MP-IST

8 Proerties of Mobile Tactical Radio Networks on VHF Bands The link between node i and j incurs a state change from time instant t-1 to t, if there is a link between node i and j at time t while none existed at time t-1; or there is no link between node i and j at time t while there was one at time t-1. The distribution of the ercentage of links that change their states across each samling interval is deicted in Figure 6. In comarison, when the node degree is around 17, i.e., ath loss margin is 130 db, on average more than 23% of the links in the network exerience state changes in the 3 sec samling interval. This amounts to about 150 links changing their state at a given time in the network cdf Percentage of Changing Links in the Network ath loss margin 130dB ass loss margin 140dB H%L Figure 6 Percentage of changing links er samling interval The above results illustrate the basic structural and dynamic network roerties of the selected tyical VHF tactical deloyment scenario. It can be seen that the network has a relatively dense neighbourhood with nodes having high average node degrees. When average network node degrees decrease, the ath ho counts will increase. To maintain the useful voice aths in the network without too many traversing hos, the node degree may need to stay quite high, e.g., about 17 to 20 nodes on average. However, the increase in data ath sectral cost is even faster than the increase of the ho counts. Hence to maintain a low cost for data communication aths, the average node degree may need to be even higher. For low node degrees, there may be an average of 6 sends for a data acket along the ath. With a high node degree and very limited link bandwidth, e.g., 20 kbs to 96 kbs of shared link rates for the next generation NATO narrowband waveforms oerating in the tyical 25 khz VHF channel, a rotocol udating the two ho neighbourhood information, e.g., the oular HELLO rotocol [6-10] may result in a fairly heavy traffic overhead in the shared neighbourhood, when node degrees are so high [13]. In general, the comrehensive exchange of link status information can be quite costly in such densely formed networks. Even when the node degree decreases, e.g., if the ath loss margin is reduced to 130 or 135 db, the number of links that need to reort an udate would increase, which roduces more data in the udate messages [1, 13]. It should also be noted that this network configuration concentrates the highest node degrees on the commander nodes. As shown in Figure 7, which deicts the average node degrees of each of the 38 nodes in the network, the commander nodes, numbered as node 33, 34 and 35, have much higher average node degree comared to the other nodes. In fact, the commander nodes become the major relay nodes for network aths. On one hand, this fortifies the connections between the commander nodes and the rest of the network, which is desirable; but on the other hand, it leads to increased congestion and contention in the neighborhood of the commander nodes, and increases their vulnerability. This also exlains the result illustrated in the Figure 3(b). As major relay nodes, the average overall ath robabilities observed by the commander nodes reflect the overall connectivity strength among all nodes at that time. This roerty may lead to a network route and toology control strategy for this tye of network deloyment scenario that exhibits a central area. The nodes 15-8 RTO-MP-IST-092

9 Proerties of Mobile Tactical Radio Networks on VHF Bands laced in the central area will not only become the hub/relay oints, but also have visibilities on network connectivity situations. Such nodes can even be dedicated hub/central nodes if it is desirable to shield the commander nodes from the vulnerability and heavy traffic load entailed in being in the role of network hubs. Then the commander nodes may maintain a one ho strong connectivity to these central relay nodes to achieve their strong network reachability as well as network situational awareness. 5.0 CONCLUDING REMARKS Figure 7 Average node degrees for all nodes in the network In this aer, real VHF radio signal measurement data are alied to study the network roerties of mobile VHF radio networks in a tyical deloyment scenario. This is accomlished using a geometric random grah integrating tactical mobility scenarios with radio measurement data to establish a network model. The model is further extended to cature the imortant roerties of the network such as the link robability, the ath robability, the ath ho counts and the network connectivity strength. These models have identified the fundamental structural and dynamic characteristics of the network formed. The results indicate the ossible heavy overhead cost of oular networking rotocols when udating two-ho neighbour information in the dense network neighbourhood. It has also demonstrated the critical role of the central/hub nodes in such deloyment scenarios when network ho counts are small. The different ath ho count distributions for voice and data traffic call for a suitable ath selection mechanism to suort their different QoS requirements. It is evident from this study that the network roerties of mobile tactical radio networks need to be considered when designing and otimizing networking solutions. 6.0 ACKNOWLEDGEMENT This research is suorted by Defence Research and Develoment Canada. 7.0 REFERENCES [1] L. Li and T. Kunz, Efficient Mobile Networking for Tactical Radios, Proceedings of IEEE MILCOM 2009, Boston, MA, USA, Oct 2009 [2] J. Pugh, R. Bultitude, and P. Vigneron, Path Loss Measurements With Low Antennas for Segmented RTO-MP-IST

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