 Research
 Open Access
Nonparametric generalized belief propagation based on pseudojunction tree for cooperative localization in wireless networks
 Vladimir Savic^{1, 2}Email author and
 Santiago Zazo^{1}
https://doi.org/10.1186/16876180201316
© Savic and Zazo; licensee Springer. 2013
 Received: 27 October 2011
 Accepted: 17 January 2013
 Published: 9 February 2013
Abstract
Abstract
Nonparametric belief propagation (NBP) is a wellknown message passing method for cooperative localization in wireless networks. However, due to the overcounting problem in the networks with loops, NBP’s convergence is not guaranteed, and its estimates are typically less accurate. One solution for this problem is nonparametric generalized belief propagation based on junction tree. However, this method is intractable in largescale networks due to the highcomplexity of the junction tree formation, and the highdimensionality of the particles. Therefore, in this article, we propose the nonparametric generalized belief propagation based on pseudojunction tree (NGBPPJT). The main difference comparing with the standard method is the formation of pseudojunction tree, which represents the approximated junction tree based on thin graph. In addition, in order to decrease the number of highdimensional particles, we use more informative importance density function, and reduce the dimensionality of the messages. As byproduct, we also propose NBP based on thin graph (NBPTG), a cheaper variant of NBP, which runs on the same graph as NGBPPJT. According to our simulation and experimental results, NGBPPJT method outperforms NBP and NBPTG in terms of accuracy, computational, and communication cost in reasonably sized networks.
Keywords
 Probability Density Function
 Span Tree
 Receive Signal Strength
 Anchor Node
 Unknown Node
Introduction
Obtaining location estimates of each node position in wireless network as well as accurately representing the uncertainty of that estimate is a critical step for a number of applications, including sensor networks [1], cellular networks [2], and robotics [3]. We consider the case in which a small number of anchor nodes, obtain their coordinates via Global Positioning System or by installing them at points with known coordinates, and the rest, unknown nodes, must determine their own coordinates. Since we want to use energyconserving devices, with lack the energy necessary for longrange communication, we suppose that all unknown nodes obtain a noisy distance measurements of the nearby subset of the other nodes (not necessarily anchors) in the network. Typical measurement techniques [1, 4, 5] are time of arrival (TOA), time difference of arrival, received signal strength (RSS), and angle of arrival. This localization technique is well known as cooperative (or multihop) localization.
Most of the stateoftheart methods for cooperative localization estimate the point estimate of the sensor positions by applying classical least squares, multidimensional scaling, multilateration, or other optimization methods. These methods, also known as deterministic methods[1, 6–8], lack a statistical interpretation, and as one consequence do not provide an estimate of the remaining uncertainty in each sensor location. On the other hand, Gaussian probabilistic methods (such as multilateration by Savvides et al. [9], or variational method by Pedersen et al. [10]) assume a Gaussian model for all uncertainties, which may be questionable in practice. NonGaussian uncertainty is a common occurrence in realworld sensor localization problems, where typically there is a fraction of highly erroneous (outlier) measurements. This problem can be solved using nonparametric probabilistic (or Bayesian) methods [11–14], which take into account uncertainty of the measurements. They estimate the particlebased approximation of the posterior probability density function (PDF) of the positions of all unknown nodes, given the likelihood and a prior PDF of the positions of all unknown nodes. However, the main drawback of these methods is high complexity of marginalization of the joint posterior PDF, especially in largescale networks. Nevertheless, an appropriate factorization of the joint PDF using some messagepassing technique make these methods tractable. Nonparametric belief propagation (NBP), proposed by Ihler et al. [11, 12], is a wellknown particlebased message passing method for cooperative localization in wireless networks. It is capable to provide information about location estimation with appropriate uncertainty, to accommodate nonlinear models, and nonGaussian measurement errors.
However, due to the overcounting problem in the networks with loops, NBP’s convergence is not guaranteed, and its estimates are typically less accurate [15]. Our previous proposals, using NBP based on spanning trees [16] and uniformlyreweighted NBP [17], can mitigate this problem in highly connected networks, but with very small benefit comparing with NBP. Another solution is generalized belief propagation based on junction tree (GBPJT) method [18], which is a standard method for the exact inference in graphical models. In [19], nonparametric generalized belief propagation based on junction tree (NGBPJT) has been applied for the localization in a smallscale network, where it has been showed that it can outperform NBP in terms of accuracy, but with an additional cost. However, there remained two main problems: (i) how to efficiently form the junction tree in an arbitrary network, and (ii) how to decrease the number of particles. Therefore, in this article, we propose nonparametric generalized belief propagation based on pseudojunction tree (NGBPPJT). The main difference comparing with the standard method is the formation of pseudojunction tree (PJT), which represents the approximated junction tree based on thin graph. In addition, in order to decrease the number of highdimensional particles, we use a more informative importance density function, and reduce the dimensionality of the messages. As byproduct, we also propose NBP based on thin graph (NBPTG), a cheaper variant of NBP, which runs on the same graph as NGBPPJT. According to our simulation and experimental results (using measurements from indoor office environment), NGBPPJT method outperforms NBP and NBPTG in terms of accuracy, computational, and communication cost in reasonably sized networks. On the other hand, the main drawback of this method is the high cost in largescale networks.
Summary of notation
C _{ i }  Clique i 
S _{ i j }  Separator set between cliques C _{ i } and C _{ j } 
G _{ t }  Set of neighbors of node t 
${G}_{{C}_{i}}$  Set of neighbors of C _{ i } 
x _{ t }  Random variable for position of node t 
X _{ a }  Position of anchor node a 
${x}_{{C}_{i}}$  Random variable for position of C _{ i } 
${x}_{{S}_{\mathit{\text{ij}}}}$  Random variable for position of S _{ i j } 
d _{ t u }  Distance between nodes t and u 
p _{ v }  Noise distribution of the measured distance 
R  Transmission radius 
ψ _{ t }(x _{ t })  Single potential (prior) of node t 
ψ _{ t u }(x _{ t },x _{ u })  Pairwise potential between nodes t and u 
${\psi}_{{C}_{i}}\left({x}_{{C}_{i}}\right)$  Potential of clique i 
M _{ t }(x _{ t })  Belief of node t 
m _{ t u }(x _{ u })  Message from node t to node u 
${M}_{i}^{m}\left({x}_{{C}_{i}}\right)$  Belief of C _{ i } at iteration m 
${m}_{\mathit{\text{ij}}}^{m}\left({x}_{{S}_{\mathit{\text{ij}}}}\right)$  Message from C _{ i } to C _{ j } at iteration m 
${M}_{\mathit{\text{ij}}}^{m}\left({x}_{{C}_{j}}\right)$  Joint message from C _{ i } to C _{ j } at iteration m 
${q}_{{C}_{i}}^{m}\left({x}_{{C}_{i}}\right)$  Importance density function of C _{ i } at iteration m 
${X}_{{C}_{i}}^{k,m}$  Particle k from${M}_{i}^{m}\left({x}_{{C}_{i}}\right)$ 
${W}_{{C}_{i}}^{k,m}$  Weight of particle k from${M}_{i}^{m}\left({x}_{{C}_{i}}\right)$ 
${X}_{{S}_{\mathit{\text{ij}}}}^{k,m}$  Particle k from${m}_{\mathit{\text{ij}}}^{m}\left({x}_{{S}_{\mathit{\text{ij}}}}\right)$ 
${W}_{{S}_{\mathit{\text{ij}}}}^{k,m}$  Weight of particle k from${m}_{\mathit{\text{ij}}}^{m}\left({x}_{{S}_{\mathit{\text{ij}}}}\right)$ 
Background and related work
PJT
 (a)
The number of cliques should be reasonable (i.e., in the order of number of nodes).
 (b)
In order to reduce the dimensionality of the problem, each clique should include no more than three nodes.
 (c)
Since the triangulation is expensive procedure, we are going to avoid it, even if it causes the break of RIP for some small percentage of the nodes.
After these approximations, the final result represents, strictly speaking, the clique tree. However, since it is very close to the junction tree (measured by the percentage of the nodes that satisfies RIP), we name it PJT.
Thin graph formation
In order to satisfy the conditions (a) and (b), we need to decrease the number of the edges in the graph by formation of thin graph. Assuming that each edge provides the same (or sufficiently similar) amount of information, it can be done using a modified version of breadth first search (BFS) method. The standard BFS method [26] begins at randomly chosen root node and explores all the neighboring nodes. Then each of those neighbors explores their unexplored neighbors, and so on, until all the nodes are explored. In this way, there will not be a loop in the graph because all the nodes will be explored just once. Thus, the final result of BFS is the spanning tree. The worst case complexity is O(v + e), where v is the number of nodes and e is the number of edges in the graph, since every node and every edge will be explored in the worst case.
Nevertheless, the spanning tree is very coarse approximation of the original graph since it excludes a lot of edges from the graph. For example, in any spanning tree, one communication failure breaks the graph into two parts. As a consequence, we need more spanning trees in order to have reasonable accurate inference in graphical models. Therefore, we modify standard BFS method by permitting each root node to make an additional visit to the node that was already visited by some of the previous roots. All edges found by first and second visits, along with all the nodes from the original graph, represent the thin graph. In addition, the second visit will automatically form a loop, so we use it to form 3node clique. The 2node cliques can be found easily by taking all the edges that appear in thin graph, but not in any 3node clique. The worst complexity is O(v + e + v · (v  1)) ≈ O(v ^{2}), since for each of the additional visit, we need to check all previous roots (all the nodes minus one, in the worst case). The detailed pseudocode is shown in Algorithm 1, and an example of the original graph and the corresponding thin graph are shown in Figure 2a,b, respectively.
Algorithm 1. Searching for thin graph and cliques using modified BFS method
The main benefit of the thin graph is that it mainly includes 3node loops. The number of these loops, which is obviously always less than the total number of nodes, is nearly constant with respect to connectivity, so the number of cliques will nearly be constant as well. On the other hand, the main drawback is that there exist the loops which include more than three nodes.^{a} These loops should be triangulated, but we prefer to avoid it in order to keep reasonable complexity. Thus, for nnode loops (n > 3), we form maximum n 2node cliques, using each edge (which is not already subset of any 3node clique) of the loop as a clique. Another problem can be caused by the nodes which cannot determine their positions due to the possible nonrigidity of the thin graph (e.g., the nodes with less than three neighbors). However, these nodes can be still located since we bounded the estimate within its bounding box (see Section 2), created using original (not thin) graph. Therefore, the estimates will never be out of these boxes, which means that we ensured a coarse estimate in the worst case scenario. Finally, we note that anchorunknown links are not excluded, so it would be useful if the anchors are placed as close as possible to the edges of the deployment area, where the leaf nodes are expected.
PJT formation
Having defined the cliques, we can form the cluster graph by connecting all pairs of the cliques with nonempty intersection (see Figure 3a). As we already mentioned, the JT, as well as PJT, is the maximum spanning tree of the cluster graph. It can be found using, e.g., Prim’s algorithm [27], as shown in Algorithm 2. The Prim’s algorithm is a method that finds a maximum (or minimum) spanning tree for a connected weighted undirected graph, meaning that the total weight of all the edges in the final tree is maximized (or minimized). In our case, the algorithm starts with a list (i.e., CurrentList in Algorithm 2) which initially includes only randomly chosen root clique. At each step, among all the edges between the cliques in the list and those not in the list yet, it chooses the one with the maximum weight and increases the list by adding the explored clique. Finally, it stops when all the cliques are spanned. The example of PJT is shown in Figure 3b. The worst case complexity is O(e·l o g(v)) [27], but in our case the weights are binary (S _{ i j } = 1, or S _{ i j } = 2), so the execution will be very fast.
Algorithm 2. PJT formation using Prim’s algorithm
The BP/GBP methods are naturally distributed through the network which means that there is no central unit (fusion center) which will handle all computations. Thus, the proposed PJT formation should be done in a distributed way. It is already well known that there is a straightforward distributed way to form any spanning tree, so we refer the readers to [28, 29].
Having defined PJT, it remains to define the communication between neighboring cliques. Since the separator sets, between each pair of the neighboring cliques, are always nonempty, the separator nodes are responsible to perform the communication. Practically, these nodes represents the cluster heads. For example, in Figure 3b, the node 3 will request all the data from node 9, and upon receiving, it will send the data to node 10, and vice versa.
Finally, the previous approximations will likely break the RIP for some small number of the nodes. For instance, in the PJT in Figure 3b, the node 10 (due to the nontriangulated 4node loop: 3–9–5–10), and the node 7 (due to the appearance of 4node clique: 2–6–5–7) do not satisfy the RIP. Therefore, we do not have a guarantee that the belief of that node in one clique is the same as its belief in another clique [24]. Nevertheless, for cooperative localization, this is not a problem since we used the bounded boxes (see Section 2) for the initial set of particles. Regarding other applications, this method might be useful if all edges provide the same (or sufficiently similar) amount of information.
Possible alternatives
Although we provided a tractable solution for formation of the approximated junction tree, we cannot claim that it is an optimal one. In literature, there are available alternatives that could be (with some adaption) applied for this problem. For example, Dechter et al. [30] propose iterative joingraph propagation, which runs on the cluster graph with bounded cluster size, created without discarding any edges. Similar solution, thin junction tree with bounded cluster size, is available in [31]. However, for both approaches, a distributed implementation is not provided, so they cannot directly be applied for our problem. Finally, it is worth mentioning a distributed method [32], which creates a rigid subgraph from the fully connected (complete) graph in a tractable way. However, this method can be applied for cooperative localization, only if adapted for noncomplete graphs.
Nonparametric generalized belief propagation
Equations (3), (4), and (5) represent GBPJT algorithm which is valid for any arbitrary graphs. The standard BP algorithm [11] is a special case of GBPJT, obtaining by noting that the original tree is already triangulated, and has only pairs of the nodes as cliques. In that case, sets S _{ i j } are single nodes, and the marginalization is unnecessary.
where d _{ t u } represents the measured distance between nodes t and u, p _{ v }(·) the noise distribution of the measured distance, and R the transmission radius. More general model, which incorporates the probability of detection, can be found in [11, 33].
where a dynamic model p(x _{ t,τ }x _{ t,τ1}) defines the possible positions of the unknown node x _{ t,τ } in current time instant τ, given the estimated position from the previous time instant. It is also necessary to create the PJT at each time instant, except if the structure in the graph remains the same. All other computations (within the same time instant) are the same as in the static scenario. Thus, for clarity, we discard subscript τ in all equations, and focus on the static scenario. More details on mobile positioning can be found in [13, 14, 34].
Due to the high complexity, the presence of nonlinear relationships, and potentially highly nonGaussian uncertainties, GBPJT method is not appropriate for cooperative localization [11]. Thus, we need to use NGBPJT. Moreover, due to the problems explained in previous sections, we are going to use PJT instead of JT. Therefore, in following subsections, we propose NGBP based on PJT (NGBPPJT) for the arbitrary networks. Note that an analysis of NGBPJT for the smallscale network has already been provided in [19].
Drawing particles from the cliques
Computing messages
where we assumed that the position of the anchor node is perfectly known (i.e., defined by Delta Dirac function). However, if anchors’ positions are uncertain (as in [37]), the message can be computed in the same way as the messages from the unknown nodes.
Computing beliefs
According to (7), the belief of clique i is a product of its clique potential and all the messages coming into the clique. Before drawing the particles, we need to solve two problems: (i) the messages include information about different nodes within the clique, and (ii) it is intractable to draw the particles from the product.
where ${W}_{{C}_{j},\text{corr}}^{k,m+1}$ is the correction of the weights due to the MIS, ${X}_{t}^{k,m+1}$ particle from node t, m _{ a t } is the message from the anchor node a to unknown node t, and function choose chooses randomly one particle from $\left{G}_{{C}_{j}}\right$.
where we used the approximation ${M}_{j}^{m+1}\left({X}_{{C}_{j}}^{k,m+1}\right)\approx {W}_{{C}_{j}}^{k,m+1}$. The algorithm stops when $K{L}_{j}^{m+1}$ (for all j) drops below the predefined threshold. However, it is also possible to predefine the number of iterations, given diameter of the graph and transmission radius. We choose the latter approach in simulations.
The final estimation of each node within the cliques is given as the mean of the particles from the belief in last iteration. Since the most of the nodes appear in more than one clique, we simply average multiple estimates. Other options are also possible, such as choosing the belief with the smallest entropy. We summarize the NGBPPJT algorithm in Algorithm 3.
Algorithm 3. NGBPPJT method for cooperative localization
Finally, it is worth noting that a special case of NGBPPJT method is NBP method based on thin graph (NBPTG) assuming that the thin graph has only the pairs of the nodes as cliques. NBPTG is very important byproduct since it runs on the same graph as NGBPPJT, which makes this method cheaper than NBP. It also helps to understand (as shown in the following section) how much the removed edges from the original graph affect the performance of the method.
Performance evaluation
Scenario
Comparison of the computational cost (measured in MFlops)
R  NBP  NBPTG  NGBPPJT  PJT 

5m  0.56  0.56  0.56  0.007 
8m  0.71  0.59  0.67  0.011 
12m  0.82  0.62  0.73  0.013 
Comparison of accuracy and convergence
Comparison of computational and communication cost
As we already mentioned, we set the same computational cost for R = 5 m by choosing appropriate number of particles for all three methods. It was not possible to set the same cost for all methods since the cost is more sensitive to R in case of NBP method. On the other hand, NGBPPJT and NBPTG costs are less sensitive to R due to the nearly same number of edges with respect to (w.r.t.) R, in formed thin graph. We provide the average cost per node for different values of R in Table 2. We can see that the cost of NGBPPJT is the same or less for all considered values of R. We can also see that the complexity of the PJT formation is neglectable comparing with full algorithms.
Regarding communication cost, which is directly related with the battery life of the wireless devices, we provide a simplified^{f} analysis by counting elementary messages, where one elementary message is defined as a scalar value (e.g., one coordinate of one particle). We will consider the effect of transmission radius and number of unknowns, since their variations obviously affect the cost. First, we analyze the cost of PJT formation (Algorithms 1 and 2). As we can see in Figure 10, it is a linear function of transmission radius, and a quadratic function of number of unknowns. Second, we analyze the cost of all considered algorithms w.r.t. R, for two different number of unknowns. According to Figure 11, we can conclude the following

NGBPPJT significantly outperforms NBP and NBPTG methods, for reasonable number of unknowns.

Comparing with NBP, the improvement of NGBPPJT is increasing as transmission radius increasing. This is achieved thanks to the thin graph.

The cost of NBPTG is slightly less than NGBPPJT due to the redundancy in PJT graph (i.e., when the same node appears in more than one clique).

Increasing the number of unknowns will decrease the benefit of NGBPPJT. This is caused by quadratic dependency of PJT formation w.r.t. number of unknowns. Using results from Figures 10b and 11, we estimate that NGBPPJT will reach the same cost as NBP, for 140 unknown nodes.
Finally, we can conclude that the proposed NGBPPJT method is cheaper for reasonably sized networks. However, it can also be cheap for very largescale networks if the network is divided into regions, and one PJT created for each of them.
Experimental results
Conclusion and future work
In this article, we presented NGBPPJT, a novel message passing approach for cooperative localization in wireless networks. Since the exact formation of junction tree is intractable, we proposed the formation of PJT, which represents the approximated JT based on thin graph. In addition, in order to decrease the number of particles for NGBPPJT method, we proposed a more informative importance density function, and also reduce the dimensionality of the messages. As byproduct, we also proposed NBPTG, a cheaper variant of NBP, which runs on the same graph as NGBPPJT. According to our simulation and experimental results, NGBPPJT, outperforms NBP and NBPTG in terms of accuracy, computational and communication cost in reasonably sized networks. In addition, NGBPPJT beliefs are not overconfident as NBP beliefs, so we can obtain online more trustful information about the position uncertainty. Finally, all algorithms converge sufficiently after very small number of iterations, but the convergence is not perfect.
There remain a number of open directions for the future work. One direction could be to find an alternative method (e.g., modified variants of the methods described in Section 2), which will be tractable in largescale networks. It would be also useful to find in which graphs, and under which conditions, the benefit gained by using a NGBPPJT instead of NBP outweighs the penalty caused by discarding edges. Moreover, an important research line is to investigate if there is some cheaper (nonparticlebased) message representation, which should be capable to handle all realistic uncertainties. Finally, distributed target tracking in sensor network [42, 43] could be an interesting direction since this application requires a number of sensor nodes with known (or estimated) positions.
Endnotes
^{a}According to our empirical analysis, the number of these loops is relatively small (e.g., just one 4node loop in Figure 2b).^{b}We implicitly assumed that ${q}_{{C}_{i}}^{1}\left({x}_{{C}_{i}}\right)=0$ if the state of one of the clique nodes is out of the deployment area.^{c}In practical circumstances, we can set ϵ = 0.^{d}Note that these values do not represent the mean value and the standard deviation of the distance. They are respectively given by: ${\mu}_{d}={e}^{\mu +{\sigma}^{2}/2},{\sigma}_{d}={\mu}_{d}\sqrt{{e}^{{\sigma}^{2}}1}$. Consequently, these parameters are distance dependent.^{e}Typical values for n _{ p } are between 2 and 6 [44]. For the distance estimation, the minimum value is the worst case.^{f}Exact communication cost can only be measured by knowing the hardware specifications, especially, the amount of the bytes in the package, number of reserved bytes in package, energy required to transmit a package, etc.
Declarations
Acknowledgements
This study was supported in part by the FPU fellowship from Spanish Ministry of Science and Innovation; program CONSOLIDERINGENIO 2010 under the grant CSD200800010 COMONSENS; the European Commission under the grant FP7ICT20094248894WHERE2; the Swedish Foundation for Strategic Research (SSF) and ELLIIT.
Authors’ Affiliations
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