Coexistence Decision Making for Spectrum Sharing among Heterogeneous - PDF document

INFONET, GIST Journal Club Coexistence Decision Making for Spectrum Sharing among Heterogeneous Wireless Systems Authors: B. Bahrak, and J.M.J. Park Publication: IEEE Trans. on W.Com., Mar. 2014 Speaker: Asif Raza Short summary: In this paper authors tackle spectrum sharing with an objective of enabling coexistence among dissimilar TVWS networks. The sharing problem is defined as multi- objective optimization problem (MOOP). An algorithm to solve the MOOP has also been presented in the paper. Finally the simulation study shows the superiority of the proposed algorithm over existing coexistence decision making algorithms in terms of fairness and percentage of demand served. I. I NTRODUCTION TV whitespace (TVWS) refers to TV channels not used by licensed operators at particular location and particular time. Worldwide efforts are being initiated to utilize TVWS. As a result multiple standards have initiated steps like IEEE 802.22, IEEE 802.11, ECMA-392 etc. It is quite likely that a heterogeneous mix of secondary networks will coexist in TVWS, each with distinct operation parameters (e.g., bandwidth, transmission power, PHY and MAC techniques, etc.). Therefore, IEEE 802.19 WG has presented 802.19.1 standard to enable coexistence among heterogeneous secondary networks operating in the same region. In this paper, authors propose an algorithm called Fair Algorithm for Coexistence decision making in TV whitespace (FACT). The algorithm makes contribution in following directions: 1) Multiple constraints are used to formulate coexistence decision making algorithm. 2) Optimization problem is modeled as energy minimization problem in a modified Boltzmann machine 3) Proposed a FACT algorithm to find a Pareto optimal feasible solution II. C ONSTRAINTS F OR C OEXISTENCE D ECISION M AKING 1) Contiguous Channels The allocation of contiguous channels enables channel aggregation which can result in a throughput increase of more than 60% compared to the best fixed-width configuration. 2) Interference The allocation manager generates interference graph based on a node’s location, transmission power, out - of-band emission characteristics, and frequency band. An interference graph provides quantitative

information on the adjacent-channel and co-channel interference between each pair of networks within interference range of each other. The interference graph helps to find the minimum frequency separation between two interfering networks and update this value in the interference graph. 3) Fairness Following notion of fairness: spectrum allocation is considered fair if the ratio of the amount of allocated spectrum to the spectrum demand for each of the coexisting networks is the same. 4) Channel Allocation Invariability The algorithm evaluates the tradeoff between the advantages of reallocating a new block of spectrum to a network vs. the costs of reallocation. The two approaches are used for this purpose: a) A weight is assigned to each constraint to differentiate each one’s impact on the reallocation; and b) A correlation metric between the previous and the current spectrum assignments are defined, and this value is made as large as possible. The channel allocation invariability constraint prevents triggering decision-making propagation by a small change in the demand of a network and thus can help the system to reduce the channel switching and communication overhead of coexisting networks. 5) Transmission Scheduling Constraints When two networks, i and j , need to share a channel, a cost value, Cij is defined. It represents the cost of scheduling transmission durations on a channel for these networks in a scheduled repetition period. The value of Cij is determined based on the following factors. 1) Channel widths: when multiple networks need to share the channel, then a swath of spectrum that is sufficiently large to satisfy the largest channel width requirement is allocated. For example an 802.22 network operates on 6 MHz while 802.11af operates on 5 MHz wide channel. If they need to share the channel then a 6 MHz-wide channel is allocated. 2) MAC strategies : networks with compatible MAC strategies are preferred to share the channel. It is because networks with incompatible mac strategies, will result in a higher switching delay and packet error rate due to synchronization issues. 3) Transmission power : networks with comparable transmission power are preferred to share the channel as large discrepancy in transmission power between two coexisting networks can cause an asymmetric interference relation between the two networks. III. P ROBLEM F ORMULATION OF C OEXISTENCE D ECISION M AKING The CDM problem is modeled as an energy minimization problem in a modified Boltzmann machine . A. Boltzmann Machine The Boltzmann machine is a stochastic recurrent artificial neural network that combines the principles of simulated annealing with those of neural networks. The value of the neuron (network) i ’s state, S i is determined by the output of a thresholding function, f out , as: 2

  1 withprob. p       i (1) S f T ,   i out i i    0 withprob. 1 p i   Where T i , is the weighted sum of the state values of all the other neurons, T w S where w is i i j , j i j , j the connection weight between neuron i and neuron j . θ i , is an appropriate threshold for 1  and  is temperature. Thus Boltzmann T i that controls the value of S i . Probability p       i  T 1 e i i machines have a scalar value associated with each state of the network referred to as the energy , E , of the network as, 1      S  E w S S (2) i j , i j i i 2 i j i B. Problem Formulation Each neuron is denoted as a triplet ( i, j, k ). S ijk is the state of neuron ( i, j, k ), which has two possible values: 0 and 1. S ijk = 1 means that the algorithm should assign channel i at time slot j to wireless network k , and S ijk = 0 means that no channel should be assigned. Let N is number of coexisting networks, C is number of available channels and T is number of time-slots per period per channel. Let a network ‘ k ’ n number of time-slots. Let f defines the minimum frequency separation between networks k requires k kr and r. Then energy function for each of the constraints is defined as: 1) Contiguous Channels : in Boltzmann machine context, contiguous channels allocation can be expressed as: S ijk = S (i+1)jk . Thus its energy minimization is defined like:    C 1 T N  2   E S S (4)    c ijk i 1 jk    i 1 j 1 k 1 2) Interference: since the value of f indicates the minimal amount of separation needed to avoid kr adjacent-channel interference. In the context of the Boltzmann machine, this is equivalent to two neurons,       i j k , , and p j r satisfying , , i p f . To represent interference constraint using energy function kr a new variable is defined as:     1 if i p f   kr X (5) ikpr   0 otherwise This variable signifies whether assigning channel i to network k and channel p to network r in the same time slot causes interference or not. The algorithm minimizes following energy function. T C N C N   E S S X (6) I ijk pjr ikpr      j 1 i 1 k 1 p 1 r 1 3) Fairness: fairness is maximized if their ratio of spectrum demand over spectrum allocation is R  ) or    2 maximized ( 1 1 R is minimized for each network. Here R is same for all networks or k k k their variance is minimized, 3