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Joint MAC and Routing protocol for OFDMA-Based Adhoc Networks

Van Duc Nguyen1, Khanh Nguyen Quang2, Luong Pham Van3, Trung Thanh Nguyen4, Duyen Trung Ha5, and Van Binh Pham6

1,2,3,5,6School of Electronics and Telecommunications, Hanoi University of Science and Technology No.1, DaiCoViet Road, Hanoi, Vienam

4The Department of Digital Signal Processing, The Faculty of Engineering, The University of Duisburg-Essen Bismarckstrasse 81, 47057 Duisburg, Germany

1ducnv-fet@mail.hut.edu.vn, 2khanhnq1@vms.com.vn, 3phamvanluongdt12k49@gmail.com

Abstract—In this paper, a dynamic Sub-channel Assignment Algorithm (DSA) based on orthogonal frequency division multiple access (OFDMA) technology operating in time division duplexing (TDD) and a new routing protocol are proposed. The proposed dynamic Sub-channel Assignment Algorithm solves several drawbacks of existing radio resource allocation techniques in OFDM system used in ad-hoc and multi-hop networks, such as the hidden and exposed node problem, mobility, co-channels interference in frequency (CCI). The proposed routing protocol is jointed with the MAC protocol based the algorithm to ensure the mobility and multi-hop, thus the quality of service in ad-hoc and multi-hop networks is significantly improved.

Keywords-OFDMA, MAC protocol, routing protocol, ad-hoc networks, multi-hop networks

I. INTRODUCTION

A major challenge in wireless networks for multi hop communications is the co-channel interference (CCI). This interference is introduced when two different radio stations simultaneously use the same frequency. It is mainly caused by the spectrum allocated for the system being reused multiple times in TDMA network. CCI is one of the major limitations in cellular and Personal Communication Services wireless telephone networks since it significantly decreases the carrier-to-interference ratio. In addition, it makes the diminished system capacity, more frequent handoffs, and dropped calls. IEEE 802.11 Distributed Coordination function operation is based on conventional carrier mechanism (CSMA/CA) in order to prevent channel collisions, CCI and provide the communication between multiple pairs of independent mobile nodes without access points or base stations such as mobile adhoc networks [1-2].

Recently, orthogonal frequency division multiplexing (OFDM) has been intensively investigated for wireless data transmission in broadband cellular and ad-hoc networks. The multiple access technique for these networks is OFDMA [3]. The concept of this technique is to assign different users to different sub-channels in order to avoid interferences.

Dynamic Sub-channel Assignment Algorithm based on orthogonal frequency division multiple access (OFDMA)

technology operating in time division duplexing has been studied in [4-7]. However, in most of previous works, MAC layer is still transparent from physical layer.

In our previous work [7], we proposed a DSA algorithm based on orthogonal frequency division multiple access (OFDMA) technology operating in time division duplexing (TDD). This algorithm can maximize overall throughput of networks as well as solves some drawbacks existing in cellular networks such as hidden and exposed node problems. However, in this work, a concept of multi-hop networks was not considered. The performance of the proposed DSA, therefore, was not presented in such networks. In this paper we improve the DSA algorithm and apply it to the concept of multi-hop networks. In addition, we propose a simple routing protocol jointed with MAC model for ad-hoc and multi-hop networks based on OFDMA technology using the proposed DSA algorithm.

The paper is organized as follows: In section II, we briefly review sub-channels allocation methods. Session III describe the proposed DSA algorithm. In session IV, the proposed routing protocol is presented. Simulation schemes, numerical results, are discussed in section V. Finally, conclusions are drawn in session VI.

II. REVIEW OF SUB-CHANNELS ALLOCATION METHODS

Recently, we have two sub-channels allocation methods: 1. The fixed allocation method of OFDM-FDMA for multiuser communications was proposed [8]. In such method, different users will be fixedly assigned to different sub-channels. Therefore, this method has not any anti-interference mechanism. 2. The OFDM-FDMA random allocation method is based on the idle and busy of sub-channels allowing users to accounts different sub-channels [9]. However, it does not have any attention to the network interference. Once a sub-channel is selected, a user starts transmitting using the selected sub-channel. During a transmission process, if a sub-channel does not meet the required QoS, it will be released and assigned to new user. Although the method is simple and it offers an adaptive mechanism, it does not provide CCI avoidance.

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III. PROPOSED DSA ALGORITHM

Co-channel interference (CCI) is crosstalk from more than one different radio transmitter using the same frequency in wireless networks. To avoid CCI and collisions, we propose a novel channel allocation algorithm called DSA which supports simultaneous transmissions in Vehicle Ad Hoc Network among nodes. In this section, the problem of CCI in OFDMA/TDD in wireless networks is discussed in detail. Then, we present the proposed Dynamic Subchannel Assignment (DSA) algorithm.

A. CCI in OFDMA/TDD System To illustrate the problem of CCI, a simple scenario

consisting of two base stations (BSs) and four mobile stations (MSs) is depicted in Fig.1. Let us assume an example of exchanging data among BSs and MSs as followings. The mobile station MS1

Rx, MS2Rx and MS3

Rx receive data from base station BS1

Tx, while at the same time the mobile station BS2Tx

transmits its data to the base station MS4Rx. In such scenario,

BS2Tx causes CCI to receiving data of MS1

Rx, MS2Rx and

MS3Rx; the base station BS1

Tx causes CCI to receiving data of MS4

Rx. Note that CCI only exists in TDD mode.

If a node can select appropriate sub-channels in available sub-channel set before the data transmission, CCI is minimized, thus increase the throughput of the network.

B. Sub-Channel Selection based on Busy-Signals In the previous example, CCI was introduced and it badly

affects to the receiving data of MS1Rx, MS2

Rx and MS3Rx. To

avoid this interference, MS1Rx, MS2

Rx and MS3Rx have to

broadcast a busy signal when they receive data. Before data transmission, BS2

Tx will first hear busy signals in all channels and then compares the received signal power with a predetermined threshold. If the power of busy signal in a channel is lower than the threshold, BS2

Tx can select the channel for data transmission.

C. Dynamic Sub-channel Assignment Algorithm In this section, we propose MAC frame structure for

downlink and uplink transmission based on incorporation the busy tone in OFDMA/TDD network. The proposed MAC frame is illustrated in Fig.2. The busy signal is sent an in-band signaling before data transmission. Data sub-channels are mutually orthogonal and used for transmitting OFDM symbols.

Fig.2 shows data transmitting and receiving in view point from BS and MS, respectively. The structure of the uplink frame is similar to those of the downlink frame. Each sub-frame includes a busy signal channel (an OFDM symbol) used for signaling busy tone. MAC frame length is chosen having corresponding to the time correlation of the channel. Initially, the sub-channel is selected by only the transmitter. On the other hand, the adaptive period is adjusted by both transmitter and receiver. We define two kinds of signal:

- Busy signal: transmitted only in a channel by MS

receiver. - Data signal: transmitted only by BS transmitter

The proposed algorithm includes two following steps:

1. Link initialization Link initialization occurs when mth BS need to transmit data

to kth MS in the network. Firstly, mth BS listens to all busy signals and compares each of these busy signals with a given threshold to obtain a set of available sub-channel for data transmission called set A. A sub-channel is available if the power of received busy signal is below the threshold. The mth BS will start transmit its data to receiver using a set of available

Figure 2. Structure of the proposed MAC frame

Figure 1. Co-channel interference in OFDMA / TDD system

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channel at the (i-1)th MAC frame. We propose a mathematical model (1) where ,

, 1k ml ia − denotes the sub-channel assignment for

lth sub-channel at the (i-1)th MAC frame of the link between mth BS and kth MS. If the sub-channel is assigned to the link between mth BS and kth MS, then ,

, 1 1k ml ia − = , otherwise ,

, 1 0k ml ia − = .

The outcome of this channel assignment is obtained by comparing the busy signal with the threshold as following:

, , 1, 1

ˆ1 if

0 otherwise

mk m l i thrl i

B Ia −−

⎧ ≤⎪= ⎨⎪⎩

(1)

thrI is a threshold being a measure for the interference that

this transmission would effect to other co-existing transmission. Set A is all sub-channels having channel assignment by (1).

2. Dynamic subchannel assignment At the (i-1)th MAC frame, kth MS estimates the SINR on

each sub-channel of set A. Based the given QoS requirement for the transmission, kth MS will decides to maintain or release the respective sub-channel of set A. kth MS will only broadcasts the busy signal on the remained sub-channels. If the sub-channel is specified, then ,

, 1k ml ib − is assign 1,

otherwise ,, 1k ml ib − = 0.

We propose a mathematical model (2) where ,, 1k ml ib − denotes

the reservation of lth sub-channel for the ith MAC frame. , 1kl iγ − , reqγ are SINR of estimated by kth MS and required QoS.

, , 1 , 1, 1

ˆ1 ( ) & ( )

0

m kk m l i thr l i reql i

if B Ib γ γ− −−

⎧ ≤ ≥⎪= ⎨⎪⎩ (2)

Since the ith MAC frame, the condition for the sub-channel

assignment on the desired or new link between mth BS v kth MS for subsequent MAC frames is given as follows:

, ,

, ,, 1 , 1,

ˆ1 if ( )or 1

0 otherwise

k m m k mk m l i thrl i l il i

a B I ba − −⎧ ≤ =⎪= ⎨⎪⎩

(3)

Where, ,ˆ m

l iB is the busy signal received at the mth BS on the lth sub-channel at the i th MAC frame and

,, , 1

, 11 if 0

0 otherwise

k mk m l il i

aa −

−

⎧ =⎪= ⎨⎪⎩

(4)

Since the ith MAC frame, set A includes all maintained

sub-channels (set B of (i-1)th MAC frame) and new respective

sub-channels if the power of their received busy signal of are below the threshold.

IV. PROPOSED ROUTING PROTOCOL DESCRIPTION

In ad-hoc and multi-hop networks, a node transmits it’s data to a destination node via an optimal route. In order to find the optimal route, routing is implemented. In this section, we propose a new routing protocol which can be easily jointed to MAC protocol based the DSA algorithm to ensure mobility and multi-hop in ad-hoc multi-hop network.

Routing protocols working in wireless networks depend on the radius of coverage of nodes. The connection among all nodes changes fast over time. The proposed routing protocol is based on shortest path finding algorithms and the radius of coverage of nodes in network. The principal cost of this protocol is distance among nodes.

Before describing the flow chart of this protocol, we define:

- Ri is the radius of coverage of node in network - The network is supposed to be a graph with nodes

and edges. - The connection among nodes in the coverage is

described in a connection matrix.

We propose the mathematic model for the connection matrix:

( ){ tan ( , ) if tan ( , ) min ,0 Otherwise

i jdis ce i j dis ce i j R RijM ≤=

(5) Where Mij and distance(i,j) are the connection state and the

distance between the ith node and the jthnode, respectively. The proposed routing protocol finds the route using the

two following steps: Step1: Build connection matrix based on the radius of

coverage of all nodes in the network Step2: The connection matrix is input of shortest path

finding algorithms to find the route matrix that describe the best route from sources to destinations. The shortest path finding algorithm used in our routing protocol is dijkstra algorithm shortest path algorithm, see [11].

V. SIMULATION MODEL

In the simulation scheme, OFDM parameters are selected as in Table I. 16-QAM modulation is selected for all sub-channels.

Table I. OFDM SYSTEM PARAMETERS

Parameters Values

Bandwidth (B) 20 MHz

Sampling interval (ta=1/B) 50 ns

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Parameters Values

FFT length (NFFT) 256

OFDM symbol duration (TS) 12.8µs

Guard interval (TG) 2µs

Frequency ( cf ) 1.9 GHz

Modulation scheme 16-QAM

SINRmin ( reqγ ) 16 dB

A. Throughput of Single-hop adhocNetworks 1) Analysis Model

We consider a model including two transmitter-receiver pairs depicted in Fig.3 where Tx and Rx are in transmission mode and the receive mode, respectively. TDD mode is used. In this model, the distance between two nodes in each pair is constant and these pairs move in opposite directions.

2) Numerical Results

In this evaluation, the throughput terms of number of selected sub-channels is considered for two methods: OFDM-FDMA fixed allocation, and proposed DAS algorithm. It can be seen from Fig.4 that the throughput of network using proposed DSA depends on the value of different threshold Ithr as well as the distance between pairs in the network. When the distance between pairs is high, proposed method demonstrates a high performance. Since in OFDM-FDMA fixed allocation method, sub-carries are fixedly allocated to users, its throughput is constant and much less than one of proposed DSA method. On the other hand, the proposed method offers CCI avoidance mechanism based on busy tone signaling. The selection of sub-channel consider to given threshold, thus the throughput depends on the selection of threshold. From the results in Fig 4, the suitable threshold for the proposed method is -60 dBm.

B. Throughput of Multihop adhocNetworks 1) Analysis Model

Mobility and multi-hop are two principal characters of multi-hop ad-hoc networks. Here, the best route from source node to destination node is found via the proposed protocol in session IV. We provide an analysis model for evaluating throughput via numerical simulation. The proposed DSA algorithm and routing protocol are implemented on vehicles network VANET [12]. This network includes 6 mobile stations MS indexed from 1 to 6 with the initial state depicted in Fig.5. MS1, MS5, and MS6 are running step by step while the other mobile stations are fixed. The running step length is set 10 metres. MS5 and MS6 run in the same direction being opposite the running direction of MS1. While the mobile stations are running, MS1 is transmitting its data to MS5. Before the transmission of a step, routing finds a path to transmit data. Fig.6 draws an example of route from MS1 to MS5 via intermediate nodes MS2, and MS3. After each step, throughput of whole network is obtained.

Figure 3. A simple circumstance for single hop ad-hoc network

Figure 4. Number of selected sub-channels with different thresholds and

conventional OFDM-FDMA Figure 5. Initial state of networks used in simulation in the case of ad-hoc

multi-hope network

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2) Numerical Results Fig.7 plots the length of route from MS1 to MS5 with

respect to running step. We truncate the process into three groups A, B and C. In each group, we can see that lengths of routes seem to be the same. When the length of a route is high, it means that the number of intermediate nodes also high. Therefore, data is transmitted through many sections. In some sections, some sub-channels do not meet the QoS. Hence, the channel is not full-fill, thus make the decrease of throughput (in Fig.8). This conclusion is backed-up from group A and B where lengths of routes are high and throughput is small. On the other hand, when length of a route is small, the number of intermediate nodes is also small. Data can be directly transmitted from source to destination or through few intermediate nodes. In such case, the effect of interferences, expose node, and hidden node is reduced, and channel is full-fill. Consequently, throughput of network is significantly increased. It can be seen from group C, the length of route at each step is small and throughputs would, therefore, be high.

Throughput is also depends on the Ithr which decides a sub-channel be selected for transmission data or not. When Ithr is small, the probability that a sub-channel be selected is high. Hence, throughput is also high. The effect of Ithr is clear when the length of a route is high (in A and B group), since it affects to the selection of sub-channel in many intermediate sections. On the other hand, when mobile stations are close (in group C), the role of Ithr is not so important. Therefore, throughputs seem to be similar for three cases of Ithr of -20, -40, -60 dBm.

VI. CONCLUSION

In this paper, we present a dynamic Sub-channel Assignment Algorithm based on orthogonal frequency division multiple accesses (OFDMA). A concept of ad-hoc and multi-hop networks based on the DSA algorithm is simulated with the proposed routing protocol. The proposed algorithm has interference avoidance mechanism, so the throughput of the network can be maximized. The problems of cellular network such as hidden and expose node are solved. By using the proposed routing, the mobility of ad-hoc and multi-hop network is ensured. In the future, we consider designing a new routing protocol using a parameter of MAC protocol based on this algorithm to more optimize QoS of ad-hoc and multi-hop network.

ACKNOWLEDGEMENT

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 102.02-2011.15.

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Figure 8. Throughput of network

Figure 7. Length of route from MS1 to MS5 found by proposed routing

protocol

Figure 6. Example of a path found by our proposed method from MS1to MS5

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