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Abstract— CDMA is most widely used standard for multiuser system in wireless adhoc network. CDMA standards presents set of protocols for power control, BER control and multi user detection system in adhoc network. Therefore behevior of MANET depends upon the modeling of cdma based MAC layer, which also attributes to the performance of the network. In this paper we present a means of improving the performance of the network by adopting a cross layer based fair scheduling algorithm. We consider a losy fading channel and show that by appropriate scheduling technique, latency and losses and be significantly reduced. .

Index Terms—MANET, Cross Layer, CDMA, Bandwidth Estimation, QOS , Scheduling

I. INTRODUCTION

[A]Problem Analysis Mobile adhoc network is an infra structureless network which works in a collaborative mode between the neighbouring nodes. A path is determined on the fly when any node( a source) wants to communicate with any other node. It is a packet switched network with it’s own distinct set of protocols for network and MAC layer. We will first try to understand how the network adopts the CS-CDMA standards. Consider the following diagram.

Figure 1: Architecture and Concept

Let us consider that node A, B, C and D are four arbitary wireless node of adhoc network. A wants to unicast a packet to D. In no way does, nodes in such a network can have a fixed position or IP address. Therefore routing a packet directly to D is impossible for A. in CDMA standards each node is allocated with a unique PN (Pseudo Noise) identification number. When A needs to transmit to D, it encodes the data with the PN code of D (known as spreading) and then just broadcasts the wireless data. This data is reached to all it’s neighbors. C and B tries to read the message by decoding the data with their respective PN code (Despreading). But because the PN code does not match they again forward the packets to their neighbors. Therefore even though theoretically it is a unicast communication between node A and D, in reality it is a breadcast scenario.

Many, such communication session may go on at the same time incorporating same or different set of nodes or group of nodes. Therefore there is a possibility that the messages( wireless signals) may get corrupted if they mix with each other. Therefore CDMA presents a scheme by means of which we can generate PN sequences which are orthogonal to each other( phase shifted by certain degree or PN bits shifted using a sequence like a gold sequence). Due to the orthogonality of PN codes probability of errors in transmission is reduced to a great deal. But due to huge broadcast overload, the possibility of exhausting the bandwidth increases a great deal. Due to this broadcast the transmitted data encounters substantial Attenuation before it reaches the destination. They also suffer variable delays depending upon the distance and the location of the neighbours. Hence it is important that an appropriate scheduling algorithm be designed so that the bandwidth utilization is maximum and trandmission latency is minimum.

As it is been elaborated that CDMA based communication relies on broadcasting, it is essential for the nodes to probe the channel continuesly and determine if it can transmit the data or not. This is also known as carrier sensing. Due to CS-CDMA standard, effective bandwidth at any node is essentially the total available bandwidth minus the bandwidth used by neighbouring nodes.

Cross layer Fair scheduling for MANET with 802.11 CDMA Channels

1Rekha Patil, 2A. Damodaram,3Rupam Das 1Assistant Professor, Computer Science and Engineering Department,P D A College of Engineering, Gulbarga,

India 2Professor, Computer Science and Engineering Department,JNTU College of Engg., JNT University, Hyderabad,

India 3 Member,Technical Team, Intergrated Ideas, Gulbarga,India

E-mail: 1rekha.patilcse@gmail.com, 2damodarama@gmail.com,3rupam@integratedideas.co.in

978-1-4244-4570-7/09/$25.00 ©2009 IEEE

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In this paper we will adopt a Network layer scheduling technique which should be based on the link information obtained by the MAC layer. Forwarding of the packets may even be scheduled at the application layer by utilizing this information.

[B]Problem Formation

In this section, we will discus the need of scheduling, by analyzing the characteristics of CS-CDMA in MANET. The roots of the near-far problem lies in the fact that unlike FDMA and TDMA channels which can be completely orthogonal, CDMA codes suffer from nonzero cross-correlation between codes. A system is called time-synchronous if all signals originate from the same transmitter, as in the case of the downlink of a cellular CDMA network4. In here, synchrony is manifested in two ways. First, different transmissions that are intended for different receivers will have a common time reference. Second, from the viewpoint of a given mobile terminal, all signals (intended or not) propagate through the same paths, and thus suffer the same time delays. In synchronous systems, it is possible to design completely orthogonal spreading codes. In fact, in the IS-95 standard for cellular CDMA networks , each user of the channel is assigned a Hadamard (or Walsh) code. These codes are orthogonal and are used to “channelize” the available bandwidth. On the other hand, a system is called time-asynchronous if signals originate from multiple transmitters, as in the case of the uplink of cellular networks and also in MANETs. The reasons behind the naming are twofold. First, since signals originate from different transmitters, it is generally not feasible to have a common time reference for all the transmissions that arrive at a receiver. Second, these transmissions propagate through different paths; thus, they suffer different time delays. In an asynchronous system, it is not possible to design spreading codes that are orthogonal for all time offsets. In this case, the cross-correlation between codes cannot be neglected. In fact, codes that are orthogonal in synchronous systems (e.g., Hadamard codes) exhibit high cross-correlation when not perfectly synchronized. Instead, PN codes that are designed specifically to have low cross-correlation are used. While the code design problem is crucial in determining the system performance, of greater importance is the problem of nonzero cross-correlation of the PN codes. Unintended transmissions add nonzero MAI during the dispreading at a receiver. The near-far problem is a severe consequence of MAI, whereby a receiver who is trying to detect the signal of the ith transmitter may be much closer in distance to, say, the jth transmitter than the ith transmitter. When all transmission powers are equal, the signal from the jth transmitter will arrive at the receiver in question with a sufficiently larger power than that of the ith transmitter, causing incorrect decoding of the ith transmission (i.e., a secondary collision). In order to avoid such collisions, and due to the fact that pure orthogonal codes can not be formed in this case, we require a suitable scheduling algorithm.

II. RELATED WORK

[1] proposes a CDMA-based power controlled medium access protocol for mobile ad hoc networks (MANETs). Unlike previously proposed protocols, ours accounts for the multiple access interference (MAI), thereby addressing the notorious near-far problem that undermines the throughput performance in MANETs. Channel-gain information obtained from overheard RTS and CTS packets over an out of band control channel is used to dynamically bound the transmission power of mobile terminals in the vicinity of a receiver. By properly estimating the required transmission power for data packets, the proposed protocol allows for interference-limited simultaneous transmissions to take place in the neighborhood of a receiving terminal.

In [2] a scheduling algorithm for CDMA systems is presented which is a trade-off between two extreme ways of scheduling: C/I based and Round-Robin scheduling. The simulation results indeed display that the advantages of both these extremes have been combined in the new algorithm: a good fairness, comparable to that of the Round-Robin scheduling, together with almost the same power gain as reached in the C/I based scheduling.

[3] proposes a Load-based Queue Scheduling algorithm (LBQS) for MANET. Nodes thoroughly consider their own load states when forwarding packets. The priorities of packets are assigned according to the current node’s load level. When nodes are leisure, they should help other nodes to construct route first. In order to avoid network transmission delay increasing and packets losing, nodes should delay or forbid the construction of new route passing through them when their load level is high.

[4] contributes a cross-layer DTN routing approach based on the observation that application-layer orders from a MANET’s leader also control workers’ mobility and ability to forward messages. authors approach attempts to minimize deadline misses and energy consumption by scheduling worker tasks considering both application- and network-layer needs. Simulations demonstrate performance benefits of our approach in a variety of scenarios.

[5] introduces radio link stability and shows how cross-layer cooperation leads to higher throughput and lower delays by avoidance of unstable links.

III. METHODOLOGY

Scheduling is generally adopted in the downlink in a infrastructured CDMA network. A downlink is defined as a link in which a base station forwards the packets to the mobile

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user. In MANET, there is no base station. Therefore downlink can not be appropriately defined. For the synonymy of the study we will assume each node as base station when they are in the forwarding mode. In figure 1 Node B and C will be considered as independent base stations and the transmission from B to D will be considered as downlink. Usually, the scheduling is based on either the current channel conditions (C/I based scheduling) or on simple Round- Robin mechanisms. While the latter is based on the principle of fair-share, channel-dependent scheduling mechanisms tend to be unfair because receiving nodes which are close to the sending node are served more often than others. This implies that those users that need least power, or that can send at the highest data rate, are scheduled for transmission. Sending at low power reduces interference and as a result increases system capacity. So does sending at higher data rate. So, users far away will hardly be served by the Node B.

Generally the transmitted signals are modulated signals, which are modulated with high frequency carrier( generally sinusoidal signal) signals. The power of such signals varies from a source node to a destination node based on the aspects like the fading over distance and multipath reception, fading caused by energy absorption of the signals by the intermediate nodes etc. But study shows that the power value of the signals always varies around a mean value of their recent history. We define a relative power as one with actual transmitted power and the received power separated by a standarad deviation.

Mac layer collects the information about the percentage of signal deviation from a node to all reachable neighbours. Now its is quite easily understandable that the node with maximum deviation or the power loss possibility will have the highest probability of losing a packet and will encounter highest latency if it is transmitted from this node. Therefore packets to such nodes will be scheduled first. Network layer will collect the information of power deviation of the neighbours from the MAC layer and device the scheduling algorithm.

Propagation Model Fading in mobile communication systems may be divided into two different types, fast and slow fading. In this chapter we give the formulas used to compute the transmit power at the Node B. The loss is defined as follows: L = A + S + R with A the attenuation (path loss), S the long-term fading and

R the short-term fading. The attenuation is described by the Okamura-Hata propagation reference model for suburban areas: A(x) =129.4 + 10 log10(x),

with a path loss exponent of β=3.52 and x the distance to the Node B in kilometers we only consider an attenuation fading here and consider long term and short term loos to be 0.

Hence L=A.

If E0 is the signal transmitted by node 0 and E1 is the signal received by node1 then

E1=E0-A(dEo-E1)

Where the term dEo-E1 denotes the distance between the node 0 and 1. This formulae can be generalized to each and every pair of MANET.

Here the term 10log(x) is the attenuation term or the loss term. We will call this loss term as loss exponent Le. Every node will keep an entry of the Le of it’s neighbor.

Figure 2: Propagation Model Now consider the following routing scenario,

We assume that node B is serving as intermediate nodes for three nodes. Packets are transmitted by left nodes with an order presented by the the numbers in the arrow. Node c should receive the packet as the packet to C is the first packet to arrive. But B puts the packets in a queue. From the loss exponent, it determines that D has the maximum loss exponent, therefore packet to D is transmitted before packet to C or E. This scheduling guarantees that node D receives the packets prior to node E and C and there is no significant data loss in D. It is an improvement over the priority scheduling as in the case of priority scheduling node D will be put in waiting and the delay in that node will be substantial.

IV. SIMULATION

We simulate the technique in OMNET++ simulator. Here each layer is been made as independent module. We have adopted AODV protocol and is the module which links all the modules. A cross layer interface is build which enables message exchange between all the layers directly (bypassing intermediate layers). We vary node density and packet transfer rate and measure the network performance. Parameters used for the simulation is listed bellow.

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The algorithm is as given bellow.

N Nodes are randomly placed over an area A marked by X and Y diamensions Each node generates hello packets to their neighbor with equal Power E. Upon receiving of hello packets, each node calculates the Le K pair of source and destinations are selected. Path is established by using AODV from all K sources to K destinations.

This K path now has S common nodes( any node which is common to more than one path will be considered as S) which will also be scheduling nodes. Initialize the traffic Source.( we consider CBR traffic with packet rate Pr 4092 packets/sec) Initialize simulation time t. start transmit from 0 source to k source and back to 0 till all data is finished. at S nodes schedule the packets. Other nodes simply forwards the packets to next hops. Total latency is calculated.

We change N/A(Network Density), K/N(Path density), Pr and measure average Data Delivery Ratio, Latency and Queue Delay.

V. RESULTS

Figure 3: Node Density v/s Throughput

Node density is defined as number of nodes par unit square area. The more nodes are closer, the more they will affect to packet collision. In general, as node density increases,

throughput decreases due to losses suffered by collision. But due to appropriate scheduling, even at higher node density, throughput is maintained at a good level.

Figure 4: Pause Time v/s Throughput

Pause time is the time interval for which a node does not move. If the pause time is less, the topology will be fixed. Hence throughput will be substantially high. With higher mobility the throughput decreases. But due to proper scheduling, as the channel is shared fairly, the throughput is high even at moderate mobility. But for very high mobility(low pause time) throughput is observed low. This is due to fast reconfiguration of the network.

Figure 5: Pause Time v/s Latency of Proposed and AODV protocols.

Result 3(Figure 5) depicts pause time v/s latency. As the pause time increases( mobility decreases) latency also comes down. It is due to the fact that once network characteristics are determined, it will be fixed for some interval of time, which will stabilizes the performance.

VI. CONCLUSION

CS-CDMA is a widely used standard for MANET. It presents many challenges for QOS due to excessive channel sharing. In this paper we proposed cross layer mechanism to overcome the challenges. Simulation shows that average latency is always consistent. Simulation also shows that by adopting cross layer approach to determine the order of the nodes to which the packets will be schedule give a very high throughput. This is

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attributed by the fact, the paths where loss is maximum, packets are scheduled with high priority. In case there are packet looses and retransmission is necessary, still the nodes get enough time to serve those nodes. Because by the time loss is detected, all the other nodes are finished. The technique can be further improved by adopting a suitable bandwidth estimation mechanism as one of the parameters for scheduling.

REFERENCES

[1] Alaa Muqattash and Marwan Krunz, CDMA-Based MAC Protocol for Wireless Ad Hoc Networks [2] Irene de Bruin1, Geert Heijenk1,2 , Magda El Zarki3 & Jasmine Lei Zan3, Fair Channel-Dependent Scheduling in CDMA Systems[3] Zhigang Chen1, Zhihui Ge2, Ming Zhao3, A Load-based Queue Scheduling Algorithm for MANET

[4] Jos´e Brustoloni, Sherif Khattab, Christopher Santamaria, Brian Smyth and Daniel Moss´e, Integrated Scheduling of Application- and Network-Layer Tasks in Delay-Tolerant MANETs

[5] KAMBIZ HOMAYOUNFAR, CROSS-LAYER MANET ROUTING ALGORITHM BASED ON RADIO LINK STABILIT

[6] Samarth H. Shah, Kai Chen, Klara Nahrstedt, Available Bandwidth Estimation in IEEE 802.11-based Wireless Networks,,Department of Computer Science University of Illinois at Urbana-Champaign

[7] IEEE standard for wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specification, June 1997. [8] R. S. Prasad�M. Murray_ C. Dovrolis�K. Claffy,”Bandwidth estimation: metrics, measurement techniques, and tools””