A Queueing Analysis of Polling Based for Mobile Multihop Relay WiMAX Networks

1Mohd Daud Alang Hassan, 2Habibah Hashim, 3Darmawaty Mohd Ali & 4NurHidayat Ahmad

Faculty of Electrical Engineering, Universiti Teknologi MARA, Shah Alam, Malaysia. 1mohddaud106@ppinang.uitm.edu.my,

2habib350,3darma504 @salam.uitm.edu.my,4hidayat.ahmad88@ymail.com

Abstract- By employing a relay station (RS) on the WiMAX networks, it offers an extended coverage and improved capacity over wireless broadband. However the mobile station (MS) such that connected to the RS has effect the system performance due to increase of the MS numbers. In WiMAX the base station (BS) adopts the centralized-based polling mode where the best way to guarantee the delay bound is by polling in every frame. Through the polling approach, the BS allocated dedicated or shared resources periodically to each MS, which can be used to request resources. Therefore in centralized-based polling MAC a few parameters become important factor to determine the performance of IEEE802.16j. This paper analyze the queue based polling for multi-hop relay in WIMAX networks in order to minimize the average polling delay while increase network throughput, based on tandem queue analysis. The product-form solution applied is valid since the models related to M/M/1. The numerical results showed that numbers of poll is affect by analyzed under various number of MS. Keywords – queueing; Multihop relay, polling; tandem queue, delay

I. INTRODUCTION

Multi-hop relaying is a part of the standard which currently being develops for wireless broadband system [1]. The IEEE 802.16j MMR (mobile multi-hop relay) was proposed to gain coverage extension and throughput enhancement through introduces of a relay station (RS). It will play important role in current broadband wireless networks such as WiMAX or LTE by facilitating either advantage (capacity increase/throughput or coverage extension) at reduced cost, which alternative to using Base Station (BS) in high density or expanded deployments. Furthermore with RS reduce the power consumptions for mobile station (MS), thus improving service delivery zones and overcoming patched coverage. In multi-hop relay (MR) networks, the base station (BS) may be replaced by a multi-hop relay BS (MR-BS) and one or more relay station (RS). The signaling between the MR-BS and mobile station (MS) are relayed by the RS thereby extending the coverage and performance of the system in areas where RS are deployed. MR-BS will supervise each RS. Fig. 1 shows the MMR WiMAX network. The RS may be fixed in location (i.e, attached to a building) or in the

case of an access RS, it may be mobile (i.e, traveling with transportation vehicle). Research on multi-hop relay over network coverage and capacity are done in various perspectives [2-5]. Most of them special attention on the bandwidth allocation and in band relay, while in [6], the author analyze the polling based for uplink WiMAX using Markov chain. A queuing model analysis for polled service [7] only discuss for WiMAX/IEEE802.16 based wireless network. In [8] the authors describe the cost effective coverage extension by using two sectorized cellular approach while in [9] a scalable QoS scheduling architecture was introduce in MMR. The performance of MMR through frame efficiency by varying the number of RS was investigated in [10] which more focus in single-frame and multi-frame for coverage. There are no previous work has specify on the analysis for MMR WiMAX in term of queuing over the unicast polling-based. The focus of this contribution is to analyze the performance of MMR using tandem queue for average polling delay, moreover the throughput by deriving the analytical open M/M/1 queue.

Fig. 1. Mobile multi-hop relay network scenario

We present the theoretical analysis of the queueing of polling based in MMR. The analysis focuses on the effects of the number of polls and mobile station (MS). The remainder of this paper is organized as follows. In Section II, define the relay technologies and its frame. The network model and analysis for MMR WiMAX is present in Section III. In Section IV, we present the analysis result with respect to few parameters, followed by conclusion in Section V.

2013 Fifth International Conference on Computational Intelligence, Communication Systems and Networks

978-0-7695-5042-8/13 $26.00 © 2013 IEEE

DOI 10.1109/CICSYN.2013.71

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II. AN OVERVIEW OF RELAY TECHNOLOGIES

Standard IEEE802.16j define the RS in two mode; transparent and non-transparent which is differ in term of usage. The transparent mode increases the throughput where facilities capacity increases within the BS coverage area. It does not forward framing information, and hence do not extend outer MR-BS coverage and has lower complexity. Mean while in the non-transparent transmits the entire control signal/information thus far RSs generate their own framing information or forward those provided by the MR-BS as well as data packet and extends coverage area as well as enhancing the cell capacity. Two different modes of resource allocation and scheduling in relay technologies. The first is centralized scheduling where the BS determines the scheduler and the scheduling packets are transmitted in a collision-free way within the scheduling control sub-frames. Another word all scheduling for all nodes in MMR cell takes place in the MR-BS. The transparent relay mode operates by this centralized scheduling. The second mode is distributed which is RSs have some autonomy and can make scheduling decision for the nodes which they communicate with. The non-transparent relay has a choice to operate these distributed or centralized depending on scheduling approach. The modification were required to support relay network architecture due to standard 802.16e is design for single hop wireless networks. The BS divides the timeline into contiguous frame, each of which further consists of a downlink (DL) and uplink (UL) sub-frame [11]. The DL and UL sub-frames shall include at least one access zone (AZ) and may include one or more relay zone (RZ) to enable RS operating in either transmit or receive mode. Frame structure configuration for MMR WiMAX systems depends on the type of relaying. For transparent RSs is hence fixed which consists of an Access Zone (AZ) and a Transparent Zone (TZ) in the DL subframe, while an AZ and a RZ for the UL subframe. In AZ for DL subframe, BS sends out data burst to RS and MS, respectively, whereas in TZ, RS forwards data burst receive from BS in AZ to MSs. BS and RS will never transmit simultaneously, therefore no cross interference exits between BS and RS. The frame structure for non-transparent is shown in Fig. 2. DL sub-frame is partitioned into an AZ and RZ. In AZ, both BS and RSs transmit data bursts and control information to MS simultaneously in each sector; while in RZ, BS sends data bursts to RSs (for forwarding to MSs in the next DL AZ). Note that BS’s AZ is 2 symbols longer than a RS due to the transmission-to- reception transmission gap (R-TTG) of a RS. In the UL, the BS receives MSs transmission both direct in AZ and relayed in RZ.

Fig. 2. MAC frame for non-transparent

There are two types of non-transparent relay in IEEE802.16j which is Time-division Transmit and Receive (TTR) relay and Simultaneously Transmit and Receive (STR) relay. In TTR two approaches for supporting relaying: allow one or more RS or MB-RS frames to group into a multi-frame with a repeating pattern of allocated relay zones. The MR-BS and RS, are assign to transmit, receive or being idle in each of the relay zone within the multi-frame. The second approach enables the single frame structure consisting of more than one relay zone. The MR-BS and RS are assigned to transmit, receive, or be idle in each relay zone within the frame. The TTR relay acts in the AZ as BS and relay in RZ. In our works, we are concentrated on non-transparent due to support larger coverage areas and hence used to provide increase coverage. MAC frame structure in non-transparent mode using the single-frame method for multi-hop relaying are depicts on figure below.

III. NETWORK MODEL ANALYSIS FOR MULTIHOP RELAY WIMAX

This part presents the analysis of a parameter that affects the system performance. A number of polls (NOP) are one of the causes besides access range, data rates (arrival rate, service rate) or numbers of MS. The MR-BS adopts the centralized polling mode where the best way to guarantee the delay bound is by polling in every frame. It allocates dedicated or shared resources periodically to each MS, which can used to request resources. The MS is then allocated with enough bandwidth and if the MS does not have data to transmit, then the MS just keep silent. Fig. 3 depicts the topology of MMR.

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Fig. 3. Mobile multi-hop relay network model

In the polling-based Mobile Multi-hop Relay WiMAX network, the number served MSs is an important parameter that significant affects the network performance. The centralized BS polls some of the served MSs in within every frame period, and then will cause polling delay. Furthermore every MSs that registered in WiMAX will send the Constant Bit Rate (CBR) using UGS service for internet service without applied polling access. Consequently the WiMAX`s traffic load increase as the number of served MS increase. We model a polling based multi-hop relay WiMAX as two-stage tandem queues in queuing systems network which object of each type move from one service point to another. This tandem refers to an arrangement of objects in which the objects are lineup one behind the other, all facing the same direction and multiple job classes, one after the other and an arriving packet undergoes each job class before leaving the system[12]. In Fig. 4, the first system behaves like M/M/1 system with arrival rate λ, and service rate µ1. The departure from this system is the arrivals to the second system. But knowing that these departure from a Poisson stream with rate λ. Thus the second system also behaves like an M/M/1 system with arrival rate λ and service rate µ2.

Fig. 4. Two stages -Markovian queues in tandem

The departure from the first system form the arrivals to the second system, but the state of the first system (N1 = n1) is independent of the previous sequence of departure from it and it is these departure which define the arrivals to the second system. Thus if N1 & N2 represent the numbers of served MSs in the two systems in then steady state p(n1, n2) = Pr (N1 = n1, N2 = n2) = Pr(N1 = n1) Pr(N2 = n2) (1) therefore from result for the M/M/1 queue, pn =

nρρ )1( −

p(n1, n2) = 212211 )1()1( nn ρρρρ −− ; n1, n2 ≥ 0 (2)

where 1ρ = λ/µ1 and 2ρ = λ/µ2 and 0 < 21, ρρ < 1

by summing over n1, obtained that

Pr (N2 = n2) = 2

22 )1( nρρ− and E [N2] = 2

2

1 ρρ−

(3)

similarly

Pr (N1 = n1) = 111)1( nρρ− and E [N1] =

1

1

1 ρρ−

(4)

E[N1+N2] =∑ ∑ −−+1 2 221121

21 )1()1)((n n

nnnn ρρρρ

=

∑ ∑∑ ∑

−−

+−−

2 1 11222

1 2 22111

12

21

)1(.)1(

)1(.)1(

n nnn

n nnn

n

n

ρρρρ

ρρρρ

= 1

1

1 ρρ−

+ 2

2

1 ρρ−

= E[N1] +E[N2] (5) As the result p(n1, n2) = Pr(N1 = n1) Pr(N2 = n2) , the so-called product-form solution, is valid if the two system are M/M/1. Refer to [6], frame structure for UL sub-frame are divided by into 3 parts: initial ranging, bandwidth request and data transmission periods. For the sake of simplicity these analyses assume that part of UL for AZ and RZ are combined. We focus on the UL frame which numbers of poll (NOP) affect the performance as in Fig. 5.

Fig. 5. Generic frame structure at MR-BS

The frame period, Tf above consists of TDL and TUL where the UL sub-frame divides into initial ranging, bandwidth request and transmission data. The average polling delay of multi-hop relay WiMAX depends on number of polls (NOP), denote . A BS polls MSs by using UL-MAP in the downlink sub-frame and the MSs can request bandwidth through the specified slot in the up-link sub-frame. Thus, the average polling delay for unicast polling, T ave of multi-hop relay WiMAX is as below,

324

T ave = Tf . φ1

. E [N1 + N2] (6)

= ( T DL + T UL ) . φ1 . E [N1 + N2]

= (TDL+TIR+TBW +φ .Tp+Tdata). ⎥⎦

⎤⎢⎣

⎡−

+− 2

2

1

1

11.1

ρρ

ρρ

φ

where; TDL – downlink subframe duration time TUL – uplink subframe duration time TIR – initial ranging period for uplink subframe TBW – bandwidth request period for uplink subframe TP – polling period for MSs to send bandwith request φ - number of polls N1 & N2 – numbers of MS in the two system in the steady state Here the number of polls φ is inverse to the average polling delay. Therefore more the number of polls obviously reduce the average polling delay; indirectly it will increase bandwidth waste for polling and reduces available bandwidth for data transmission. For analyzing the network throughput, we assume that the frame ready for transmission with probability, pt at every MSs and also piggyback bandwidth is not active. As state earlier the work using unicast polling, there should not have collision. A successful transmission occurs after each packet is polled. The total numbers of packets that want to be sent denoted by A = ( E [N1] + E [N2] )* pt (7) which is the expected numbers of MSs in the network times the packet transmission probability and the polled

probability is written by ppoll =

⎟⎟⎠

⎞⎜⎜⎝

⎛φN1

=Nφ

(8)

In general the throughput can be regarded as the overall output by averaging all cycles of busy period and idle period of the uplink sub-frame:

S =IB

TU+

= IB

U+

(9)

where: B: mean length of busy period I: mean length of idle period T: length of a frame transmission, T=1 U: probability of a successful transmission in a busy period (B+I) is the average length of a time cycle

Here U can be determined by, U = (TIR + TBW + φ . TP) . R + ppoll . A . E[L] (10) Consequently, the network throughput is

S = up

pollPBWIR

TLEApTTT ][... +++ φ

(11)

This can be shown that the number of polls, total number of served MSs and packet transmission probability obviously affect the throughput of the MMR system.

IV. NUMERICAL RESULTS The results of the average polling delay are evaluated under different numbers of mobile station (MSs) and numbers of polls (NOP). In these analysis the frame period is 5 ms while the MSs are assume to arrive at multi-hop WiMAX network to a Poisson distribution with arrival rate λ = 0.3. Table 1 summarizes the parameters for analysis. Fig. 6 to Fig. 10 illustrates the result of the multi-hop relay analysis based on the tandem queue analysis for WiMAX. Fig. 6 demonstrates the average polling delay analysis of different NOP ranging from 1 to 11 under various numbers of MSs ranging from 20 to 100.

TABLE I. PARAMETER FOR ANALYSIS

Parameters Values

Number of polls (NOP), 1 ~ 20

Numbers of mobile station (MS) 1~ 120

Packet transmission probability, 0.5 ~ 0.9

Frame periods, Tf 5 ms

Data rates, R 50 Mbps

Total number of slots in frame, Lf 2500

Total number of slot in DL & UL sub-frame 1250

Polling Bandwidth slots 3

Required bandwidth slots 32

MS mean service time, µ 0.5

The performance of the system which the average polling delays of analysis over different number of polls ranging from 1 to 11 under various numbers of MSs ranging from 20 to 100. As shows in Fig. 7, the average polling delay with the same numbers of MSs decrease when the NOP are increase. Furthermore, average polling delay of analysis for MMR as similar to the standard WiMAX as in [6], where the number of MS increase concomitant with the average polling delay. On contrast, average polling delay decrease exponentially when the NOP increase. Thus it’s shown that

325

NOP acts as an important factor to the multi-hop relay WiMAX networks. There also shown that in the high number of polls may increase since number of MSs added in system. The throughput for MMR analysis is shown in Fig. 8. Here we set transmission probability to be 0.5 which is moderate traffic load. Under this condition, the throughput of the same number of MS increases when the NOP is increasing. This is due to higher NOP indicate higher successful transmission rate. We observed that when increase the packet transmission probability the value of throughput also increase. Higher the probability means more traffic in the network.

No. of MSs

20 40 60 80 100

Aver

age

pollin

g de

lay

0.0

0.2

0.4

0.6

0.8

1.0

1.2

NOP=1NOP=3NOP=5NOP=7NOP=9NOP=11

Fig. 6. Average polling delay vs no. of MSs.

Fig. 7. Average polling delay of different NOP and no. of MSs.

From Fig. 9 we examine the throughput is slight decrease when the number of mobile station increases, but very too small changes. For instance as NOP=1 the throughput is

268.060 Kbps when number of MS is 5 and 268.015 Kbps for number of Ms is 40. Furthermore throughput is saturation when it reaches number of MS for 75 and above. That means the throughput is no effect after one point maximum number of MS.

5 10 20 40 75 100

No. of MSsTh

roug

hput

(kbp

s)

0

100

200

300

400

500

NOP=1NOP=3NOP=5NOP=7NOP=9NOP=11

Fig. 8. Throughput of analysis of different MSs

V. CONCLUSIONS Mobile multihop relay (MMR) WiMAX network is adopted to extend cell coverage, to enhance link throughput and increase network reliability, while supporting service requirement. In this paper we introduced the queuing analysis for MMR based on tandem queue and investigated the performance of network. In polling scheme the number of polls is an important parameter that takes notice based on the total number of mobile station. The performance is presented in term of average polling delay and throughput by varying number of poll and number of mobile station. As a future work, the investigation for polling algorithm in MMR which can provide bandwidth efficiency, decrease polling overhead and substantially manages the delay bound.

326

REFERENCES

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