A Distributed Cooperative MAC Protocol for QoS Improvement … · A Distributed Cooperative MAC Protocol 1145 the WiMedia MAC supports a distributed MAC approach, it is suitable for

Wireless Pers Commun (2013) 69:1143–1164DOI 10.1007/s11277-012-0625-3

A Distributed Cooperative MAC Protocol for QoSImprovement and Mobility Support in WiMediaNetworks

Jin-Woo Kim · Kyeong Hur · JangWoo Kwon

Published online: 18 April 2012© Springer Science+Business Media, LLC. 2012

Abstract A Distributed Medium Access Control (D-MAC) protocol based on UWB forhigh-rate Wireless Personal Area Networks is specified by the WiMedia Alliance. D-MACprotocol is suitable for ubiquitous connection in home networks, military/medical applica-tions due to its inexpensive cost, low power consumption, high data rate, and distributedapproach. In contrast to IEEE 802.15.3, D-MAC makes all devices have the same function-ality. And its networks are self-organized and provide devices with functions such as accessto the medium, channel allocation to devices, data transmission, quality of service and syn-chronization in a distributed manner. D-MAC fundamentally removes the problems of thecentralized MAC approach revealed at IEEE 802.15.3 MAC by adopting a distributed archi-tecture. However, the current D-MAC can’t prevent QoS degradations, occurred by mobilenodes with low data rate due to bad channel status, which cause critical problems in QoSprovisioning to isochronous streams and mobile applications. Therefore, we propose a dis-tributed cooperative MAC protocol for multi-hop WiMedia networks using virtual MIMOlinks. Based on instantaneous Channel State Information among WiMedia devices, our pro-posed protocol can intelligently select the transmission path with higher data rate to provideadvanced QoS with minimum delay for real-time multimedia streaming services.

J.-W. KimResearch Institute of Information Science and Engineering, Mokpo National University,Dorim-ri, Cheonggye-myeon, Muan-gun, Jeollanam-do, Mokpo 534-729, Koreae-mail: [email protected]

K. Hur (B)Department of Computer Education, Gyeongin National University of Education,Gyesan-Dong San 59-12, 45 Gyodae-Gil, Gyeyang-Gu, Incheon 407-753, Koreae-mail: [email protected]

J. KwonDepartment of Computer and Information Engineering, INHA University,100 Inha-ro, Nam-gu, Incheon 402-751, Koreae-mail: [email protected]

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1144 J.-W. Kim et al.

Keywords UWB · Cooperative MAC · Distributed MAC · WPAN · WiMedia ·Home networks

AbbreviationsUWB Ultra Wide-BandD-MAC Distributed Medium Access ControlDRP Distributed Reservation ProtocolWPAN Wireless Personal Area Network

1 Introduction

The Ultra Wide-Band (UWB) technologies are being feverishly developed in the technicalcommunity and will enable extremely high rate, short-range wireless networks. UWB devicesare expected to operate at rates of up to 0.5 Gb/s and communicate with other devices at a shortrange communication, thus enabling high-speed wireless personal area networks (WPANs).Currently, the general applications of UWB networks replace all the USB wires connectedbetween PC and various external devices (e.g., printer, monitor, scanner, keyboard, externalmass storage device, etc.) and remove all wires that connect all devices configuring the homenetworks such as HDTV, DVD player, home theater system, etc.

The MAC for the high rate WPAN can be designed in two approaches of the centralizedapproach and the distributed approach. One example of the centralized MAC approach isIEEE 802.15.3 protocol [1]. The MAC makes devices form a piconet which consists of apiconet coordinator (PNC) and several devices. A PNC provides devices with functions toadmit the network access of a device, to allocate a channel (time slot) to transfer data toanother device, and to synchronize in its own piconet. The centralized architecture of IEEE802.15.3 can cause several problems in mobile environment. Because the PNC controls mostnetwork operations, the disappearance of PNCs by mobility or device failure causes severeproblems. In that case, rest devices in WPAN should re-elect a new PNC, which wastes lotsof time and energy. As a result, the quality of service (QoS) of all data streams cannot beguaranteed during the PNC re-election procedure. It can be more serious problems to isoch-ronous streams. Moreover, as the IEEE 802.15.3 protocol is based on a centralized TDMA,overlapping among two more piconets can degrade the network performance significantly.When two devices connected to different PNCs are within the range of each other due tomobility or radio environment changes, and unfortunately use the same time slots, corre-sponding PNCs may be not aware of the overlapping piconets since the PNCs are not withinthe range of each other and also not within the range of the interfering device in the otherpiconets. Consequently, the centralized MAC approach in WPANs has critical problems inQoS provisioning to isochronous streams and mobile applications.

The WiMedia Alliance has specified a Distributed Medium Access Control (D-MAC)protocol based on UWB for WPANs [2]. The WiMedia D-MAC protocol is suitable forubiquitous connection in home networks, military/medical applications, etc due to its inex-pensive cost, low power consumption, high data rate, and distributed approach. In contrastto IEEE 802.15.3, D-MAC makes all devices have the same functionality, and networks areself-organized and provide devices with functions such as access to the medium, channelallocation to devices, data transmission, quality of service, synchronization in a distributedmanner. D-MAC fundamentally removes the above three problems of the centralized MACapproach revealed at IEEE 802.15.3 MAC by adopting the distributed architecture. Because

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A Distributed Cooperative MAC Protocol 1145

the WiMedia MAC supports a distributed MAC approach, it is suitable for wireless meshnetwork.

Meanwhile, multi-input multi-output (MIMO) antenna system can increase the channelcapacity and reliability and reduce transmission energy in fading and time-varying channel[3]. However, it is infeasible to integrate multiple antennas on a small mobile device due toits cost and size constraint. Thus, the cooperative communication is proposed to construct avirtual MIMO system that multiple neighboring nodes configure a virtual antenna array inrecent years. A combination of intelligent cooperation among mobile nodes and distributedspace time block coding (DSTBC) scheme can significantly improve the network perfor-mance such as reliability, throughput, and energy-efficiency. The existing wireless systemssupport multiple data rate according to channel conditions. This multiple data rate capabilitysupported by physical layer is generally exploited by direct rate adaptation scheme. An effi-cient rate adaptation scheme should not only obtain the accurate channel state information(CSI), but also respond to the time-varying channel rapidly. Direct rate adaptation scheme canbe divided into two categories (i.e. sender-based and receiver-based). The auto rate feedbackprotocol [4] is sender-based rate adaptation scheme that adjust the next transmission rateusing the history of previous transmissions. A receiver-based auto rate (RBAR) protocol [5]is proposed in which receivers estimate the instantaneous CSI through measuring the signalstrengths of the modified control frames and determine the transmission rate. However, theseprotocols would not help much when direct channel condition between sender and receiveris poor.

CoopMAC and rDCF [6,7] protocols based on the IEEE 802.11 DCF were proposed toreduce the throughput degradation occurred by mobile nodes with low data rate. However,these protocols can’t exploit the diversity and multiplexing advantage of MIMO systemsand cooperative protocol. Since the existing cooperative MAC protocols [6–18] are mostlyproposed for only one-hop network, they don’t consider the interference that is occurred bymultiple transmission pair or mobile nodes in multi-hop network environment. Also, sincethey are contention based protocol, they are not suitable for isochronous service such asmultimedia streaming.

Therefore, the motivation of our research is to design a cooperative D-MAC protocolwhere the central network coordinator doesn’t exist. Proposed cooperative D-MAC proto-col adaptively selects suitable transmission modes and relay nodes according to the channelquality at each transmit/receive node pair to maximize the WiMedia system throughput.Most importantly, cooperative D-MAC protocol is backward-compatible with the WiMediaD-MAC standard [2] so that it can coexist with traditional WiMedia networks.

The key features of our proposed cooperative D-MAC protocol include as following.Firstly, this protocol is fully distributed and does not depend on global time synchronizationamong nodes and the central network coordinator which controls the entire network. Thisfeature simplifies the network operation complexity. And both the transmission path andits transmission mode are decided by MAC layer in the cooperative D-MAC. But, they aretypically decided at the routing layer. By this way, our proposed protocol can be fast andadaptive to high-mobility environment. Lastly, proposed protocol also prevent the interfer-ence occurred by broadcast nature in wireless multi-hop networks. This feature overcomesa significant shortcoming of traditional cooperative approaches designed for single hop net-works. The cooperative schemes designed for single hop networks increase the collisionand the cost by retransmission between relay nodes when they are applied into multi-hopenvironment.

In this paper, we proposed the cooperative D-MAC protocol through an optimal relaydecision process to avoid the interference at the multi-hop UWB network environment. And

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we focused on distributed reservation protocol (DRP) in the WiMedia standard that canguarantee QoS for real-time multimedia services [2]. Thus we proposed a new CoopDRPIE (Information Element) conveyed in beacon frames. And then, by using the CoopDRPIE, a new cooperative DRP negotiation process is executed at each device to select a datatransmission path with higher data rate and minimum interference, through the optimal relaydecision process. The remainder of this paper is organized as follows. The related work isdescribed in Sect. 2. In Sect. 3, we presented background and problem description. Detailedprotocol description for our proposed protocol is given in Sect. 4. Performance Analyses ofproposed protocol are provided in Sect. 5. Finally, Sect. 6 concludes this paper.

2 Related Works

Recently, extensive research has been carried out for the physical layer of cooperative com-munication [8–14] by researchers in information theory and communication communities.In [13], the error probability performances of three multiuser detection algorithms, namelythe conventional detector, the linear minimum mean square error (MMSE), and the adap-tive MMSE are evaluated and compared. It was shown that the linear MMSE offers slightlybetter performances than the conventional one. The adaptive MMSE eliminates the needfor on-line computation of the impulse responses of the matched filters. The performancesachieved by the adaptive MMSE are not as good as the ones obtained by the linear MMSE,but still better than the ones of the conventional receiver. In [14], a turbo encoding tech-nique used in a CDMA multiuser system with conventional decoding and showed that it isefficient when the receiving is achieved in perfect conditions (equal amplitude uncorrelatedusers). In this case the turbo encoding compensates the effects the other users, the resultsobtained being close to a single user system. Also it was shown that if the cross-correlationbetween users is not very large, the turbo encoding is able to partially correct part of theerrors. However, many researches are still needed for practical high layer protocol to realizeeffective cooperative communications. Cooperative communications require many uniquefeatures in MAC, which should be distributed and cooperative for a wireless multi-hop net-work environment.

In [6,7], two similar protocols, called CoopMAC and rDCF, based on the IEEE 802.11DCF was proposed to mitigate the throughput bottleneck caused by low-data-rate nodes. Forlow data rate nodes, they allow a high-rate node to help a low-rate node through two-hoptransmission. However, two protocols using table-based proactive relay selection scheme isnot suitable for time varying channels. In [15], a multi-layer approach has been proposed,which configure virtual multiple-input single-output (VMISO) link to a receiver on the rout-ing table and as far as possible to the transmitter. Also, distributed space-time block coding(DSTBC) is utilized to support transmission over a long distance in mobile ad hoc networks.In [16], cooperative MAC (CMAC) was proposed to minimize the energy consumption inthe network by selecting the relays based on the ranging information. One clear shortcomingis the overhead incurred by the large number of exchanged coordination packets. In [17], tofully take advantage of cooperative diversity, maximum ratio combiner (MRC) is utilized bythe receiver to combine the data packets transmitted by sender and relay.

In [18], a novel cooperative relay-based auto rate MAC protocol (CRBAR) was proposedto adapt to dynamical channel variation and network topology. In [18], the relay candidatesadaptively select themselves as the relay nodes and determine the relay scheme and transmis-sion rates based on the instantaneous channel measurements. Also, the receiver can improveits capability to decode the original frame by combining the information from both the sender

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and the relay. Similar to CRBAR, a cooperative triple busy tone multiple access (CTBTMA)[19] scheme was proposed.

Adaptive modulation and coding (AMC) and multimode transmission are scheduledtogether according to the channel condition to improve the network throughput. The useof busy tones helps to solve collisions in a cooperation scenario and to address the optimalhelper selection problem.

In similar to CTBTMA, protocols proposed in [20,21] let neighbor(s) (overhearing thepacket) retransmit the packet instead of the source node when the NACK is detected. In [20],multiple relays operating over orthogonal time slots was proposed based on a generalizationof hybrid-automatic repeat request (ARQ) that do not need retransmitted packets to comefrom the original source radio but could instead be sent by relays that overhear the trans-mission. In [21], Zhao et al. proposed a node cooperative automatic repeat request (ARQ)scheme for wireless ad hoc networks, which is suitable for mobile wireless channels withhigh and correlated frame-error profile.

In [25], a novel fully adaptive distributed cooperative medium access control (ADC-MAC) protocol is proposed for vehicular networks. ADC-MAC leverages new handshak-ing messages such as HRTS (Helper-Request-To-Send) and HCTS (Helper-Clear-To-Send),which are used for cooperative relay activity coordination. With this RTS-CTS-HRTS/HCTStriangular handshake, the transmit/receive pair can now choose the most suitable helperfor assistance during data transmissions. Cooperative Automatic Repeat reQuest protocol(C-ARQ) is proposed in [26]. Since C-ARQ initiates the cooperative transmission only whenthe direct transmission fails, unnecessary occupation of channels by relay nodes and wasteof system resources are avoided. However, these researches focuses on one hop networks.Thus, they can’t avoid the interference occurred by broadcast nature in wireless multi-hopnetworks.

Recent studies on cooperative scheme mainly focused on the narrow band cases with cen-tralized implementations. These proposed schemes, when applied for distributed implemen-tations, are not straightforward. In this paper, we propose a distributed cooperative schemeconsidering wireless multi-hop network in mobile environment.

3 Background and Problem Description

The proposed protocol is an efficient MAC scheme that makes use of MAC layer cooperationfor reliable communication. Before explaining the proposed protocol, this Section describesbackground and problem description to understand WiMedia protocol and proposed protocol.Section 3.1 explains the WiMedia protocol structure. Section 3.2 explains the acknowledge-ment scheme assumed in this paper. Section 3.3 explains problem occurred by broadcastnature in wireless multi-hop networks.

3.1 WiMedia Structure

D-MAC operates per a time unit of a superframe. The superframe has a fixed length of time,and it is split into a plurality of time windows called time slots. The time slot is also calleda medium access slot (MAS). The superframe consists of 256 MASs. The length of thesuperframe is 65.536 ms, and the length of each MAS is 256 µs.

In Fig. 1, each superframe starts with a beacon period (BP), which extends over one ormore contiguous MASs. A BP consists of beacon slots, and each device sends its own beaconin a non-overlapping beacon slot with others. Thus, devices need to search free beacon slots

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Beacon Period

MAS

Data Period

Superframe

Beacon Slot

Beac

on

Bea

con

Bea

con

Bea

con

Beac

on

…

DRP Reservation

Block

DRP Reservation

Block

PCABlock

PCABlock

Fig. 1 Superframe structure in D-MAC

that are unused in the beacon period so as to send their beacons. The remainder of MASs inthe superframe are used to transfer data, and it is called a data period. The length of a BP maybe less than that of a data period. A data period is divided into two types of MAS blocks. Acontention-based protocol works during the one MAS block, and a reservation-based proto-col works during another MAS block. The contention-based protocol is known as PrioritizedContention Access (PCA), and it is similar to IEEE 802.11e for multiple prioritized classes.The PCA provides differentiated and distributed contention access to the medium for fouraccess categories (ACs) in a device for asynchronous traffic transmissions. The PCA offersdifferentiated priorities to the four ACs for CSMA/CA-based medium access, respectively.

The reservation-based protocol is the Distributed Reservation Protocol (DRP). The DRPenables devices to reserve one or more MASs that the device can use to communicate withone or more neighbors. All devices which use the DRP for data transmission or receptionshould announce their reservations by including DPR IEs in their beacons. Reserved MASsmean the set of MASs in which the DRP provides reservation owner devices with exclusiveaccess to the medium. Since DRP is a contention free channel access scheme, it has theimportant role to guarantee the QoS to isochronous traffic such as real-time streaming. It isused by devices to negotiate and reserve bandwidth. Also, the DRP enables devices to reserveone or more MASs (Media Access Slot-each 256 µs) that the device can use to communicatewith one or more neighbors. A reservation of MASs guarantees a period of time for transmis-sion during which the reservation owner has exclusive access to the medium. A device thatwishes to establish a reservation negotiates the channel time with its communication peer.There is no need for a central entity that controls the reservation process. In DRP, a devicecan only establish a reservation during the MASs that are not being used by another existingreservations. All devices that use the DRP for transmission or reception shall announce theirreservations by including DRP IEs (Information Elements) in their beacons.

3.2 Block Acknowledgement (B-ACK)

WiMedia MAC protocols can use three acknowledgement schemes: no acknowledgement(No-ACK), immediate acknowledgement (Imm-ACK) and block acknowledgement (B-ACK). In the case of No-ACK scheme is vulnerable to packet loss. In Imm-ACK, everyframe is acknowledged with an ACK frame after a short inter frame space (SIFS) duration.Acknowledgement time out takes place similar to automatic repeat request (ARQ) mecha-nisms. However, Imm-ACK mechanism causes the mount of overhead to acknowledge a burstof received traffic such as multimedia streaming. To mitigate this overhead inefficiency, aB-ACK scheme has been proposed in WiMedia standard and IEEE 802.11e standard [22–24].

The B-ACK mechanism allows a source device to transmit multiple frames and to receivea single acknowledgement frame from the recipient indicating which frames were receivedand which need to be retransmitted. The efficiency of the B-ACK scheme comes from the

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Frame 1 Frame 2 Frame 3 Frame 4

B-ACK

MISFSIFS

DRP reservation block (TDRP)

SIFS

Fig. 2 Example of some frame transactions according to B- ACK policy

S1

D1

R11

R12

S2

D2

R21

R22

Data

Data

Data

Data

ACK

ACK

Fig. 3 Example of interference occurred by adjacent transmission pair

fact that the overhead is greatly reduced, because only one ACK is used for all the frames inreserved transmission duration. Thus, I apply B-ACK policy to our protocol. Figure 2 showssome frame transactions according to B-ACK scheme in DRP reservation block.

3.3 Problem Description

Since most of the existing researches mainly focus on single transmission between source anddestination in single hop network, they don’t consider the interference occurred by anothertransmission pairs or mobile node. In practical network environment, however, since severalWPANs coexists in the same space or data transmission is carried out in multi-hop network,multiple sources transmit to multiple destinations simultaneously. In this case, interferencewill occur and greatly decrease the link quality. Figure 3 shows the example of interferenceoccurred by adjacent transmission pair. In Fig. 3, source node S1 determines the relay nodeR12 among nodes that enable relaying the data frame without consideration of another trans-mission pair. If Source node S1 and another source node S2 simultaneously transmit the dataframe to their own destinations, the interference occurs between relay nodes R12 and R21.Also, there can occur the collision by node’s mobility. Figure 4 shows the collision occurredby node’s mobility.

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S1 D1

R1

S2

D2

R2

Data Data

Data

Data

ACK

ACK

Fig. 4 The example of the collision occurred by node’s mobility

In Fig. 4, when relay node R2 moves in radio range of destination node D1, the colli-sion can occur between D1 and R2. These interference and collision can greatly degrade thethroughput and reliability of network. Thus, we propose the cooperation scheme consideringthe mobility and the interference by adjacent nodes in this paper.

4 Protocol Description

In this paper, we provide details of the distributed cooperative MAC protocol. The proposedprotocol utilizes new CoopDRP IE and MAS Availability IE, and is needed a new cooperativeresource reservation procedure and relay decision scheme.

4.1 Relay Decision

WiMedia Specifications provides the Link Feedback IE that advertises information on thedata rate and transmit power level of neighboring nodes. Since all devices in WiMedia net-work include the Link Feedback IE in its own beacon frame, they can be used to select apotential relay node that can relay data transmission. Figure 5 shows the frame format of theLink Feedback IE.

The DevAddr field is set to the address of the source device for which the feedback isprovided. The Transmit Power Level Change field is set to the change in transmit power levelthat the recipient recommends to the source device. The Data Rate field is set to the data ratethat the recipient device recommends that the source device use.

After archiving all information for data rate between neighbor device and target device,source device determine the transmission scheme spending the minimum time. In this paper,we will use three transmission schemes: direct transmission (DT), relay transmission (RT),cooperative transmission (CT). Figure 6 shows the example of three transmission schemes.

In Fig. 6, if the link quality between source node S1 and destination node D1 is goodenough, DT is employed. If the link quality between S1 and D1 is poor and the link qualityof intermediate node, R1 is good enough, R1 is selected as relay node and RT is employed.Also, if the link quality between S1 and D1 is poor and the link quality of adjacent nodes R11

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Element IDLength (=3*N)

Link 1 Link N

Data RateDevAddrTransmit Power Level

Change

1 octet 1 octet 2 octets 2 octets

b0~b15 b16~b19 b20~b23

Fig. 5 The frame format of the Link Feedback IE

S1 D1Data

ACK

S1 D1

R11

R12

Data Data

ACK

S1 D1

R1

Data Data

ACK

Data Data

(a)

(b)(c)

Fig. 6 The example of three transmission schemes. a Direct transmission (DT), b relay transmission (RT), ccooperative transmission (CT)

and R12 is good enough, R11 and R12 are selected as relay nodes and they transmit distributedspace time coded data frames to destination node D1 simultaneously.

To determine transmission scheme, source node have to calculate the total transmissiontime to transmit data frames from source to destination. Their transmission times are calcu-lated as follows:

TDT = Nf · 8Lh

Rmim+ Nf · 8Lp

Rdirect+ (Nf − 1) · TMISF + TSIFS (1)

TRT = 2 · Nf · 8Lh

Rmin+ Nf · 8Lp

RSR+ Nf · 8Lp

RRD+ 2 · {(Nf − 1) · TMISF + TSIFS} (2)

TCT = 2 · Nf · 8Lh

Rmin+ 2 · Nf · 8LP

min(RSR, RRD)+ 2 · {(Nf − 1) · TMISF + TSIFS} (3)

where Nf is the number of data frame and Lh and Lp are the size of MAC header and payload.Also, Rmin is the minimum data rate supported by WiMedia PHY and Rdirect is the data ratefor direct transmission between source and destination. RSR and RRD are the data rate fromsource to relay, and from relay and destination. And the TSIFS and TMISF are interframe space

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Element ID Length (=32) 2-hop DRP Availability Bitmap

1 octet 1 octet 0~32 octets

Fig. 7 A format of proposed 2-hop DRP availability IE

interval defined by the WiMedia specification. Since DSTBC scheme transmits copies of datausing the same modulation scheme and the same data rate for data combining in receiver node,relay nodes must transmit data frames using the same data rate in CT. Before the source nodereserves wireless medium to transmit data frames to destination node, it decides the trans-mission scheme to enable to reduce the total transmission time. If the link quality betweensource node and destination node is good enough and the total transmission time cannot bereduced via relay node compared to the direct transmission, i.e. TDT >max(TCT, TRT) directtransmission will be adapted. If source node finds that the total transmission time can bereduced via relay node compared to direct transmission and a higher data rate or a equaldata rate between relay node and destination node can be supported compare to a data ratebetween source node and relay node, i.e. TCT > TRT, it will be adapted the CT. Otherwise,it will decide the RT.

Once source node decides the transmission scheme, it carries out the resource reservationprocedure.

4.2 Resource Selection Considering Interference and Mobility

WiMedia specification provides the DRP Availability IE to indicate its view of the currentutilization of resources. WiMedia nodes can be aware of existing neighbors’ resource utili-zations through the DRP Availability IE included in beacon frame. However, since the DRPAvailability IE can’t reflect the mobility or interference occurred by devices out of communi-cation range, I need new resource selection scheme considering the mobility and interference.In previous section, we proposed the 2-hop DRP Availability IE generated by receiving andcombining all DRP Availability IEs and DRP IE from all neighbor devices in communicationrange. The 2-hop DRP Availability IE depicted in Fig. 7 includes a bitmap field of 256 bitslong, one bit per each MAS in a superframe. If the corresponding resource is available for aresource reservation in 2-hop range area from a device, each bit is set to ‘one’, otherwise itis set to ‘zero’.

Since nodes can be aware of the information of resource utilization out of communica-tion range through proposed 2-hop DRP Availability IE, it can select the safe channel frominterference and mobility of outside nodes. Figure 8 shows the scenario of resource selectionconsidering mobility and interference from outside nodes.

In Fig. 8a, source node S2 can’t be aware of the information of channel utilization usedby transmission link between S1 and D1 since they communicate with each other out of S2’srange. If S2 decides the relay node R22 regardless of interference or mobility from D1, thelink quality between S2 and D2 greatly degrades. However, if source node S2 can obtain theinformation of channel utilization used by S1 and D1, it can decide the relay node that isn’tinfluenced by interference and mobility of node D1. Figure 8b, c shows the example of relayand channel selection considering mobility and interference of node D1. In Fig. 8b, if thetotal transmission time via relay node R22 is smaller than total transmission time via relaynode R21, source node S2 selects R22 as relay node and reserves channel difference from the

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R22

S2

R12

R11

D2

D1 IS1 R21

Data

ACK

Data

2-hop DRPAvailability IE

DRPAvailability IE

R22

S2

D2

I R21 R22

S2

D2

I R21

Data

Data

ACKACK

Data

Data

(a)

(b) (c)

Fig. 8 The scenario of resource or relay selection considering mobility and interference. a The propagationof resource utilization information, b the example of resource selection considering mobility and interference,c the example of relay selection considering mobility and interference

ReservationType

StreamIndex

ReasonCode

ReservationStatus

OwnerConflict

Tie-breakerUnsafe Reserved

b3~b5 b6~b8 b9 b10 b11 b12 b13

Zone Bitmap MAS Bitmap

2 octets 2 octets

LinkFeedback

type

Element ID Length DRP Allocation #Priority

1 octet1 octet 1 octet

DestinationDevAddr

2 octets 2 octets 4 octets

RelayDevAddr 1

RelayDevAddr 2DRP Control

2 octets 2 octets

b0~b2

Offset

1 octet

b15~b16

Fig. 9 The format of CoopDRP IE

channel used by transmission pair via R11. If the total transmission time via relay node R22

is equal to total transmission time via relay node R21 or available channel is insufficient, S2

will select R21 as relay node.

4.3 Resource Reservation Procedure

If source node selects DT, it carries out DRP negotiation procedure defined by WiMedia Spec-ifications. However, if cooperative communication is determined, source node constructs theCoopDRP IE. Figure 9 shows the format of CoopDRP IE.

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Table 1 Reservation type fieldencoding

Value Reservation type

0 Alien BP

1 Hard

2 Soft

3 Private

4 PCA

5 Relay transmission

6 Cooperative transmission

7 Reinforcement

Table 2 Reason code fieldencoding

Value Code

0 Accept

1 Conflict

2 Pending

3 Denied

4 Modified

5 Canceled

6 Warning

7 Reserved

The Reservation Type field is set to the type of reservation and is encoded as shown inTable 1.

In Table 1, encoding values 0–4 are defined by WiMedia standard, and we will use thereserved value for CoopDRP IE. The Stream Index field identifies the stream of data to besent in this reservation. The Reason Code is used by a reservation target to indicate whethera DRP reservation request was successful or not, and it is encoded as shown in Table 2. ThisReason Code is set to ‘zero’ (i.e., ‘Accepted’), in a CoopDRP IE sent by a source node duringnegotiation. Table 2 shows the Reason Code encoding.

The Reservation Status bit shows the status of the resource reservation process. The Res-ervation Status bit is set to ‘zero’ in a CoopDRP IE for a reservation that is under negotiationor in conflict, while it is set to ‘one’ by a node granting or maintaining a reservation referredto as an established reservation. The Owner bit is set to ‘one’ if the device transmitting theCoopDRP IE is a source node, or to ‘zero’ if the device transmitting the CoopDRP IE is adestination node or relay node. The Conflict Tie-breaker bit is set to a random value of zeroor one when the reservation request is made. For all CoopDRP IEs that represent the samereservation, the Conflict Tie-breaker bit is set to the same value. The Unsafe bit is set to ‘one’if any MASs identified in the DRP Allocation fields are considered in excess of reservationlimits. Link Feedback Type field indicates the type and format of link feedback that the nodeshall provide in its control frames. The Priority field indicates the priority to guarantee theQoS of application such as multimedia streaming. The Offset field indicates the beginning ofMAS reserved for Destination node. The Relay DevAddr field indicates the address of Relaynode. If the selected transmission scheme is RT, the Relay DevAddr 2 field is reduced. TheDRP Allocation field contains the information of reserved MAS for RT or CT.

After constructing the CoopDRP IE, source broadcasts its own beacon frame includingthe CoopDRP IE. It sets the Reservation Status bit to zero and the Reason Code to Accepted

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in the CoopDRP IE. For new streams, the Stream Index is set to a value that is currentlynot used with Destination DevAddr and Relay DevAddr. To reserve additional resource foran existing stream, the Stream Index is set to the value used for the existing stream. Onreception of a CoopDRP IE, relay and destination nodes set the Owner bit in DRP Controlfield to zero. If a resource reservation is granted, destination node and relay node set theReservation Status bit to one and the Reason Code to Accepted. If the resource reservationcannot be granted due to an overlap with resource used by other transmission pair, destinationor relay node set the Reservation Status bit to zero and the Reason Code to Conflict. If theresource selected by source node overlaps with resource used by transmission pair out ofrange of node, node sets the Reason Code to Warning. Then they include DRP AvailabilityIE and Resource Alloaction IE in its own beacon. All devices that receive the CoopDRP IEare aware of devices associated with cooperative transmission and utilize resources exceptreserved resources. On reception of a DRP Availability IE and Resource Allocation IE in abeacon, the source node selects non-overlapped resources based on the resource reservationinformation provided by DRP Availability IE and Resource Allocation IE. After selectingnon-overlapped resources, the source node transmits the beacon including the CoopDRP IEthat describes the proposed reservation. To confirm the resource reservation, the source nodesets Reservation Status bit to one in the CoopDRP IE in its beacon after receiving a beaconfrom the destination node and relay node that contain a corresponding CoopDRP IE withReservation Status bit set to one. To terminate a resource reservation, the source node setsthe Reservation Status bit to zero and the Reason Code to appropriate value. Then the sourcenode removes the CoopDRP IE from its beacon frame. The flow charts at source node S,destination node D, and relay node R are shown in Figs. 10, 11, and 12.

4.4 Resource Reinforcement According to Link State

While transmission is proceeding via relay node, link quality at relay node can be degradeddue to the mobility of relay node or obstacle. To maintain the QoS of multimedia stream-ing, source node needs to configure new path. Since the information of link quality can beobtained through the Link FeedBack IE, source node can sense a change of link quality atrelay node and search a potential relay node that can provide better link quality. Once itsenses a potential relay node to provide higher data rate, source node transmits the Coop-DRP IE set the Reservation Type field in DRP Control field to Reinforcement and RelayDevAddr field to the address of potential relay node. At this time, source node equally setsup the DRP Allocation field in CoopDRP IE with resource used by current transmission link.The resource reservation procedure for reinforcement is same with the resource reservationprocedure mentioned in the previous subsection. If the resource reservation request for rein-forcement is confirmed, the existing transmission link is discarded and newly configuredtransmission link is utilized. Figure 13 shows the example of reinforcement.

5 Performance Evaluation

It will be shown in the following that proposed protocol always outperforms legacy WiMe-dia protocol with the given parameter setting in various considered scenarios except whenonly one potential relay exists. In fact, the spatial diversity among potential relays cannotbe exploited with no choice of the relay. For simplicity, the diversity gain Dgain is assumedto be 5 dB for all the modulation schemes. In simulations, the source nodes always havedata packets to send. A WiMedia network is considered with three rates available: 53.3, 160

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Idle

TransmitData?

Calculate the TotalTransmission Time

RelayAvailable?

Transmit DRP IETransmit

CoopDRP IE

Reason Code from D== Accept?

Transmit Data

Reason Code from D== Accept?

Reason Code from R== Accept?

Reason Code from R ==Warning?

Reason Code from R== Conflict?

Available anotherresource?

Yes

Yes

YesYes

YesYes

Yes

Yes

Select the new relaynode

No

NoNo

No

No

No

No

Fig. 10 Flow chart at source node S

and 480 Mbps. The frame arrival rate at each sender is high enough to saturate the networkwith a payload size 4,095 bytes. The distance between each sender-receiver pair is set to be20 m. The number of flows is set to be 5. The transmission power of a device is fixed to−41.25 dBm/MHz and the packet size transmitted in a beacon group is fixed to 2,048 bytes[2]. In the WiMedia D-MAC performance analysis, WiMedia PHY/MAC parameters in theWiMedia standard [2] are considered and are found in Table 3.

Figure 14 shows the simulation result of our proposed protocol. As shown in Fig. 14, theproposed scheme outperforms the direct rate adaptation scheme of WiMedia protocol. The

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Fig. 11 Flow chart at relay nodeR Receive the resource

reservation request

Set the Reason Code toAccepted

Is requestedresource overlapped?

Transmit the replyTransmit the DRPAvailability IE and

Resource Allocation IE

Set the Reason Code toConflict

Yes

No

Receive the resourcereservation request

Set the Reason Code toAccepted

Is requestedResource overlapped

with neighbors?

Transmit the reply

Transmit the DRPAvailability IE and

Resource Allocation IE

Set the Reason Code toConflict

Yes

No

Is requestedResource overlappedwith outside node?

Set the Reason Code toWarning

Yes

No

Fig. 12 Flow chart at destination node D

transmissions via two high-rate hops can deliver faster when the direct channel condition canonly support low-rate transmissions on average. Furthermore, the proposed scheme signif-icantly increases the throughput compared to legacy WiMedia protocol with an increasingnumber of potential relays since they can exploit the spatial diversity of the potential relays.

We investigated throughput performance of proposed protocol according to variable nodedensities and packet sizes. It is shown in Fig. 15 that with the increase of node density, thecooperative D-MAC protocol increases its throughput significantly. As is explained before,

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R22

S2

D2

R21ACK

Data

Data

Coo

pDR

PIE

R22

S2

D2

R21ACK

Data

Data

Fig. 13 The example of reinforcement

Table 3 WiMedia PHY/MACparameters

Parameter Value

Symbol length 312.5 ns

Preamble length Standard preamble: 9.375 µs

Header length 3.75 µs

MIFS 1.875 µs

SIFS 10 µs

Maximum frame payload size 4,095 octets

Maximum Beacon period length 96 beacon slots

Beacon slot length 85 µs

Superframe length 256 × MAS length

MAS length 256 µs

Limitation of total MAS legnth 112 MASs

Fig. 14 Throughput according to the number of potential relays

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Fig. 15 Effect of frame size

Fig. 16 Throughput according to wireless channel status (BER)

the cooperative D-MAC has the effect of interference avoidance. Increase of node density inFig. 15 means the increase of interference in multi hop networks. So our cooperative D-MACoperated more efficiently in Fig. 15. More specifically, as the number of nodes increases, thelikelihood of finding a better relay also increases. Figure 15 also shows how the throughputperformance of proposed protocol varies with each packet size. Throughput performance ofproposed protocol is improved in proportion to packet size because the BER (Bit Error Rate)is set to a small value of 10−5. Under a low BER, a large payload can be transmitted withmore bits in a single transmission that increases the throughput. However, as the payload sizein a packet increases, the frame error rate may be significantly increased at a high BER.

Throughput performance in the WiMedia D-MAC environment where twenty devicesoperate according to BER indicating current wireless channel status is shown in Fig. 16.As the channel status becomes worse, throughput is decreased. However, when using theproposed protocol, the throughput decrease is less than that of the legacy D-MAC protocol.In the result of proposed protocol, the throughput is more degraded than others at the period

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Fig. 17 Energy consumption comparison according to the number of devices

from BER 10−4 to BER 10−3. This result shows that there exists a threshold value where thecooperative D-MAC protocol cannot compensate the throughput decrease due to the harshwireless channel status even though it performs cooperative relay transmissions to find stablechannels.

Equation (4) explains the energy consumption required for data transmissions within asuperframe in WiMedia D-MAC system with No-Ack policy [27]. Ptx, Prx, and Pidle are thepower consumed at data transmissions, at data receptions, and at idle states within a super-frame, respectively. Ntx and Nrx denote the number of transmitted PSDUs (PHY Service DataUnit) and that of received PSDUs in a DRP reservation block. TPSDU denotes the requiredtime delay during transmitting or receiving a PSDU. TMIFS and TSIFS are time length ofMIFS (Minimum Inter-Frame Spacing) and SIFS (Short Inter-Frame Spacing) defined in theWiMedia D-MAC Standard [2], respectively. NDRP denotes the number of DRP reservationblocks in a superframe.

ESuperframe = [Ptx · TPSDU · Ntx + Prx · TPSDU · Nrx + Pidle · TMISF · (Ntx + Nrx)

+Pidle · TSIFS] · NDRP (4)

Figure 17 shows the ratio variation of EProposed_scheme/ELegacy_D−MAC according to the num-ber of WiMedia D-MAC devices. EProposed_scheme is the ESuperframe value of the proposedprotocol and ELegacyD−MAC denotes that value of the legacy D-MAC protocol. As shown inFig. 17, the proposed protocol shows the superior energy saving performance to the legacyD-MAC protocol. Furthermore, the ratio of the energy consumption decreases as the num-ber of WiMedia D-MAC devices increases. This result can be explained from a situationthat there occurs more interference causing retransmissions during communications betweenthe nodes as the number of nodes increases. In this case, by performing cooperative relaytransmissions via stable channels with few interference, energy consumption at each nodedecreases due to reduced retransmissions in the proposed protocol. Furthermore, becausemultiple relay nodes can share the role of relay transmission, the entire energy consumptionalso can be reduced the number of nodes increases.

Figure 18 shows the comparison of average delay performance comparison between pro-posed protocol and the legacy D-MAC standard. Delay for each packet transmission consistsof two parts, the queuing delay and the service delay. Service delay is defined as the time

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Fig. 18 Number of potential relay node versus service delay

Fig. 19 Throughput as a function of maximum speed v

from when a packet becomes the head-of-line packet in the queue to the time the packet isreceived. In this simulation, the packet length is set to 2,048 bytes. From Fig. 18, it is obvi-ous that the end-to end delay in proposed protocol is much shorter and stable in proportionto the node density. This result is caused by the reason that proposed protocol decides thetransmission path and its relay mode. So it can quickly respond to the channel change due tohigh-mobility or fast fading channel conditions.

The comparison of mobility performance between proposed protocol and the legacyD-MAC standard is shown in Fig. 19. Note that node mobility affects not only the loca-tion of a node but also the wireless channel condition of channel coherence time. In Fig. 19,we assumed a typical indoor environment with nodes moving at slow walking speed belowν = 1 m/s where the channel variation occurs frequently. In that situation, proposed coop-erative D-MAC can select adaptively the most suitable transmission path and its mode fordata transmission in accordance with the channel status between the transmit/receive nodes.Therefore, throughput of proposed protocol outperforms the legacy D-MAC standard.

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6 Conclusions

In this paper, we proposed the cooperative D-MAC protocol through an optimal relay deci-sion process to avoid the interference at the multi-hop UWB network environment. Based oninstantaneous CSI among WiMedia devices, our proposed protocol can intelligently selectthe transmission path with higher data rate to provide advanced QoS with minimum delayfor real-time multimedia streaming services. The key features of our proposed cooperativeD-MAC protocol include as following. Firstly, this protocol is fully distributed and does notdepend on global time synchronization among nodes and the central network coordinatorwhich controls the entire network. This feature simplifies the network operation complex-ity. And both the transmission path and its transmission mode are decided by MAC layerin the cooperative D-MAC. But, they are typically decided at the routing layer. By thisway, our proposed protocol can be fast and adaptive to high-mobility environment. Lastly,proposed protocol also prevent the interference occurred by broadcast nature in wirelessmulti-hop networks. This feature overcomes a significant shortcoming of traditional coop-erative approaches designed for single hop networks. The cooperative schemes designedfor single hop networks increase the collision and the cost by retransmission between relaynodes when they are applied into multi-hop environment. The simulation results show thatproposed protocol can enhance throughput performance and improve energy efficiency byavoiding interference. From the simulation results for throughput and energy consumption,it is shown that the performance of the proposed protocol is superior to that of the legacyWiMedia D-MAC protocol. And the proposed protocol is compatible and can be directlyapplied with small overhead to the current WiMedia standard system.

Acknowledgments This work was supported in part by Priority Research Centers Program through theNational Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology(2011-0022980) and in part by Basic Science Research Program through the National Research Foundationof Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (2010-0002366).

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Author Biographies

Jin-Woo Kim is currently an Research Professor in the Research Insti-tute of Information Science and Engineering at the Mokpo NationalUniversity. In 2011, he received his Ph.D. in Electronics and ComputerEngineering at Korea University, Seoul, Korea. His research interestsinclude WPANs, embedded-system, ad-hoc and sensor networking, andubiquitous computing.

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Kyeong Hur is currently an Associate Professor in the Department ofComputer Education at Gyeongin National University of Education,Republic of Korea. He was senior researcher with Samsung AdvancedInstitute of Technology (SAIT), Korea from September 2004 to August2005. He received a M.S. and Ph.D. in Department of Electronics andComputer Engineering from Korea University, Seoul, Korea, in 2000and 2004, respectively. His research interests include; computer net-work designs, next generation Internet, Internet QoS, and future All-IPnetworks.

JangWoo Kwon received the B.S. degree in electronic Eng. fromINHA University in 1990, the M.E. and Ph.D. degree in electronicengineering from INHA University in 1992 and 1996, respectively.In 1992 he was a visiting Researcher at Department of Biomedi-cal Engineering of Tokyo University, Tokyo, Japan. From 1996 to1998 he was a deputy director of Korea Industrial Property Office(KIPO) where his responsibility was to examine patents. From 1998to 2009 he was an Associate Professor of Department of ComputerEngineering at Tongmyoung University, Pusan, Korea. He had been aDean of Research Institute for Information Eng. Tech. at TongmyoungUniversity from 2002 to 2006. From 2010 to 2012 he was an Associ-ate Professor of Department of Computer Eng. at Kyungwon Univer-sity, Kyeong-gi Province, Korea. Since 2006, he has been a Director ofHuman Resource Development Division of National IT industry pro-motion agency of Korea. Currently, his research area is in sensor net-works and human computer interaction using biomedical signals. Forthe last 20 years he has been working in biomedical signal analysis andits recognition using artificial intelligence.

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A Distributed Cooperative MAC Protocol for QoS Improvement … · A Distributed Cooperative MAC Protocol 1145 the WiMedia MAC supports a distributed MAC approach, it is suitable for