Dynamic Channel Scheduling for UWB-Based WPAN

KHALED AMAILEF, JIM WUUniversity of Western SydneySchool of Computing and IT

Penrith South DC, NSW 1797 SydneyAustralia

kamailef@cit.uws.edu.au, jimwu@cit.uws.edu.au

MAHBUB HASSANUniversity of New South Walse

School of Computer Science and EngineeringANZAC PDE, NSW 2052 Sydney

Australiamahbub@cse.unsw.edu.au

Abstract: Wireless Personal Area Network (WPAN) has gained a lot of attention in industry recently. Ultra-Wideband (UWB) presents itself as a good alternative physical layer of WPAN, for both high and low data rateapplications. Here, we present an algorithm that can utilize the available channels in UWB systems. The algorithmis known as Dynamic Channel Scheduling Algorithm (DCSA). The proposed algorithm is based on a distributeddynamic channel allocation technique to distribute the channels among neighboring piconets. The purpose of thealgorithm is to increase the spectral Efficiency (SE) and the Quality of Service(QoS) of the system. We also presentthe algorithm in details and a numerical example to describe the algorithm is given.

Key–Words: Scheduling algorithm; UWB; WPAN; OFDM; Spectral efficiency; DCSA.

1 Introduction

Future wireless systems are expected to provide otherinformation services beyond voice, such as videoconferencing, interactive media, real-time Internetgames, etc., at anytime, anywhere [1]. Throughoutthe world, the demand for wireless communicationsystems has increased exponentially in the past fewyears. However, as any other technologies, a numberof issues should be considered when implementingwireless technology. Providing high-capacity trans-mission is one of the most important issues in wirelesscommunications [2]. Because the radio spectrumavailable for wireless services is an extremely scarceresource, efficient frequency utilization is becomingincreasingly important. Resource allocation schemesand scheduling policies are critical to achieving thesegoals. These facts motivate research in techniquesthat better utilize the radio spectrum than existingsystems do today. The goal is ultimately to increasethe spectral efficiency and Quality of Service [3].

Designing an efficient and effective schedulingalgorithm is not a trivial task, because wirelesscommunications pose special problems that do notexit in wired networks, such as limited bandwidth,high error rate, and transmission link variability. Thequality of a wireless link is time-dependent as well aslocation-dependent [4]. A good scheduling schemeshould be able to utilize the time-varying channelconditions of users to achieve higher utilization ofwireless resources.

In this paper, we consider IEEE 802.15.3 standardbased on Ultra-Wideband(UWB). Despite the goodfeatures that UWB provide such as high data rate(>100 Mbs), low power emission in short range (10meters), a number of challenges must be addressed[4]. One of the important issues is multiple channelscheduling problems. We propose a scheduling algo-rithm for IEEE 802.15.3 MAC protocol that utilizesthe multiple channels available in UWB networkin indoor environment. The proposed schedulingmechanism uses a distributed dynamic channelallocation algorithm to distribute the channels amongneighboring piconets. The scheduling is based ondynamic traffic demand. Distributed dynamic chan-nel allocation allows each piconet to decide the setof channels based on the information available locally.

The rest of the paper is organized as follows. Wediscuss background in Section 2. In Section 3, weprovide the problem description. In Section 4, we dis-cuss the previous work. We describe our proposedalgorithm in Section 5. Finally, we present our con-clusions in section 6.

2 Background

In this section, we describe related information on thenetwork architecture of WPAN, IEEE 802.15.3 stan-dard [5] and UWB.

Proceedings of the 5th WSEAS International Conference on Signal Processing, Istanbul, Turkey, May 27-29, 2006 (pp215-221)

2.1 Network Architecture

Wireless Personal Area networks (WPANs) are de-fined as networks that are used to transmit informationover relatively short distances (10 meters). The con-nections via WPAN require little or no infrastructure.The IEEE has started standardizing the WPANs tech-nologies in the IEEE 802.15.3 working group. TheIEEE 802.15.3 piconet components are shown in Fig1. A piconet is a collection of devices consisting ofone master or piconet Controller (PNC) and slave de-vices (DEVs). Each piconet can support only onePNC (master) and up to 255 devices (slaves). Slavesin IEEE 802.15.3 piconet communicate on a peer-to-peer basis.

3 Background

In this section, we describe related information on thenetwork architecture of WPAN, IEEE 802.15.3 stan-dard [5] and UWB.

3.1 Network Architecture

Wireless Personal Area networks (WPANs) are de-fined as networks that are used to transmit informationover relatively short distances (10 meters). The con-nections via WPAN require little or no infrastructure.The IEEE has started standardizing the WPANs tech-nologies in the IEEE 802.15.3 working group. TheIEEE 802.15.3 piconet components are shown in Fig1. A piconet is a collection of devices consisting ofone master or piconet Controller (PNC) and slave de-vices (DEVs). Each piconet can support only onePNC (master) and up to 255 devices (slaves). Slavesin IEEE 802.15.3 piconet communicate on a peer-to-peer basis.

Figure 1: 802.15.3 piconet elements.

3.2 IEEE 802.15.3a Standard

The standard provides simple ad-hoc communication.Timing for data transmission in the piconet is based on

the superframe (SFs). A superframe consists of threeparts:

• The beacon: Used to broadcast control informa-tion including channel time allocation (CTA) forthe current superframe. The beacon is also usedfor the devices to synchronize with the PNC.

• The contention access period (CAP): Used forauthentication request/response, channel time re-quest and other command.

• The contention free period (CFP): Used forasynchronous and isochronous data stream. Italso contains management timeslots (MTS).

3.3 Ultra-Wideband(UWB)

The UWB systems are based on the Impulse Radio(IR) technology[6]. It is emerging as a potential tech-nology to support high data rate wireless transmis-sion in indoor environment[7]. The IEEE 802.15.3study group has been established to study using UWBtechnology as alternative physical layer transmissiontechnique [8]. UWB signal has a fractions bandwidth20% of its center frequency, or has a 10-dB band-width equal to or larger than 500 MHz at low power(-41.3 Dbm/MHz) in an unlicensed spectrum from 3.1to 10.6 GHz[12], where the fraction bandwidth is de-fined as[9]:

η =2(fH − fl)HH − fL

(1)

Where, fH and fL are respectively defined as thehighest and lowest frequencies in the transmissionband.

One approach to designing a UWB system basedon OFDM is to combine the modulation techniquewith a multibanding approach [10], which divides thespectrum into several sub-bands, whose bandwidthis approximately 500 MHz. The transmitted OFDMsymbols are time-interleaved across the subbands.An advantage of this approach is that the averagetransmitted power is the same as a system designed tooperate over the entire bandwidth. Other advantagesof multibanding include processing the informationover much smaller bandwidth (approximately 500MHz), which reduces power consumption and lowerscost, improving spectral flexibility and worldwidecompliance[11].

Proceedings of the 5th WSEAS International Conference on Signal Processing, Istanbul, Turkey, May 27-29, 2006 (pp215-221)

4 Problem Description

In a wireless network, the available transmissionchannel has to be used by several nodes and asthe number of nodes sharing the wireless mediumincreases, the amount of bandwidth available to eachnode drops. It is vital to allocate communicationchannels efficiently because the bandwidth is lim-ited[14], so they should be reused as much as possible.

To our best knowledge, the IEEE 802.15.3 MACis not specifically designed for UWB. As a result, anumber of issues must be considered when attemptingto utilize UWB with IEEE 802.15.3 MAC:

• It is difficult for an UWB radio to detect when an-other DEV is transmitting. This may make muchof signaling difficult or impractical to implement.It may also means that CAP’s may be zero lengthas multiple access is less of possible than for con-ventional narrow-band systems.

• The high data rate supported by UWB will allowa large number of devices to be in a piconet. Cur-rently the maximum number of unique DEV IDin a piconet is 255, which will potentially limitthe capacity of the UWB system.

• High-data-rate systems require efficient channelmanagement to ensure high throughput. To max-imize throughput, a physical-layer aware MACshould allow larger packet size and minimalspacing between frames from the same devicetype.

5 Previous Work

Several algorithms have previously been studied forsolving the channel scheduling problem. This sectionincludes an overview of the existing schedulingalgorithms and their limits.

Turner designed the Horizon Scheduling algo-rithm [11]. In this algorithm, a scheduler only keepstrack of the so-called horizon for each channel, whichis the time after which no reservation has been madeon that channel. The scheduler assigns each arrivingdata burst to the channel with the latest horizon aslong as it is still earlier than the arrival time of thedata burst. The horizon scheduling algorithms resultsin low bandwidth utilization and a high loss rate.This is due to the fact that horizon algorithm simplydiscards all the void intervals.

Xiong et al. [12] proposed a channel schedulingalgorithm called LAUC-VF (Latest Available UnusedChannel with Void Filling). LAUC-VF keeps trackof all void intervals and assigns a burst arriving attime r a large enough void interval whose startingtime Si is the latest but still earlier than r. thisyield a better bandwidth utilization and loss ratethan Horizon Algorithm. However, even the bestknown implementation of LAUC-VF has a muchlonger execution time than the Horizon Schedulingalgorithm, especially when the number of voids mis significantly larger than the available channels k.For example, a straightforward implementation of theLAUC-VF algorithm takes O(m) time to schedule aburst. The time complexity becomes O(Bm) whenthere are B different delays. Searching for a suitablevoid interval in this way might take a longer timethan that is allowed by the offset time of a burst, thusresulting in a failed reservation.

Chin et al. [13] proposes a UWB-based alterna-tive PHY for Low Rate WPAN system and shows thesystem performance when the UWB transceiver oper-ates in the first 500MHz band, 3.1 3.6GHz, 3.6GHz,within the The Federal communications Commission(FCC) allocated UWB band. Extensive simulationshows that the proposed UWB based Alternate PHYlayer can support 802.15.4 250kbps 16-byte packettransmission over WPAN range of 5m with 10%Packet Error Rate (PER). In general, it is also ob-served that PER increases over longer communicationrange and over longer channel delay spread. However,multiple piconets within the same coverage area werenot tested to utilize the multiple available channels.

6 Dynamic Channel Scheduling Al-gorithm (DCSA)

In this section, we present the basic idea behind theproposed scheduling mechanism. The algorithm em-ploys a distributed dynamic channel allocation algo-rithm to efficiently allocate channels to neighboringpiconets.

6.1 Data Structure

We consider multiple PNCs. There are a set of chan-nels denoted by Spectrum(Ω). The channel with low-est frequency has the minimum order, while the chan-nel with highest frequency has the maximum order.Each PNC in the system maintains the following localvariable:

Proceedings of the 5th WSEAS International Conference on Signal Processing, Istanbul, Turkey, May 27-29, 2006 (pp215-221)

• N : Denotes the number of PNCs in the system,such that i,j∈ N .

• PNC: Denotes the ID number of PNC in thesystem. That is PNC=PNC1, PNC2, · · ·,PNCN

• Λi: Denotes the set of channels are currently al-located to PNCi.

• r: Denotes the channel which is chosen byPNCi such that r ∈ Ω.

• Ui: Denotes the set of channels currently used byPNCi, such that Ui ⊆ Λi.

• >i: Denotes the set of channels that can be trans-ferred to the other neighbors.

• ψi: Denotes the set of channels PNCi is holdingfor its neighboring. At any time,ψi

⋂Ui is empty

and ψi ⊆ Λi.

The spectral efficiency (given in b/s/MHz per piconet)is defined as[16]:

SE =β ×G

Ω×W ×m2b/s/MHz/m2 (2)

Where G is defined as the offered traffic per piconet,Ω corresponds to the number of channel per piconet,W is the bandwidth per channel(MHz), and β is thenumber of users..

The PNCs use the following type of message tocommunicate with each other:

REQUEST(r,tsi,i): Indicates that sendingPNCi is requesting to obtain a channel, r is therequired channel, tsi is the timestamp of PNCi atthe time of generating the request.

REPLY(j,Uj ,>j ,ψi): Indicates that sendingPNCi is responding to the request from receivingPNCj .

CONFIRM(r): Indicates that the sending PNCj

has sent an REFUSE(r) or AGREED(r) message togrant PNCi’s request to borrow channel r.

RELEASE(r): Indicates that PNCj needs torelease channel r from >i and ψi sets.

When PNCi needs a channel to support thetraffic load, PNCi starts the algorithm as follows:

Step 1: first it makes sure that if there is anyallocated a channel still not used. Let Unusedi = Λi -

Ui -ψi, if Unusedi 6= φ (null set), Then it selected theone with the highest frequency to support the trafficload, and set Ui=Ui+r, where r is the selectedchannel, and the algorithm stopped.

Step 2: Otherwise, this means that all allocatedchannel for PNCi are being utilized. It sends arequest to all its neighbors(it invokes the procedureRequestChannel which is given in Algorithm 1), set atimer and wait for reply(PNCj invokes the procedureReceiving request which is given in Algorithm 2).Each request message has a timestamp. The messagewith the lower timestamp has the higher priority.

Step 3: After receiving a reply messages from itsneighbors, each reply contains Ui, ψi and >i. PNCi

begins to compute the set of channel that can be bor-row. Let Unusable=Ui

⋃ψi

⋃Uj

⋃ψj

• If Freei = Ω - Unusable 6=φ, then a channelr∈Freei with the highest order is selected tosupport the system. Set Ui=Ui+r, and setΛi=Λi+r.

• If Freei = Ω-Unusable =φ, there is no a freechannel from the other PCNs can be transferred.Freei =

⋃∀j>i If Freei 6=φ, then select r∈Freei

with the highest order is selected.Set Ui=Ui+r,and set Λi=Λi+r. otherwise the algorithm hasto be stopped.

Step 4: To guarantee that the channel is notallocated to any other simultaneous request, thePNCi sends a CONFIRM(r) to its neighboring(itinvokes the procedure Confirming request which isgiven in Algorithm 3). If all PNCj reply with aAGREED(r), then the channel can be used by PNCi

and PNCj needs to release channel r (topj and ψj-it invokes the procedure Releasing a channel whichis given in Algorithm 4) . Otherwise, a REFUSE(r)message will received, and the channel can not beused by PNCi, set Ui=Ui-r, and set Λi=Λi-r. Ifthere are no more free channels, the algorithm has tobe stopped.

6.2 Numerical example

The algorithm can better be explained by the follow-ing example. We consider the number of channels are1,2,3,4,5,6,7,8 channels, the set of PNC is definedas PNC= PNC1 ,PNC2, PNC3 , the channelbandwidth is 500MHz, the number of users(β) is 64,and the coverage area is within 10m2.

Proceedings of the 5th WSEAS International Conference on Signal Processing, Istanbul, Turkey, May 27-29, 2006 (pp215-221)

1. When PNC1 needs a channel to support thetraffic load, the following scenario is considred:

- PNC1: Let Λ1=1,2, U1=1,2, >1=φ.PNC2: Let Λ2=3,4,5,6, U2=3,4,5,>2=6.PNC3: Let Λ3= 7 ,8 , U3=7,>3=8.Let Unused1 =Λ1 - U1 ->1.Unused1=1,2-1,2-φ=φ.

- if Unused1=φ, it sends a request messageto each of its two neighbors(PNC2 andPNC3).

- if Unused1 6= φ, it picks a channel r ∈Unused1 with the highest order to sup-port the traffic load. Set U1=U1+r, setΛ1=Λ1+r

2. When PNC2 and PNC3 receive a request mes-sage from PNC1:

- All neighboring PNC send a reply message in-cluding Uj , >j where j ∈ 2,3.PNC2 reply with U2=3,4,5, >2=6,ψ2=φ.PNC3 reply with U3=7,>3=8, ψ3=φ.

3. After PNC1 receives a reply messages from eachneighbor:

- Compute Unusable=Ui⋃

ψi⋃

Uj⋃

ψj

where i=1 and j=2,3.Unusable=1,2⋃φ

⋃3,4,5⋃φ⋃ 7⋃φ

Unusable=1,2,3,4,5,7.

- Compute Free1 = Ω - UnusableFree1 = 1,2,3,4,5,6,7,8-1,2,3,4,5,7Free1 = 6,8.

- PNC1 sends a CONFIRM(r) message toPNC2 and PNC3, where r=6,8

4. When PNC2 and PNC3 receive a CONFIRM(r)message from PNC1:

- If r∈Uj OR r ∈>j then send REFUSE(r) toPNCi, where i=1 , j=2,3 and r=6,8in our example the above condition is nottrue.

- Both PNC2 and PNC3 send a AGREED(r)message to PNC1, set ψj=ψj+r, and thechannels 6,8 can be used by PNC1. Inthis case, we assume a PNC1 will select6 firstly.

- ψ2=ψ2+6=6. U1=U1+rU1=1,2+6, U1=1,2,6Λ1=Λ1+r Λ1=1,2,3+6,Λ1=1,2,3,6

5. When PNC2 receives a RELEASE(r) fromPNC1: >j=>j-r, ψj=ψj-r, where j=2 andr=6>2=φ, ψ2=φ.

6.3 Spectral Efficiency and ThroughputClculations

Calculations for the spectral effiiency (SE) is pre-sented in Table I based on Eq.2, for the case of Band-width (W) =500, 300, 200MHz, the number of chan-nels (Ω)=4,5,6,7, the number of users (β)=64, and thecoverage area within 10m2.

Table 1: SUMMARY OF Spectral Efficiency(SE)

Spectral Efficiency (SE)

N 500MHz 300MHz 200MHz4 0.0960 0.1600 0.24005 0.1024 0.1706 0.25606 0.1280 0.2133 0.32007 0.1462 0.2438 0.3657

4 5 6 70.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Channels(C)

Spe

ctra

l Effi

cien

cy(S

E)(

bits

/s/H

z)

500MHz300MHz200MHz

Figure 2: Spectral Efficiency VS number of channels

Fig. 2 shows the system spectral efficiency whena number of channels is used. It has been observedthat increasing the number of channels per piconet in-creases the spectral efficiency (SE). For example, thespectral efficiency (SE) is calculated over 500MHzchannel, for channels Ω=5 and 6, the offered trafficper piconet G=300Mbps, the number of users (β)=64and the coverage area =10m2, respectively, 0.1024and 0.1280b/s/Hz/m2, even though it increases thethroughput. The system throughput is shown in Fig. 3as a function of channels.

Proceedings of the 5th WSEAS International Conference on Signal Processing, Istanbul, Turkey, May 27-29, 2006 (pp215-221)

4 5 6 7400

450

500

550

600

650

700

750

800

Channels (N)

Thr

ough

put (

Mbp

s)

110Mbps

Figure 3: Throughput VS number of channels

7 Conclusion

The characteristics of Ultra-Wideband (UWB) offernew solution and opportunities for resource manage-ment. In this paper, a scheduling algorithm was pro-posed. The algorithm based on a distributed dynamicchannel allocation technique to utilize the multiplechannels available in UWB. It is concluded that in-creasing the number of channels or borrowing a num-ber of channels increases the spectral efficiency (SE)even though it increases throughput. As a future work,we intend to investigate the fair sharing of the UWBchannel within a picont.

Algorithm 1 Pseudo-code of Requesting a channel1: Procedure RequestChannel(tsi)2: send REQUEST(r,tsi,i) to every j∈N ; wait UNTIL

REPLY (j,Uj ,>j ,ψi))is received from each j∈ N ;3: Unusable=Ui

⋃ψi

⋃Uj

⋃ψj

4: if Freei = Ω - Unusable 6=φ then5: channel r∈Freei with the highest order is selected6: else7: Freei =

⋃∀j>i

8: if Freei 6=φ then9: channel r∈Freei with the highest order is se-

lected10: else11: STOP12: end if13: end if14: set Ui=Ui+r set Λi=Λi+r15: send GRANTED(r) to all PNCj and wait for reply16: if all PNCj reply with AGREED) then17: channel r can be used and STOP18: else19: PNCj reply with REFUSE message20: Ui=Ui-r, Λi=Λi-r21: end if

Algorithm 2 Pseudo-code of Receiving request1: Procedure ReceiveRequest(r,tsi,i)2: if PNCj does not initiate the algorithm OR tsi < tsj

then3: send REPLY(j,Uj ,>j ,ψi) to PNCi

4: else5: defers sending a REPLY to PNCi

6: end if

Algorithm 3 Pseudo-code of Confirming request1: Procedure ConfirmRequest(r)2: if r in Ui OR r ∈>i then3: send REFUSE(r) to PNCi

4: else5: send AGREED(r) to PNCi, set ψj=ψj+r6: end if

Algorithm 4 Pseudo-code of Releasing a channel1: Procedure ReleaseChannel(r)2: >j=>j-r, ψj=ψj-r

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