Throughput enhancement using synchronization and threedimensional resource allocation
 HyukChin Chang^{1}Email author and
 Saewoong Bahk^{1}
https://doi.org/10.1186/168714992011150
© Chang and Bahk; licensee Springer. 2011
Received: 1 February 2011
Accepted: 31 October 2011
Published: 31 October 2011
Abstract
Emerging multimedia applications require more bandwidth and strict QoS requirements. To meet these in wireless personal area networks, WiMedia multibandorthogonal frequency division multiplexing (MBOFDM) has been designed while consuming lowtransmission power. In this article, we increase the wireless bandwidth of the standard MBOFDM scheme three times using device synchronization, and consider resource allocation policies to deal with the increased bandwidth. Then, we apply the proposed allocation policies with some operation rules to support prioritized QoS traffic. Extensive simulations verify that the synchronized MBOFDM triples the throughput of the standard MBOFDM, and the considered allocation policies with the considered operation rules run effectively as desired.
1 Introduction
Wireless technologies have been evolved to support data rates of up to a few hundreds of Mbps for high data rate and QoS services such as voice over internet protocol, internet protocol television, and wireless universal serial bus. Typically, the communication range for such high data rates is within a few tens of meters that covers home or office environments, where wireless personal area network (WPAN) technology provides the communication with high data rate, lowtransmission power consumption, and low cost [1]. WiMedia alliance has standardized the PHY and medium access control (MAC) layers for multibandorthogonal frequency division multiplexing (MBOFDM) of high data rate WPAN based on ultra wide band (UWB), called ECMA (European Computer Manufacturers Association)368 [2].
Supporting multimedia traffic with QoS requirements over wireless environments is still an important issue in the resource management. Besides, emerging highquality video applications such as full highdefinition multimedia contents require more bandwidth. The MAC is a key layer to meet tight QoS requirements and achieve high throughput [3–7].
WiMedia MAC has two wireless channel access policies: contentionfree distributed reservation protocol (DRP) like time division multiple access (TDMA) and contentionbased prioritized contention access (PCA) with priorities like IEEE 802.112007 [8]. DRP is designed to support QoS for isochronous streams such as multimedia contents [9–11], and PCA to support a random channel access for asynchronous services [12, 13]. In [3], DRP and PCA are used together to assign I, B, and P frames in H.264/AVC (MPEG4 Part 10) to the wireless resource. In this article, we only consider contentionfree DRP to support QoS traffic.
In [14], two analytical models for resource assignment in WiMedia MAC are proposed: subframefit and isozonefit reservations. The subframefit scheme only uses request sizes and delay requirements, whereas the isozonefit scheme does block sizes and locations recommended in [15]. They also suggest improvements to the isozonefit algorithm by introducing crossisozone allocation and ondemand compaction.
Adaptive multiuser (MU) spectrum allocation methods have been investigated in [16, 17]. They allow users to share available resources by exploiting the effective signaltointerference plus noise ratio and priority level, depending on throughput, delay, and packet error rate. They apply crosslayer approaches for the PHY and MAC layer designs that use the channel state information and service differentiation.
The WiMedia standard adopts MBOFDM where signal transmission uses only one of the three bands at a symbol time. This means that the standard scheme does not exploit the bandwidth fully. In this article, we increase the wireless bandwidth three times using the three bands together, which is enabled by synchronizing devices in a piconet. This provides the benefit of increasing the number of multimedia flows to be serviced at a time. To deal with the enlarged bandwidth in supporting various QoS traffic types, we consider appropriate resource allocation policies too.
The remainder of the article is organized as follows. In Section 2, we briefly overview WiMedia PHY and MAC, and propose the synchronized MBOFDM in Section 3. We consider the resource allocation algorithms to deal with the enlarged bandwidth in Section 4. Then, we apply the proposed allocation policies with some operation rules for prioritized QoS traffic support in Section 5, and present simulation results in Section 6, followed by concluding remarks in Section 7.
2 Background
We overview the WiMedia specification with regard to PHY and MAC layers, MBOFDM, and timefrequency code (TFC).
2.1 WiMedia PHY and MAC
ECMA specified WiMedia PHY and MAC, called ISO (International Organization for Standardization)based ECMA368 [2]. The standard uses the spectrum between 3.1 and 10.6 GHz and supports the data rates of 53.3, 80, 106.7, 160, 200, 320, 400, and 480 Mbps. The spectrum is divided into 14 bands with each bandwidth of 528 MHz. The consecutive three bands form one band group except the last two bands that form the last fifth group. And frequencydomain and timedomain spreading, forward error correction with convolutional codes are used.
2.2 MBOFDM
2.3 TFC
Timefrequency codes for band group 1 in ECMA368 [2]
TFC number  Types  Band ID for TFC  

1  TFI  1  2  3  1  2  3 
2  TFI  1  3  2  1  3  2 
3  TFI  1  1  2  2  3  3 
4  TFI  1  1  3  3  2  2 
5  FFI  1  1  1  1  1  1 
6  FFI  2  2  2  2  2  2 
7  FFI  3  3  3  3  3  3 
8  TFI2  1  2  1  2  1  2 
9  TFI2  1  3  1  3  1  3 
10  TFI2  2  3  2  3  2  3 
Collision probabilities between TFCs in band group 1
TFC #  14  5  6  7  8  9  10  Avg. prob. 

14  1/3  1/3  1/3  1/3  1/3  1/3  1/3  1/3 
5  1/3  1  0  0  1/2  1/2  0  1/3 
6  1/3  0  1  0  1/2  0  1/2  1/3 
7  1/3  0  0  1  0  1/2  1/2  1/3 
8  1/3  1/2  1/2  0  1/2  1/4  1/4  1/3 
9  1/3  1/2  0  1/2  1/4  1/2  1/4  1/3 
10  1/3  0  1/2  1/2  1/4  1/4  1/2  1/3 
3 System model
We model MU MBOFDM to exploit three bands at each symbol time. To realize this model in a piconet, we propose to synchronize three concurrent transmissions at each MAS boundary time to overcome the clock drift. Moreover, we consider imperfect synchronization and some issues in applying the model for multipiconet environments.
3.1 MU MBOFDM
The conventional MBOFDM uses only one band among the three in a band group at each symbol time. However, the synchronization of devices in a piconet can make it possible to use three bands concurrently, thereby tripling the wireless bandwidth compared to the standard scheme. The synchronization helps to avoid interference from other devices in a piconet. The MU MBOFDM selects a TFC in TFI or TFI2, and shifts it by some OFDM symbol times to create two or three orthogonal TFCs that can be used together.
Specifically, the numbers of the shift are 0, 1, 2 at TFC1 and TFC2, 0, 2, 4 at TFC3 and TFC4, and 0, 1 at TFC8, TFC9, and TFC10. ^{b}The use of three shifted TFCs brings the gain of the frequency diversity.
3.2 Synchronization
In the BP, every node in a piconet is awake and broadcasts its own beacon at its predetermined slot. Each node maintains a table of timing differences between the actual arrival times of each neighbor's beacon by simply synchronizing with the slowest device in the BP. The expected arrival time is calculated based on the BPST.
In the DTP, concurrent transmissions should be synchronized to avoid inter symbol interference between consecutive adjacent transmissions. One OFDM symbol time is 312.5 ns with the fast Fourier transform time of 242.42 ns and the zeropadded suffix duration of 70.08 ns, of which function is to overcome the multipath effect and give time for frequency hopping [2].
where mClockAccuracy is the clock drift set to 20 PPM (parts per million) and SyncInterval the synchronization time of a device in the DTP. MaxDrift is about 2.62 μ s for each transmission pair in a superframe if all the nodes are synchronized in the BP.
There is a guard interval, i.e. mGuardTime = 12 μ s, between two adjacent MAS boundary times to overcome the clock drift in the conventional MAC policy. Consequently, RX nodes are ready to listen to signals prior to mGuardTime at their reserved MAS boundary times.
However, concurrent transmissions scheduled at the same MAS with differently shifted TFCs can arrive prior to the MAS boundary time within mGuardTime simultaneously, when all the devices are synchronized in the BP. To solve this problem, TXRX pairs have to listen first to the hopping pattern for the duration of OFDM symbol time, and then transmit their signals according to their scheduled hopping patterns. Each TXRX pair already knows the hopping patterns of other TX nodes from hearing beacons in the BP.
3.3 Implementation
The frame structure has the PLCP protocol data unit (PPDU) that consists of physical layer convergence protocol (PLCP) preamble, PLCP header, and PHY service data unit. The PLCP preamble has two distinct parts: a unique synchronization sequence and a channel estimation sequence. It helps the receiver in timing synchronization, carrieroffset recovery, and channel estimation. In our proposed MUsynchronization, TX0 with 0 shift starts to transmit a PPDU first based on its local timer and the other nodes, i.e. TX1, TX2, RX0, RX1, and RX2, start to listen to the synchronization sequence of TX0 for the synchronization with their local timers. The transmitters, TX1 and TX2, start to transmit their PPDUs with their shifted TFCs after the synchronization. Then, the TXRX pairs can communicate synchronously.
3.4 Imperfect synchronization
where D_{max} is the maximum distance between two nodes in a piconet, and c is the speed of light. The timing offset between a TX and the other TX, measured at an RX, is d_{prop} ∈ [0, d_{prop,max}].
3.5 Effects of imperfect synchronization
Though the timing offset can be mitigated by the ranging capability of WiMedia UWB, it cannot be removed perfectly. We consider using the zeropadded prefix in an OFDM symbol time to absorb such timing offset. The zeropadded suffix duration of 70.08 ns in a OFDM symbol serves to mitigate the effects of multipath and give a guard time for the band switch, pBandSwitchTime (= 9.47 ns). And the indoor communication range for multimedia traffics is generally within a few meters, resulting in d_{rng,max} to be below a few ns delay, e.g. 10 ns at 3 m.
However, the received signal will be degraded if the effects of multipath and the propagation delay are not mitigated sufficiently by the zeropadded suffix duration. In this case, the TXRX pair lowers their transmission rate based on packet error rate in practice to overcome the effects of imperfect synchronization, requiring more wireless resources. Therefore, this imperfect synchronization degrades the network throughput.
3.6 Multipiconet environments
In normal operation, there is no interference in a piconet if all the nodes are synchronized and scheduled in the MU MBOFDM. But the interference is not avoidable if the network is heavily loaded in a multipiconet environment. It happens when some bands are occupied again by neighboring piconets at a given time.
In the conventional MBOFDM, several methods such as transmit power control, band group change, TFC change, and exclusive time reservation have been proposed to mitigate the interferences from neighboring networks [18–21]. In this context, we use the band group change to avoid the interferences from other piconets. Our scheme can support 14 concurrent users at a time in the UWB spectrum, i.e. a user per band, without creating interference.^{c}
Different from ours, the standard scheme can support five users at a time, i.e. a user per band group. This means our scheme can accommodate about three times more users than the standard scheme. To apply our scheme to a multipiconet environment, we need to adopt a solution to a distributed vertex coloring problem with five colors, i.e. a different color for each band group. Several solutions to this problem have been proposed and analyzed [22–24]. The detailed discussion about the coloring problem is beyond the scope of this article.
4 Resource allocation
In this section, we review the conventional 2dimensional (2D) resource allocation scheme to assign 256 MASs in MBOFDM, and consider 3dimensional (3D) allocation schemes to deal with the increased 3 × 256 MASs in MU MBOFDM.
4.1 Conventional 2D resource allocation
The 2D structure of 16 × 16 MASs in a superframe has been proposed for MAS allocation [15]. The contiguous 16 MASs are grouped into an allocation zone, called zone. There are 16 zones in column. We denote the zones by ${\mathcal{Z}}_{0}$. to ${\mathcal{Z}}_{15}$. ${\mathcal{Z}}_{0}$ is reserved for BP, and the other 15 zones are grouped into four subsets, called isozones. We denote the set of zones with isozone j by ${\mathcal{I}}_{j}$ that has 2 ^{ j } zones. That is, ${\mathcal{I}}_{0}=\left\{{\mathcal{Z}}_{8}\right\}$, ${\mathcal{I}}_{1}=\left\{{\mathcal{Z}}_{4},{\mathcal{Z}}_{12}\right\},$${\mathcal{I}}_{2}=\left\{{\mathcal{Z}}_{2},{\mathcal{Z}}_{6},{\mathcal{Z}}_{10},{\mathcal{Z}}_{14}\right\}$, and ${\mathcal{I}}_{3}=\left\{{\mathcal{Z}}_{1},{\mathcal{Z}}_{3},{\mathcal{Z}}_{5},{\mathcal{Z}}_{7},{\mathcal{Z}}_{9},{\mathcal{Z}}_{11},{\mathcal{Z}}_{13},{\mathcal{Z}}_{15}\right\}$.
Since an MAS has the duration of 256 μ s, each zone is separated by 4.096 ms from each neighboring one. Higherindexed isozones are used to support services with smaller service intervals, i.e. tight QoS requirements. For instance, the service intervals of ${\mathcal{I}}_{0}$ and ${\mathcal{I}}_{3}$ are 16 × 4.096 and 2 × 4.096 ms, respectively. When a flow with QoS requirements enters the network, it indicates its service requirements by an isozone number and the number of required MASs in a superframe.
4.1.1 2D MAC policy
where P_{2D} is the number of assigned MASs in the 2D MAC policy, r_{ i } the number of requested MASs in ${\mathcal{I}}_{i}$ specified by a QoS flow, and x the number of selected MASs in each zone with ${\mathcal{I}}_{j*}$. Note that the assigned MASs can be more than the requested MASs because of the symmetric assignment property. The MASs to be allocated are evenly distributed over the zones with the same requested isozone for the convenience of future reservation.
4.2 3D resource allocation
Against the standard 2D allocation of 16×16 MASs, our proposed allocation schemes handle the 3D structure of 3 × 16 × 16 MASs. This structure comes from the MU MBOFDM that uses the three bands. We denote the three superframes with 0, 1, and 2 shift(s) of OFDM symbol time by SF_{0}, SF_{1}, and SF_{2}, respectively.^{d} This implies that the standard MBOFDM uses SF_{0} only.
In the 2D MAC policy, if there are not enough MASs in the requested isozone of a superframe, each TX node searches for MASs from other higherindexed isozones. In our MU MBOFDM scheme, as we have the 3D resource structure, we can consider three types of resource assignment policies: SF (SuperFrame)first policy tries available resources sequentially from equal and next higherindexed isozones in SF_{0} first, IZ (IsoZone)first policy tries the requested isozone first over the three SF s, and SIZ (SharedIZ) policy tries resources from all the isozones and SF s exhaustively. When a resource request is given, SIZ policy can partially assign MASs from an isozone and then additional MASs from other isozones over the three SF s. We explain these three policies in detail.
4.2.1 SFfirst policy
where P_{ SF }is the number of assigned resources, ${\mathcal{A}}_{SF}$ the set of available isozones j with each SF, and m_{j,l}the available MASs with ${\mathcal{I}}_{j}$ and SF_{ l }.
4.2.2 IZfirst policy
where P_{ IZ } is the number of assigned resources and ${\mathcal{A}}_{IZ}$ the set of available isozones with each SF.
4.2.3 SIZ policy
This policy tries cross isozones for MAS allocation if the requested MASs cannot be allocated to one isozone of an SF. This is a simply extended version of the 2D crossIZ allocation scheme for 3D allocation [14]. Given the resource request r_{ i } , it will be allocated to isozone j(≥ i) that uses the minimum sum of MASs, while meeting the QoS requirements.
where P_{ SIZ } is the number of assigned resources, ${\mathcal{A}}_{SIZ}$ the set of feasible combinations of x_{ j } , M_{ j } the maximum of available MASs in ${\mathcal{I}}_{j}$ over the three SF s, and x_{ j } the number of selected MASs with ${\mathcal{I}}_{j}$ in the selected SF.
5 Resource allocation for prioritized QoS traffic
In this article, we consider video with low quality (VL), video with high quality (VH), and best effort (BE), and assume that VL has priority over VH.^{e} BE has no priority and requirements, and simply tries to take all the available MASs that are unassigned to VL and VH.
5.1 Priority support
The resource allocation policy for QoS flows can be preemptive or nonpreemptive: a policy is preemptive if a QoS flow can be interrupted by another QoS flow, and nonpreemptive otherwise.
5.1.1 Preemptive policy
Each QoS flow of VL or VH is assigned to at least one SF with available isozones. We simply dedicate one SF to VL and two SF s to VH, and use the following rules for preemptive QoS operation with ownership.

VL owns SF_{0}, VH owns SF_{1} and SF_{2}, but BE has no dedicated SF.

VL and VH occupy any available SF and preempt BE.

VL can preempt VH in SF_{0}, but VH cannot preempt VL in SF_{1} and SF_{2}.

An existing owner of each SF cannot be preempted by other traffic types.
We also consider preemptive QoS operation without ownership.

VL and VH occupy any available SF without dedicated SF and preempt BE.

VL can preempt VH over three SF s.
5.1.2 Nonpreemptive policy
All the QoS flows of VL and VH can use three SF s without being preempted by next incoming QoS flows. However, BE flows still can be preempted by QoS flows.
5.2 BE service support
All the unassigned MASs can be allocated for BE services. Incoming BE flows share available MASs with other existing BE flows in a fair manner, and do not follow the symmetric assignment property.
where P_{ CSF } (N, n) is the number of MASs to be assigned over the three SF s for BE flow n, ${\mathcal{S}}_{i}$is the number of elements in the set ${\mathcal{S}}_{i}$, ⌊x⌋ is a floor function which maps x to the largest integer not greater than x. And b_{ n } is a binary variable having 0 or 1 when the input x of ⌊x⌋ is not an integer, and 0 otherwise. One MAS will be assigned to BE flow n starting with 1, i.e. b_{ n } = 1, till the remaining MASs are empty if the input x is not an integer.
After calculating P_{ CSF } (N, n), each BE flow n occupies resources in a descending order of N_{ q } in ${S}_{{N}_{q}}$, i.e. ${\mathcal{S}}_{3}$, ${\mathcal{S}}_{2}$, and ${\mathcal{S}}_{1}$. When two or three MASs are available at a given time, a lowindexed SF is selected.
The first arriving flow 1 in Figure 11a transmits through six MASs sequentially, i.e. SF_{2}, SF_{0}, SF_{0}, SF_{0}, X, SF_{0}, X, SF_{0}, where X indicates 'not available' MAS at the given time. The number of assigned MAS for flow 1 is 6.
The second arriving flow 2 in Figure 11b has the same number of assigned MASs with flow 1 according to (9): P_{ CSF } (2, 1) = 5 and P_{ CSF } (2, 2) = 5. At MAS 6 the resource on SF_{2} cannot be assigned to any flow because a BE flow can transmit through only one MAS at a time.
Finally, flow 3 in Figure 11c requests resources and then we get P_{ CSF } (3, 1) = 4, P_{ CSF } (3, 2) = 4 and P_{ CSF } (3, 3) = 3 from (9). Therefore, the CrossSF allocation policy guarantees the fairness of each BE flow.
6 Simulation results
In simulations, QoS flows are generated with uniformly distributed delay requirements in [10] ms. Each QoS flow comes with a requested isozone corresponding to the delay requirement. VL and VH flows have a uniformly distributed MAS requirement in [2, 10] and in [10], respectively. The maximum number of MASs in an SF, i.e. M, is set to 240. We ran the simulations 1,000 times with MATLAB [25] and averaged out the results.
6.1 Case of VL traffic only
If we only consider the requested MASs without the symmetric assignment, about 40 flows are supportable at maximum in the standard MBOFDM. We compare the throughput of the proposed MU MBOFDM with that of the standard MBOFDM. Then, we measure the ratio of redundant MASs and the number of blocked flows for each assignment policy.
6.1.1 Throughput
6.1.2 Redundant MASs
In the SFfirst policy, the ratio of redundant MASs starts to decrease at the loads of 26 and 54 flows where the policy starts to allocate resources with each additional SF. The IZfirst policy shows lower ratio than the SFfirst policy as the IZfirst policy assigns MASs to the requested isozone as much as possible. The SIZ policy has the least ratio of redundant MASs. The use of multiple isozones over the three SF s in this policy reduces redundant MAS allocation compared to that of single isozone over the SFfirst and IZfirst policies.
The ratio of redundant MASs in the SIZ policy starts to decrease at about 80 flows. We can explain this as follows. First, higherindexed resources, i.e. having shorter service intervals, become candidates for the resource assignment more frequently. Therefore, higherindexed resources tend to be consumed earlier than lowerindexed resources. This tendency causes higherindexed flows to be blocked more often compared to lowerindexed flows. Second, lowerindexed resources have a lower number of symmetric zones according to (4). This leads this policy to have the lowest ratio of redundant MASs at above 80 flows.
6.1.3 Blocked flows
6.2 Case of VL, VH, and BE traffics
Flows of VL, VH, and BE are generated with the equal probability. We apply the SIZ policy with the preemptive policy for VL and VH flows, and the CrossSF allocation policy for BE flows. Then, we measure throughput and numbers of serviced, blocked, and dropped flows.
6.2.1 Throughput
Even when the network is heavily crowded by all the traffic types, BE flows still have a chance to use fragmented MASs that are left over from the allocation for VL and VH flows.
6.2.2 Serviced flows
6.2.3 Blocked flows
6.2.4 Dropped flows
7 Conclusion
In this article, we have modeled the MU MBOFDM using synchronization that provides the merit of three concurrent transmissions for a band group in a piconet. As a result, the proposed MU MBOFDM triples network throughput compared to the conventional MBOFDM. Then, we considered three 3D resource allocation policies to handle the expanded MAS resources: SFfirst, IZfirst, and SIZ policies. The simulations showed that the SIZ policy performs the best in terms of throughput, redundant MASs, and blocked flows.
We also investigated some operation rules with resource allocation policies to support prioritized QoS traffic in the MU MBOFDM. Extensive simulations presented that the proposed QoS support rules operate well in terms of throughput and numbers of serviced, blocked, and dropped flows.
Declarations
Acknowledgment
This study was supported by the "Samsung Electronics Semiconductor Business".
Endnotes
^{a}In the fifth band group, two bands are available for frequency hopping.
^{b}In band group 5, the numbers of the shift are 0, 1 only at TFC8.
^{c}This does not mean that 14 users are the maximum that can be admitted. For instance, the maximum length of BP is 96 beacon slots which support 94 users (two beacon slots are reserved for signaling) in a piconet at maximum.
^{d}In this case, we consider TFC1 and TFC2 in band groups 1, 2, 3, and 4.
^{e}We have not considered resource allocation for voice traffic because its bandwidth requirement is too small. The allocation of even an MAS is too much for a voice traffic. Note that the SIZ policy cannot allocate one MAS in pieces over three SFs.
Authors’ Affiliations
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