Opportunistic Nonorthogonal Packet Scheduling in Fixed Broadband Wireless Access Networks

In order to mitigate high cochannel interference resulting from dense channel reuse, the interference management issues are often considered as essential part of scheduling schemes in ﬁxed broadband wireless access (FBWA) networks. To that end, a series of literature has been published recently, in which a group of base stations forms an interferer group (downlink transmissions from each base station become dominant interference for the users in other in-group base stations), and the scheduling scheme deployed in the group allows only one base station to transmit at a time. As a result of time orthogonality in transmissions, the dominant cochannel interferers are prevented, and hence the packet error rate can be improved. However, prohibiting concurrent transmissions in these orthogonal schemes introduces throughput penalty as well as higher end-to-end packet delay which might not be desirable for real-time services. In this paper, we utilize opportunistic nonorthogonality among the in-group transmissions whenever possible and propose a novel transmission scheduling scheme for FBWA networks. The proposed scheme, in contrast to the proactive interference avoidance techniques, strives for the improvements in delay and throughput e ﬃ ciency. To facilitate opportunistic nonorthogonal transmissions in the interferer group, estimation of signal-to-interference-plus-noise ratio (SINR) is required at the scheduler. We have observed from simulations that the proposed scheme outperforms the reference orthogonal scheme in terms of spectral e ﬃ ciency, mean packet delay, and packet dropping rate. Copyright


INTRODUCTION
Fixed broadband wireless access (FBWA) [1,2] is recognized to be a promising alternative technology to existing copper line asymmetric digital subscriber loop (ADSL) [3,4] and hybrid fiber-coaxial (HFC) [5] cable broadband services for its fast, simple, and less expensive deployment. However, efficient system planning and resource allocation policies are warranted for such systems, because in addition to the challenges posed by the dynamic nature of wireless links, interference resulting from aggressive channel reuse is a major design concern. Therefore, resource allocation strategies play a major role for the successful evolution of FBWA. In this paper, we focus on one of the most important aspects of resource allocation, packet scheduling.
Wireless scheduling techniques [6][7][8][9][10] have emerged as tailored versions of wireline scheduling to cope with the dynamic nature of wireless links. To account for cochannel interference, it is common to consider the issues of interference management as an integral part of scheduling techniques in FBWA networks [11][12][13][14][15][16]. In our previous works [12,13], we have shown that a very effective means of managing interference is to employ coordinated orthogonal transmissions among dominant interferers achieved by inter-base station (BS) signaling. The main idea of this scheme is to group a number of BSs (termed as interferer group) that are dominant interferers to each other and to schedule transmission orthogonally so that only one BS in the group transmits at a particular time. This scheme is composed of two independent scheduling disciplines and hence named as intrasector and intersector scheduling (ISISS) [13].
High end-to-end packet delay is the main drawback of the ISISS scheme. Packet delay is an important quality-ofservice (QoS) parameter for a variety of delay-sensitive applications, which is directly related to the throughput for a given data rate. Therefore, improving throughput and delay in an orthogonal scheduling scheme is essential. In this paper, we propose a novel scheduling scheme that improves both packet delay and resource utilization in terms of area spectral efficiency. The performance of the proposed scheme is compared to that of a reference scheme adapted from basic ISISS [13]. This reference scheme is named as intrasector and orthogonal intersector scheduling with fixed modulation (ISOISS-FM).
Proposed scheme in this paper considers interference management issues, integrates adaptive modulation and coding (AMC), and makes channel-state-based scheduling decisions to enhance network performance. We investigate the performance of the proposed scheme in two steps. First, we introduce AMC instead of fixed modulation and evaluate the performance of the scheme. The resulting scheme is still orthogonal, while it makes channel-state-based scheduling decisions. This intermediate scheme is named as intrasector and orthogonal intersector scheduling with adaptive modulation and coding (ISOISS-AMC). Investigation of this intermediate scheme quantifies the performance gain achieved from the use of AMC in an orthogonal scheme. We then employ opportunistic nonorthogonality in transmissions, where multiple cochannel BSs are allowed to transmit simultaneously. This final scheme is named as intrasector and opportunistic nonorthogonal intersector scheduling with adaptive modulation and coding (ISONOISS-AMC). Basically, if a number of cochannel BSs transmit simultaneously, each becomes interferer for the users in other BSs. The idea is that if the interference levels (hence the SINRs) are predicted and are transparent to each BS in the group, then every BS in the interferer group would potentially be able to transmit simultaneously with its feasible AMC mode in the presence of others being interferers.
Opportunistic scheduling, in general, implies a scheduling mechanism that exploits channel variations and schedules a user having the best channel condition at the time of interest [17]. However, according to the context of our study in this paper, opportunistic nonorthogonal scheduling means exploitation of channel variations among a group of mutually interfering BSs and scheduling concurrent in-group transmissions opportunistically based on the mutual interference situation.
The proposed scheme in contrast to the widely studied proactive interference avoidance techniques predicts the interference and achievable SINR on the fly. It then decides whether or not concurrent transmissions in the interferer group should be allowed at a particular instant. This reactive interference-aware scheduling scheme allows controlled in-group interference, which functions adaptively in an optimistic manner yielding the capability of improving throughput and the delay. The details of the proposed scheme are illustrated in Section 3.
Similar notion of concurrent cochannel transmissions based on terminal classifications has been previously considered in the enhanced staggered resource allocation (ESRA) scheme [14]. However, the time slot allocation in that scheme is static, which might result in low resource utilization especially for bursty traffic such as in FBWA. The proposed scheme in this paper, on the contrary, is dynamic in nature, adaptive according to the channel state, and optimistic.
The intermediate and proposed schemes are more prone to packet errors compared to the reference ISOISS-FM, primarily because the predicted SINRs in these schemes do not account for the out-of-group interference. We define parameter interference compensation guard to offset overestimation in the predicted SINR. This guard acts as a method of protecting the in-group transmissions to a certain degree from out-of-group interference. The effect of interference compensation guard on the performance of proposed ISONOISS-AMC scheme has also been investigated.
The rest of this paper is organized as follows. Section 2 describes the reference ISOISS-FM scheme. The intermediate ISOISS-AMC and proposed ISONOISS-AMC schemes are illustrated in Section 3. Section 4 describes system model. Simulation results are presented in Section 5 followed by conclusions in Section 6.

REFERENCE SCHEME: ISOISS-FM
A downlink time-division multiple-access (TDMA) system in a hexagonal six-sectored nine-cell network as shown in Figure 1(a) is considered in the reference as well as in the intermediate and proposed schemes. It is assumed that a frequency reuse plan with a reuse factor of 1/6 is employed in the network. The shaded sectors 1 (e.g., sector 1 in Figure 1) in all cells use the same frequency band. It should be noted here that an alternative assignment technique for sectors,  such as the rotational or staggering approach used in [11] or [14], is also possible in order to reduce intersector interference, especially for lower network loading. The rationale behind the assignment used in this study, where cochannel sectors are positioned in a line, is to investigate the worstcase intersector interference scenario. However, the proposed scheduling scheme can be employed with any other frequency planning to enhance the performance in addition to what can be obtained by the static frequency assignment alone. We assume that base stations and subscriber station (SS) terminals are equipped with directional antennas with 60 • and 30 • beamwidths, respectively. The SS antennas are pointing towards the serving BSs. The effective gains of BS transmit and SS receive antennas are considered to be 20 dB (10 dB main and −10 dB side lobe) and 10 dB (5 dB main and −5 dB side lobe), respectively. We have considered wraparound interference model such that an interferer BS position is taken to be at a place from where it contributes the maximum interference for the SSs in the BS of interest (see [18] for details). Figure 1(b) shows the positions of the interferer BSs for the SSs in BS I. Base station sets {J, K} and {I , J , K , I , J , K } are potential ingroup and out-of-group interferers for the SSs in BS I, respectively. A similar approach can be followed to find out the positions of interferers for SSs in other BSs. It can easily be conceived that as a result of combined effects of the antenna directivities, gains, and relative positions of the cells, the downlink transmissions from BSs I and Jwill be the two most dominant interferers for the SSs in BS K. Similarly, BS I and wraparound BS K (considered to be at the left of BS I) would be the most dominant interferers for the SSs in BS J. Moreover, wraparound BSs J and K are the most dominant interferers for SSs in BS I. Following these arguments, BSs I, J, and K form an interferer group. Similarly, BSs {I , J , K } and {I , J , K } form two other interferer groups in the network.
The scheduling scheme (reference, intermediate, or proposed) is employed in each interferer group as shown in Figure 2 for interferer group {I, J, K}. The in-group BSs ex-change information with each other as illustrated in the figure. The intrasector scheduling discipline decides the service order of each SS inside the sector, while the intersector discipline determines the service order among different BSs in the group to ensure orthogonal or opportunistic nonorthogonal transmissions in the interferer group. As the contributions of the schemes lie in the intersector scheduler, for simplicity the first-come-first-serve (FCFS) principle is considered as the intrasector discipline in the reference system as well as in the intermediate and proposed schemes.
Transmissions use fixed 16-quadrature amplitude modulation (16-QAM) bit-interleaved coded modulation (BICM) with a coding rate of 1/2 in the reference ISOISS-FM scheme. Base stations in the interferer group exchange traffic-related information, such as the arrival times of the packets (with the packet lengths) arrived in previous data frame duration. Therefore, each BS in the group is aware of the arrival times of the packets of its own queue as well as the packets of the queues of the other BSs in the group. The intersector scheduler checks the arrival times of the head-of-line (HOL) packets in all three queues in the group and selects the candidate packet to be transmitted that has the earliest arrival time; for example, in group {I, J, K}at a particular instant, where w is the BS that wins the service opportunity at that instant, and t i a , t j a, and t k a are the arrival times of the HOL packets at BSs I, J, and K destined to SSs i, j, and k, respectively.

DESCRIPTIONS OF THE INTERMEDIATE AND PROPOSED SCHEMES
Schematically, the reference, intermediate, and proposed schemes are alike in the sense that they all are composed of two independent schedulers (intrasector and intersector). The main difference is in the function of the intersector schedulers and modulation (fixed or adaptive). The intermediate and proposed schemes make channel-state-based scheduling decisions and employ AMC based on the predicted SINR for transmissions towards particular SSs. In this section, we provide an overview of the SINR estimation first, and then we describe how the intersector schedulers work in ISOISS-AMC and ISONONISS-AMC schemes.

SINR estimation and BS information exchange
In order for the intermediate and proposed schemes to be able to execute link-state-based scheduling decisions and employ AMC, SINR would have to be estimated at each BS. For the nine-cell network shown in Figure 1(a), every transmission will have eight potential interferers. Let us consider the scenario shown in Figure 1(b). The SINR of a received packet at SS i served by BS I can be expressed as

EURASIP Journal on Wireless Communications and Networking
where P t is the fixed transmit power. The first term in the denominator is the summation of interference from in-group BSs (IG) and the second term expresses the total interference from out-of-group BSs (OG). For the given scenario, Parameter G i I is the link gain between the serving BS I and SS i. Parameters G i x and G i y are the link gains to the desired SS i from the interfering in-group and out-of-group BSs, respectively. These link gain parameters include the effect of antenna gains at the BS and the SS terminals, as well as the propagation loss (including shadowing and fading) of the link. In (2), P i N is the average thermal noise computed at the receiver of SS i.
We note that all BSs do not necessarily transmit simultaneously because of either algorithm dictation or empty queues. The parameters A x and A y in (2) denote activity factors which take value of 1 if the interferer BS is transmitting and 0 if it is idle. An expression similar to (2) is applicable for the SINR at any SS in other BSs.
The link gain parameters are monitored at the SS terminal and reported back to the serving BS from where they are exchanged among in-group BSs by inter-BS signaling. For example, SS i in the interferer group of {I, J, K} keeps track of G i I , G i J , and G i K , and reports this information to the serving BS I as often as necessary. BS I shares this information with in-group BSs J and K. It is important to note that the channel changes slowly because of the fixed SS locations; this yields low Doppler shifts in FBWA networks. Therefore, link state reporting does not have to be very frequent, which makes it completely feasible in such systems.
Since the inter-BS signaling is performed only among ingroup interferers, BSs do not have knowledge about the outof-group interference, and hence the estimated SINRs do not include the second denominator term in (2). The estimated SINRs for orthogonal ISOISS-AMC scheme, γ i O , and for opportunistic nonorthogonal ISONOISS-AMC scheme, γ i ONO , for SS i are given as follows: From (3), we see that only the link gains from the serving BSs to desired SSs, for example, (4). The number of in-group interference contributing terms in the denominator of (4) equals the number of in-group BSs transmitting simultaneously, minus one.

Intersector scheduler in the intermediate ISOISS-AMC scheme
Similar to ISOISS-FM scheme, this scheme is orthogonal as well; however, it employs AMC instead of fixed modulation and makes channel-state-based scheduling decisions as opposed to the arrival-time-based decisions in ISOISS-FM. At where w is the BS that wins the scheduling opportunity. The selected BS predicts the SINR according to (3) or a similar expression. Using this estimated SINR, the feasible AMC mode is chosen from Table 1 and the packet is scheduled for the instant. It should be noted that the modulation schemes listed in Table 1 are the mandatory schemes for downlink transmissions recommended by the 802.16 a standard [1].

Intersector scheduler in the proposed ISONOISS-AMC scheme
Using estimated SINRs from (4), the intersector scheduler finds a combination of concurrent transmissions that gives the highest aggregate throughput efficiency. If queues of all in-group BSs are nonempty, there are seven possible combinations of transmissions at a particular instant. For example, all three BSs transmit (1 choice) or two BSs transmit (3 choices), or only one BS transmits (3 choices). We note that the last 3 choices are only available transmission options in ISOISS-AMC. For each combination, first, the SINRs are estimated from exchanged information as discussed. Then, the spectral efficiency for each transmission is calculated. Finally, the aggregate spectral efficiency for the combination of simultaneous transmissions is predicted. Let us illustrate the steps for the first combination when all three BSs I, J, and K have potential to transmit concurrently to respective SSs i, j, and k. Each reception will have two in-group interferers. Therefore, according to (4) the estimated SINR at SS i's packet, given I, J, and K are transmitting simultaneously, is Similarly, for BSs J and K, γ j|(I,J,K) ONO and γ k|(I,J,K) ONO can be found in a straightforward manner.
From these estimated SINRs, the achievable AMC modes, and corresponding spectral efficiencies η I , η J , and η K can be obtained from Table 1. Then, the aggregate spectral efficiency Γ I,J,K for the combination is calculated from the following relation: where t i d , t j d , and t k d are the transmission durations for BSs I, J, and K's packet determined by the packet length and AMC modes as discussed later. The longest transmission time among all three transmission durations is denoted as Similarly, aggregate spectral efficiencies for other combinations, namely Γ I,J , Γ J,K , Γ K,I , Γ I , Γ J , and Γ K , can be calculated. Service opportunity is granted to the combination of BSs that gives highest aggregate spectral efficiency according to the following: where w is the set of BSs transmiting concurrently. We note here that packets in different BSs take different lengths of frame time due to the variability of packet size, modulation level, and coding rate. In order to avoid excessive interference, a new scheduling event cannot be made until the largest transmission time t r of the previous event elapses.

Out-of-group interference guard
An effort has been made in order to avoid out-of-group interference as much as possible in all simulated scheduling schemes by using groupwise time partitioning in the frame. The frame is partitioned into three subframes (SFs), indexed as SF1, SF2, and SF3 from start to the end of the frame. BSs in the interferer group {I, J, K} schedule their traffic with the subframe sequence of {SF1, SF2, SF3}, while, group {I , J , K } and {I , J , K } use the sub-frames in the sequences of {SF2, SF3, SF1} and {SF3, SF1, SF2}, respectively. Clearly, this technique is effective as long as the arriving traffic in each group is such that it can be accommodated into 1/3 of the frame. However, the system must be designed for loaded network where out-of-group interference is inevitable.
SINR estimations discussed in Section 3.1 do not take the out-of-group interference into account. As a result, the estimations are optimistic, which might result in higher packet error rate. To investigate the effects of out-of-group interference on network performance, we consider an out-of-group interference guard while making SINR estimations. Let us denote that 50th percentile value of the error between the actual and estimated SINR is φ(l) (dB), which is a function of the network loading l users/sector. There could be numerous ways to find this error in a real network. For example, the network can be equipped with a mechanism to track outof-group interference from history. However, in this study, we find this error from simulations as follows. First, a set of SINRs for different loading values is noted in the presence of out-of-group interferers. Then, a second set is generated where the out-of-group interferers are neglected. Now, the difference of the 50th percentile SINR (dB) of these two sets gives φ(l). Table 2 shows different φ(l) values for different network loading levels obtained in the ISONOISS-AMC scheme. We investigate the effect of this guard only for the proposed scheme. The amount of error φ(l) (dB) is subtracted from (4) (dB) to obtain the expected SINR in ISONOISS-AMC. The estimated SINR with guard at SS i's packet, given I, J, and K are transmitting simultaneously, is ONO,guard = 10 log 10 However, while employing this guard is expected to improve the packet error rate performance of the proposed scheme, it will lower the throughput, as the scheduler chooses the AMC modes more conservatively. Therefore, this interference guard can be regarded as a system design parameter to be adjusted according to desired tradeoff between the packet error rate and throughput efficiency.

A note on implementations
It should be mentioned that in a practical deployment scenario, a single BS would qualify as a member of three independent interferer groups for the above-described setting. Therefore, there is an issue of resolving the conflicts that might arise from the commands of three different groups. Our focus in this paper is to present the basic concept of opportunistic nonorthogonal scheduling; nevertheless, we state a number of solutions to this issue. First, the interferer groups can be determined in such a way that each BS can only be a member of only one interferer group. This deployment 6 EURASIP Journal on Wireless Communications and Networking solution would result in some degradation in performance in terms of overall network interference; however, this solution would still control in-group interference for a subset BSs in the group. Secondly, even when a BS is a member of different interferer groups and receives different commands, a second tier of the control scheme (e.g., the majority rule algorithm) can be employed to resolve the conflicts. For instance, when a BS is a member of three groups, it can only transmit when the decisions from two or more groups go in favor of transmissions. Table 3 summarizes system parameters used in this study.

SYSTEM MODEL
The path-loss model has been taken from [20,21]. For a transmitter-receiver (T-R) separation of d meters the largescale path-loss (in linear scale) PL including shadowing is given by the following relation: where n is the propagation exponent (we have taken n = 3.75 for 50-meter antenna height in terrain type C; see [20,21] for details). Parameter d 0 is the close-in reference distance considered to be 50 m, f is the operating frequency in MHz, λ is the operating wavelength related to speed of light c and operating frequency f , and h r is the receiver antenna height in meters which is considered to be 3 meters. Parameter X σ is a Gaussian distributed random variable with a mean of 0 and a standard deviation of σ used for shadowing. We have considered independent lognormal random variables with a standard deviation of 8 dB for shadowing. Time-correlated flat Rayleigh fading with Doppler frequency of 2.0 Hz has been considered in this study, where the Doppler spectrum S( f ) is given by the following equation [20,21]: In the above, To evaluate the proposed scheme, real-time video traffic is used in this study. Two interrupted renewal process (2IRP) sources are superimposed to model the user's video traffic in the downlink transmission as indicated in [22]. The average packet rate of one 2IRP generator is 126.3 packets per second determined from parameters given in Table 4. The length of packets is assumed to be variable and is uniformly distributed between 250 to 550 bytes. Therefore, the average downlink data rate for each SS is 404.16 kbps.
End-to-end packet delay is the summation of queuing delay and packet transmission delay. Packet transmission delay depends on the packet size L p , symbol rate of the transmission channel r s , modulation level M, and coding rate r c , and is expressed as We assume asynchronous transmission such that interferers may arrive or leave anytime during the transmission time of a packet of interest. Therefore, SINR varies, and the packet experiences different bit error rates at different segments of the packet. The number of erroneous bits in a segment s is given by the product of the probability of the bit error in the segment Pr b(s) and the number of bits corresponding to the segment length N b(s) . The total number of bits in error in the packet N e can be written by the following relation: where S is the total number of segments in that packet experiencing different SINR. The total number of erroneous bits is used to decide whether the packet is received correctly. In simulations, we Mahmudur Rahman et al.  assume that a packet is considered to be in error if more than 1% of the total bits present in the packet are erroneous. Retransmissions of erroneous packets by automatic repeat request (ARQ) are not considered in this study. The frame length is considered to be 5 milliseconds. Packets are scheduled in a frame-by-frame basis at the start of every frame. Any packet arriving at current frame time will have to wait at least until the start of the next frame.

SIMULATION RESULTS
The performance of the proposed scheduling scheme ISONOISS-AMC is evaluated by comparing it with that of the reference scheme ISOISS-FM in terms of the essential network performance parameters such as packet error rate, area spectral efficiency, packet dropping rate, and the mean end-to-end packet delay. Also, the performance of ISOISS-AMC is shown in order to quantify the benefits of employing AMC alone. These performance metrics are functions of network loading and are observed against the number of SSs per sector (varied from 4 to 24).
The packet error rate is the ratio of the number of erroneous packets to the total packets received during the simulation period. The area spectral efficiency is expressed as the correctly received information bits per second per Hz per sector. Packet is dropped from the BS queue when the queuing delay exceeds 195 milliseconds. The delay constraint is assumed to be 200 milliseconds (For interactive video, such as videoconferencing) with a 5-milliseconds safety margin provided to ensure that every packet received by the SS meets the delay requirement. We express packet dropping rate in packets per frame per sector. The mean end-to-end delay measure does not include the delays of the dropped packets in the queue at transmitter side.  The network simulation is executed in real time, using OPNET [19] Modeler and wireless module, and the statistics are taken over a long enough time for the observed parameters to converge. It should be noted that shadowing for a particular SS does not change over simulation time as the SS location is fixed. At any loading, a set of shadowing values is assigned for all SSs (randomly placed) in the network. During the course of simulation time, neither the locations of SSs nor the shadowing values are changed. For any particular SS, fading is correlated and it changes over time. Therefore, performed simulation is Monte Carlo in the time axis, but not for SS locations and shadowing. However, statistics are collected in sectors of all nine cells in the network, and hence there is a certain degree of averaging with respect to the SS locations. Figure 3 shows the percentage of the scheduling decisions that yields into 1, 2, and 3 (all) in-group BSs transmissions in ISONOISS-AMC scheme. We observe that around 35% of the time, the scheme is capable of using opportunistic nonorthogonality in transmissions (all three BSs transmit 5% of the time and any 2 BSs transmit 30% of the time) giving higher aggregate spectral efficiency than single transmission. Figure 4 compares the packet error rate performance of the proposed, reference, and intermediate schemes. The modulation and coding level used in the reference ISOISS-FM scheme is more robust than the channel-state-based chosen AMC modes in the proposed ISONOISS-AMC scheme. Also, increased number of packets in the air results in increased number of out-of-group interferers in ISONOISS-AMC scheme. Consequently, the packet error rate in proposed scheme is higher. The packet error rate of ISOISS-AMC fall in between the reference and proposed schemes as ISOISS-AMC suffers less from interference in comparison to   ISONOISS-AMC. However, when out-of-group interference guard is considered in ISONOISS-AMC, packet error rate is reduced drastically and the resulting error rate is comparable to that of ISOISS-AMC.
We present area spectral efficiency and net throughput in Figures 5 and 6, respectively. Although packet error rate is high, ISOISS-AMC and ISONOISS-AMC show tremendous improvements in terms of area spectral efficiency and net throughput. This is because the intermediate and proposed schemes are capable of using much higher AMC modes whenever possible in comparison to 16-QAM with a coding rate of 1/2 mode used in ISOISS-FM; therefore, a larger num-  ber of packets per frame can be transmitted in these schemes. While the area spectral efficiency in ISOISS-FM is limited by around 0.6 bps/Hz/sector, the proposed ISONOISS-AMC shows an area spectral efficiency of around 2.2 bps/Hz/sector at the network loading of 24 SSs per sector. ISOISS-AMC delivers spectral efficiency of around 1.65 bps/Hz/sector at the same loading. At this loading value, around 3 times higher area spectral efficiency and throughput are achieved in the ISONOISS-AMC compared to those obtained in the ISOISS-FM. Improvements in ISONOISS-AMC compared to ISOISS-AMC are solely due to the benefits of in-group opportunistic multiple transmissions. As employing out-ofgroup interference guard in the proposed scheme led the schedulers to choose AMC modes conservatively, the area spectral efficiency and net throughput are reduced slightly. However, while packet error rates are similar in ISONOISS-AMC with guard and in ISOISS-AMC, the former achieves much higher area spectral efficiency and net throughput. Figure 7 illustrates the mean end-to-end delay. We observe that the delay reaches the threshold 200 milliseconds for a loading level as low as 6 SSs per sector in the ISOISS-FM scheme. Because of less efficient AMC mode usage, fewer packets get transmitted per frame in the ISOISS-FM scheme. As a result, the queue length grows even at very low loading levels such as 5 or 6 SSs per sector, causing high mean end-to-end delay. In ISONOISS-AMC, on the other hand, the queues grow at much higher loading levels, as the proposed scheme is able to use efficient AMC modes, and it allows concurrent transmissions among in-group BSs. Therefore, we notice a much better delay performance in the proposed scheme compared to the reference scheme. For instance, for a mean delay of 50 milliseconds, ISOISS-FM supports only 4 SSs, while ISONOISS-AMC is able to support 16 SSs in a sector. Once again, the mean end-to-end delay in ISOISS-AMC falls between those in ISOISS-FM and  in ISONOISS-AMC as expected. Observed improved delay performance in ISONOISS-AMC compared to ISOISS-AMC is due to the simultaneous in-group transmissions in the ISONOISS-AMC scheme. When out-of-group interference guard is used in ISONOISS-AMC, the mean end-to-end delay increases slightly, however, it is always less than that in ISOISS-AMC.
The comparison of packet dropping rate is shown in Figure 8. ISONOISS-AMC shows much better performance than ISOISS-FM in terms of packet dropping rate for the same reasons as for the delay. The packet dropping rate in the intermediate scheme ISOISS-AMC is lower than that obtained in ISOISS-FM and higher than that observed in ISONOISS-AMC.
It is observed that the performances of ISOISS-AMC and ISONOISS-AMC are comparable until the loading level of 12 users/sector. This is due to the fact that at this point of loading, ISOISS-AMC becomes fully loaded and packets start to drop, while ISONOISS-AMC still has some capacity left in the frame. The difference in performance increases as the loading values grow further beyond this point. Simulations are prevented from going beyond 24 users/sector due to the long simulation time needed. However, the trends of the performance curves show that the benefits in ISONOISS-AMC are even higher at higher loading than presented here.

CONCLUSIONS
The benefits of combining link-state-based scheduling decisions, AMC, and opportunistic nonorthogonal transmissions in fixed broadband wireless access networks have been investigated in this paper. A reference orthogonal scheduling scheme that makes arrival-time-based scheduling decisions and uses fixed modulation, namely ISOISS-FM, has been adapted from [13]. The intermediate scheme, ISOISS-AMC, is still orthogonal, while it makes link-state-based scheduling decisions and uses AMC. Finally, the proposed interference-aware scheme, ISONOISS-AMC, makes linkstate-based scheduling decisions, employs AMC, and allows controlled in-group interference in order to improve throughput and packet delay.
It has been observed that the area spectral efficiency in ISONOISS-AMC is around three times higher than that in ISOISS-FM. Moreover, higher throughput results in significant improvements in end-to-end packet delay and packet dropping rate in ISONOISS-AMC. To quantify the benefits of AMC alone, we also have studied ISOISS-AMC, which outperforms the reference scheme in terms of area spectral efficiency, net throughput, mean end-to-end delay, and packet dropping rate. The proposed ISONOISS-AMC achieves up to 33% better area spectral efficiency than the intermediate ISOISS-AMC scheme. This improvement is solely due to the opportunistic nonorthogonal transmissions in the proposed scheme.
While the proposed scheme shows performance improvements in terms of area spectral efficiency, delay, and packet dropping rate, it experiences higher packet error rate due to increased number of uncontrolled out-of-group interferers. However, when out-of-group interference guard is used in ISONOISS-AMC, the packet error rate becomes comparable to that observed in ISOISS-AMC. Nevertheless, if even 10% packet error rate is allowed by the upper layer, the proposed ISONOISS-AMC can support as many as 16 SSs per sector with mean packet delay of around 50 milliseconds and the reasonable packet dropping rate, while ISOISS-FM supports only 4 SSs. For the similar packet error rate and mean end-to-end delay, the ISOISS-AMC scheme can accommodate 13 SSs per sector.   To exploit these potentialities, simulation modeling and theoretical analyses have to be complemented by real experiences (e.g., experiences/measurements on real prototypes) which provide both a direct evaluation of ad hoc networks and, at the same time, precious information for a realistic modeling of these systems.
The need for more experimental activities stimulated a new community of researchers combining theoretical research on ad hoc networking with experiences/measurements obtained by implementing ad hoc network prototypes. This community is growing as witnessed by several workshops organized worldwide. In particular, this special issue is tightly coupled with the REALMAN workshop series, http://www.cl.cam.ac.uk/realman. REALMAN workshops constitute a unique forum for presenting and discussing measurement studies and experiences based on real ad hoc networks testbeds and prototypes. Following the success of the first edition (REALMAN 2005), REALMAN 2006 will be the satellite workshop of the most prestigious symposium for mobile ad hoc networks, ACM MobiHoc 2006.

Aims and scope of the Special Issue
The aim of this special issue is to present, from an experimental perspective, architectures and protocols for implementing (mobile) ad hoc networks. This special issue constitutes a unique forum to present measurements/experiences/lessons obtained by implementing ad hoc networks testbeds and prototypes. The special issue will investigate the whole protocol stack from enabling technologies to middleware and applications.
Original contributions are solicited, related to system and protocol development, measurement, and testing, in all areas related to ad hoc networking. Specifically, topics of interest include, but are not limited to: • Ad hoc networks applications • System prototypes and experiences

Call for Papers
Sensor networks have been researched and deployed for decades already; their wireless extension, however, has witnessed a tremendous upsurge in recent years. This is mainly attributed to the unprecedented operating conditions of wireless sensor networks (WSNs), that is, • a potentially enormous amount of sensor nodes, • reliably operating under stringent energy constraints.
The wireless sensor networks' virtually infinite degrees of freedom have ignited feverish research activities, having led to thousands of publications, white papers and patents in less than a decade, with new contributions emerging on a daily basis. The rich mathematical and technical toolboxes already available from the design of wireless cellular and ad hoc systems clearly aided the birth of new ideas tailored to the problems in WSNs.
To date, and this may very well change in forthcoming years, the main problem in deploying WSNs is their dependence on scarce battery power. A main design criterion is hence to extend the lifetime of the network without jeopardizing reliable and efficient communications between the sensor nodes as well as from the nodes to one or several data sinks. A prominent example of today's non-optimized WSN deployment experiences is that the start-up alone costs the network half of its battery power.
Optimizing every facet of the communication protocols is hence vital and imperative; such stringent design requirements can be met by a plethora of approaches, for example, optimizing each layer of the protocol stack separately (traditional) or jointly (cross-layer), for each node separately (traditional) or for an ensemble of nodes (distributed and cooperative), and so forth. This has led to copious novel distributed signal processing algorithms, energyefficient medium access control and fault-tolerant routing protocols, self-organizing and self-healing sensor network mechanisms, and so forth.
In the light of the above, the main purpose of this special issue is twofold: • to obtain a coherent and concise technical synthesis from the abundance of recently emerged material in the area of WSNs, • to promote novel approaches in analyzing, designing, and optimizing large-scale WSNs, preferably inspired by approaches from other disciplines, such as physics or biology.
As for the first one, very few papers are currently available which synthesize the large amount of fairly dispersed technical contributions; a coherent exposure, also touching upon open research issues, will certainly be appreciated by the academic and industrial research community. As for the second one, we believe that novel approaches, potentially inspired by entirely disjoint disciplines, may help considerably in dealing with networks of thousands of nodes.

Topics of Interest
Topics of interest in the area of energy-constraint WSNs include (but are not limited to): • Network capacity w/o imperfections • Joint source and channel coding • Cooperative and distributed signal Data fusion and dta aggregation processing • Novel PHY, MAC, and network paradigms • Cross-layer and cross-functionality design • Security, robustness, and reliability • Self-healing, self-stabilization, and self-organization • Applications, architectures, and topologies • (Macroscopic) information flows • Physically and biologically inspired approaches

Call for Papers
The adoption of multiple-antenna techniques in future wireless systems is expected to have a significant impact on the efficient use of the spectrum, the minimisation of the cost of establishing new wireless networks, the enhancement of the quality of service, and the realisation of reconfigurable, robust, and transparent operation across multitechnology wireless networks. Although a considerable amount of research effort has been dedicated to the investigation of MIMO systems performance, results, conclusions, and ideas for future directions remain fragmental. Recent trends in MIMO research include reconfigurable multiple-antenna transceivers, cross-layer optimisation and efficient radio resource management for smart antenna networks, antenna technologies for reconfigurable multiple-antenna terminals, and smart antenna deployment issues.
The objective of this special issue is to invite contributions on the most recent developments and promising future directions in the field, with emphasis on reconfigurable transceiver design, efficient resource management, realistic performance evaluation, and implementation aspects.
Topics of interest include (but are not limited to):   The enormous available bandwidth, the wide scope of the data rate/range trade-off, as well as the potential for very-lowcost operation leading to pervasive usage, all present a unique opportunity for UWB systems to impact the way people and intelligent machines communicate and interact with their environment.
The aim of this book is to provide an overview of the state of the art of UWB systems from theory to applications.
Due to the rapid progress of multidisciplinary UWB research, such an overview can only be achieved by combining the areas of expertise of several scientists in the field.
More than 30 leading UWB researchers and practitioners have contributed to this book covering the major topics relevant to UWB. These topics include UWB signal processing, UWB channel measurement and modeling, higher-layer protocol issues, spatial aspects of UWB signaling, UWB regulation and standardization, implementation issues, and UWB applications as well as positioning.
The book is targeted at advanced academic researchers, wireless designers, and graduate students wishing to greatly enhance their knowledge of all aspects of UWB systems. For any inquiries on how to order this title please contact books.orders@hindawi.com