An adaptive framework is proposed for multi-user video transmission over orthogonal frequency division multiple access (OFDMA) systems. Utilizing the channel knowledge, a two-step heuristic sub-carrier assignment algorithm is proposed to achieve unequal error protection for the video data. The approach also solves the fairness issue among different users that may be caused by varying channel quality on different sub-carriers. Meanwhile, multi-user channel gain is also achieved. The proposed framework significantly improves the video transmission quality with little extra computational complexity and system overhead.

In this paper, we propose an adaptive video transmission scheme to achieve unequal error protection in multichannel ad hoc networks. In our scheme, video data is divided into high priority (HP) and low priority (LP) portions, and mobile nodes have two channels for video transmission. A channel quality metric, busy time ratio (BTR), is employed to characterize the channel quality. The channel with better BTR metric is used for HP data, and the other for LP. Further, adaptive load control and delay-constrained queue management schemes are proposed to improve performance. Simulation results demonstrate the performance of video streaming is greatly enhanced in multichannel multi-hop ad hoc networks.

Opportunistic transmission scheduling schemes improve system capacity by taking advantage of independent time varying channels in wireless networks. In the design of such scheduling schemes, the fairness criterion plays an important role in the tradeoff of total system capacity and the achievable throughput of individual users. To meet different fairness demands with a unified opportunistic scheduling scheme, in this paper, we have extended the well known opportunistic scheduling scheme PFS into αPFS, which satisfies arbitrary fairness demands, varying from proportional fairness to maxmin fairness, through adjusting the parameter α. To further improve the achievable diversity gains of αPFS, we extend the αPFS scheme into an αPFS-P scheme. Performances of αPFS and αPFS-P are studied and compared. As demonstrated in the simulation results, both αPFS and αPFS-P can achieve adjustable fairness criteria, varying from proportional fairness to max-min fairness. Compared with αPFS, αPFS-P achieves higher diversity gains with degraded short term performance, which is still better than the performance of PFS.

We propose a hybrid multiple-input-multiple-output (MIMO) architecture to implement unequal error protection (UEP) for video delivery in MIMO systems. On the transmitter side, some of the transmit antennas are used to implement transmit diversity for delivering video data with high priority, while others are employed for spatial multiplexing to transmit video data with low priority. On the receiver side, a hybrid signal detection mechanism is adopted to separate and decode the mixed data. The goal is to exploit the diversity gain to provide better protection to the high priority data, while transmitting the low priority data with spatial multiplexing to achieve high data rate. Analysis and simulation results demonstrate that the proposed UEP mechanism significantly enhances the quality of video reception with high spectrum efficiency.

In this paper, we propose a new scheduler for orthogonal frequency-division multiplexing (OFDM) wireless communication systems, called channel-condition and packet-length dependent packet generalized processor sharing (CPLD-PGPS) scheduler. Based on PGPS, the CPLD scheduler considers both the physical channel condition and the length of packets, and optimally allocates the sub-carriers to different users. The total transmit power is adoptively allocated to each subcarrier. With this scheduler, the system can achieve better system BER performance, and correspondingly superior PER performance. The system throughput is improved, the required bandwidth is guaranteed, and long term fairness for all traffic in the system is provided. In order to reduce the complexity, a simplified algorithm is proposed, which maintains the system throughput as in the original scheduler, and guarantees the system performance with properly set system parameters. The superior performance of the proposed schedulers is demonstrated by simulation with multimedia traffic

In this paper, we present an accurate frequency offset estimation method for frequency-selective channels. Through the iterative use of the chirp z-transform (CZT) algorithm, an accurate frequency offset estimator is proposed, approaching the Cramer-Rao bound (CRB) even at low signal-to-noise ratio (SNR). Further, the estimation can he achieved within one block of training sequence, thus avoiding the transmission of repetitive known blocks as is usually required in many conventional methods. Meanwhile, the overall complexity is acceptable. More importantly, the CZT operation can utilize the fast Fourier transform (FFT) structure that is favourable for digital signal processor (DSP) implementation. Simulation results show that two or at most three iterations of the CZT computation are sufficient for an accurate frequency offset estimation in the SNR range from 0 dB to 30 dB.

A novel receiver-initiated soft-state probabilistic multicasting protocol (RISP) for mobile ad hoc network is proposed in this paper. RISP introduces probabilistic forwarding and soft-state for making relay decisions. Multicast members periodically initiate control packets, through which intermediate nodes adjust the forwarding probability. With a probability decay function (soft-state), routes traversed by more control packets are reinforced, while the less utilized paths are gradually relinquished. In this way, RISP can adapt to node mobility; at low mobility, RISP performs similar to a tree-based protocol; at high mobility, it produces a multicast mesh in the network. Simulation results show RISP has lower delivery redundancy than mesh-based protocols, while achieving higher delivery ratio. Further, the control overhead is lower than other compared protocols.

To maximize system throughput and guarantee the quality of service (QoS) of multimedia traffic in orthogonal frequency division multiplexing (OFDM) systems with smart antennas, a new packet scheduler is introduced to consider QoS requirements, packet location in the frame, and modulation scheme. In OFDM, several consecutive subcarriers are grouped as a frequency subband. Each subband in a frame can be reused by several users with smart antennas. In this paper, based on the best-fit algorithm proposed for TDMA and the physical features of OFDM, a new packet scheduler is proposed to allocate different BER-classified traffics into the frame. Adaptive modulation is also applied in the scheduler. When compared with existing schedulers, our scheduler achieves higher system capacity with much reduced complexity. The use of adaptive modulation further enhances the system capacity. Simulation results demonstrate that as the traffic load increases, the new scheduler has much better performance in system throughput, average delay, and packet loss rate.

The original power controlled multiple access (PCMA) protocol does not support service differentiation. In this paper, we extend PCMA to form a new media access control protocol supporting service differentiation in mobile ad hoc networks. To support QoS, we first introduce the in-station access category concept in 802.11e to PCMA. For service differentiation between access categories, our major contribution is to propose a sender-initiated busy tone based mechanism that allows a user to gain quick channel access. This quick access mechanism is only performed when the number of access failures exceeds a threshold. An access category with higher priority is assigned a lower threshold for easier channel access, and vice versa. Through analysis and simulation, we demonstrate that our protocol can provide better quality of service than 802.11e in terms of throughput, delay, loss, and fairness.

Multicasting protocols deliver data packets from a source node to multiple receivers, and serve a very important function in mobile ad-hoc networks (MANETs). In this paper, a novel receiver-initiated soft-state probabilistic multicasting protocol (RISP) for MANETs is proposed. RISP is inspired by the ant colony's route-seeking mechanism, in which an individual ant chooses the optimal path to its destination through cooperation with others in a totally distributed manner. Imitating the behaviour of ants in nature, RISP introduces probabilistic forwarding and soft-state for making relay decisions that are automatically adaptive to node mobility in MANETs. Compared with other protocols, we show by computer simulations that RISP has lower delivery redundancy, while achieving higher delivery ratio at all mobility scenarios. Furthermore, RISP has lower control overhead.

In this paper, we propose a new scheduler for orthogonal frequency-division multiplexing (OFDM) wireless communication systems, called channel-condition and packet-length dependent packet generalized processor sharing (CPLD-PGPS) scheduler. The CPLD scheduler considers the condition of the physical channel and the length of packets at the same time, and optimally allocates the subcarriers in a frame. With this scheduler the system can achieve better system BER performance, and correspondingly superior PER performance. The system throughput is improved, at the same time the required bandwidth is guaranteed, and long term fairness for all the traffic in the system is provided. In order to reduce the algorithm complexity, a simplified CPLD is proposed, which maintains the system throughput as in the original scheduler, and guarantees the system performance with properly set system parameters. The superior performance of the proposed scheduler is demonstrated by simulation results.

We provide a sub-channel partitioning based unequal error protection (UEP) scheme for a space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) system. In such a scheme, video data is partitioned into high-priority (HP) and low-priority (LP) layers according to the importance of the data. At the receiver side, OFDM sub-channels are partitioned into high-quality (HQ) and low-quality (LQ) groups according to the estimated channel qualities. Based on the feedback of sub-channel partitioning results, the transmitter assigns HP and LP video data to the corresponding HQ and LQ sub-channels. Through theoretical analysis, we show there is indeed a significant BER difference between the HQ and LQ sub-channels, which can be exploited by UEP. Based on the analysis, we provide a criterion for determining the appropriate transmission power. Through computer simulations, we show that the proposed scheme offers significant performance gain compared to conventional methods. We also demonstrate that the scheme is the least sensitive to channel estimation errors among all compared schemes, and is hardly influenced by the Doppler spread. The feedback overhead can also be reduced with almost no performance penalty by bundling several neighboring sub-channels together and assigning them to the same group.

Many space-time codes (STC) have been proposed to enhance the performance of wireless communications in flat fading channels. All of them rely on the knowledge of the channel, and are hence affected by the channel estimation errors. In this paper, we investigate STC robustness under imperfect channel knowledge. We first define the concept of "closeness" by comparing the BER under channel estimation errors with that under perfect channel knowledge, aiming to characterize STC performance degradation due to imperfect channel knowledge. Then the robustness of STC can be compared by their "closeness" to perfect results. We find that for systems with two and three transmit antennas, the space time block codes (STBC) are always more robust to channel estimation errors than space time trellis codes (STTC). With the increase of receive diversity, all STC become more robust to the channel estimation errors. For STTC, as the number of trellis states increases, the codes become less robust to the channel estimation errors. We also compare the BER performance of STC in the presence of channel estimation errors. For the two-transmit-antenna system, the performance of STBC is always better than that of the 4-state STTC, but is always worse than 16-state STTC. For systems with three transmit antennas, the BER performance of STTC is much better than that of STBC.

In this work, we introduce the idea of adaptive sub-channel allocation based unequal error protection (ASCA-UEP) to a space-time block coded orthogonal frequency division multiplexing (STBC-OFDM) system. In such a system, UEP is realized by adaptively allocating and transmitting high-priority and low-priority video data over high-quality and low-quality sub-channels, respectively. Further, we propose two ASCA-UEP schemes in a time division duplex (TDD) system: a receiver-based scheme and a transmitter-based scheme. Analysis and simulation results demonstrate that ASCA-UEP greatly enhances the quality of video reception, and the transmitter-based scheme is more robust to uplink channel noise than the receiver-based scheme, and is thus preferred when the receiver is power-constrained and the transmitter has sufficient power.

The problem of super-resolution time delay estimation in multipath environments is addressed in this paper. Two cases, active and passive systems, are considered. The time delay estimation is first converted into a sinusoidal parameter estimation problem. Then the sinusoidal parameters are estimated by generalizing the multiple signal classification (MUSIC) algorithm for single-experiment data. The proposed method, referred to as the MUSIC-type algorithm, approximates the Cramer-Rao bound (CRB) in terms of the mean square errors (MSKs) for different signal-to-noise ratios (SNRs) and separations of multipath components. Simulation results show that the MUSIC-type algorithm performs better than the classical correlation approach and the conventional MUSIC method for the closely spaced components in multipath environments.

In this paper, we study the uplink performance of an orthogonal frequency division multiplexing (OFDM) wireless system where multiple antennas are utilized at the base station (BS). Further, capacity can be greatly enhanced through spatial division multiple access (SDMA), so that several users can transmit packets simultaneously to the BS. The system performance is determined by various transmission techniques, including methods for channel estimation, modulation, as well as channel coding. Conventional parameters such as packet error rate (PER) and bit error rate (BER) are unable to reflect the actual system performance because no consideration is given to the overheads incurred by the transmission techniques. Therefore, we are motivated to propose a novel concept called effective throughput to characterize the capacity available to users by incorporating all these factors. The effective throughput for a user can be viewed as the average number of successfully received data bits in an OFDM symbol after excluding erroneously received packets and the overheads due to channel estimation and coding. It also directly relates to the transmission delay of a user packet. The system effective throughput is the aggregated effective throughput of all users. Simulation results demonstrate that effective throughput can serve as a useful and more meaningful benchmark parameter in optimizing system performance.

We study the slotted ALOHA with multiple random access channels, the so called multi-channel ALOHA (MC-ALOHA). It is well known that single-channel ALOHA (SC-ALOHA) is unstable. Not surprisingly, MC ALOHA is also unstable. A stabilization algorithm for MC-ALOHA has been proposed in [D. Shen and V.O.K. Li, 2002], in which the pseudo-Bayesian algorithm in SC-ALOHA was extended to achieve stabilized MC-ALOHA. The idea is to estimate the number of attempting users so that user transmission probability can be adjusted accordingly. In this paper, we give a theoretical analysis on the algorithm performance for cases with limited and unlimited number of users by assuming perfect estimate. The theoretical results are validated by simulation, which shows the stabilization algorithm performs close to a system with perfect estimate. The simulation results also show that the performance of the stabilized algorithm is much better than the non-stabilized algorithm. With the stabilized algorithm, the system is always stable when the new packet arrival rate is less than system capacity. Even when the arrival rate is higher than capacity, system throughput can still be guaranteed.

This paper investigates the performance of coded orthogonal frequency division multiplexing (OFDM) systems with receiving adaptive array through which multiple users with single-element transmitting antenna are supported simultaneously in spatial division multiple access (SDMA). We characterized the performance of an OFDM/SDMA systems by effective throughput, which is essentially the average number of data bits in an OFDM symbol after considering the erroneous packet transmissions and modulation scheme by excluding the overhead from coding and pilots for channel estimation. Optimization of system operating parameters can be achieved through the maximization of effective throughput. The focus of the performance of coded OFDM/SDMA systems. Through extensive computer simulation, we show that using more pilots always improves bit error rate (BER) performance, but may reduce effective throughput. The optimal number of pilots together with the modulation scheme can be determined by maximizing the effective throughput for given operating signal-to-noise ratio (SNR). It is also shown that the system performance degrades gradually with the increase of users. For a system with a six-element adaptive array, the effective throughput with 5 users is lower than that with 4 users for a certain range of SNR. This indicates that the maximal number of users supportable by the system should consider the effective throughput.

Multiple access based on orthogonal frequency division multiplexing (OFDM), or OFDMA, enables multiple users to simultaneously access the media by using different subcarriers. This leads to the convenient realization of multi-channel ALOHA, in which each user transmits with a group of subcarriers. In this paper, we first introduce the multi-channel slotted ALOHA algorithm to OFDM, which is called OFDMA-based multi-channel ALOHA (OMC-ALOHA). Since ALOHA is an unstable algorithm, we show OMC-ALOHA is also unstable. To solve this stability problem, we extend the pseudo-Bayesian algorithm to achieve stabilized OMC-ALOHA.

The capacity of code-division multiple-access (CDMA) networks is an important issue, especially when CDMA is used for broadband wireless networks to support high speed data traffic. In this paper, based on a CDMA cell model developed in our prior work, we develop accurate and easily calculable estimations for the capacity of CDMA over large scale fading channels. We show that user bandwidth demand, transmission power and outage requirements have significant impact on network capacity. The above factors can be traded-off with each other, which is also demonstrated by our numerical results.

We investigate the CDMA capacity in the support of heterogeneous traffic. The analysis is based on a CDMA cell model developed in our prior work, which provides a comprehensive yet simple foundation for theoretical analysis. Then, we develop accurate and easily calculable estimations for the capacity of CDMA networks with traffic heterogeneity over large scale fading channels. We show that the user bandwidth demand, transmission capability and outage requirement have significant impact on network capacity. The above factors can be traded-off with each other, which is also demonstrated by our numerical results.

We study the admission control of heterogeneous traffic over the reverse link of broadband CDMA networks. With traffic heterogeneity, users can have diverse QoS requirements such as transmission rates, signal-to-interference ratio (SIR), and outage probability. On the other hand, individual users experience different levels of path loss and fading. All those factors should be considered while admitting a user. Our algorithm uses a concept called effective interference bandwidth to characterize the capability of a user to support user bandwidth demand. According to our previous work, the total bandwidth resource in a CDMA cell is divided into bandwidth available to user traffic for in-cell users, and interference bandwidth for other-cell interference. User effective interference bandwidth characterizes the maximal interference a user can tolerate, and is thus a limiting parameter of cell capacity. Basically, the cell capacity is approximately the total cell bandwidth minus the user effective interference bandwidth. Since the effective interference bandwidth term varies from user to user, each user imposes a different limit on the cell capacity. If a user with high effective interference bandwidth is admitted, it is possible that future users can be blocked, thus increasing the blocking probability. By requiring users with limited effective interference bandwidth, we optimize the overall blocking probability. In designing the admission algorithm, we first evaluate the expected average traffic in a cell. This value is used to compute the maximal allowable effective interference bandwidth. Then user effective interference bandwidth is compared with maximal allowable value for admission decision. Only users with interference bandwidth not exceeding the maximal allowable level are granted admission. Numerical results show that this scheme performs better than not checking user effective interference.

As CDMA is to be used for the third generation wireless networks to provide integrated services, it is important to study how the quality of service, such as packet loss and delay, should be guaranteed. To accomplish this, we develop a cell-wise model in which user's resource consumption is characterized, and the constraint between system bandwidth and resource consumptions of users is given. This model incorporates important factors that affect user's quality of service, including the aggregation of user traffic, user the power constraint, path loss and interference. Based on this model, we develop an admission control procedure. We further study transmission schemes using one and two channels. Numerical results show the two-channel scheme can have more users admitted, especially under heavy interference from other cells and when users' traffic is bursty.

Proactive management holds the promise of taking corrective actions in advance of service disruptions. Achieving this goal requires predictive models so that potential problems can be anticipated. Our approach builds on previous research in which HTTP operations per second are studied in a Web server. As in this prior work, we model HTTP operations as two subprocesses, a (deterministic) trend subprocess and a (random but stationary) residual subprocess. Herein, the trend model is enhanced by using a low-pass filter. Further, we employ techniques that reduce the required data history, thereby reducing the impact of changes in the trend process. As in the prior work, an autoregressive model is used for the residual process. We study the limits of the autoregressive model in the prediction of network traffic. Then we demonstrate that long-range dependencies remain in the residual process even after autoregressive components are removed, which impacts our ability to predict future observations. Last, we analyze the validity of assumptions employed, especially the normality assumption.