Enhanced Multi-Antenna Technologies on OFDM for Future Mobile Communication Systems

Various types of multi-antenna techniques have been discussed to realize high speed data transmission in mobile communication systems. Those multiple antenna systems obtain space diversity gain by utilizing multi-paths as independent channels from transmitter and receiver. One important issue to utilize the powerful diversity gains of those multi-antenna technologies is how to create independent channels between transmitter and receiver. Almost all of conventional multi-antenna technologies have been discussed with assuming that there is not any correlation among channels between transmitter and receiver. However in realistic world this assumption is not always true and the correlation among channels becomes very high. This high correlation drastically degrades the performance of those multi-antenna technologies. In this paper we present an enhanced transmission diversity method to solve the problem of high correlation among channels. The following contents of this paper mainly focus on the Space Time Turbo Coding in OFDM systems, however the concept of the presented method can be adopted in various types of multi-antenna systems. This revised version was published online in August 2006 with corrections to the Cover Date.     

Radio Resource Allocation for Scalable Video Services Over Wireless Cellular Networks

Good quality video services always require higher bandwidth. Hence, to provide the video services e.g., multicast/broadcast services (MBSs) and unicast services along with the existing voice, internet, and other background traffic services over the wireless cellular networks, it is required to efficiently manage the wireless resources in order to reduce the overall forced call termination probability, to maximize the overall service quality, and to maximize the revenue. Fixed bandwidth allocation for the MBS sessions either reduces the quality of the MBS videos and bandwidth utilization or increases the overall forced call termination probability and of course the handover call dropping probability as well. Scalable video coding (SVC) technique allows the variable bit rate allocation for the video services. In this paper, we propose a bandwidth allocation scheme that efficiently allocates bandwidth among the MBS sessions and the non-MBS traffic calls (e.g., voice, unicast, internet, and other background traffic). The proposed scheme reduces the bandwidth allocation for the MBS sessions during the congested traffic condition only to accommodate more calls in the system. Instead of allocating fixed bandwidths for the MBS sessions and the non-MBS traffic, our scheme allocates variable bandwidths for them. However, the minimum quality of the videos is guaranteed by allocating minimum bandwidth for them. Using the mathematical and numerical analyses, we show that the proposed scheme maximizes the bandwidth utilization and significantly reduces the overall forced call termination probability as well as the handover call dropping probability.     

多播OFDM系统中基于调控因子的资源分配

多媒体多播系统中的动态资源分配技术以其有效利用系统资源,及满足具有差异性信道条件的多用户需求而受到业内广泛研究和重视。针对MRA_LCG算法和MRA_GAT算法存在的弊端,提出了MRA_FCF算法,利用灵活可控的速率均衡因子使得高速率多媒体信息向低速率上分流。仿真结果表明,所提MRA_FCF算法获得的吞吐量明显优于MRA_LCG算法的,且同时在保证组内用户公平性方面比MRA_GAT算法具有优势。     

Cross-Layer Architecture for Adaptive Video Multicast Streaming Over Multirate Wireless LANs

Multicast video streaming over multirate wireless LANs imposes strong demands on video codecs and the underlying network. It is not sufficient that only the video codec or only the underlying protocols adapt to changes in the wireless link quality. Research efforts should be applied in both and in a synchronized way. Cross layer design is a new paradigm that addresses this challenge by optimizing communication network architectures across traditional layer boundaries. This paper presents cross-layer architecture for adaptive video multicast streaming over multirate wireless LANs where layer-specific information is passed in both directions, top-down and bottom-up. The authors jointly consider three layers of the protocol stack: the application, data link and physical layers. The authors analyze the performance of the proposed architecture and extensively evaluate it via simulations. The results show that the real-time video quality of the overall system can be greatly improved by cross-layer signaling.     

双选择性信道条件下OFDM系统的频域Turbo均衡

未来OFDM系统将工作在高载频、高容量、高移动速率的条件下,传统的单抽头频域均衡器将不再适用。该文基于双选择性信道的ICI分析,提出了一种用于OFDM系统的频域Turbo均衡(Frequency-domain Turbo Equalization, FTE)算法。仿真表明该算法具有BER性能好,计算复杂度低的优点,能够很好地抑制ICI。     

基于H.264 SVC和OFDM的可分级视频传输方法

提出了一种基于H.264 SVC国际编码标准和OFDM传输体制的可分级视频传输方法.实验结果表明,与传统的对等保护方法相比,该方法在不同的无线衰落信道下能够得到更好的峰值信噪比(PSNR),同时也具有更好的主观显示效果.     

OFDM系统中视频传输的调度算法研究

针对OFDM系统,提出了一种基于视频内容的跨层调度方案。该方案采用了基于生存期的包排序策略,根据视频包的生存期大小进行排序,保证视频包能在生存期内发送到接收端,减少丢包数目。同时,采用跨层设计的思想,综合考虑了信道状况和视频包特性,如视频包生存期、重要性、解码器采用的错误掩藏方法,提出改进的比例公平调度算法,不仅有效地利用了多用户分集来进一步提高数据吞吐量,也充分考虑了视频的重要性和时延约束。实验结果表明,采用内容感知的跨层调度算法,解码端的视频质量得到有效提高,从而可以提高主观感知质量。     

Blind Receiver Design for OFDM Systems Over Doubly Selective Channels

We develop blind data detectors for orthogonal frequency-division multiplexing (OFDM) systems over doubly selective channels by exploiting both frequency-domain and time-domain correlations of the received signal. We thus derive two blind data detectors: a time-domain data detector and a frequency-domain data detector. We also contribute a reduced complexity, suboptimal version of a time-domain data detector that performs robustly when the normalized Doppler rate is less than 3%. Our frequency-domain data detector and suboptimal time-domain data detector both result in integer least-squares (LS) problems. We propose the use of the V-BLAST detector and the sphere decoder. The time-domain data detector is not limited to the Doppler rates less than 3%, but cannot be posed as an integer LS problem. Our solution is to develop an iterative algorithm that starts from the suboptimal time-domain data detector output. We also propose channel estimation and prediction algorithms using a polynomial expansion model, and these estimators work with data detectors (decision-directed mode) to reduce the complexity. The estimators for the channel statistics and the noise variance are derived using the likelihood function for the data. Our blind data detectors are fairly robust against the parameter mismatch     

Multi-Antenna AF Two-Way Relaying Over Nakagami-m Fading Channels

We present the performance of multi-antenna selective combining amplify-and-forward (SC-AF) two-way relaying systems over independent and identically distributed (i.i.d) Nakagami- $m$ fading channels. The outage probability and symbol error probability of our relaying system are derived in closed-form. In order to get additional insights into the impact of system parameters, we consider the analysis of system at high signal-to-noise (SNR) regime. Through over high SNR analysis, we assume that the fading channels are independent and non-identically distributed (i.n.i.d). Also, there is no assumption on the fading parameter, $m$ . Moreover, some special cases of practical interest (e.g., Rayleigh fading channels, and single relay system) are also examined. Subsequently, we define an optimization problem using some approximations and then solve it analytically. Lastly, we present numerical simulations to check our analytical formulas.     

Dynamic Bandwidth and Carrier Allocation for Video Broadcast/Multicast Over Multi-Cell Environments

Recently, broadcast/multicast over cellular networks has been actively discussed over commercial wireless mobile terminals. Compared to conventional terrestrial or satellite broadcasting systems, the quality-of-service (QoS) for edge users is an important issue due to inter-cell interference over multi-cell environments. In this paper, we introduce a dynamic bandwidth and carrier allocation (DBCA) technique by fully utilizing different visual importance of each layer in multi-layer video for broadcast/multicast services when the number of users is limited over macro/micro/femto cell environments. To ensure an acceptable video quality for edge users, the bandwidth and the loading ratio are dynamically controlled to enhance the utility through the radio resource control in accordance with the visual importance. The simulation results show that DBCA exhibits much better QoS by sending visually more important data with high priority in the cell border region.     

Bit Interleaved Time-Frequency Coded Modulation for OFDM Systems Over Time-Varying Channels

Bit Interleaved Time-Frequency Coded Modulation for OFDM Systems Over Time-Varying Channels Orthogonal frequency-division multiplexing (OFDM) is a promising technology in broadband wireless communications with its ability in transforming a frequency selective fading channel into multiple flat fading channels. However, the time-varying characteristics of wireless channels induce the loss of orthogonality among OFDM sub-carriers, which was generally considered harmful to system performance. In this paper, we propose a bit interleaved time–frequency coded modulation (BITFCM) scheme for OFDM to achieve both time and frequency diversity inherent in broadband time-varying channels. We will show that the time-varying characteristics of the channel are beneficial to system performance. Using the BITFCM scheme and for relatively low maximum normalized Doppler frequency, a reduced complexity Maximum Likelihood (ML) decoding approach is proposed to achieve good performance with low complexity as well. For high maximum normalized Doppler frequency, the inter-carrier interference (ICI) can be large and an error floor will be induced. To solve this problem, we propose two ICI mitigation schemes by taking advantage of the second order channel statistics and the complete channel information, respectively. It will be shown that both schemes can reduce the ICI significantly.     

Internet Multicast Video Delivery

Internet video delivery has been motivating research in multicast routing, quality of service (QoS), and the service model of the Internet itself for the last 15 years. Multicast delivery has the potential to deliver a large amount of content that currently cannot be delivered through broadcast. IP and overlay multicast are two architectures proposed to provide multicast support. A large body of research has been done with IP multicast and QoS mechanisms for IP multicast since the late 1980s. In the past five years, overlay multicast research has gained momentum with a vision to accomplish ubiquitous multicast delivery that is efficient and scales in the dimensions of the number of groups, number of receivers, and number of senders. This work presents an overview of the issues facing both IP and overlay multicast and the approaches that researchers are taking to solve them. Many of these approaches take advantage of a rich interface, beyond a single rate video stream, between the coding and delivery mechanisms. The semantics of this interface is an important question for future research and we discuss this with insight from experience on delivery technologies.     

Network-Adaptive Rate Control and Unequal Loss Protection with TCP-Friendly Protocol for Scalable Video Over Internet

Video communication with Quality of Service (QoS) is an important and challenging task. To have QoS provision at application level in the current best-effort Internet, rate control, congestion control and error control are several effective approaches. In this paper, we propose a new network-adaptive rate control and Unequal Loss Protection (ULP) scheme in conjunction with TCP-friendly congestion control for scalable video streaming. Our proposed approach is capable of simultaneously controlling congestion and packet loss occurred across the Internet. More specifically, we first dynamically estimate the available network bandwidth on the fly. Then, TCP-friendly congestion control is performed to smoothly adjust the sending rate for transmission of continuous media. Considering the characteristic of scalable video, unequal loss protection at packet level is adopted for different video layers while performing congestion control. Consequently, a fixed-length and priority-based packetization scheme is introduced to enhance the capability of loss protection and improve the efficiency of network-bandwidth utilization. Moreover, Rate-Distortion (R-D) based bit allocation is proposed to minimize the expected end-to-end distortion. Simulation results demonstrate the effectiveness of our proposed scheme.     

Doppler Spread Estimation for Broadband Wireless OFDM Systems Over Rician Fading Channels

In this paper, we present a new Doppler spread estimation algorithm for broadband wireless orthogonal frequency division multiplexing (OFDM) systems with fast time-varying and frequency-selective Rayleigh or Rician fading channels. The new algorithm is developed by analyzing the statistical properties of the power of the received OFDM signal in the time domain, thus it is not affected by the influence of frequency-domain inter-carrier interference (ICI) introduced by channel variation within one OFDM symbol. The operation of the algorithm doesn’t require the knowledge of fading channel coefficients, transmitted data, or signal-to-noise ratio (SNR) at the receiver. It is robust against additive noise, and can provide accurate Doppler spread estimation with SNR as low as 0 dB. Moreover, unlike existing algorithms, the proposed algorithm takes into account the inter-tap correlation of the discrete-time channel representation, as is the case in practical systems. Simulation results demonstrate that this new algorithm can accurately estimate a wide range of Doppler spread with low estimation latency and high computational efficiency.     

Advances in Scalable Video Coding

Scalable video coding is attractive due to the capability of reconstructing lower resolution or lower quality signals from partial bit streams. This allows for simple solutions in adaptation to network and terminal capabilities. Different modalities of scalability are specified by video coding standards like MPEG-2 and MPEG-4. This paper gives a short overview over these techniques and analyzes in more detail the encoder/decoder drift problem, which is the major reason why scalable coding has been significantly less efficient than single-layer coding in most of these implementations. Only recently, new scalable video coding technology has evolved, which seems to close the gap of compression performance compared to state of the art single-layer video coding. New methods of efficient enhancement layer prediction were developed to improve traditional (motion-compensated hybrid) scalable coders, providing more flexible compromises on the drift problem. As a new technology trend, motion-compensated spatiotemporal wavelet coding has matured which entirely discards the drift and allows most flexible combinations of spatial, temporal, and signal-to-noise ratio (SNR) scalability with fine granularity over a broad range of data rates.     

Scalable Multicast Provisioning in DiffServ with MPLS Labeling

The phenomenal growth of multimedia applications imposes scalable and efficient network support. The DiffServ and MPLS architectures were developed to provide QoS. The combination of both architectures presents a very attractive strategy to backbone network providers. However, integrating native IP multicasting with MPLS supports DiffServ is a quite comple issue. Major problems are: the lack of labels in MPLS networks, the core routers simplicity in DiffServ and the multicast state scalability problems, since it requires routers to keep a forwarding state for every multicast tree passing through it. In addition, the number of states grows with the number of groups. Under such circumstance, we propose an hybrid label aggregation algorithm in order to solve multicast scalability problem and provide a solution for multicast in MPLS support DiffServ. In the proposed scheme, one label is assigned per multicast groups (logical aggregation) and different multicast groups sharing the same output interface in a router are aggregated locally (physical aggregation). Also, in order to support the proposed algorithm, we propose a separate treatment and labels space (prime numbers range) for multicast traffic. The proposed solution allows consuming fewer labels, reducing the forwarding table and consequently the total packet processing delay.     

The Diversity Gain and ICI Suppression for OFDM Systems Over Doubly Selective Channels

How to compensate the detrimental impact of intercarrier interference (ICI) on BER performance is the key issue of signal detection for orthogonal frequency division multiplexing (OFDM) systems in doubly selective channels. In this paper, we propose an optimal linear ICI suppression algorithm with successive detection based on oversampling at the receiver, by which the OFDM systems can benefit from doubly selective channels for achieving both the multipath diversity and Doppler diversity. To reduce the additional computational complexity caused by oversampling, a low complexity implementation method for this algorithm is also presented. Simulations results show that the proposed algorithm can enhance the BER performance with the increase of the Doppler spread and the oversampled factor.     

可伸缩视频编码标准中的差错控制

可伸缩视频编码标准（SVC）是由ISO/IEC的MPEG专家组和ITU-T的VCEG专家组联合组成的联合视频专家组制定的对H.264/AVC视频编码标准的可伸缩性扩展。SVC通过对时间分辨率、空间分辨率和质量等参数的可伸缩性来适应不同网络环境下用户对视频资料的分辨率、帧率、质量的不同需求,是目前解决这一问题的最好方法之一。由于信道传输中大量存在的衰减、误码和数据丢失,差错控制显得十分重要,因而两种有效的对抗措施——错误弹性编码和错误隐藏技术被引入到SVC中：一部分是直接从H.264中继承而来;还有一部分则是利用了可伸缩视频的自身特性而提出的。文中将对SVC中一些有代表性的错误弹性和错误隐藏技术进行介绍,并给出部分实验结果来展示这些技术带来的性能提升。