Multi-mode Transmission for the MIMO Broadcast Channel with Imperfect Channel State Information

This paper proposes an adaptive multi-mode transmission strategy to improve the spectral efficiency achieved in the multiple-input multiple-output (MIMO) broadcast channel with delayed and quantized channel state information. The adaptive strategy adjusts the number of active users, denoted as the transmission mode, to balance transmit array gain, spatial division multiplexing gain, and residual inter-user interference. Accurate closed-form approximations are derived for the achievable rates for different modes, which help identify the active mode that maximizes the average sum throughput for given feedback delay and channel quantization error. The proposed transmission strategy is combined with round-robin scheduling, and is shown to provide throughput gain over single-user MIMO at moderate signal-to-noise ratio. It only requires feedback of instantaneous channel state information from a small number of users. With a feedback load constraint, the proposed algorithm provides performance close to that achieved by opportunistic scheduling with instantaneous feedback from a large number of users.Comment: 25 pages, 10 figures, submitted to IEEE Trans. Commun., March 201

Adaptive Multicell 3D Beamforming in Multi-Antenna Cellular Networks

We consider a cellular network with multi-antenna base stations (BSs) and single-antenna users, multicell cooperation, imperfect channel state information, and directional antennas each with a vertically adjustable beam. We investigate the impact of the elevation angle of the BS antenna pattern, denoted as tilt, on the performance of the considered network when employing either a conventional single-cell transmission or a fully cooperative multicell transmission. Using the results of this investigation, we propose a novel hybrid multicell cooperation technique in which the intercell interference is controlled via either cooperative beamforming in the horizontal plane or coordinated beamfroming in the vertical plane of the wireless channel, denoted as adaptive multicell 3D beamforming. The main idea is to divide the coverage area into two disjoint vertical regions and adapt the multicell cooperation strategy at the BSs when serving each region. A fair scheduler is used to share the time-slots between the vertical regions. It is shown that the proposed technique can achieve performance comparable to that of a fully cooperative transmission but with a significantly lower complexity and signaling requirements. To make the performance analysis computationally efficient, analytical expressions for the user ergodic rates under different beamforming strategies are also derived.Comment: Accepted for publication in IEEE Transaction on Vehicular Technolog

Mobile and Wireless Communications

Mobile and Wireless Communications have been one of the major revolutions of the late twentieth century. We are witnessing a very fast growth in these technologies where mobile and wireless communications have become so ubiquitous in our society and indispensable for our daily lives. The relentless demand for higher data rates with better quality of services to comply with state-of-the art applications has revolutionized the wireless communication field and led to the emergence of new technologies such as Bluetooth, WiFi, Wimax, Ultra wideband, OFDMA. Moreover, the market tendency confirms that this revolution is not ready to stop in the foreseen future. Mobile and wireless communications applications cover diverse areas including entertainment, industrialist, biomedical, medicine, safety and security, and others, which definitely are improving our daily life. Wireless communication network is a multidisciplinary field addressing different aspects raging from theoretical analysis, system architecture design, and hardware and software implementations. While different new applications are requiring higher data rates and better quality of service and prolonging the mobile battery life, new development and advanced research studies and systems and circuits designs are necessary to keep pace with the market requirements. This book covers the most advanced research and development topics in mobile and wireless communication networks. It is divided into two parts with a total of thirty-four stand-alone chapters covering various areas of wireless communications of special topics including: physical layer and network layer, access methods and scheduling, techniques and technologies, antenna and amplifier design, integrated circuit design, applications and systems. These chapters present advanced novel and cutting-edge results and development related to wireless communication offering the readers the opportunity to enrich their knowledge in specific topics as well as to explore the whole field of rapidly emerging mobile and wireless networks. We hope that this book will be useful for students, researchers and practitioners in their research studies

다중사용자 다중안테나 시스템을 위한 SVD 기반의 유니터리 프로세싱

학위논문 (박사)-- 서울대학교 대학원 : 전기정보공학부, 2013. 8. 이광복.다중 사용자 다중 안테나 프로세싱은 동시에 여러 명의 사용자에게 서비스를 제공함으로써 커다란 셀 용량 증대를 얻을 수 있는 기술로 지난 10년 간 3GPP-LTE Advanced, IEEE 802.16m, IEEE 802.11ac 등의 차세대 무선 통신 표준에 상당한 관심을 받아왔다. 이 학위 논문에서는 현실적인 구현을 고려하여 적은 계산량과 다양한 무선 통신 시스템에 적용 가능성을 지닌 다중 사용자 다중 안테나 프로세싱 방법을 제안한다. 특히 다른 두 무선 통신 시스템인 셀룰라 시스템과 무선랜 시스템에 적용하여 제안하는 다중 사용자 다중 안테나 프로세싱 방법의 평균적인 총 전송률을 평가하고 이를 part I과 part II에 각각 설명한다. 학위 논문의 part I에서는 셀룰라 시스템에 초점을 맞춘다. 셀룰라 시스템은 사용자의 채널 정보를 피드백하기 위해 적은 양의 피드백 비트가 할당되어 있다. 현실적인 구현 가능성을 높이기 위해 적은 계산량을 필요로 하는 선형 빔포밍 다중 사용자 다중 안테나 프로세싱 방법을 제안한다. 제안하는 선형 빔포밍 다중 사용자 다중 안테나 프로세싱 방법은 선호빔 색인 피드백, 사용자 선택 알고리즘, 빔포밍 매트릭스 형성 방법이 포함되어 있다. 먼저, 선호빔 색인 피드백 방법은 특히 적은 양의 피드백을 사용하는 시스템에서 사용자의 채널 상태와 인접 사용자와의 간섭의 영향에 관한 정보를 효과적으로 기지국에게 전달한다. 또한, 사용자 선택 방법은 사용자의 수가 기지국의 안테나 수 보다 많은 경우 다중 사용자 다이버시티를 활용하여 평균적인 총 전송률을 향상시킨다. 마지막으로 빔포밍 매트릭스 형성 방법은 사용자로부터의 피드백 정보를 바탕으로 SVD 동작을 통해 쉽게 빔포밍 매트릭스가 계산되기 때문에 기존 다중 사용자 다중 안테나 프로세싱에 비해 계산상의 복잡도를 크게 줄일 수 있다. 시뮬레이션을 통한 수치 결과를 통해 제안하는 SVD 기반의 다중 사용자 다중 안테나 프로세싱 방법이 기존 프로세싱 방법에 비해 더 높은 평균 총 전송률 얻을 수 있다는 것을 확인하였다. part II에서는 AP가 다중 사용자 다중 안테나 기술을 통해 여러 사용자에게 동시에 데이터를 전송할 수 있는 IEEE 802.11ac 기반의 무선랜 시스템이 고려되었다. 고려된 무선랜 시스템은 part I에서의 셀룰라 시스템과는 달리 사용자 채널 정보 피드백을 위해 많은 양의 피드백 비트가 할당되었으며, Givens rotation이라는 효과적인 피드백 방법을 사용하여 피드백 정보의 오버헤드를 줄일 수 있다. 그 결과 적은 양의 피드백 할당에 의해 야기되는 채널 양자화 오류를 무시할 수 있다. 그러나 무선랜 시스템에서는 심각한 성능 저하를 일으키는 긴 피드백 지연이 (현실적으로 200 ms이상) 발생할 수 있다. 이와 더불어 무선랜 표준에서 다중 사용자 다중 안테나 프로세싱 구현을 위해 새롭게 정의한 그룹 식별 및 사용자 스케쥴링으로 인해 기존에 셀룰라 시스템에 제안되었던 사용자 선택 방법 등을 직접 적용하기 힘들다. part II에서는 이러한 새로운 내용을 고려하여 효과적이고 현실적인 사용자 스케쥴링 방법을 제안한다. 제안한 스케쥴링 방법은 적은 계산량을 가지고 있으며 평균 총 전송률을 향상시킬 수 있다. 시뮬레이션을 통한 수치 결과를 통해 사용자 스케줄링을 통한 제안한 SVD 기반의 다중 사용자 다중 안테나 프로세싱 방법은 기존의 프로세싱에 비해 훨씬 좋은 성능을 나타냄을 보여주고, 특히 작은 SNR 영역과 긴 피드백 지연 환경에서 상당한 성능 이득이 있음을 확인하였다.Over the last decade, multiple-user multiple-input multiple-output (MU-MIMO) processing has gained considerable attention in the wireless communication standards such as 3GPP-LTE Advanced, IEEE 802.16m, and IEEE 802.11ac. MU-MIMO processing is capable of simultaneously supporting multiple users and therefore attains large cell capacity increase. In this dissertation, MU-MIMO processing with low-computational complexity and applicability to diverse wireless communication systems is proposed for practical interests. For evaluating average sum-rate of the proposed MU-MIMO processing, two different wireless communication systems, cellular systems and wireless local area network (WLAN) systems, are considered in Parts I and II. In Part I of this dissertation, we focus on cellular systems where low feedback bits are allocated to report user channel information. For practical downlink MU-MIMO processing, we propose a linear beamforming MU-MIMO processing with low-computational complexity that includes preferred-beam index feedback, user selection algorithms, and beamforming matrix construction method. The preferred-beam index feedback efficiently conveys information on both the channel states of users and the effect of interuser interference especially in low-rate feedback environments. The proposed user selection algorithms exploits multiuser diversity to improve average sum-rate for the case when the number of users exceeds the number of transmit antennas. The proposed beamforming matrix construction method easily computes unitary beamforming matrix based on the feedback information using singular value decomposition (SVD) operation, which results in significant computational complexity reduction compared to the conventional methods. Simulation results show that the proposed SVD-based unitary MU-MIMO processing achieves higher average sum-rate particularly at low-rate feedback, while the computational complexity is kept reasonable. In Part II of this dissertation, IEEE 802.11ac-based WLAN systems are considered where Access Point (AP) can transmit multiple data streams to different users in parallel by MU-MIMO processing. Unlike cellular systems, the considered WLAN systems assign high-rate bits to feedback user channel information and utilize an efficient feedback mechanism using Givens rotation that reduces the overhead of feedback information. As a result, a channel quantization error caused by low-rate feedback could be negligible. In WLAN systems, however, there may be long feedback delay, more than 200 ms in reality, that leads to severe performance degradation. In addition, WLAN systems are difficult to directly apply conventional user selection algorithms including the proposed one in Part I since group identification (ID) and user scheduling that WLAN standard newly defines for MU-MIMO processing should be considered. Based on these features, we propose an efficient and a practical user selection algorithm with low-computational complexity. Simulation results also present that the proposed MU-MIMO processing combined with the proposed user selection algorithm considerabley outperforms conventional MU-MIMO processing such as zero-forcing beamforming (ZFBF) especially in low signal to noise ratio (SNR) region and/or long feedback delay.Abstract Contents List of Figures List of Tables 1 Introduction 1.1 SVD-Based Unitary MU-MIMO Processing in Cellular Systems 1.2 SVD-Based Unitary MU-MIMO Processing in WLAN Systems 1.3 Outline of Dissertation 2 SVD-Based Unitary MU-MIMO Processing in Cellular Systems 2.1 System Model 2.2 Proposed SVD-Based Unitary MU-MIMO Processing 2.2.1 User Feedback 2.2.2 User Selection 2.2.3 Construction of Unitary Beamforming Matrix 2.2.4 User Data Decoding 2.3 Simulation Results 2.4 Summary 3 SVD-Based Unitary MU-MIMO Processing in WLAN Systems 32 3.1 System Model 3.2 Proposed SVD-Based MU-MIMO Processing 3.2.1 Channel Sounding and User Feedback 3.2.2 User Grouping and User Scheduling 3.3 Simulation Results 3.4 Summary 4 Conclusion and Future Work 58 4.1 Conclusion 4.2 Future WorkDocto

Algorithm design for scheduling and medium access control in heterogeneous mobile networks

Mención Internacional en el título de doctorThe rapid growth of wireless mobile devices has led to saturation and congestion of wireless channels – a well-known fact. In the recent years, this issue is further exacerbated by the ever-increasing demand for traffic intensed multimedia content applications, which include but are not limited to social media, news and video streaming applications. Therefore the development of highly efficient content distribution technologies is of utmost importance, specifically to cope with the scarcity and the high cost of wireless resources. To this aim, this thesis investigates the challenges and the considerations required to design efficient techniques to improve the performance of wireless networks. Since wireless signals are prone to fluctuations and mobile users are, with high likelihood, have difference channel qualities, we particularly focus on the scenarios with heterogeneous user distribution. Further, this dissertation considers two main techniques to cope with mobile users demand and the limitation of wireless resources. Firstly, we propose an opportunistic multicast scheduling to efficiently distribute or disseminate data to all users with low delay. Secondly, we exploit the Millimeter-Wave (mm-Wave) frequency band that has a high potential of meeting the high bandwidth demand. In particular, we propose a channel access mechanism and a scheduling algorithm that take into account the limitation of the high frequency band (i.e., high path loss). Multicast scheduling has emerged as one of the most promising techniques for multicast applications when multiple users require the same content from the base station. Unlike a unicast scheduler which sequentially serves the individual users, a multicast scheduler efficiently utilizes the wireless resources by simultaneously transmitting to multiple users. Precisely, it multiplies the gain in terms of the system throughput compared to unicast transmissions. In spite of the fact that multicast schedulers are more efficient than unicast schedulers, scheduling for multicast transmission is a challenging task. In particular, base station can only chose one rate to transmit to all users. While determining the rate for users with a similar instantaneous channel quality is straight forward, it is non-trivial when users have different instantaneous channel qualities, i.e., when the channel is heterogeneous. In such a scenario, on one hand, transmitting at a low rate results in low throughput. On the other hand, transmitting at a high rate causes some users to fail to receive the transmitted packet while others successfully receive it but with a rate lower than their maximum rate. The most common and simplest multicasting technique, i.e., broadcasting, transmits to all receivers using the maximum rate that is supported by the worst receiver. In recent years, opportunistic schedulers have been considered for multicasting. Opportunistic multicast schedulers maximize instantaneous throughput and transmit at a higher rate to serve only a subset of the multicast users. While broadcasting suffers from high delay for all users due to low transmission rate, the latter causes a long delay for the users with worse channel quality as they always favor users with better channel quality. To address these problems, we designed an opportunistic multicast scheduling mechanism that aims to achieve high throughput as well as low delay. Precisely, we are solving the finite horizon problem for multicasting. Our goal is that all multicast users receive the same amount of data within the shortest amount of time. Although our proposed opportunistic multicast scheduling mechanism improves the system throughput and reduces delay, a common problem in multicast scheduling is that its throughput performance is limited by the worst user in the system. To overcome this problem, transmit beamforming can be used to adjust antenna gains to the different receivers. This allows improving the SNR of the receiver with the worst channel SNR at the expense of worsening the SNR of the better channel receivers. In the first part of this thesis, two different versions of the finite horizon problem are considered: (i) opportunistic multicast scheduling and (ii) opportunistic multicast beamforming. In recent years, many researchers venture into the potential of communication over mm-Wave band as it potentially solves the existing network capacity problem. Since beamforming is capable to concentrate the transmit energy in the direction of interest, this technique is particularly beneficial to improve signal quality of the highly attenuated mm-Wave signal. Although directional beamforming in mm-Wave offers multi-gigabit-per-second data rates, directional communication severely deteriorates the channel sensing capability of a user. For instance, when a user is not within the transmission coverage or range of the communicating users, it is unable to identify the state of the channel (i.e., busy or free). As a result, this leads to a problem commonly known as the deafness problem. This calls for rethinking of the legacy medium access control and scheduling mechanisms for mm-Wave communication. Further, without omni-directional transmission, disseminating or broadcasting global information also becomes complex. To cope with these issues, we propose two techniques in the second part of this thesis. First, leveraging that recent mobile devices have multiple wireless interface, we present a dual-band solution. This solution exploits the omni-directional capable lower frequency bands (i.e., 2.4 and 5 GHz) to transmit control messages and the mm-Wave band for high speed data transmission. Second, we develop a decentralized scheduling technique which copes with the deafness problem in mm-Wave through a learning mechanism. In a nutshell, this thesis explores solutions which (i) improve the utilization of the network resources through multicasting and (ii) meet the mobile user demand with the abundant channel resources available at high frequency bands.This work has been supported by IMDEA Networks Institute.Programa Oficial de Doctorado en Ingeniería TelemáticaPresidente: Ralf Steinmetz.- Secretario: Carlos Jesús Bernardos Cano.- Vocal: Jordi Domingo Pascua

OPTIMIZING RADIO RESOURCE MANAGEMENT IN VERY BAD CHANNEL CONDITIONS

Radio resource management is one of the most important parts of modern multi-user wireless communication systems. The main reason for this importance comes from the fact that the radio resources, such as bandwidth and power, are scarce. For instance, UMTS systems use 5MHz bandwidth for voice as well as data services. The optimum usage of the radio resource guarantees the highest efficient utilization of wireless networks. To optimize the radio resources, the transmitters need to estimate the channel conditions. This channel estimation is done by using pilot signal from the receiver. There are usually small delays between the measurements and the radio resource allocation. When the channel is highly correlated, this delay will not affect the performance, because the channel will not be significantly changed between the time of measurement and the time of transmission. However, if the mobile speed is high or the channel is very high dynamic, the correlation becomes very low. This is due to the timevarying nature of the channel. We call channels with very low correlation in time as bad condition channels. In this thesis we discuss this extremely important topic. The tools for analyzing bad condition channels are also proposed and discussed. Two power control algorithms to mitigate the low correlation of channels have been proposed. Our algorithms are validated through several simulations.fi=Opinnäytetyö kokotekstinä PDF-muodossa.|en=Thesis fulltext in PDF format.|sv=Lärdomsprov tillgängligt som fulltext i PDF-format