Backhaul-Aware Caching Placement for Wireless Networks

As the capacity demand of mobile applications keeps increasing, the backhaul network is becoming a bottleneck to support high quality of experience (QoE) in next-generation wireless networks. Content caching at base stations (BSs) is a promising approach to alleviate the backhaul burden and reduce user-perceived latency. In this paper, we consider a wireless caching network where all the BSs are connected to a central controller via backhaul links. In such a network, users can obtain the required data from candidate BSs if the data are pre-cached. Otherwise, the user data need to be first retrieved from the central controller to local BSs, which introduces extra delay over the backhaul. In order to reduce the download delay, the caching placement strategy needs to be optimized. We formulate such a design problem as the minimization of the average download delay over user requests, subject to the caching capacity constraint of each BS. Different from existing works, our model takes BS cooperation in the radio access into consideration and is fully aware of the propagation delay on the backhaul links. The design problem is a mixed integer programming problem and is highly complicated, and thus we relax the problem and propose a low-complexity algorithm. Simulation results will show that the proposed algorithm can effectively determine the near-optimal caching placement and provide significant performance gains over conventional caching placement strategies.Comment: 6 pages, 3 figures, accepted to IEEE Globecom, San Diego, CA, Dec. 201

Multirate Multiband Hybrid MC/MC-DS CDMA for Ultra Wideband Transmission

最新的無線個人區域網路標準802.15 3a採用超寬頻技術來傳輸多媒體資訊。因應其高且多重速率的要求，促使我們考量如何在已開放的頻寬中作規劃。而多頻帶傳輸被視為可降低硬體複雜度的方法之一，並它同時充釵U種多重存取系統架構於其夠寬的子頻帶中。在本文中，我們建構一種混合多載波/多載波-直序展頻之分碼多工系統於每一子頻帶中，此系統除了利用時域展頻之外也用了頻域展頻。至於多速率傳輸則使用已知的多碼存取和可變展碼長度存取技術來達成。此外我們提出去相關和干擾消除多戶偵測接收機用以克服高速傳輸使用者所遭受到來自等效用戶的多重存取干擾。而這兩種接收機又分別可用結合時頻域和分離時頻域之方法來實現。從模擬結果得知，當我們在增加時域或頻域展碼時，建議使用多碼存取技術配合結合時頻域去相關接收機。若因考量硬體複雜度而採用分離時頻率接收機，則使用可變展碼長度存取技術將是較佳的方法。Ultra wideband (UWB) transmission is adopted in the specific of 802.15 3a for wireless personal area network (WPAN) and applied to multimedia communications. The requirement of high and multiple data rates motivates us to consider how to plan the feasible transceiver structure on available ultra wide bandwidth. Multiband trans-mission technology, which divides the available bandwidth into several conjunctional subbands and transmits information through one subband at one time, is presented here to simplify the hardware complexity and still offer enough wide subbands for multiple access schemes. The novel hybrid MC/MC-DS CDMA structure, which exploits joint time and frequency domain spreading, is constructed in each subband. In another de-signed consideration of multirate communication, we choose the known multirate ac-cess schemes – multicode access and variable spreading length access. Furthermore, we propose two kinds of multiuser detection receivers, to decorrelate and cancel MAI from the effective users of a high rate user due to multipath channel. Two possible imple-mentations, joint and separate TF-domain, are considered for each kind of proposed re-ceivers. From the simulation, we recommend adopting MCD access scheme using joint TF-domain decorrelating receiver whether increasing spreading code assignment on T-domain or F-domain and using joint TF-domain interference cancellation receiver to avoid arithmetic computations of crosscorrelation matrix of TF-domain spreading codes. Considering the issue of the complexity, which means to use separate TF-domain re-ceiver, we prefer adopting VSL access scheme whether increasing spreading code as-signment on T-domain or F-domain.Chapter 1 Introduction 1 Chapter 2 Background 3 2.1 Ultra Wideband Channel Description 3 2.2 UWB TH-PPM Impulse Radio 5 2.3 Multiband Transmission 6 2.3.1 Benefits 6 2.3.2 Design Considerations 7 2.4 Subband Multirate Multiple Access Schemes 8 2.4.1 Multirate DS CDMA 8 2.4.2 Multirate MC CDMA 10 2.4.3 Multirate MC DS CDMA 13 Chapter 3 Multirate Multiband Hybrid MC/MC DS CDMA 17 3.1 Multiband Hybrid MC/MC DS CDMA 17 3.2 Slowly Fading Frequency Selective Channel Model 19 3.3 Multirate Schemes 19 3.3.1 Multicode Access 20 3.3.2 Variable Spreading Length Access 22 3.4 Proposed High Rate Decorrelating Receiver Structures 24 3.4.1 Joint TF-Domain Decorrelating Receiver 25 3.4.2 Separate TF-Domain Decorrelating Receiver 27 3.4.3 Issue of Complexity 31 3.4.4 Performance Results 31 3.5 Proposed High Rate Interference Cancellation Structures 35 3.5.1 Joint TF-Domain Interference Cancellation Receiver 36 3.5.2 Separate TF-Domain Interference Cancellation Receiver 37 3.5.3 Issue of Complexity 39 3.5.4 Performance Results 39 Chapter 4 Overall System Simulations 45 4.1 Different Ratio of Multiband to Users 45 4.1.1 Decorrelating Receiver 46 4.1.2 Interference Cancellation Receiver 49 4.1.3 Tradeoff 51 4.2 Multiple TF-Domain code assignments 55 4.2.1 Decorrelating Receiver 55 4.2.2 Interference Cancellation Receiver 58 4.2.3 Tradeoff 61 Chapter 5 Conclusion and Future Works 65 Bibliography 6

Downlink user capacity of massive MIMO under pilot contamination

Pilot contamination has been regarded as a main limiting factor of time division duplexing (TDD) massive multiple-input-multiple-output (Massive MIMO) systems, as it will make the signal-to-interference-plus-noise ratio (SINR) saturated. However, how pilot contamination will limit the user capacity of downlink Massive MIMO, i.e., the maximum number of users whose SINR targets can be achieved, has not been addressed. This paper provides an explicit expression of the Massive MIMO user capacity in the pilot-contaminated regime where the number of users is larger than the pilot sequence length. This capacity expression characterizes a region within which a set of SINR requirements can be jointly satisfied. The size of this region is fundamentally limited by the pilot sequence length. Furthermore, the scheme for achieving the user capacity, i.e., the uplink pilot training sequences and downlink power allocation, has been identified. Specifically, the generalized Welch bound equality sequences are exploited and it is shown that the power allocated to each user should be proportional to its SINR target. With this capacity-achieving scheme, the SINR requirement of each user can be satisfied and energy-efficient transmission is achieved in the large-antenna-size (LAS) regime. The comparison with two non-capacity-achieving schemes highlights the superiority of our proposed scheme in terms of achieving higher user capacity. Furthermore, for the practical scenario with a finite number of antennas, the actual antenna size required to achieve a significant percentage of the asymptotic performance has been analytically quantified. © 2002-2012 IEEE