Joint quantization strategy for uplink cooperative cellular interference alignment with limited feedback

Coordinated multi-point transmission/reception (CoMP), in which base stations cooperation during the downlink, has been considered as a attractive way of achieving higher performance and mitigating interference, especially for terminals at cell edges. In this paper, distributed precoding for multicell-MISO downlink CoMP is considered. Most previous work in the area is on the assumption that base stations have knowledge of both data symbols and full channel state information (CSI), which leads to heavy exchange of channel state information and signal information over backhaul links . In this paper, several distributed precoding strategies (Distributed MRT, distributed ZF, distributed LVSINR and DVSINR) for multicell-MISO downlink CoMP are analyzed, which supposes that each base station only has local CSI and the precoders can be devised at each BS in a distributed manner, and Monte Carlo simulations demonstrate the performance of such strategies.

The traditional limited feedback scheme can use imperfect channel state information to achieve interference alignment (IA) in a fully connected situation. But these solutions could no longer be used directly in heterogeneous networks where partial connectivity is considered. To solve this problem, this article proposes an IA scheme based on limited feedback with differentiation for the two‐tiered heterogeneous network. First, users who are affected by different types and numbers of interference and corresponding base stations are divided into different units according to the partial connectivity of heterogeneous networks. Then, we design interference alignment algorithm for each unit separately. Next, considering the imperfect channel state information, we derive the leakage of interference caused by the limited feedback, and give expressions for the upper bound of sum rate loss in various units. Finally, dynamic feedback bit allocation is carried out to each unit with the goal of minimizing the throughput loss in the limited feedback system. The simulation results show that the proposed IA scheme can eliminate interference well and efficiency of limited feedback can be improved through dynamic feedback bit allocation, so that system can obtain higher sum rate than other schemes.

Coordinated multi-point (CoMP) transmission is an effective means of improving network throughput in heterogeneous cellular networks (HetNets). However, its performance is seriously weakened if imperfect coordination happens between base stations (BSs). Many prior CoMP works do not consider inter-cell overhead message delays such that a seemingly astonishing CoMP throughput gain is attained. In this paper, the quantization error and delay that actually exist in overhead messages was modeled and we developed a much tractable SIR model based on the stochastic geometry framework. We proposed adaptive CoMP that is applied to downlink zero-forcing beamforming (ZFBF) and it can mitigate the interference from the coordinated cells with delayed overhead messages. The bounds on the complementary cumulative distribution function (CCDF) of the SIR of a user are characterized such that the average throughput of a user is able to be analytically evaluated. Numerical results show that the proposed adaptive CoMP scheme can make the throughput gain very robust to the overhead delay and thus significantly increase the throughput even when BSs are not perfectly coordinated.

Mobile WiMAX is a broadband technology that is capable of delivering triple play services (voice, data, and video). However, mobility in WiMAX system is still an issue when the mobile station (MS) moves and be handovered between base stations (BSs). This issue can cause unnecessary neighboring BS scanning and association, handover delay, and MAC overhead which may affect real-time applications. In this paper, the MS movement direction prediction (MMDP) based MS scanning to overcome the mobile WiMAX handover issues is presented. In this scheme, only potential BS candidates will be chosen for scanning purpose. Hence, the MS scanning process reputation will be reduced with these shortlisted BS candidates instead of scanning all neighbors BSs. Simulation results show that the proposed scheme reduces total scanning delay and scanning interval duration by 25% and 50% respectively.

Two-way relaying reduces the loss in spectral efficiency caused in a conventional half-duplex relay due to two channel uses per data unit transmitted to the destination. Two-way relaying is possible when two nodes exchange data simultaneously through a relay. In the case of cellular systems, data exchange between base station (BS) and users (UE) is usually not symmetric, e.g., a user (UE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> ) might have uplink data to transmit during multiple access (MAC) phase, but might not have downlink data to receive during broadcast (BC) phase. This asymmetry in data exchange will reduce the gains of two-way relaying. In the case of infrastructure relays, where there are multiple users communicating through a relay, we propose that the BC phase following the MAC phase of UE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> be used by the relay to transmit downlink data to a second user (UE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ). Conventional two-way relaying with symmetric MAC and BC phases must now be modified to asymmetric MAC (BS → RS ← UE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> ) and BC phases (BS ← RS → UE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ), respectively. This will result in UE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> not being able to cancel the back-propagating interference in the usual way. We design precoders using conventional zero-forcing and linear minimum-mean-square-error criteria to mitigate the back-propagating interference at UE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> for an amplify-and-forward (AF) relay. We also propose a novel precoder appropriate for the asymmetric two-way relaying. The sum-rate performance of the proposed precoder is shown to be better than the conventional precoders.

Mobile WiMAX is a broadband technology that is capable of delivering triple play services (voice, data, and video). However, mobility in mobile WiMAX system is still an issue when the mobile station (MS) moves and its connection is handed over between base stations (BSs). In the handover process, scanning is one of the required phases to find the target BS. During the handover scanning process, the MS must synchronize with all the advertised neighbour BSs (nBSs) to select the best BS candidate for the incoming handover action. Without terminating the connection between the SBS and MS, the SBS will schedule the scanning intervals and sleep-intervals (also called interleaving interval) to MS for the handover scanning. However, during the scanning interval period, all the coming transmissions will be paused. Therefore, the redundant or unnecessary scanning of neighbouring BS cause delay and MAC overhead which may affect real-time applications. In this paper, the MS movement direction prediction (MMDP) based handover scanning scheme is introduced to overcome the mobile WiMAX handover scanning issue. It based on dividing the BS coverage area is into zones and sectors. According to the signal quality; there are three zones, no handover (No-HO), low handover (Low-HO) and high handover (High-HO) zones respectively and six sectors. In this scheme, only two BSs can become candidates; the two that the MS moves toward them will be chosen as the candidate for the handover scanning purpose. Hence, the handover scanning process repetition will be reduced with these two shortlisted BS candidates instead of scanning all nBSs. Thus, MMDP will reduce scanning delay and the number of exchange messages during the handover scanning comparing to the conventional scanning scheme. Although, the MMDP may need an extra computational time, the prediction and scanning process will be finished before the MS reach the High-HO zone, which mean the end-user's running application will be affected. Simulation results show that the proposed MMDP scheme reduces the total handover scanning delay and scanning interval duration by 25 and 50 % respectively. Also, the size of scanning message is reduced, which leads to reduced signalling overhead.

In modern wireless networks, dynamically optimizing the association between base stations (BSs) and users is a promising solution for performance improvement. However, a frequently changing BS-user association introduces heavy burden for network management. In this paper, we seek for balancing the flexibility and stability of the BS-user association in a green downlink MISO network. In addition to minimizing the transmit power and controlling the number of active BSs, we further consider reducing the users' switching frequency among BSs. This problem is a nonlinear mixed-integer program, and challenging to solve. As a compromise, we see for some efficient approximate solution to it by reformulating it as a convex sparse problem. An efficient algorithm is then developed to enhance the sparse solution, based on the iterative reweighted l 1 technique. The effectiveness of the proposed algorithm was validated by numerical simulations.

Interference alignment (IA) is a suggested technique for multiple input multiple output (MIMO) based multipath interference channels to achieve large multiplexed gain. CSI (Channel state information) is required by IA schemes to design precoding vectors at the transmitter side. There will be significant capacity overhead in feedback link. This paper concentrates on the MIMO channels which are having overtime-correlated differential CSI feedback employed by IA design. Utmost multiplexing gain is obtained by the lowest amount of differential feedback. Because of restricted differential feedback, the interference in the upper bound is avoided by using IA system through differential CSI feedback. The analytical performance of the proposed algorithm is done using simulation results and achieved better Sum rate by using normal differential feedback and by using differential feedback through the Lloyd's algorithm.

Network MIMO is a technology being developed towards lifting the interference limits on wireless network through collaborative transmissions from multiple base stations. However its great potential is shadowed by the stringent requirement of high speed/capacity backbones facilitating information exchange, including data, control signals, and channel state information amongst the base stations. In this paper, we propose a user-selective collaboration scheme to mitigate the backbone requirement by reducing the information exchange required. We define a certain threshold for users in each base station and only those users exceeding the threshold will be served by multiple base stations collaboratively. The other users will be served by a single base station as in a conventional non-collaborative transmission. The threshold is determined in such a way that the users with collaborative transmissions enjoy maximized sum rates whereas the others without collaborative transmissions sacrifice only marginal potential collaboration gain in terms of achievable maximized sum rates. As the amount of information to be exchanged will be a square function of the number of users served by the collaborative base stations, the user-selective scheme can reduce the backhaul overhead significantly.

In the presence of perfect channel state information (CSI), the achievable degrees of freedom (DoF) in wireless interference networks can be linearly scaled up with the number of users. Achievability is based on the idea of interference alignment (IA). However, in the presence of imperfect CSI, the sum rate becomes degraded, and full DoF may no longer be achievable. In this paper, the authors propose novel least squares (LS)- and minimum mean square error (MMSE)-based IA schemes that adaptively design beamformers by relying on the availability of imperfect CSI and knowledge of the channel estimation error variance in advance. Interestingly and unlike the other robust algorithms, the proposed adaptive schemes do not impose extra computational complexity compared to their nonadaptive counterparts. It is shown that the LS-based IA is able to outperform interference leakage minimization algorithms under both perfect and imperfect CSI. Furthermore, the authors compare the performance of the proposed MMSE-based IA with maximum signal-to-interference-plus-noise ratio (Max-SINR) algorithm. The authors show that while under perfect CSI, the MMSE-based IA achieves the same performance as that of Max-SINR, the former outperforms the latter under CSI mismatch. Meanwhile, it is shown that the proposed MMSE-based IA needs less CSI to be available and has less computational complexity compared to Max-SINR.

It is very important that the radar sensor network has autonomous capabilities such as self-managing, etc. Quite often, MIMO interference channels are applied to radar sensor networks, and for self-managing purpose, interference management in MIMO interference channels is critical. Interference alignment (IA) has the potential to dramatically improve system throughput by effectively mitigating interference in multi-user networks at high signal-to-noise (SNR). However, the implementation of IA predominantly relays on perfect and global channel state information (CSI) at all transceivers. A large amount of CSI has to be fed back to all transmitters, resulting in a proliferation of feedback bits. Thus, IA with limited feedback has been introduced to reduce the sum feedback overhead. In this paper, by exploiting the advantage of heterogeneous path loss, we first investigate the throughput of IA with limited feedback in interference channels while each user transmits multi-streams simultaneously, then we get the upper bound of sum rate in terms of the transmit power and feedback bits. Moreover, we propose a dynamic feedback scheme via bit allocation to reduce the throughput loss due to limited feedback. Simulation results demonstrate that the dynamic feedback scheme achieves better performance in terms of sum rate.

In this paper, we conduct performance evaluation for Ultra-Dense Networks\n(UDNs), and identify which modelling factors play major roles and minor roles.\nFrom our study, we draw the following conclusions. First, there are 3\nfactors/models that have a major impact on the performance of UDNs, and they\nshould be considered when performing theoretical analyses: i) a multi-piece\npath loss model with line-of-sight (LoS) and non-lineof- sight (NLoS)\ntransmissions; ii) a non-zero antenna height difference between base stations\n(BSs) and user equipments (UEs); iii) a finite BS/UE density. Second, there are\n4 factors/models that have a minor impact on the performance of UDNs, i.e.,\nchanging the results quantitatively but not qualitatively, and thus their\nincorporation into theoretical analyses is less urgent: i) a general multi-path\nfading model based on Rician fading; ii) a correlated shadow fading model; iii)\na BS density dependent transmission power; iv) a deterministic BS/user density.\nFinally, there are 5 factors/models for future study: i) a BS vertical antenna\npattern; ii) multi-antenna and/or multi-BS joint transmissions; iii) a\nproportional fair BS scheduler; iv) a non-uniform distribution of BSs; v) a\ndynamic time division duplex (TDD) or full duplex (FD) network. Our conclusions\ncan guide researchers to downselect the assumptions in their theoretical\nanalyses, so as to avoid unnecessarily complicated results, while still\ncapturing the fundamentals of UDNs in a meaningful way.\n

The mobile IEEE 802.16e (WiMAX) environment has been considered a promoting wireless video multicasting technology. For example, mobile digital TV, which is one of video streaming service, can be supported in this environment. In order to support energy-consuming service like this, we need to operate some energy-saving mechanisms in the environment. In the IEEE 802.16e standard, some energy-saving mechanisms called “sleep mode operations” have been defined. Sleep mode operations can make connections between base station (BS) and mobile station (MS) enter sleep mode. While all of the connections enter sleep mode, MS can enter an energy-saving state. But in the standard, for real-time connections, it does not mention that how to switch a connection between awake mode and sleep mode. And the sleep mode operation for real-time connections can be more efficient about energy-saving and the Quality of Service (QoS) can be higher. So we have proposed “Two-Timer Mechanism” which can make real-time connections switch between awake mode and sleep mode, upgrade the efficiency of sleep mode operation, and improve the QoS. In this thesis, after a general structure of WiMAX MAC layer is analyzed, our Two-Timer Mechanism is designed and implemented in media access control (MAC) layer of WiMAX BS on the C platform provided by Institute for Information Industry (III). The conformance testing shows that MOB-SLP-RSPs conform to the standard. So it can work sleep mode operations on real-time connections successfully.

We consider relay selection technique in a cooperative cellular network where user terminals act as mobile relays to help the communications between base station (BS) and mobile station (MS). A novel relay selection scheme, called Joint Uplink and Downlink Relay Selection (JUDRS), is proposed in this paper. Specifically, we generalize JUDRS in two key aspects: (i) relay is selected jointly for uplink and downlink, so that the relay selection overhead can be reduced, and (ii) we consider to minimize the weighted total energy consumption of MS, relay and BS by taking into account channel quality and traffic load condition of uplink and downlink. Information theoretic analysis of the diversity-multiplexing tradeoff demonstrates that the proposed scheme achieves full spatial diversity in the quantity of cooperating terminals in this network. And numerical results are provided to further confirm a significant energy efficiency gain of the proposed algorithm comparing to the previous best worse channel selection and best harmonic mean selection algorithms.

In this paper, a new scheme of downlink co-channel interference (CCI)\ncancellation in OFDM cellular networks is introduced for users at the\ncell-edge. Coordinated symbol transmission between base stations (BS) is\noperated where the same symbol is transmitted from different BS on different\nsub-carriers. At the mobile station (MS) receiver, we introduce a soft decision\nmaximum likelihood CCI canceler and a modified maximum ratio combining (M-MRC)\nto obtain an estimate of the transmitted symbols. Weights used in the combining\nmethod are derived from the channels coefficients between the cooperated BS and\nthe MS. Simulations show that the proposed scheme works well under\nfrequency-selective channels and frequency non-selective channels. A gain of 9\ndB and 6 dB in SIR is obtained under multipath fading and flat-fading channels,\nrespectively.\n