Tilak Rajesh Lakshmana

Partial Joint Processing for Frequency Selective Channels

In this paper, we consider a static cluster of base stations where joint processing is allowed in the downlink. The partial joint processing scheme is a user-centric approach where subclusters or active sets of base stations are dynamically defined for each user in the cluster. In frequency selective channels, the definition of the subclusters or active set thresholding of base stations can be frequency adaptive (per resource block) or non-adaptive (averaged over all the resource blocks). Frequency adaptive thresholding improves the average sum-rate of the cluster, but at the cost of an increased user data interbase information exchange with respect to the non-adaptive frequency thresholding case. On the other hand, the channel state information available at the transmitter side to design the beamforming matrix is very limited and rank deficiency problems arise for low values of active set thresholding and users located close to the base station. To solve this problem, an algorithm is proposed that defines a cooperation area over the cluster where the partial joint processing scheme can be performed, frequency adaptive or non-adaptive, for a given active set threshold value.

Partial joint processing with efficient backhauling using particle swarm optimization

In cellular communication systems with frequency reuse factor of one, user terminals (UT) at the cell-edge are prone to intercell interference. Joint processing is one of the coordinated multipoint transmission techniques proposed to mitigate this interference. In the case of centralized joint processing, the channel state information fed back by the users need to be available at the central coordination node for precoding. The precoding weights (with the user data) need to be available at the corresponding base stations to serve the UTs. These increase the backhaul traffic. In this article, partial joint processing (PJP) is considered as a general framework that allows reducing the amount of required feedback. However, it is difficult to achieve a corresponding reduction on the backhaul related to the precoding weights, when a linear zero forcing beamforming technique is used. In this work, particle swarm optimization is proposed as a tool to design the precoding weights under feedback and backhaul constraints related to PJP. The precoder obtained with the objective of weighted interference minimization allows some multiuser interference in the system, and it is shown to improve the sum rate by 66% compared to a conventional zero forcing approach, for those users experiencing low signal to interference plus noise ratio.

Precoder Design With Incomplete Feedback for Joint Transmission

A centralized coordinated multipoint downlink joint transmission in a frequency division duplex system requires channel state information (CSI) to be fed back from the cell-edge users to their serving BS, and aggregated at the central coordination node for precoding, so that interference can be mitigated. The control signals comprising of CSI and the precoding weights can easily overwhelm the backhaul resources. Relative thresholding has been proposed to alleviate the burden; however, this is at the cost of reduction in throughput. In this paper, we propose utilizing the long-term channel statistics comprising of pathloss and shadow fading in the precoder design to model the statistical interference for the unknown CSI. In this regard, a successive second-order cone programming (SSOCP)-based precoder for maximizing the weighted sum rate is proposed. The accuracy of the solution obtained is bounded with the branch and bound technique. An alternative optimization framework via weighted mean square error minimization is also derived. Both these approaches provide an efficient solution close to the optimal, and also achieve efficient backhauling, in a sense that the precoding weights are generated only for the active links. For comparison, a stochastic approach based on particle swarm optimization is also considered.

Scheduling for backhaul load reduction in CoMP

Coordinated multi-point (CoMP) transmission has received a lot of attention, as a way to improve the system throughput in an interference limited cellular system. For joint processing in CoMP, the user equipments (UEs) need to feed back the channel state information (CSI), typically to their serving base stations (BSs). The BS forwards the CSI to a central coordination node (CCN) for precoding. These precoding weights need to be forwarded from the CCN to the corresponding BSs to serve the UEs. In this work, a feedback load reduction technique is employed via partial joint processing to alleviate the CSI feedback overhead. Similarly, to achieve backhaul load reduction due to the precoding weights, scheduling approaches are proposed. The state of the art block diagonalization solution is compared with our proposed constrained and unconstrained scheduling. Our main contribution is the method of choosing the best subset of the BSs and UEs at the CCN that yields the best sum rate under the constraint of efficient backhaul use. In particular, with constrained scheduling, the choice of a smaller subset proportionally reduces the backhaul load. Simulation results based on a frequency selective WINNER II channel model, show that our proposed constrained scheduling outperforms the block diagonalization approach in terms of the average sum rate per backhaul use.

Frequency allocation in non-coherent joint transmission CoMP networks

In this paper, we study the problem of joint transmission (JT) in coordinated multipoint (CoMP) networks from a new point of view where the system performance is optimized via frequency allocation for 5G small cells. Moreover, we investigate the implementation of hybrid automatic repeat request (HARQ), as an efficient scheme facing the feedback load problem in CoMP setups. The results are obtained for the cases with slow and fast fading conditions. Considering the channel state information (CSI) only at the receiver, we show that at low and medium signal to noise ratios (SNRs) sharing the frequency resources between users outperforms the case when the frequency resources are dedicated under non-coherent JT-CoMP setting. We find that the maximum long term throughput is achieved by either sharing the entire frequency resources between the users or allocating each user in a disjoint dedicated frequency resource. These extreme cases show the best performance in the SNR region of interest. Finally, as demonstrated analytically and numerically, HARQ feedback increases the long term throughput and reduces the outage probability substantially, with an affordable average delay.

Partial Joint Processing with Efficient Backhauling in Coordinated Multipoint Networks

Joint processing between base stations is a promising technique to improve the quality of service to users at the cell edge, but this technique poses tremendous requirements on the backhaul signaling capabilities, such as the distribution of channel state information and the precoding weights to the base stations involved in joint processing. Partial joint processing is a technique aimed to reduce feedback load, in one approach the users feed back the channel state information of the best links based on a channel gain threshold mechanism. However, it has been shown in the literature that the reduction in the feedback load is not reflected in an equivalent backhaul reduction, unless additional scheduling or precoding techniques are applied. The reason is that reduced feedback from users yields sparse channel state information at the Central Coordination Node. Under these conditions, existing linear precoding techniques fail to remove the interference and reduce backhaul, simultaneously, unless constraints are imposed on scheduling. In this paper, a partial joint processing scheme with efficient backhauling is proposed, based on a stochastic optimization algorithm called particle swarm optimization. The use of particle swarm optimization in the design of the precoder promises efficient backhauling with improved sum rate.

On the potential of broadcast CSI for opportunistic Coordinated Multi-Point transmission

Coordinated Multi-Point transmission is a promising technique to improve the performance of the users at the cell-edge. To achieve this, in case of a centralized approach, users need to unicast the quantized channel state information (CSI), typically to the anchor base station (BS), and then each BS forwards this information to a central coordination node for precoding and scheduling. In the case of a decentralized approach, users broadcast the quantized CSI such that the coordinating BSs could simultaneously receive the CSI. The advantage of a decentralized approach is that it does not require a central coordination node, thereby not imposing stringent latency constraints on the backhaul. The CSI transmission over the erroneous feedback channel in the uplink gives rise to precoding loss and scheduling loss. In the decentralized framework, the feedback errors could result in BSs receiving a different version of the CSI. In this work, we propose a decentralized opportunistic scheduling approach, which only requires a minimal sharing of scheduling information between BSs. The results show that the sum rate achieved with the proposed method is comparable to that of the centralized approach even when there is a high bit error probability introduced by the feedback channel. We also show that when the bit error probabilities in the feedback channel are less than 10-4, the decentralized approach achieves the sum rate of the centralized approach.

Resource and Mobility Management in the Network Layer of 5G Cellular Ultra-Dense Networks

The provision of very high capacity is one of the big challenges of the 5G cellular technology. This challenge will not be met using traditional approaches like increasing spectral efficiency and bandwidth, as witnessed in previous technology generations. Cell densification will play a major role thanks to its ability to increase the spatial reuse of the available resources. However, this solution is accompanied by some additional management challenges. In this article, we analyze and present the most promising solutions identified in the METIS project for the most relevant network layer challenges of cell densification: resource, interference and mobility management.

Improved Local Precoder Design for JT-CoMP With Periodical Backhaul CSI Exchange

Joint transmission (JT) in coordinated multipoint (CoMP) systems can be used to significantly improve the data rate of the cell-edge users (UEs) via cooperation of base stations (BSs). In a frequency division duplex system, the UEs need to feedback the channel state information (CSI) to its strongest BS. In a distributed JT-CoMP setup, the exchanging of CSI can occur periodically over the backhaul. Any feedback of CSI would need to trigger immediate exchange of CSI among the BSs to preserve the gains of JT-CoMP. We propose to utilize the newly available local CSI to locally improve the precoding performance. This is performed in-between the triggered periodic CSI exchange between BSs. We characterize the performance between exchanging and not exchanging the CSI for local precoder design (LPD) in terms of the average sum rate with UE mobility and different feedback intervals. We solve the decentralized LPD for weighted sum rate maximization with partial new CSI, and show that significant part of the JT-CoMP gains can still be preserved.

A Low-Complexity Semi-Analytical Approximation to the Block Error Rate in Nakagami-m Block Fading Channels

There are few analytical formulas that can be used for calculating the block error rate (BLER) in block fading channels. Thus, an estimate of the BLER is often obtained using numerical methods. One such method is the threshold method which assigns 0 or 1 to the instantaneous BLER given the signal to noise ratio (SNR) level. It has been shown that utilizing such a method results in an accurate approximation of the BLER in Nakagami-m block fading channels for a wide range of m. In this work, we consider a recently proposed simple method of obtaining the threshold and study the effect of adopting different physical layer and channel parameters on that threshold. We show that, while the value of this threshold depends on the modulation, coding, and block size, it is almost unaffected by the m parameter of Nakagami-m channels for a wide range of practical values. In addition, for a given modulation and coding method, the threshold is shown to be a simple function of block size. As a result, the computational complexity required to obtain the threshold can be significantly reduced.