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

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

<p>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.</p> <p>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.</p

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

Coordinated MultiPoint Transmission with Incomplete Information

The demand for higher data rates and efficient use of various resources has been an unquenchable thirst across different generations of cellular systems, and it continues to be so. Aggressive reuse of frequency resources in cellular systems gives rise to intercell interference which severely affects the data rate of users at the cell-edge. In this regard, coordinated multipoint (CoMP) is one of the ways to mitigate interference for these cell-edge users. In the downlink, joint transmission (JT) CoMP involves the cooperation of two or more geographically separated base stations to jointly transmit to these users by treating the interfering signal as useful signal.To realize the gains of JT-CoMP in a frequency division duplex system, the users need to feedback the channel state information (CSI) to its serving base station. This needs to be aggregated at the central coordination node for mitigating interference via precoding. However, the process of aggregation poses tremendous burden on the backhaul. One of the ways to reduce this burden is to use relative thresholding, where the users feed back the CSI of only those links that fall within a threshold relative to the strongest base station. The side effect of thresholding results in limited or incomplete CSI for precoding. Efficient backhauling is achieved when the quantity of CSI available for certain links at the central coordination node be correspondingly equivalent to the quantity of precoding weights generated for the same links. The incomplete CSI poses problems for the simple zero-forcing precoder to mitigate interference and also achieve efficient backhauling. In this thesis, the main problem of simultaneously mitigating interference and achieving efficient backhauling is addressed with a layered approach. Our physical (PHY) layer precoding approach solves the problem and allowes the medium access control (MAC) layer scheduler to be simple. The PHY layer precoding algorithms such as successive second order cone programming are proposed using convex optimization in [Paper A], and particle swarm optimization based on stochastic optimization is proposed in [Paper B]. Also, we exploit the use of long term channel statistics for the incomplete CSI and characterize the promising performance of the proposed precoder using numerical bounds. Based on our results, we observed that the swarm algorithm struggles with the increase in the problem size. The MAC layer approach exploits scheduling to solve the problem keeping a simple PHY layer zero-forcing precoder [Paper C]. Our proposed constrained scheduling approach provides the best tradeoff in terms of average sum rate per backhaul use compared to other MAC layer techniques. These results can be applied to a variant of the baseband hotel, a centralized architecture. In a distributed architecture, the CSI is exchanged periodically between the base stations over the backhaul for JT-CoMP. Any CSI feedback update from the user must be immediately exchanged over the backhaul to preserve the gains of JT-CoMP. We propose an improved decentralized local precoder design where the base station with new local CSI can design the local precoding weights in between the CSI exchange between base stations [Paper D]. With our approach some of the gains of JT-CoMP can still be preserved without the need to burden the backhaul

Efficient Backhauling in Cooperative MultiPoint Cellular Networks

The efficient use of the spectrum in cellular systems has given rise to cell-edge user equipments (UEs) being prone to intercell interference. In this regard, coordinated multipoint (CoMP) transmission is a promising technique that aims to improve the UE data rates. In a centralized network architecture, the users need to feed back the channel state information (CSI) to its anchor base station (BS). The CSI is then forwarded to a central coordination node (CCN) for precoder design to jointly mitigate interference. However, feeding back the CSI consumes over-the-air uplink resources as well as backhaul resources. To alleviate this burden, the quantity of CSI being fed back is limited via relative thresholding. That is, the CSI feedback is limited to those BSs whose signal strength fall above a threshold relative to the strongest BS. Moreover, with limited CSI, efficient backhauling of the precoding weights is necessary, as the user data is routed based on the path taken by the precoding weights from the CCN to the corresponding BSs. The focus of this thesis is mainly on a physical (PHY) layer and a medium access control (MAC) layer approach for reducing the backhaul load in a CoMP system, with minimal penalty on the potential CoMP gains. Furthermore, broadcasting the CSI in a decentralized network architecture is considered in order to reduce backhaul latency. In the PHY layer approach, the precoder design is based on stochastic optimization such as particle swarm optimization (PSO). This method has no constraints on the scheduling of the UEs. The PSO based precoder design was also applied to field measurement data with CSI imperfections due to prediction errors and quantization errors. It was found to perform the best compared to other robust precoders de- veloped in the EU FP7 ARTIST4G consortium. With the MAC layer approach, a simple zero forcing precoder is assumed, which focuses on how to schedule the UEs in such a way that they achieve the backhaul load reduction. Lastly, the decentralized network architecture is explored, where the UEs broadcast the CSI. The BSs coordinate by sharing minimal scheduling information, thereby achieving data rates comparable to the centralized network architecture. In this thesis, the backhauling is defined to be efficient when the total number of CSI coefficients aggregated at the CCN is equal to the total number of precoding weights for a given time-frequency resource, in a centralized architecture with the PHY layer approach. In the MAC layer approach, the total number of precoding weights is less than or equal to the total number of CSI coefficients. In the decentralized network architecture, the CCN does not exist. The BSs can coordinate over a less stringent backhaul, thereby reducing the backhaul load and latency

Efficient Backhauling in Cooperative MultiPoint Cellular Networks

The efficient use of the spectrum in cellular systems has given rise to cell-edge user equipments (UEs) being prone to intercell interference. In this regard, coordinated multipoint (CoMP) transmission is a promising technique that aims to improve the UE data rates. In a centralized network architecture, the users need to feed back the channel state information (CSI) to its anchor base station (BS). The CSI is then forwarded to a central coordination node (CCN) for precoder design to jointly mitigate interference. However, feeding back the CSI consumes over-the-air uplink resources as well as backhaul resources. To alleviate this burden, the quantity of CSI being fed back is limited via relative thresholding. That is, the CSI feedback is limited to those BSs whose signal strength fall above a threshold relative to the strongest BS. Moreover, with limited CSI, efficient backhauling of the precoding weights is necessary, as the user data is routed based on the path taken by the precoding weights from the CCN to the corresponding BSs. The focus of this thesis is mainly on a physical (PHY) layer and a medium access control (MAC) layer approach for reducing the backhaul load in a CoMP system, with minimal penalty on the potential CoMP gains. Furthermore, broadcasting the CSI in a decentralized network architecture is considered in order to reduce backhaul latency.In the PHY layer approach, the precoder design is based on stochastic optimization such as particle swarm optimization (PSO). This method has no constraints on the scheduling of the UEs. The PSO based precoder design was also applied to field measurement data with CSI imperfections due to prediction errors and quantization errors. It was found to perform the best compared to other robust precoders de- veloped in the EU FP7 ARTIST4G consortium. With the MAC layer approach, a simple zero forcing precoder is assumed, which focuses on how to schedule the UEs in such a way that they achieve the backhaul load reduction. Lastly, the decentralized network architecture is explored, where the UEs broadcast the CSI. The BSs coordinate by sharing minimal scheduling information, thereby achieving data rates comparable to the centralized network architecture.In this thesis, the backhauling is defined to be efficient when the total number of CSI coefficients aggregated at the CCN is equal to the total number of precoding weights for a given time-frequency resource, in a centralized architecture with the PHY layer approach. In the MAC layer approach, the total number of precoding weights is less than or equal to the total number of CSI coefficients. In the decentralized network architecture, the CCN does not exist. The BSs can coordinate over a less stringent backhaul, thereby reducing the backhaul load and latency

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

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

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. 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 inthe design of the precoder promises efficient backhauling with improved sum rate