Physical layer control signals in cellular communications serve the purpose of delivering physical layer control messages in a timely fashion to support cell radio resource management and data transmissions between the network and the mobile users. This article describes the evolution of 3GPP LTE in control channel design. The legacy LTE control channels (Release 8) have proven to be well-designed robust channels for signaling physical layer messages. However, emerging wireless communication transmission technologies continue to challenge the traditional design of LTE control channels. We have reached a point where a totally new design of the control channels has to be provided in order for the new transmission features to be introduced into LTE. In this article we first give an overview of the legacy LTE control channel, and present the new challenges. We then describe in depth the new solutions provided by LTE (Release 11). Finally, we address the limitations of the new design, which are recently identified in certain newly-emerging applications, in particular, the machine-type communications. A new round of evolution is hence imperative. New solutions provided by next release of LTE (Release 13) are presented.

The Evolution of LTE Physical Layer Control Channels

This paper studies the achievable rate and power allocation to improve the uplink (UL) spectrum efficiency in a Long-Term Evolution Advanced (LTE-A) cooperative cellular network with the deployment of Type-II in-band decode-and-forward (DF) relay stations (RSs). The physical-layer UL transmission technology is based on single-carrier frequency-division multiple access (SC-FDMA) with frequency-domain equalization (FDE). Different from the downlink (DL) orthogonal FDMA system, signals on all subcarriers in the SC-FDMA system are transmitted sequentially rather than in parallel; thus, the user's achievable rate is not simply the summation of the rates on all allocated subcarriers. Moreover, each user equipment (UE) device has its own transmission power constraint instead of a total power constraint at the base station in the DL case. Therefore, the UL resource allocation problem in the LTE-A system is more challenging. To this end, we first derive the achievable rates of the SC-FDMA system with two commonly used FDE techniques, namely, zero-forcing (ZF) equalization and minimum-mean-square-error (MMSE) equalization, based on the joint superposition coding for cooperative relaying. We then propose optimal power allocation schemes among subcarriers at both the UE and RS to maximize the overall throughput of the system. Both theoretical analysis and numerical results demonstrate that our proposed power allocation schemes can drastically improve system throughput.

Uplink Achievable Rate and Power Allocation in Cooperative LTE-Advanced Networks

Visible light communication (VLC) is a wireless communication technology in which data is transferred over the huge and unused visible light spectrum. Light emitting diodes (LEDs) are used in data transmission and hence their nonlinearities including limited dynamic range lead to a considerable impact on the performance of VLC systems. Single-carrier frequency division multiple access (SC-FDMA) is commonly used to decrease the peak to average power ratio and hence has been introduced for single user VLC systems to mitigate the effect of nonlinearities. In this paper, we investigate the performance of a SC-FDMA VLC system for the scenario of dowlink multiuser data transmission. We consider optimizing the total transmission power and the subcarriers power allocation for a VLC access point to maximize the minimum achievable data rate of the VLC users under transmit power constraint. While calculating the achievable rate, we consider the effect of signal clipping due to the LED nonlinearities.

Power allocation for downlink multi-user SC-FDMA visible light communication systems

Over-the-air intercell signaling is one of the most effective means for fast, distributed, and dynamic intercell interference management (ICIM), which is the key to maintaining the quality of service (QoS) and fairness in most interference-dominant networks, such as femtocell networks However, the design of this type of signaling is challenging In this paper, we address these challenges and propose an effective solution, which we refer to as single-tone signaling (STS) The proposed STS possesses many highly desirable and crucial properties for ICIM, such as strong resistance to interference, deep coverage, low overhead, and most of all, the lack of the need for dedicated resources In addition, STS provides an effective means of estimating interference channels for advanced interference management The effectiveness of STS is demonstrated through applications to a femtocell network

Single-Tone Signaling Design for Intercell Interference Management With Applications to Femtocell Networks

The femtocell technology is a promising concept for deploying the next generation of wireless networks. One of the most critical issues concerning femtocell deployment is the interference between femtocells and macrocells. Interference alignment (IA) is a technique that tackles the interference management problem in a macrocell/femtocell heterogeneous network (HetNet). Using IA, macrocell users (MUs) design precoders to align their signals in such a way that they do not cause interference at the femtocell base stations (FBSs). Nevertheless the availability of MIMO dimensions limit the extent of alignment and hence the residual interference degrades the performance of femtocell tier. In this paper, we design a novel two-level precoder at the MU to reduce the overall leaked interference at the FBS in uplink transmissions. IA precoding done in the first level aligns the leakage signals from MUs into the interference subspace of FBSs. In the second level, leakage based precoding further reduces the strength of the aligned interference signals. This helps to reduce error in the femtocell tier and hence provides better quality of service (QoS) guarantees. The performance advantages of the proposed precoding technique is validated through simulation studies.

Two-level precoding based interference reduction in uplink for MIMO femtocell networks

Orthogonal frequency division multiple access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA) are the two major multiple access schemes for 4G wireless communications. In long term evolution (LTE) the downlink multiple access scheme is based on OFDMA and the uplink multiple access scheme is based on SC-FDMA. In this letter, we derive the fundamental performance difference between OFDMA and SC-FDMA, and present the general performance comparison between them. Analytical results show that OFDMA performance upper bounds SC-FDMA in the sense of capacity. The conclusion is further confirmed by numerical results.

Performance Analysis of OFDMA and SC-FDMA Multiple Access Techniques for Next Generation Wireless Communications

In this paper, we review the signal-to-leakage-plus-noise ratio (SLNR) transmit precoding criterion and the active antenna selection (AAS) strategy in a multi-user MIMO system. We then address the limitations of the original SLNR and AAS schemes and provide a solution that generalizes the SLNR precoding model by incorporating the antenna-receiver structure into the optimization process to further improve the multi-user MIMO performance under various receiver structures.

An Improved Leakage-Based Precoding Scheme for Multi-User MIMO Systems

Differential codebook precoding is known as a capacity-achieving precoding scheme when the channel variation is slow. However, at medium to high channel variation speeds the differential codebook optimized for low channel variation speeds performs poorly compared to other precoding schemes, thereby limiting its application to practical systems. In this paper, we apply an alternating codebook technique to differential precoding to improve its robustness to high channel variability. The basic idea is creating multiple (instead of single) differential codebooks optimized for various channel variation speeds, and alternating these codebooks over time providing a virtually larger differential codebook, containing various precoding matrices optimized for different speeds, without the need for additional feedback overhead. Simulation results show that the proposed alternating differential codebook scheme not only retains the superior performance of differential precoding at low channel variation rate but, at the same time, reduces the sensitivity to high speed channel variations, resulting in a more practical precoding scheme.

Chunliang Yang

Papers

Performance Analysis of OFDMA and SC-FDMA Multiple Access Techniques for Next Generation Wireless Communications

An Improved Leakage-Based Precoding Scheme for Multi-User MIMO Systems

Limited feedback MIMO precoding via alternating multiple differential codebooks