Toni Levanen

Radio Interface Evolution Towards 5G and Enhanced Local Area Communications

The exponential growth of mobile data in macronetworks has driven the evolution of communications systems toward spectrally efficient, energy efficient, and fast local area communications. It is a well-known fact that the best way to increase capacity in a unit area is to introduce smaller cells. Local area communications are currently mainly driven by the IEEE 802.11 WLAN family being cheap and energy efficient with a low number of users per access point. For the future high user density scenarios, following the 802.11 HEW study group, the 802.11ax project has been initiated to improve the WLAN system performance. The 3GPP LTE-advanced (LTE-A) also includes new methods for pico and femto cells interference management functionalities for small cell communications. The main problem with LTE-A is, however, that the physical layer numerology is still optimized for macrocells and not for local area communications. Furthermore, the overall complexity and the overheads of the control plane and reference symbols are too large for spectrally and energy efficient local area communications. In this paper, we provide first an overview of WLAN 802.11ac and LTE/LTE-A, discuss the pros and cons of both technology areas, and then derive a new flexible TDD-based radio interface parametrization for 5G local area communications combining the best practices of both WiFi and LTE-A technologies. We justify the system design based on local area propagation characteristics and expected traffic distributions and derive targets for future local area concepts. We concentrate on initial physical layer design and discuss how it maps to higher layer improvements. This paper shows that the new design can significantly reduce the latency of the system, and offer increased sleeping opportunities on both base station and user equipment sides leading to enhanced power savings. In addition, through careful design of the control overhead, we are able to improve the channel utilization when compared with LTE-A.

Performance evaluation of IEEE 802.11ah and its restricted access window mechanism

In this paper we provide an analytical model to compute the throughput and energy consumption of IEEE 802.11ah, the new Sub-1 GHz WiFi standard. The analytical model assumes known collision and error probabilities and applies to both basic and RTS/CTS access mechanisms. Comparison with simulation results shows that the model is extremely accurate in predicting the system throughput and energy consumption. We also investigate the IEEE 802.11ah system including the new restricted access window (RAW) mechanism, and compare it to the basic scheme. The obtained results show that the RAW mechanism can provide substantial improvements in the system performance, in terms of throughput, packet delay and energy consumption, in particular in highly-loaded dense network scenarios. These findings affirm and substantiate the prospects of IEEE 802.11ah as one of the key enabling technologies for wide-scale low-cost and energy-efficient M2M deployments and IoT applications in the future.

Radio interface design for ultra-low latency millimeter-wave communications in 5G Era

The projected growth of mobile data traffic requires the 5G wireless systems to support at least 1000 x larger area throughput than the existing 4G solutions. This requires ultra-dense local area networks combined with millimeter-wave communications to provide high spatial multiplexing gain and wide bandwidths for multi-gigabit peak data rates. In this paper, we extend our 5GETLA reference design for 5G small cell network radio interface in 3-10 GHz carrier frequencies towards millimeter-wave communications and discuss separate solutions for both line-of-sight and non-line-of-sight scenarios. The non-line-of-sight frame design achieves frame duration equal to 0.1 ms which is one hundredth of the LTE frame duration. The line-of-sight design is also considered as a good candidate especially for small-distance indoor wireless access or inband backhaul and is particularly optimized in terms of ultra-low latency with frame duration equal to 0.05 ms, achieving the strictest physical layer latency requirements set for 5G communications.

Low latency radio interface for 5G flexible TDD local area communications

This paper presents a low latency radio interface design for future 5G local area communications that provides transmission latencies less than 1 ms while providing sufficient spectral efficiency. We concentrate on the excellent latency aspects of the proposed 5GETLA radio interface and discuss the factors leading to very low latency and high energy efficiency. In addition, we study two different radio interface parameterizations and compare their total overheads and achievable transmission times.

Efficient Fast-Convolution Implementation of Filtered CP-OFDM Waveform Processing for 5G

This paper investigates the use of effective and flexible fast-convolution (FC) filtering scheme for multiplexing OFDM resource blocks (RBs) in a spectrally well-localized manner. The scheme is able to effectively suppress interference leakage between adjacent RBs, thus supporting asynchronous operation and independent waveform parametrization of RBs. This is considered as an important feature in 5G waveform development for effectively supporting diversified service characteristics. Our approach is applicable for cyclic prefix or zero prefix based OFDM and the corresponding OFDM based single-carrier (SC-FDMA) waveforms. It is also possible to generate and process traditional Nyquist pulse shaping based single-carrier waveforms and filter bank multicarrier waveforms using the same FC processing engine, accommodating different waveforms simultaneously in different RBs. Our case study is based on proposed numerology for 5G cm-wave communications utilizing flexible time-division duplexing principle. While using RBs of 160 OFDM subcarriers, it is enough to deactivate 3 to 5 subcarriers out of each RB as guardbands to effectively suppress interference leakage between RBs.

Efficient Fast-Convolution-Based Waveform Processing for 5G Physical Layer

This paper investigates the application of fast-convolution (FC) filtering schemes for flexible and effective waveform generation and processing in the fifth generation (5G) systems. FC-based filtering is presented as a generic multimode waveform processing engine while, following the progress of 5G new radio standardization in the Third-Generation Partnership Project, the main focus is on efficient generation and processing of subband-filtered cyclic prefix orthogonal frequency-division multiplexing (CP-OFDM) signals. First, a matrix model for analyzing FC filter processing responses is presented and used for designing optimized multiplexing of filtered groups of CP-OFDM physical resource blocks (PRBs) in a spectrally well-localized manner, i.e., with narrow guardbands. Subband filtering is able to suppress interference leakage between adjacent subbands, thus supporting independent waveform parametrization and different numerologies for different groups of PRBs, as well as asynchronous multiuser operation in uplink. These are central ingredients in the 5G waveform developments, particularly at sub-6-GHz bands. The FC filter optimization criterion is passband error vector magnitude minimization subject to a given subband band-limitation constraint. Optimized designs with different guardband widths, PRB group sizes, and essential design parameters are compared in terms of interference levels and implementation complexity. Finally, extensive coded 5G radio link simulation results are presented to compare the proposed approach with other subband-filtered CP-OFDM schemes and time-domain windowing methods, considering cases with different numerologies or asynchronous transmissions in adjacent subbands. Also the feasibility of using independent transmitter and receiver processing for CP-OFDM spectrum control is demonstrated

New Spectrally and Energy Efficient Flexible TDD Based Air Interface for 5G Small Cells

In this paper we introduce a new flexible time division duplexing based radio interface for future 5G small cell communications. We describe the benefits of the new design to achieve high energy efficiency, high spectral efficiency and low latency at the same time. We also compare our reference design against LTE-A assuming rank 8 DL SU-MIMO transmission and show that we our design can achieve more than 29% lower total overhead and up to 90% lower round trip time.

Improved performance analysis for superimposed pilot based short channel estimator

In this paper we study the MSE performance of a short ML channel estimator in a discontinuous block fading channel using superimposed pilots. The earlier analytical MSE estimates that we have seen were not concerned with ideal feedback or discontinuous block-wise transmission. In addition, we are interested in the scenario where we use shorter channel estimator than the true channel length. In this paper, we present solutions for these modeling problems and obtain improved analytic MSE estimates.

Iterative signal processing for mitigation of analog-to-digital converter clipping distortion in multiband OFDMA receivers

In modern wideband communication receivers, the large input-signal dynamics is a fundamental problem. Unintentional signal clipping occurs, if the receiver front-end with the analog-to-digital interface cannot respond to rapidly varying conditions. This paper discusses digital postprocessing compensation of such unintentional clipping inmultiband OFDMA receivers. The proposed method iterativelymitigates the clipping distortion by exploiting the symbol decisions. The performance of the proposed method is illustrated with various computer simulations and also verified by concrete laboratory measurements with commercially available analog-to-digital hardware. It is shown that the clipping compensation algorithm implemented in a turbo decoding OFDM receiver is able to remove almost all the clipping distortion even under significant clipping in fading channel circumstances. That is to say, it is possible to nearly recover the receiver performance to the level, which would be achieved in the equivalent nonclipped situation.

Fast-convolution filtered OFDM waveforms with adjustable CP length

This paper investigates the application of flexible and effective fast-convolution (FC) filtering scheme for multiplexing OFDM physical resource blocks (PRBs) in a spectrally well-localized manner. The scheme is able to suppress interference leakage between adjacent PRBs, thus supporting independent waveform parametrization for different PRBs, as well as asynchronous multiuser operation. These are considered as important features in the 5G waveform development. This contribution focuses on the parametrization alternatives and constraints, which are mostly due to the forward and inverse discrete Fourier transform (DFT/IDFT) lengths feasible for practical implementation. Special attention is on cyclic prefix (CP) OFDM system dimensioning with adjustable CP length while the overall symbol duration is fixed. It is demonstrated that a wide range of alternative CP-lengths are usually available through the FC-filtered OFDM scheme.