Wireless Communications

A cellular base station serves a multiplicity of single-antenna terminals over the same time-frequency interval. Time-division duplex operation combined with reverse-link pilots enables the base station to estimate the reciprocal forward- and reverse-link channels. The conjugate-transpose of the channel estimates are used as a linear precoder and combiner respectively on the forward and reverse links. Propagation, unknown to both terminals and base station, comprises fast fading, log-normal shadow fading, and geometric attenuation. In the limit of an infinite number of antennas a complete multi-cellular analysis, which accounts for inter-cellular interference and the overhead and errors associated with channel-state information, yields a number of mathematically exact conclusions and points to a desirable direction towards which cellular wireless could evolve. In particular the effects of uncorrelated noise and fast fading vanish, throughput and the number of terminals are independent of the size of the cells, spectral efficiency is independent of bandwidth, and the required transmitted energy per bit vanishes. The only remaining impairment is inter-cellular interference caused by re-use of the pilot sequences in other cells (pilot contamination) which does not vanish with unlimited number of antennas.

Noncooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas

Multiple-input multiple-output (MIMO) technology is maturing and is being incorporated into emerging wireless broadband standards like long-term evolution (LTE) [1]. For example, the LTE standard allows for up to eight antenna ports at the base station. Basically, the more antennas the transmitter/receiver is equipped with, and the more degrees of freedom that the propagation channel can provide, the better the performance in terms of data rate or link reliability. More precisely, on a quasi static channel where a code word spans across only one time and frequency coherence interval, the reliability of a point-to-point MIMO link scales according to Prob(link outage) ` SNR-ntnr where nt and nr are the numbers of transmit and receive antennas, respectively, and signal-to-noise ratio is denoted by SNR. On a channel that varies rapidly as a function of time and frequency, and where circumstances permit coding across many channel coherence intervals, the achievable rate scales as min(nt, nr) log(1 + SNR). The gains in multiuser systems are even more impressive, because such systems offer the possibility to transmit simultaneously to several users and the flexibility to select what users to schedule for reception at any given point in time [2].

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Scaling Up MIMO: Opportunities and Challenges with Very Large Arrays

The quintessential goal of sensor array signal processing is the estimation of parameters by fusing temporal and spatial information, captured via sampling a wavefield with a set of judiciously placed antenna sensors. The wavefield is assumed to be generated by a finite number of emitters, and contains information about signal parameters characterizing the emitters. A review of the area of array processing is given. The focus is on parameter estimation methods, and many relevant problems are only briefly mentioned. We emphasize the relatively more recent subspace-based methods in relation to beamforming. The article consists of background material and of the basic problem formulation. Then we introduce spectral-based algorithmic solutions to the signal parameter estimation problem. We contrast these suboptimal solutions to parametric methods. Techniques derived from maximum likelihood principles as well as geometric arguments are covered. Later, a number of more specialized research topics are briefly reviewed. Then, we look at a number of real-world problems for which sensor array processing methods have been applied. We also include an example with real experimental data involving closely spaced emitters and highly correlated signals, as well as a manufacturing application example.

Two decades of array signal processing research: the parametric approach

It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

On the capacity of a cellular CDMA system

We develop an information-theoretic model to explain and predict the coding gains of the incremental redundancy over the Chase combining scheme. From this analysis, we identify a new approach to improve the spectral efficiency of the conventional incremental redundancy scheme, which uses the same modulation and coding formats and, most notably, the same amounts of channel resources for retransmissions as those for the initial transmissions. Depending on the relative channel conditions between the transmissions, fewer incremental redundant bits than those employed in the first transmission might be enough to guarantee high probability of successful decoding. Hence, we propose a new scheme that adapts the modulation and coding formats according to simple feedback information to minimize unnecessary channel usage. Our simulation results demonstrate system throughput gains up to 25% in certain conditions can be achieved by the proposed scheme without modifying the existing WCDMA specifications.

Adaptive incremental redundancy [WCDMA systems]

According to certain embodiments, a method for selecting beam-reference signals for channel-state information reference-signal transmission by a wireless device is provided. The method includes receiving, from a network node, a plurality of beam-reference signals. The wireless device performs measurements on the plurality of beam-reference signals. Based on the performed measurements, a preferred beam-reference signal is selected from the plurality of beam-reference signals. A subset of the plurality of beam-reference signals that have a relationship with the preferred beam-reference signal is identified. Information identifying the subset of the plurality of beam-reference signals is transmitted to the network node.

Systems and methods for selecting beam-reference signals for channel-state information reference-signal transmission

We consider the problem of data transmission over a linear-filter channel with an unknown filter response. Traditionally, the various operations (equalization, detection, decoding) carried out by the receiver of a system that operates over an unknown channel assume access to high-quality estimates of the channel parameters. Such estimates are typically computed by a separate device for channel identification. In contrast, this paper focuses on combined channel estimation, equalization, and decoding. We also study the problem of code design, assuming such combined decoding. More precisely, we investigate nonlinear block coding and present a design criterion and a design algorithm, under the assumptions that a statistical description of the channel is available, and that the structure of the decoder is known to the transmitter. Numerical simulations demonstrate that the new scheme is capable of outperforming four different benchmark schemes.

Code design for combined channel estimation and error protection

This paper presents outdoor field experimental results showing the achievement of 3-Gbps throughput performance based on 400-MHz bandwidth transmission when applying carrier aggregation with 4 component carriers and 4-by-4 single-user multiple-in multiple-out multiplexing in the 15-GHz frequency band in the downlink of 5G cellular radio access. A new radio interface with time division duplexing and radio access based on orthogonal frequency-division multiplexing is implemented in a 5G testbed to support ultra-high speed transmission with low latency. Experimental results in an outdoor open-space parking area show that the peak throughput is 2.8 Gbps in front of a base station (BS) antenna with a high reference signal received power (RSRP) although rank 2 is selected due to the high antenna correlation. The results also show that the average throughput of 2 Gbps is achieved 120 m from the BS antenna. In a courtyard enclosed by building walls, 3.6 Gbps is achieved in an outdoor-to-outdoor environment with a high RSRP and in an outdoor-to-indoor environment where the RSRP is lower due to the penetration loss of glass windows, but a multipath rich environment contributes to actualizing a low antenna correlation.

Field experiments on 5G radio access using 15-GHz band in outdoor small cell environment

A user equipment unit (30) and a base station node (28) which is configured for operation with a synchronous HARQ protocol and with capability of sending data on an E-DCH channel either (1) in a nominal mode in single transmission time intervals of a predetermined length, or (2) in an extended mode in a pseudo transmission time interval The pseudo transmission time interval comprises a first transmission time interval in which the data is transmitted and a second transmission time interval in which the data is re-transmitted The second transmission time interval is consecutive to the first transmission time interval, and the first transmission time interval and the second transmission time interval are each of the (same) predetermined length

Stefan Parkvall

Papers

Adaptive incremental redundancy [WCDMA systems]

Systems and methods for selecting beam-reference signals for channel-state information reference-signal transmission

Code design for combined channel estimation and error protection

Field experiments on 5G radio access using 15-GHz band in outdoor small cell environment

Autonomous transmission for extended coverage