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

A UE and a method therein for transmission power control of uplink transmissions are provided. The method comprises receiving (810), through signalling a configuration indicating at least one reference signal, RS, and a reference transmission power level for each indicated RS. The method further comprises measuring (820)received power of the indicated at least one RS, and for each measured received power, determining (830)a pathloss, PL, based on the measured received power and the reference transmission power level for each measured received power. The method further comprises determining (840)an uplink transmission power based on at least one determined PL.

A user equipment and a method therein for transmission power control of uplink transmissions

The present invention relates to radio communication in a cellular network and in particular to the sharing of a frequency spectrum with another network. The object is to enable robust communication in the shared band. The solution is to allocate a third band for communication in a cell. A L1/L2 control channel may be transmitted in the third band only, whereas data can be transmitted in the shared band. The L1/L2 control channel support the data communication in the shared band. The invention relates to a method and a radio base station.

Method and radio base station for effective spectrum utilization

This paper discusses the requirements on future radio access and, based on the requirements, proposes a framework for such a system. The proposed system based on orthogonal frequency-division multiplexing supports very low latencies and data rates up to 100 Mb/s with wide area coverage and 1 Gb/s with local area coverage. Spectrum flexibility is identified as one main requirement for future systems, and the proposed framework can be deployed in a wide range of spectrum allocations with bandwidths ranging from 2.5 to 100 MHz. Multihop relaying, useful to extend the range for the high data rates, and multiple-antenna configurations are integral parts of the framework. A packet-centric approach is taken for the dataflow processing, implying that the scheduling mechanism and the retransmission protocol operate on complete packets rather than segments thereof, thus allowing for cross-layer optimization. Finally, numerical evaluations are provided, illustrating the feasibility of future very wideband radio access.

A future radio-access framework

Different frequency reuse values are associated with different channels, e.g., different types of channels in a cellular communications system. For a high speed downlink shared type of channel, the frequency reuse may be greater than one in order to achieve higher data rates. On the other hand, the frequency reuse may be set to one for other channel types, e.g., dedicated channels including both uplink and downlink dedicated channels. Alternatively, the frequency reuse may also be set greater than one for channels in addition to the downlink shared channel, e.g., one or more dedicated channels, while a frequency reuse of one is deployed for one or more uplink dedicated channels. Using different frequency reuse values reduces inter-cell interference, particularly at cell borders, while still maintaining existing soft handover schemes for dedicated channels if one or more of the channels is a code division multiple access (CDMA) type of channel.

A cellular radio communication system with frequency reuse

The problem of near-far resistant time-delay estimation in an asynchronous direct-sequence code-division multiple-access system is considered, and by better exploiting the structure of the problem, estimators superior to previously known techniques are obtained. For a typical numerical example, a gain in signal-to-noise ratio of approximately 3 dB is obtained.

Stefan Parkvall

Papers

A user equipment and a method therein for transmission power control of uplink transmissions

Method and radio base station for effective spectrum utilization

A future radio-access framework

A cellular radio communication system with frequency reuse

An improved MUSIC algorithm for estimation of time delays in asynchronous DS-CDMA systems