Per Zetterberg

The spectrum efficiency of a base station antenna array system for spatially selective transmission

In this paper we investigate the spectrum efficiency gain using transmitting antenna arrays at the base stations of a mobile cellular network. The proposed system estimates the angular positions of the mobiles from the received data, and allows multiple mobiles to be allocated to the same channel within a cell. This is possible by applying a transmit scheme which directs nulls against co-channel users within the cell. It is shown that multiple mobiles per cell is an efficient way of increasing capacity in comparison with reduced channel reuse distance and narrow beams (without directed nulls). The effect of the spatial spread angle of the locally scattered rays in the vicinity of the mobile is also investigated. >

Capacity Limits and Multiplexing Gains of MIMO Channels with Transceiver Impairments

The capacity of ideal MIMO channels has a high-SNR slope that equals the minimum of the number of transmit and receive antennas. This letter analyzes if this result holds when there are distortions from physical transceiver impairments. We prove analytically that such physical MIMO channels have a finite upper capacity limit, for any channel distribution and SNR. The high-SNR slope thus collapses to zero. This appears discouraging, but we prove the encouraging result that the relative capacity gain of employing MIMO is at least as large as with ideal transceivers.

Experimental investigation of TDD reciprocity-based zero-forcing transmit precoding

We describe an implementation of TDD reciprocity based zero-forcing linear precoding on a wireless testbed. A calibration technique which self-calibrates the base-station without the need for help from other nodes is described. Performance results in terms of downlink channel estimation error as well as bit error rate (BER) and signal to interference noise and distortion ratio (SINDR) are presented for a scenario with two base-stations and two mobile stations, with two antennas at the base-stations and a single antenna at the mobile-station. The results show considerable performance improvements over reference schemes (such as maximum ratio transmission). However, our analysis also reveals that the hardware impairments significantly limit the performance achieved. We further investigate how to model these impairments and attempt to predict the SINDR, such as what would be needed in a coordinated multipoint (CoMP) scenario where scheduling is performed jointly over the two cells. Although the results are obtained for a MISO scenario the general conclusions are relevant also for MIMO scenarios.

Inter-and intrasite correlations of large-scale parameters from macrocellular measurements at 1800 MHz

The inter- and intrasite correlation properties of shadow fading and power-weighted angle spread at both the mobile station and the base station are studied utilizing narrowband multisite MIMO measurements in the 1800 MHz band. The influence of the distance between two base stations on the correlation is studied in an urban environment. Measurements have been conducted for two different situations: widely separated as well as closely located base stations. Novel results regarding the correlation of the power-weighted angle spread between base station sites with different separations are presented. Furthermore, the measurements and analysis presented herein confirm the autocorrelation and cross-correlation properties of the shadow fading and the angle spread that have been observed in previous studies.

WiFi fingerprint indoor positioning system using probability distribution comparison

Positioning services are increasingly used for applications such as navigation, advertising and social media. While outdoor navigation based on GPS and/or cellular systems works well, indoor navigation is a much tougher challenge. This paper presents a new indoor positioning method based on Wi-Fi fingerprints, i.e. RSSI measurements from multiple Wi-Fi access points. During an offline phase, fingerprints are collected at known positions in the building. This database of locations and the associated fingerprints are called the radio map. In the online mode, the current Wi-Fi fingerprint probability distributions are compared with those of the radio map. The user location is estimated by calculating a weighted average of the three offline positions that best match the online measurements. Experiments show that our technique is superior to other proposed methods and reaches a median error of 2.4m.

Optimal coordinated beamforming in the multicell downlink with transceiver impairments

Physical wireless transceivers suffer from a variety of impairments that distort the transmitted and received signals. Their degrading impact is particularly evident in modern systems with multiuser transmission, high transmit power, and low-cost devices, but their existence is routinely ignored in the optimization literature for multicell transmission. This paper provides a detailed analysis of coordinated beamforming in the multicell downlink. We solve two optimization problems under a transceiver impairment model and derive the structure of the optimal solutions. We show numerically that these solutions greatly reduce the impact of impairments, compared with beamforming developed for ideal transceivers. Although the so-called multiplexing gain is zero under transceiver impairments, we show that the gain of multiplexing can be large at practical SNRs.

Cost drivers and deployment scenarios for future broadband wireless networks - key research problems and directions for research

The most prominent problem in providing anywhere, anytime wideband mobile access is the towering infrastructure cost as it is basically proportional to the bandwidth provided. We provide a simple, initial, analysis of the various infrastructure cost factors. This analysis shows that, contrary to what one may expect, the infrastructure cost is not dominated by electronic equipment, but rather by other deployment related costs (towers, wiring, building, network connections) and maintenance costs. In the paper some novel architectural approaches for future wideband mobile access focusing on these dominant cost factors are described and the related key research issues are discussed.

Performance of multiple-receive multiple-transmit beamforming in WLAN-type systems under power or EIRP constraints with delayed channel estimates

Downlink beamforming in a WLAN-type system employing access points and mobiles equipped with multiple antennas and associated receivers and transmitters are considered. The beamforming aims at maximizing the performance under constraints on transmit power or equivalent isotropic radiated power (EIRP). Solutions for the two constraints are derived and investigated using simulated and measured channels. Our simulation and experimental results shows that performance gains of 8-10 dB when using four directional transmitter antennas and two receive antennas (as compared with a base-line one-transmit two-receive), are possible under both constraints. For simulated channels, a delay between channel estimation and use of the same channel of up to 10% of the (inverse of the) Doppler frequency only degrades performance some tenths of a dB. In our measurements, very small degradations are seen with delays of up to 130 ms. The measurements were made under relatively stationary conditions with only occasionally moving people. Two different strategies for updating the beamforming vectors: sounding and ping-pong, are also considered in the paper.

Wideband Spatial Channel Model for MIMO Systems at 5 GHz in Indoor and Outdoor Environments

This work presents a spatial channel model developed within the EC WINNER project. The channel model is mainly based on double-directional measurement campaigns carried out in the 5 GHz ISM band with bandwidths of up to 120 MHz. The model is based on a unified structure for indoor and outdoor environment. It covers 6 different propagation scenarios, namely indoor small office, indoor hall, urban microcell, urban macrocell, suburban macrocell, and rural environment. Both line-of-sight (LOS) and non-line-of-sight (NLOS) propagation conditions are modeled

Receiver I/Q Imbalance: Tone Test, Sensitivity Analysis, and the Universal Software Radio Peripheral

The problem of determining the gain imbalance, quadrature skew, and local oscillator leakage of contemporary radio frequency receivers by tone test is considered. A least squares approach for indirect estimation of the sought-for parameters is proposed, which is linear in six out of its seven parameters. The performance of the method, particularly its accuracy as a function of measurement time, imbalance parameters, and signal-to-noise ratios, is investigated. The theoretical predictions of the performance are illustrated by Monte Carlo simulations and experimental data that are obtained from testing several universal software radio peripheral (USRP) receivers. This paper shows that, for the studied off-the-shelf receivers, gain imbalance and quadrature skew may accurately be predicted (i.e., < 0.1 dB and <, respectively) by employing baseband data covering only a handful of full periods of the excitation stimuli. Using short records of data, this paper also shows that local oscillator leakage adding a bias term in the receiver baseband data may suffer from a systematic bias error on the order of 15 dB in the estimate. To obtain local oscillator leakage estimates with an uncertainty on the order of 1 dB, the measurement time has to be increased by two orders of magnitude.