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We present a simulation-based assessment of the performance potential of distributed MIMO (D-MIMO), multi-user MIMO (MU-MIMO) and particularly the combined D/MU-MIMO operation, for which we extend previously published scheduling and beamforming principles. The assessment study reveals that, when optimizing average user throughput performance, D-MIMO, while fruitless when used in isolation, is very effective when intelligently combined with MU-MIMO. Alternatively, when optimizing the cell edge performance, MU-MIMO, while also shown to be ineffective when used in isolation, is in fact very valuable when accompanied by a suitable configuration of D-MIMO. As an illustrative example, when using a jointly optimised configuration of D/MU-MIMO in a highly loaded urban deployment scenario, a 121% cell edge performance gain can be attained over a scenario using only D-MIMO, and even a demonstrated 153% gain over a scenario where only the MU-MIMO feature is available.

It is common practice to use nonlinear, high-efficient power amplifiers to transmit non-constant envelope signals using concepts like LINC (linear amplification using nonlinear components). Traditional systems use a power combiner to reconstruct the non-constant envelope RF signal that is fed into the antenna. In this paper, we will investigate the integration of the power combiner with the radiator, resulting in a LINC power-combining antenna concept. Our novel concept uses two polarizations where the full reconstruction of the signal is done at the receiver side. System-level measurements at 2.45 GHz with 32-QAM signals show that our power-combining antenna concept out-performs traditional LINC implementations.

The impact of directional antennas and multiantenna beamformers on radio transmission is formulated in terms of the gain of the Rician -factor, the reduction of the root-mean-squared delay spread, and the gain of the signal-to-noise ratio at the receiver for Rician fading channels in multipath environments. The analysis is based on a double-directional channel model. For the analytical formulation, the joint channel spectrum is assumed to be decomposable into separate spectra in time and angular domains. By way of illustration, closed-form expressions for the impact of hypothetical cosine-shaped antenna patternsand conventional beamformers are derived for channels with uniform angular spectra and an exponential decaying delay spectrum. The impact factors are explicitly related to the antenna beamwidth and the number of antenna elements. In addition, the effect of misalignment between the antenna main beam and the direct path is included in the analysis. The quantitative analysis given in this paper is important for radio system design, particularly for the design of antennas and multiantenna beamformer configurations.

This article reviews, discusses, and proposes crucial 60-GHz radio design options. First, it discusses the choice of integrated circuit technology for the radio frequency (RF) part and presents the status quo concerning standardization. Then, it describes the 60-GHz channel-propagation characteristics and proposes antenna solutions, as well as the architecture of the RF part, including a channelization scheme and frequency synthesizer architecture. With regard to the baseband part, we discuss the use of multicarrier modulation and compare it to single-carrier modulation. Finally, we demonstrate the consequences of the various design options as judged from link-budget calculations.

Orthogonality between the subcarriers of an orthogonal frequency division multiplexing (OFDM) system is affected by phase noise, which causes inter-carrier interference (ICI). The distribution of this interference term is studied in this paper. The distribution of the ICI for large number of carriers is derived and it is shown that the complex Gaussian approximation, generally applied in previous literature, is not valid and that the ICI term exhibits thicker tails. An analysis of the tail probabilities confirms these finding and shows that bit-error probabilities are severely underestimated when the Gaussian approximation for the ICI term is used, leading to too optimistic design criteria. Results from a simulation study confirm the analytical findings and show the validity of the limit distribution, obtained under the assumption of a large number of subcarriers, already for a modest number of subcarriers.

Wideband channel measurements at the 60-GHz frequency band in a reflective environment with different antenna heights are presented. The log-distance model of the normalized received power (NRP) is used to fit the measured data for both line-of-sight (LOS) and nonline-of-sight (NLOS) cases. Also, the impact of the biconical antennas and the reflective environment on the channel properties is analyzed and explained. Results show that the most favorable NRP values are found in the case that the Tx and Rx antennas are at the same height. Also, in most cases, the scatter paths have more contributions on the total received power than the direct path. In addition, in the LOS case, the NRPs become less dependent on the Tx-Rx separation owing to the combined effect of the antenna and the reflective environment. In the NLOS region, the contribution level of the strongest path becomes less dominant with increasing virtual Tx-Rx separation. The analysis of these results can be instructive for the deployment of a 60-GHz system in a reflective environment.

A novel system concept is presented to transport microwave signals over an in-house multimode graded-index polymer optical fiber network, in order to feed the radio access points in high-capacity wireless LANs. By employing optical frequency multiplying, the network's intrinsically limited bandwidth is overcome. The feasibility of this concept to carry data at several hundreds of Mbit/s speed for various microwave signal formats at carrier frequencies in the tens of GHz range is shown. The concept enables cost-effective system implementation, and easy upgrading by offering data signal transparency. It can readily be integrated with other system technologies such as wired Gigabit Ethernet in a single multi-service in-house polymer optical fiber network.

This article addresses basic issues regarding the design and development of wireless access and wireless LAN systems that will operate in the 60 GHz band as part of the fourth-generation (4G) system. The 60 GHz band is of much interest since this is the band in which a massive amount of spectral space (5 GHz) has been allocated worldwide for dense wireless local communications. The article gives an overview of 60 GHz channel characteristics and puts them in their true perspective. In addition, we discuss how to achieve the exploitation of the abundant bandwidth resource for all kinds of short-range communications. The main tenor is that an overall system architecture should be worked out that provides industry with plenty of scope for product differentiation. This architecture should feature affordability, scalability, modularity, extendibility, and interoperability. In addition, user convenience and easy and efficient network deployment are important prerequisites for market success. This article discusses these features and indicates a number of key research topics.

An Automatic Repeat Request (ARQ) scheme is proposed for a wireless ATM LAN based on the MEDIAN concept. Simulations have been performed to demonstrate the consequences of ARQ inclusion in terms of required buffer capacity and resulting cell end-to-end delay. In particular, the behaviour of the ARQ-enhanced system for real-time traffic under non error-free conditions has been examined. The main result of the simulations is that, due to specific properties of the MEDIAN wireless ATM LAN, the proposed ARQ scheme is suitable for real-time ATM services, while yielding virtually error-free links. Next to low cell delay, it features low overhead and low processing complexity whereas it can be readily combined with a simple and efficient MAC scheme.

The influence of transmitter and receiver phase noise (PN) on the performance of a multiple-input multiple-output orthogonal frequency division multiplexing (MIMO OFDM) based communication system is analyzed. It is shown that in the case of frequency flat Rayleigh fading, the influence of receiver and transmitter PN is equal. In the case of independent Rayleigh fading, however, the impact of the receiver PN is shown to depend on the ratio between the number of transmit and receive branches.

This work addresses the blueprint of short-range wireless systems supporting data rates of 100 Gb/s and beyond. A number of usage models are identified for such ultra-high data rates, including wireless communication within electronic equipment enabling possibilities such as wireless reconfigurable chip-to-chip communication. The 300 GHz band spanning about 55 GHz of contiguous bandwidth is identified as the most suitable candidate to accommodate these bandwidth-demanding applications. Furthermore, the main bottleneck issues are discussed: power consumption and antenna integration. With the help of basic link budget considerations, we indicate the technical feasibility of the proposed concept with compact low-cost antenna solutions. Finally, we discuss the overall system architecture to be standardized and indicate a number of key research topics.

Coherent wideband frequency-domain measurements of the complex frequency response of millimeter wave indoor radio channels are discussed. In addition, results of measurements performed in a 2 GHz band centered around 58 GHz will be presented. It is shown that a 40 dB dynamic range and a 400 ns aliasing-free range are sufficient for a correct estimation of the rms delay spread from the measurement data. >

A propagation model for the prediction of indoor radio channel characteristics at millimetre-wave frequencies, based on geometrical optics is described. This model has been shown to agree well with wide-band measurements carried out at Eindhoven University of Technology [8]. Simulation results for 4th and 6th order reflection are compared with corresponding measured data obtained in small and medium size rooms. The influence of objects located inside the room superstructure, on the indoor radio channel characteristics, is analysed. The agreement between measured and simulated results, confirms the validity of the proposed simulation method.

Millimeter-wave reflectometry is a potentially interesting method to determine properties of human skin tissue. With this method an open-ended waveguide probe is pressed against the skin which then measures the reflection coefficient in the millimeter-wave frequency range. Ideally, the probe is put perpendicular on the skin. In practice, the angle between probe and skin surface will deviate from the exact 90 degrees. This could influence the measurement results. Also the pressure of the probe on the skin will vary in practice, which could also influence the results. The sensitivity for these kind of parameter variations determines the suitability for use under practical circumstances as may occur in home-care situations. In this paper, the sensitivities of three types of probes for these parameters are investigated in order to determine the most suitable candidate among three types of probes. These are the flanged probe with Teflon plug, the flanged probe without Teflon plug, and the unflanged probe with Teflon plug. The latter turns out to be the best candidate as regards sensitivity for practical circumstances.

Factors that influence the accuracy of high frequency models for indoor radio propagation at 40-60 GHz are identified and discussed. Simulation results obtained on the basis of Geometrical Optics are presented. The reliability of these results is examined by comparing them with those obtained from empirical measurements.

This paper discusses a new approach for wireless motion control. Existing wireless techniques suffer from large closed loop delays of several milliseconds, which is unacceptable in precision motion systems. These large delays are mainly caused by the protocol used, since these are optimized for transferring large amounts of data, not to minimize transmission delay. A new protocol and wireless system are proposed that reduce the closed loop transmission delay to less than 300 microseconds. The system has been verified in a real control system, and measurements show the performance, which is more than ten times better than existing techniques.

This work investigates the joint impact of quantization and clipping, caused by analog-to-digital converters (ADCs) with low bit resolutions, on single- and multi-carrier block transmission systems in wireless multipath environments. We consider single carrier block transmission with frequency domain equalization (SC-FDE) and the multi-carrier techniques OFDM and MC-CDMA. By approximating the ADC input as Gaussian distributed, the effective signal-to-noise ratio in the received signal and decision variables are derived and analyzed. The bit-error rate (BER) performance is simulated and compared for various constellations under different multipath conditions. The results reveal that frequency diversity is an effective measure to combat the joint impact of quantization and clipping. For the ADCs with moderate resolution bits, i.e., R = 5 and 4 bits, SC-FDE is shown to achieve the same performance as MC-CDMA. It is noted that due to the applied Gaussian approximation for the received signals, the derived SC-FDE results provide a lower bound on the performance of the systems under the influence of the ADC nonlinearity. This bound is tight, however, in case of rich multipath environments. © 2008 IEEE.

Electromagnetic radiation arising from RS-232 cables may contain information which is related to the original RS-232 data signals. The seriousness of eavesdropping risks is shown by estimates of bit error rates feasible with a standard radio receiver as a function of the separation distance. In addition to this, results of experimental eavesdropping are presented.

Extensive measurements are conducted in room environments at 60 GHz to analyze the channel characteristics for various channel configurations. Channel parameters retrieved from measurements are presented and analyzed based on generic channel models. Particularly, a simple single-cluster model is applied for the parameter retrieval and performance evaluation. By this model, power delay profiles are simply described by a -factor, a root-mean-squared delay spread, and a shape parameter. The considered channels are configured with the combination of omnidirectional, fan-beam, and pencil-beam antennas at transmitter and receiver sides. Both line-of-sight (LOS) and non-LOS (NLOS) channels are considered. Further, to evaluate the transmission performance, we analyze the link budget in the considered environments, then design and simulate an OFDM system with a data rate of 2 Gbps to compare the bit-error-rate (BER) performance by using the measured and modeled channels. Both coded and uncoded OFDM systems are simulated. It is observed that the BER performance agrees well for the measured and modeled channels. In addition, directive configurations can provide sufficient link margins and BER performance for high data rate communications. To increase the coverage and performance in the NLOS area, it is preferable to apply directive antennas.

The influence of nonlinear high power amplifier (HPA) at transmit side on single-and multi-carrier block transmission systems is studied in wireless mutipath environments. We consider the single carrier block transmission with frequency domain equalization (SC-FDE), OFDM and MC-CDMA. The effective signal-to-noise ratio (SNR) and SNR penalty caused by the nonlinearity are derived and analyzed. In addition, bit-error rate performance is simulated and compared for various constellations in different channel conditions. The results show that SC-FDE is a robust solution to the nonlinear effect appeared at transmitter side, compared with OFDM and MC-CDMA. MC-CDMA only achieves similar performance as SC-FDE in case of sufficiently high input power backoff, especially for lower constellations. Moreover, the SNR penalty obtained from simulations for SC-FDE and MC-CDMA has a good agreement with that analytically predicted.

This paper analyzes the frequency selectivity, which is caused by multipath effects, of indoor line-of-sight (LOS) and non-LOS (NLOS) channels based on channel measurements in the frequency band of 60 GHz. The coherence bandwidth is determined to characterize frequency selectivity figures of the channels employing the combination of omnidirectional, fan-beam and pencil-beam antennas at transmitter (TX) and receiver (RX) sides. The results quantitatively show how far directional configurations are more effective for combating multipath effect than the omnidirectional ones for LOS channels. The frequency selectivity of the fan-beam-to-fan-beam configuration is less sensitive to misaligned TX-RX beams than the fan-beam-to-pencil-beam one. In addition, when omnidirectional antennas are used at both TX and RX side, the frequency selectivity of a NLOS channel is less sensitive to the TX-RX height difference than that of a LOS channel. Moreover, coherence bandwidth is empirically related to root-mean-squared delay spread. Lastly, the results presented in this paper lead to some useful insights into antenna configurations and beamformer designs for a 60-GHz radio system.

This paper studies the influence, estimation and digital compensation of IQ imbalance at both transmitter and receiver side of a multiple-input multiple-output (MIMO) OFDM system. Hereto a preamble is designed, which enables simultaneous estimation of the channel and imbalance parameters. New estimation approaches for TX, RX and joint TX and RX IQ imbalance in MIMO OFDM systems are proposed and evaluated. Results from a numerical study show that all three proposed compensation approaches are able to significantly suppress the influence of IQ mismatch.

To study the radio propagation channel fading at 60 GHz, a 3D ray-tracing simulator is used to predict the wave propagation deterministically in a small indoor environment. Starting from the instantaneous received signals, the large-scale and small-scale fading of the line-of-sight and non-line-of-sight channels are analyzed and modeled for linear and circular polarized antenna schemes. It is shown that the large-scale fading can be well modeled by a lognormal distribution and the small-scale fading by a Rician distribution. Additionally, the multipath dispersion in time and space is analyzed by mapping the multipath waves into delay and angular domain. The analysis shows that the beamwidth of the TX-RX beams can be properly adjusted to reduce the multipath effect to a certain extent, which will provide useful information for designing 60-GHz systems with smart antennas.

This paper investigates the application of spatial shifting (SS) of partial transmit sequences (PTSs) in multiple-input multiple-output OFDM. The technique rearranges the transmit (TX) vector in such a way that subparts are transmitted on those TX branches that result in the lowest overall peak-to-average power ratio. The application of different subcarrier grouping schemes and the combination of SS with phase shifting (PS) of the PTSs is investigated. Furthermore, a transparant extension of the techniques, without extra signalling overhead, is proposed. Numerical results prove the effectiveness of the SS and the combined SS/PS approach.

The influence, digital estimation and compensation of IQ mismatch at both transmitter (TX) and receiver (RX) side of a direct-conversion based multiple antenna OFDM system are presented in this paper. The resulting IQ imbalance can be split into a frequency independent and a frequency selective part. For estimation and compensation a two-step approach is presented, combining a data-aided and decision directed approach. First the frequency independent IQ imbalance is estimated using a preamble and, subsequently, the frequency selective IQ imbalance is removed using adaptive filtering. A numerical performance analysis reveals that the presented approach effectively mitigates the influence of IQ mismatch in OFDM-based space division multiplexing systems experiencing both TX and RX IQ imbalance.

This paper investigates the performance impact of nonlinearities in the transmitter (TX) as well as the receiver (RX) of multiple-antenna OFDM systems. First different, previously proposed, nonlinearity models are reviewed and their equivalence is shown. Subsequently, analytical expressions are derived for the probability of error of multiple-input multiple-output (MIMO) OFDM systems in Rayleigh-faded channels, which show good agreement with results from a simulation study. It can be concluded from the results that for high SNR values the TX nonlinearities cause performance floors, which are independent of the MIMO configuration. For RX nonlinearities, however, the performance in this high SNR region does depend on the MIMO configuration. The results can be used to derive limits for the permissible nonlinearities in MIMO OFDM systems.

In this contribution we introduce a modeling approach for the influence of radio-impairments in (multi-antenna) wireless communication systems. This method, differently from previous approaches, regards two phases: a stage where the impairments are severe and one where the influence is less critical, which enables the inclusion of the impact of power-save-modes and transmitter/receiver mode switching. The error model includes an additive and a rotational part, both implemented in the transmitter as well as in the receiver. As an example, it is shown how phase noise maps onto the introduced error model. As a test case, a multiple-input multiple-output orthogonal frequency division multiplexing (MIMO OFDM) system based on coherent detection is regarded. The system applies zero-forcing as detection method. Analytical expression is derived for the probability of error of such a system impaired by the introduced error model. Finally, numerical results from simulations are shown, which confirm the results from the analytical study

This paper describes the wideband measurements conducted at the frequencies 58 GHz and 2.25 GHz in an indoor environment. Normalized received power (NRP) and root-mean-squared (RMS) delay spread are calculated and used to compare the characteristics of radio wave propagation in both line-of-sight and non-line-of-sight areas at the two frequencies. The results show that on top of the difference in free space, the NRP at 58 GHz is several dBs lower than at 2.25 GHz in average. This difference is also reflected in the fitted log-(virtual) distance NRP models. In the deep shadow region, the poor diffraction level at 58 GHz will reduce the power level, but meanwhile the reflected waves from walls have a strong contribution to the received signal. In addition, it is observed that the radio channel at 58 GHz shows much less time dispersion in terms of RMS delay spread. The results can be quite useful for the 60 GHz system design, which might be very different from the design of conventional systems which operate in the lower frequency bands

The influence of phase noise (PN) on the performance of a multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) based communication system is analyzed. As part of this analysis, an estimator for the common phase error (CPE) is derived, based on the maximum likelihood estimation (MLE), which was chosen for its asymptotic optimality. Since the computational complexity of the MLE procedure clearly rules out a cost effective solution, a reduced-complexity algorithm. based on least squares estimation (LSE) is designed which yields suboptimal performance. Simulation results reveal that the difference in bit-error-rate performance between systems applying the LSE or MLE of the CPE is very small. This implies that the LSE is well applicable for 802.11a based MIMO systems.

A mobility-enhanced network model is used to evaluate the terminal mobility issues in high-speed wireless asynchronous transfer mode based local area network, as considered in the ACTS project MEDIAN (Wireless Broadband Customer Premises Network/Local Area Network for Professional and Residential Multimedia Applications). Handover can be considered as an essential issue with respect to cell delay and queue size in relation to the scheduling principle used in the multi-access control (MAC). A simple handover scheme is proposed which can be realised with minor modifications in the MAC of MEDIAN like systems. Simulation results show the influence of handover on the scheduling principle in relation to the effect on the existing background traffic in the coverage areas. To guarantee a minimum bandwidth during handover it is proposed to implement traffic control functions either in the logical link control (e.g. priority mechanism) or in the MAC (e.g. number of cells in frame parameter).

The results of wide-band measurements of indoor radio channels operating in a 2 GHz frequency band centered around 58 GHz are presented. The measurements were performed with a frequency step sounding technique. From the resulting frequency domain data base, cell coverage and time delay spread with both line-of-sight and obstruction of the direct path are derived. Eight indoor environments were subjected to propagation experiments. Since these areas are well defined and described the results can be readily related to other environments.

A deterministic model for millimetre-wave indoor radio propagation is proposed, based on geometrical optics. In addition, the accuracy of the model is examined by comparing the results with those obtained from measurements.

A method is presented for dimensioning biconical horn antennas which give a near uniform coverage within a picocell. This method is applied to the synthesis of a pair of millimetre-wave biconical horn antennas for operation in indoor picocells. Results of cell coverage measurements at 42 GHz are given.

In the ACTS project MEDIAN, a wireless asynchronous transfer mode (ATM) based local area network for high-speed communication is realised. The use of the inherently unreliable wireless medium for ATM transmission calls for extra measures to secure sufficient link quality. As such we propose the selective repeat automatic repeat request (ARQ) scheme. The entities of the considered ARQ scheme are positioned in the logical link control layer of the mobility-enhanced MEDIAN network model. Simulation results show the delay for the ATM user cells introduced by retransmissions. Taken the stringent delay requirements of the wireless ATM services into account, it can be concluded that the retransmission delays have no significant influence on the total end-to-end delay whereas the link quality (in terms of cell loss ratio) exhibits a substantial improvement.

A method is presented for designing and dimensioning biconical horn antennas in such way that the level of received power does not depend strongly on the separation distance between a centralized base station and a remote radio terminal within an indoor pico-cell. Results of cell-coverage measurements at 58 GHz are presented using these antennas within eight different indoor environments. The measurement results show that, using biconical horn antennas, an overall uniform coverage can be achieved for both line-of-sight and obstructed line-of-sight topologies. >

In millimeter-wave indoor radio systems the radiation patterns of the applied antennas at the remote and base station have considerable impact on received power and channel dispersion and therefore on the performance of indoor radio communication systems. This influence is examined by simulating the high frequency propagation behavior in a typical indoor environment by using a propagation model according to Geometrical Optics. First, the accuracy of this propagation model is tested by comparing initial simulation results with measurement data obtained from a large scale measurement campaign which has been carried out in various buildings of the Eindhoven University of Technology. Next, sectoral horn antennas as well as biconical horn antennas are considered in different setups and the dependence of normalised received power and RMS delay spread on beam directivity, pointing error and wall reflectivity under both line-of-sight and non-line-of-sight conditions is determined.

Coherent wide-band frequency-domain measurements of the complex frequency response of mm-wave indoor radio channels are discussed. In addition results of measurements performed in a 2 GHz band centered around 58 GHz in three different rooms at the Eindhoven University of Technology presented. With respect to the considered multipath radio channels it is shown that a 40 dB dynamic range and a 400 ns aliasing-free range are sufficient for a correct estimation of the RMS delay spread from the measurement data.

The performance is evaluated of a Decision Feedback Equalizer (DFE) operating on a 58 GHz indoor radio channel. This evaluation is based on measured multipath profiles obtained in eight indoor environments under both line-of-sight and non-line-of-sight conditions. The results are presented in terms of maximum achievable bit rate for a specified bit error rate requirement. It is shown that the number of forward taps of the DFE and antenna directivity have a substantial influence on the maximum bit rate for a specified BER, whereas external (antenna) diversity and a high level of received power helps to overcome stringent BER requirements. For a BER -3 it is found that (single carrier) transmission at bit rates of some hundreds of Mbit/s is feasible over MM-wave indoor radio channels.

Wideband measurements of indoor radio channels operating in a 2 GHz frequency band centred around 58 GHz were performed using a frequency step sounding technique. The results are presented of cell coverage and RMS delay spreas under both line-of-sight (LOS) and obstructed (OBS) situations.

A statistical multipath model is presented to characterize the multipath propagation properties of indoor radio channels which operate in the mm-wave frequency band. The model is derived from measurement data obtained in various typical indoor environments. The results of the statistical modeling agree closely to a discrete uncorrelated scattering model. The proposed model fits our measurements well, is simple to use in performance analysis and simulation studies of mm-wave indoor radio links and is extendable to other environments.

Manuscript received June 14, 2002. The authors are with Technische Universiteit Eindhoven, NL-5600 MB Endhoven, The Netherlands Digital Object Identifier 10.1109/TAP.2003.809052 mation to the near field were performed by Dr. J. Marti-Canales under a consultancy contract with the European Space Agency (ESA). Details of the measurement instrumentation and derivation of the transformation formulas can be found in [1] and [2]. The authors regret the error.