Abu B. Sesay

Adaptive asymmetric linearization of radio over fiber links for wireless access

The biggest concern in the use of radio-over-fiber (ROF) links in wireless access is their limited dynamic range due to nonlinear distortion (NLD). In this paper, a higher order adaptive filter based nonlinearity compensation scheme is proposed. Pre-compensation is done for the downlink while post-compensation is done for the uplink to result in asymmetry with respect to complexity. This centralized signal processing is attractive in that it keeps the remote unit simple. Accurate measurements of ROF link parameters are not required with this approach because the filters are adapted from the distortion of the input/output base band signal. This technique also facilitates fast tracking of modifications and drifts in the link characteristics. Measurements and simulation results show that gradually saturating amplitude nonlinearity can be adequately linearized with some backoff from the clipping limit. A 42% backoff is required for pre-compensation to protect the laser while only a 16.7% backoff is required for post-compensation. Phase pre-compensation is accomplished with a higher accuracy than phase post-compensation.

A generalized linear quasi-ML decoder of OSTBCs for wireless communications over time-selective fading channels

We propose a novel generalized linear quasi-maximum-likelihood (quasi-ML) decoder for orthogonal space-time block codes (OSTBCs) for wireless communications over time-selective fading channels. The proposed decoder computes the decision statistics based on the channel-state information and completely removes the intertransmit-antenna interference to provide excellent diversity advantage when the channel varies from symbol to symbol. It is shown that when the channel is quasi-static, the proposed decoder is the optimum ML decoder for OSTBCs. The theoretical bit-error probabilities of the proposed decoder are given and it is shown that the proposed decoder does not exhibit error floors at high signal-to-noise ratios like the decoder proposed in and . Simulation results for various channel-fading rates are presented to verify the theoretical analysis.

Spatial correlation and capacity measurements for wideband MIMO channels in indoor office environment

This paper describes a broadband multiple input, multiple output (MIMO) channel characterization platform and capacity measurement results in indoor office environment. The MIMO testbed has been designed for broadband MIMO channel sounding, capacity measurements and for characterizing the directional-multipaths of the radio propagation channel. The MIMO channel data have been collected in 5 GHz band inside a modern office environment. Capacity results from these experiments are discussed for different propagation conditions, including non line-of-sight (NLOS) and LOS propagations with various spacings between array elements. Spatial correlations are analyzed from the measured data and a frequency selective MIMO channel model based on the correlation statistics is validated. Post-processing of the measured data with a sequential ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) algorithm is utilized to extract the directions of departures and arrivals of multipath components at the transmitting and receiving arrays, respectively. A method of obtaining broadband MIMO capacity, indirectly, from a single directional measurement of MIMO propagation channel is also proposed and verified from the measurement results.

A generalized simplified ML decoder of orthogonal space-time block code for wireless communications over time-selective fading channels

We propose a novel generalized simplified maximum-likelihood (ML) decoder of orthogonal space-time block code (OSTBC) for wireless communications over time-selective fading channels. The proposed decoder computes the decision statistics based on the channel state information (CSI) and completely removes the inter-transmit-antenna interference (ITAI) and provides diversity advantage when the channel varies from one signaling interval to another. It is shown that when the channel is quasi-static, the proposed decoder becomes the optimum ML decoder for OSTBC. We derive a tight theoretical upper bound for bit error probability of the proposed decoder. We show theoretically that the proposed decoder does not exhibit error floors at high signal-to-noise ratios (SNR). Simulation results for various channel-fading rates are presented to verify our theoretical analysis and demonstrate robust performance of the proposed decoder.

Effects of antenna height, antenna gain, and pattern downtilting for cellular mobile radio

Results of wideband path loss and delay spread measurements using high gain, high and low antenna heights with pattern tilting are presented. The measurements were done in the frequency range 905-915 MHz, at two existing cellular mobile radio (CMR) sites. Also presented are potential approaches for analyzing data from high gain antennas. It is shown that, to a large extent, existing models can be used to predict path loss for high gain antennas with downtilting. The results further support the notion that high sites together with high gain antennas and suitably selected pattern tilting can result in a significant reduction in path loss and delay spread, as well as reduction in power transmitted from the cell site and reduction in system interference.

A Hammerstein-type equalizer for concatenated fiber-wireless uplink

In optical fiber-based wireless access schemes, the radio signal is transmitted through fiber without frequency conversion radio-over-fiber (ROF). Although the fiber has adequate bandwidth, nonlinear distortion due to electrical to optical (E/O) conversion is a concern. In the uplink, the dynamic multipath wireless channel is followed by this static memoryless ROF link; this forms a Wiener system. In this paper, we propose a Hammerstein type decision feedback equalizer (HDFE) for the fiber-wireless uplink to combat the nonlinear distortion and the wireless channel dispersion. The proposed equalizer is less complex because it handles static and dynamic distortions separately. The nonlinear distortion is compensated first, reducing the power of cross modulation products significantly. Analytical results show that the lower bound of the mean squared error depends on the optical and wireless channel noise. The bit error rate (BER) performance of the HDFE for the nonlinear channel approaches the performance of a decision feedback equalizer (DFE) in a linear channel when the nonlinearity is adequately compensated.

A spread spectrum radiolocation technique and its application to cellular radio

A method for locating mobile stations in a cellular telephone system is analyzed. This method uses hyperbolic multilateration of direct sequence radio signals that are transmitted by the mobile after it has been polled. The channel that would be necessary for the radiolocation signal could be created by reusing an entire 10-MHz mobile radio bandwidth. A direct-sequence locating system has been modeled and simulated using a multipath database for a dense urban environment. Location error statistics and statistics on locating mobiles in a cellular grid are presented.<<ETX>>

Low complexity indoor wireless data links using chirp spread spectrum

This dissertation proposes the use of Chirp Spread Spectrum (CSS) modulation for the transfer of high-speed (>10 megabits per second) data in the indoor wireless environment. CSS utilizes the properties of chirp signals to combat the effects of multipath fading. The major advantage CSS has over other spectrum spreading techniques is its analog-only implementation. Two main methods for deploying CSS are described: Binary Orthogonal Keying (BOK), in which chirp signals with different characteristics represent different data symbols, and Direct Modulation (DM), where data is modulated using a conventional non-coherent modulation scheme, and then has its bandwidth spread by the chirp signal. Performance equations for the Gaussian channel are derived for both systems. Several other key system design parameters are identified and explored, including chirp windowing, occupied bandwidth, and quasi-RAKE filter configuration. Computer simulations and prototype measurements show close agreement with the developed theory. Raw bit error rates less than 10−5 at 20 megabits per second are achieved, with a bandwidth of 200 MHz in non line of sight wireless channels over 10 m with less than 2 milli-Watts of transmit power. Techniques for designing SAW chirp filters with both linear and nonlinear time-frequency responses are described, and measurements of a prototype SAW device are presented. Novel methods for enhancing the throughput of a binary CSS system are presented which increase the number of bits per symbol of the modulation CSS signal. Computer simulations show that data rates of 50 megabits per second are achievable for a single channel system using this method.

Performance of clipped OFDM signal in fiber

Combined deployment of optical fiber technology and wireless networks has great potential for increasing the capacity and quality of service. By using radio-over-fiber (ROF) technology, the capacity of optical networks can be combined with the flexibility and mobility of wireless access networks without significant cost increment. The radio-over-fiber concept means to transport information over optical-fiber by modulating the light with the radio signal. This article discusses the effects of using fiber in conjunction with wireless local area network IEEE 802.11a standard (WLAN) to distribute RF signals. To achieve high throughput 802.11a LAN uses an orthogonal frequency division multiplexing (OFDM) based multicarrier wideband modulation technique. OFDM is one of the most favored modulation techniques in the WLAN scenario due to its efficient implementation and robustness against multipath and narrowband interference. One of the biggest drawbacks of OFDM is its high peak to average power ratio (PAPR). High PAPR of OFDM makes it unusable in nonlinear systems. In this article we discuss better ways to overcome the PAPR problem of the OFDM signal which will improve its performance in fiber.

Spatial correlation measurements for broadband MIMO wireless channels

In this paper, we propose a method to determine the transmit and receive side spatial correlations for an arbitrary antenna spacing in the arrays from a directional MIMO channel measurement. A broadband MIMO channel measurement platform has been developed, which is used to obtain the angles of arrival (AOA) and angles of departure (AOD) of resolvable multipath components (MPC). The AOAs and AODs are estimated from the measured MIMO channel data by using a sequential unitary ESPRIT algorithm. The measured AOAs, AODs and corresponding powers of the MPCs are used to compute the power angle spectrum for the transmitter and receiver sides of the MIMO propagation channel. The spatial correlation matrices and MIMO capacity can then be obtained from these directional measurements. Capacity results from the correlation measurements using a stochastic broadband MIMO channel model are observed to closely match the directly measured MIMO capacity for different propagation conditions and antenna spacings.