Xavier Fernando

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.

Cognitive Wireless Sensor Networks: Emerging topics and recent challenges

Adding cognition to the existing Wireless Sensor Networks (WSNs), or using numerous tiny sensors, similar to the idea presented in WSNs, in a Cognitive Radio Network (CRN) bring about many benefits. In this paper, we present an overview of Cognitive Wireless Sensor Networks (CWSNs), and discuss the emerging topics and recent challenges in the area. We discuss the main advantages, and suggest possible remedies to overcome the challenges. CWSNs enable current WSNs to overcome the scarcity problem of spectrum which is shared with many other successful systems such as Wi-Fi and Bluetooth. It has been shown that the coexistence of such networks can significantly degrade a WSNs performance. In addition, cognitive technology could provide access not only to new spectrum, but also to spectrum with better propagation characteristics. Moreover, by the adaptive change of system parameters such as modulation type and constellation size, different data rates can be achieved which in turn can directly influence the power consumption and the network lifetime. Furthermore, sensor measurements obtained within the network can provide the needed diversity to cope with spectrum fading at the physical layer.

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.

Steady-state Markov chain analysis for heterogeneous cognitive radio networks

Cognitive radio technology has been widely researched to improve the spectrum usage efficiency. Modeling of the spectrum occupancy in a cognitive framework including licensed and unlicensed users with various traffic conditions, is a prior requirement to do the system analysis. In this paper, we develop a continuous-time Markov chain model to describe the radio spectrum usage, and derive the transition rate matrix for this model. In addition, we perform steady-state analysis to analytically derive the probability state vector. The proposed model and derived expressions are compared to the existing models, and examined through numerical analysis.

On the design of optical fiber based wireless access systems

Optical fiber based wireless access schemes receive renewed attention with the popularity of hot-spots. They increase capacity, QoS and support wideband multimedia services and have the possibility of utilizing existing fiber infrastructure. However, link design in a fiber-wireless system needs careful consideration of many factors. There are two signal to noise ratios involved, the optical SNR (OSNR) and the electrical SNR. These two form the cumulative SNR in the concatenated fiber-wireless channel. The OSNR is a function of the modulation index m, E/O, O/E conversion losses and, the fiber length. There is a 39 dB loss due to E/O and O/E conversion only in resistively matched wideband links and the OSNR rapidly decreases with fiber length. The cumulative SNR at the mobile unit decides the QoS and cell size. This SNR depends on OSNR, wireless channel path loss and the optical receiver amplifier gain. In this paper, we study the relationships between critical design parameters, such as maximum radio and optical link losses, cumulative and optical SNR and, optical amplifier gain in a fiber-based wireless system.

Estimation and equalization of fiber-wireless uplink for multiuser CDMA 4G networks

Fiber-wireless (Fi-Wi) access fronts can support 100s of Mb/s envisioned by 4G networks. However, a major issue associated with Fi-Wi links is the nonlinear distortion of the radio-over-fiber (ROF) link coupled with the multipath dispersion of the wireless channel. Estimation and subsequent equalization of the concatenated fiber-wireless channel needs to be done, especially at high bit rates. The uplink is severely affected due to large fluctuations in the radio signal. This paper proposes an estimation and subsequent equalization algorithm for the Fi-Wi CDMA uplink. The estimation employs the properties of pseudo noise (PN) sequences and the equalization uses a novel Hammerstein type decision feedback equalizer (HDFE). The estimation and equalization are performed in the presence of multiple access interference (MAI) and wireless and optical channel noise. The cumulative effects of multiuser interference, multipath dispersion, nonlinear distortion, and noise are all considered in our analysis. Correlation properties of white-noise like PN sequences enable decoupling of the linear (wireless) and nonlinear (optical) channel portions. Furthermore, we propose a unique algorithm to mitigate MAI. Numerical evaluations show a good estimation and equalization of both the linear and nonlinear channels. Bit error rate (BER) simulations show that this algorithm leaves only small residual MAI.

All-Optical Demultiplexing of WLAN and Cellular CDMA Radio Signals

Subcarrier multiplexed transmission of multimedia radio signals over fiber is often done to deliver broadband services cost effectively. These signals need to be demultiplexed, preferably in the optical domain, to avoid loss and noise due to optical-to-electrical conversion. However, it is challenging to optically isolate signals at subgigahertz range due to the need for very narrow optical bandpass filters with high selectivity and low insertion loss and distortion. We developed such a novel subpicometer all-optical bandpass filter by creating a resonance cavity using two closely matched fiber Bragg gratings. This filter has a bandwidth of 120 MHz at -3 dB, 360 MHz at -10 dB, and 1.5 GHz at -20 dB. Experimental results show that this filter optically separates two RF signals spaced as close as 50 MHz without significant distortion. This paper analytically and experimentally investigates the scenario when this filter was used with 2.4-GHz (wireless local area network) and 900-MHz (cellular wireless) radio signals. The bit-error rate of the underlying baseband data is related to the linearity and isolation of the filter.

Characteristics of directly modulated ROF link for wireless access

The radio-over-fiber (ROF) link is of much interest to provide broadband wireless access. In this paper, we discuss some characteristics of a directly modulated ROF link in CDMA and FDMA environments. These RF measurement results show that, though the ROF link has adequate bandwidth to support several radio channels, nonlinear distortion and power loss, mainly due to electrical to optical conversion process and vice versa, limit the performance. Especially, the AM-PM type nonlinearity that starts at low power levels is a bigger concern. Furthermore, the spurious free dynamic range (SFDR) decreases because of the increases with the bandwidth of the RF signal.

Radio over multimode fiber for wireless access

A radio over fiber link is a promising technology for antenna remoting applications. Typically, the radio over fiber link employs a single mode fiber. However, the signal power at the remote antenna is very small. The main reason is large power loss in the E/O and O/E convertor, but the coupling efficiency of a E/O convertor can be improved with multimode fiber (MMF), so we propose to use a ROF link with a vertical-cavity surface-emitting laser with a graded index MMF to transport optical signals. A multimode fiber has a larger core radius compared to a SMF. A larger core radius allows more optical power coupled into a fiber. With simple butt-coupling techniques, the coupling efficiency can be 90% and simplicity leads to reduction in cost of the link. Normally, the MMF is used in short distance digital applications with a bandwidth distance product of about 500 MHz.km, so it is good for local area picocells. Our approach is to transmit passband signals such as QPSK and FSK through the ROF link. Our simulation shows that a 900 MHz carrier can transport through a link of 1.22 km long. In this paper, we investigate the feasibility of using a MMF for antenna remoting in local area picocells and compare the tradeoff between coupling efficiency and bandwidth.

Spectrum sensing in cognitive radio networks: Up-to-date techniques and future challenges

Spectrum sensing is essential to secondary cognitive radio users to operate without interference to primary users. Secondary users can perform the spectrum sensing either individually or cooperatively by exchanging local observations among them. This paper presents individual spectrum sensing challenges such as hidden terminal problem, shadowing, fading, and hardware limitations. The study also discusses the benefits gained by cooperative spectrum sensing. A detailed review of up-to-date cooperative spectrum sensing techniques is given in this work along with the various sensing schemes that implement these techniques. Out review shows that cooperative spectrum sensing can improve the cognitive radio network performance by increasing spectrum efficiency and providing a better detection accuracy. However, the benefits come at the cost of increased overhead traffic, power consumption, complexity, and the need for control channels. We believe that challenges of the cooperative sensing will be the subject of many future studies yet to be done.