Wireless communications has entered a new phase. The focus is shifting from voice to multimedia services, which need wideband radio links. Furthermore, new wireless subscribers are signing up at an increasing rate demanding more capacity. Micro and pico cellular architecture is an attractive solution for both of these issues because, it increases the frequency reuse and enables wideband access. In this micro/pico cell scenario, low power radio access points (RAP's) should provide wireless access instead of conventional base stations for cost considerations. These radio access points should be robust and have low complexity and connected to the central base station via radio-over-fiber (ROF)1 links [1]. Figure 1 (left) shows such architecture and Figure 1 (right) gives a closer look at the downlink. The focus of this work is to investigate various signal processing strategies to provide a cost-effective, high performance solution for high-speed fiber based wireless access. We have been investigating signal-processing strategies to improve the performance of fiber based wireless access schemes cost-effectively.
KEYWORDS: Video, Computer programming, Video coding, Receivers, Video surveillance, Error control coding, Sensor networks, Motion estimation, Sensors, Data modeling
Low complexity encoders at the expense of high complexity decoders are advantageous in wireless video sensor
networks. Distributed video coding (DVC) achieves the above complexity balance, where the receivers compute
Side information (SI) by interpolating the key frames. Side information is modeled as a noisy version of input
video frame. In practise, correlation noise estimation at the receiver is a complex problem, and currently the
noise is estimated based on a residual variance between pixels of the key frames. Then the estimated (fixed)
variance is used to calculate the bit-metric values. In this paper, we have introduced the new variance estimation
technique that rely on the bit pattern of each pixel, and it is dynamically calculated over the entire motion
environment which helps to calculate the soft-value information required by the decoder. Our result shows that
the proposed bit based dynamic variance estimation significantly improves the peak signal to noise ratio (PSNR)
performance.
In a microwave fiber wireless communication system, reducing optical carrier-to-sideband ratio can improve the receiving sensitivity and dynamic range of the system. However, at lower modulation frequencies corresponding to cellular radio (900 MHz), personal communication system (1.8 GHz) and IEEE 802.11g (2.4 GHz), separating the signals from the carrier and reducing carrier power in optic domain is a rather challenging task. In this paper, we have presented a new method to improve the transmission efficiency in microwave fiber-optic link by suppressing the optical carrier using a sub-picometer bandpass filter. The filter we designed and fabricated has a −3dB bandwidth of 120 MHz, capable of even filtering out a microwave signal as low as 900 MHz. We compared the transmission performance between filtered signals and unfiltered signals. The experimental results showed that the receiver sensitivity was improved significantly, 4.4dB for the 900MHz signal, and up to 8.7dB for the 1.8GHz at bit-error-rate of 10-9, by optical carrier suppression.
Subcarrier multiplexed transmission of cellular (900 MHz),
personal communications systems (1.8 GHz) and wireless LAN (2.4
GHz) over the fiber has interesting applications. These multi
channel radio over fiber links can connect enhanced wireless
hot-spots that will support high speed wireless LAN services or
low speed cellular services to different customers from the same
antenna. Optical pre-filtering of SCM signals allows the use of
inexpensive photodetector and increases network flexibility with
fiber based optical filters. However, realizing optical
demultiplexing at such low frequencies necessitates optical
filters with high selectivity and low insertion loss. In this
paper, we implemented a fiber wireless access system, where
demultiplexing of subcarrier multiplexed cellular and WLAN signals
was demonstrated in optical domain using a sub-picometer bandpass
filter. Our novel fiber Bragg grating based bandpass filter has a
bandwidth of 120 MHz at -3dB, 360 MHz at -10 dB and 1.5 GHz at -20
dB respectively. We experimentally verified that this filter
could adequately isolate signals at as low as 900 MHz from 2.4
GHz. Experimental results show that the designed all optical
demultiplexer provides about 25 dB isolation between 900 MHz and
2.4 GHz radio signals.
This paper proposes an innovative approach for noise cancellation at infrared (IR) wireless receivers. Ambient noise due to artificial lighting and the sun has been a major concern in infrared systems. The background induced shot noise typically has a power from 20 to 40 dB more than the signal induced shot noise and varies with time. Due to these changing conditions, infrared wireless receivers experience high level of non-stationary noise. The objective of the work mentioned in this paper is to develop digital signal processing algorithms at the infrared wireless system to combat high power non-stationary noise. The noisy signal is decomposed using a joint time and frequency representation such as wavelets and wavelet packets, into transform domain coefficients and the lower order coefficients are removed by applying a threshold. Denoised version is obtained by reconstructing the signal with the remaining coefficients. In this paper, we evaluate different wavelet methods for denoising at an infrared wireless receiver. Simulation results indicate that if the noise is uncorrelated with the signal and the channel model is unavailable the wavelet denoising method with different wavelet analyzing functions improves the signal to noise ratio (SNR) from 4 dB to 7.8 dB.
Infrared (IR) is a new medium for wideband wireless applications especially at indoors. Infrared has abundant bandwidth and does not require FCC approval enabling rapid deployment. Intensity modulated systems do not experience multipath fading. However, health regulations limit the laser output power [IEC 825]. Photodetectors with large photosensitive areas have high sensitivity but they also have high junction capacitance that limits the bandwidth. High ambient noise is another concern in infrared systems. Spreading the spectrum, which enables low power transmission, can be used to overcome these limitations. In this paper, we evaluate the performance of a direct spread CDMA type infrared wireless system in a multiuser environment considering: multi-user interference and inter-symbol-interference assuming different channel models. According to the simulation a BER of 10-7 can be achieved with a moderate bit rate when, the channel impulse response exponentially decays. The maximum bit rate, however, depends on the mean square value of the channel dispersion. Lab prototype agrees with the theory and shows ambient light is the biggest concern in a pico cellular environment with few users.
Microwave Fiber Optic (MFO) links have attracted much attention recently with their application in wireless access. When a wireless link is in series with the optical link, nonlinear distortion (NLD) of the MFO link becomes the biggest concern. The linearity requirement is high due to the large variations in the RF power through the link. Laser intrinsic nonlinearity is usually the major concern in a directly modulated link. However, in wireless applications significant amplification is required at the antenna site, which introduces additional NLD. In this paper a higher order adaptive fiber is proposed to model the entire MFO link considering all cascaded nonlinearities. The FIR filter runs at the baseband symbol rate and trains itself from the input/output amplitude and phase relationships of the microwave modulated symbols. Thus no accurate knowledge of the link physical parameters is required. The powerful recursive least square algorithm converges quickly, tracking any modification or drift in the link parameters. Simulation results show that, third order Volterra adaptive filters are adequate to model measured AM-AM and AM-PM characteristics of a MFL link under steady state conditions. The link consists of a directly modulated InGaAsP DFB laser and PIN diode receiver with a high gain amplifier.
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