- Research
- Open Access
Ultra-wideband outdoor communication characteristics with and without traffic
- Chien-Ching Chiu^{1}Email author,
- Chien-Hung Chen^{2},
- Shu-Han Liao^{1} and
- Ting-Chieh Tu^{1}
https://doi.org/10.1186/1687-1499-2012-92
© Chiu et al; licensee Springer. 2012
- Received: 8 July 2011
- Accepted: 7 March 2012
- Published: 7 March 2012
Abstract
The BER performance for ultra-wideband (UWB) outdoor communication in LOS and NLOS environments with and without traffic is investigated. We obtain the impulse responses of the UWB outdoor environment by both 2.5D SBR-Image method and inverse Fourier transform techniques. The 2.5D SBR-Image method is first considered for two-dimensional environment simulated without heights of obstacles by ray tubes. Then, heights of the obstacles are taken into consideration between the transmitters and receivers. If the height of ray is lower than that of obstacles, the ray is neglected for the receivers. This effectively reduces the simulating time. By using the impulse response of multipath channels, the BER performance for binary pulse amplitude modulation communications over the radio UWB system is evaluated. We have performed computer simulations in LOS and NLOS environments with and without traffic in dense building areas. Numerical results have shown that the multipath effect caused by moving vehicles in the outdoor LOS and NLOS environments has a great impact on BER performance. Rake receivers are used to improve the outage probability. The relationship between traffic and BER performance is investigated; meanwhile, the characteristics of LOS and NLOS outdoor UWB environments are analyzed. Our investigation results can help improve planning and design of the UWB system.
Keywords
- UWB
- multipath
- BER
- outage probability
- RMS delay spread
1. Introduction
When the Federal Communications Commission agreed in February 2002 to allocate the 7500 MHz bandwidth for unlicensed use of ultra-wideband (UWB) communications in the 3.1-10.6 GHz frequency region [1], the UWB technology has received wide research attention ever since. There are two task groups for Ultra UWB systems. One is the 802.15.3a for high data rate (100 M/bps) and short operation distance (10 m). The other one is 802.15.4a for lower data rate (2 M/bps) and longer propagation distance (up to 100 m) [2].
UWB system offers many potential advantages, such as high resolution in multipath, reducing fading margins in link budget analysis, allowing for low transmit powers and low complexity [3, 4]. All wireless systems must be able to deal with the challenges of operating over a multipath propagation channel, where objects in outdoor environment can cause multiple reflections and shadow effect. The bit error rate (BER) degradation is caused by intersymbol interference (ISI) due to a multipath propagation arising from radio wave reflections by buildings, vehicles, trees, and even pedestrians. Recently, UWB has become an alternative physical layer candidate for TG4a in 802.15. In this task group, the emphasis is on lower data rates with larger propagation range in outdoor environment. In this study, we shall focus on this task group and investigate the outdoor environment performance [5, 6]. We shall use ray tracing techniques and inverse Fourier transform to obtain the impulse for UWB outdoor communications and evaluate the BER performance for binary pulse amplitude modulation (BPAM) communications over radio (IR) UWB systems with and without traffic. Section 2 presents channel modeling and system description. Section 3 shows numerical results. Conclusions are drawn in Section 4.
2. Channel modeling and system description
2.1. Channel modeling
The following two steps are used to model the multipath radio channel.
(1) Frequency responses for sinusoidal waves using the SBR/Image technique
The SBR/Image method can deal with high-frequency radio wave propagation in the complex indoor environment [7, 8]. It conceptually assumes that many triangular ray tubes are shot from the transmitting antenna (TX), and each ray tube, bouncing and penetrating in the environment is traced in the indoor multipath channel. If the receiving antenna (RX) is within a ray tube, the ray tube will produce image contributions to the received field at the RX, and the corresponding equivalent source (image) can be determined. By summing all contributions of these images, we can obtain the total received field at the RX. In the real environment, external noise in the channel propagation will be considered. The depolarization yielded by multiple reflections, refraction, and first-order diffraction, is also taken into account in our simulations. Note that the different values of dielectric constant and conductivity of materials for different frequencies are carefully considered in channel modeling.
For each ray tube bouncing and penetrating in the environment, we check whether reflection and penetration times of the ray tube are larger than the number of maximum reflection N_{ref} and maximum penetration N_{pen}, respectively. If not, we check whether the receiver falls within the reflected ray tube. If yes, the contribution of the ray tube to the receiver can be assumed to be emitted from an equivalent image source. In other words, a specular ray going to receiver is assumed to exist in this tube and this ray can be thought as launched from an image source. Moreover, the field diffracted from illuminated wedges of the objects in the environment is calculated by uniform theory of diffraction [10]. Note that only first diffraction is considered in this article, because the contribution of second diffraction is very small in the analysis.
where p is the path index, N_{p} is the number of paths, f is the frequency of sinusoidal wave, θ_{ p }(f) is the p th phase shift, and a_{ p }(f) is the p th amplitude. Note that the channel frequency response of UWB systems can be calculated by Equation (1) in the frequency range of UWB for both desired and interference signals.
(2) Inverse Fast Fourier Transform (IFFT) and Hermitian Processing
The frequency response can be transformed to the time domain by using the inverse Fourier transform with the Hermitian signal processing [11]. By using the Hermitian processing, the pass-band signal is obtained with zero padding from the lowest frequency down to direct current (DC), taking the conjugate of the signal, and reflecting it to the negative frequencies. The result is then transformed to the time domain using IFFT [12]. Since the signal spectrum is symmetric around DC, the resultant doubled sideband spectrum corresponds to a real signal in the time domain.
where l is the path index, α_{ l } is the amplitude of l th path and τ_{ l } is the time delay of the l th path. δ(.) is the Dirac delta function [13]. The goal of channel modeling is to determine the α_{ l } and τ_{ l } for a transmitter-receiver location in the system. The impulse response function of the station for a transmitter-receiver location is computed by the following two steps: step one is the obtaining of frequency response for sinusoidal waves by the SBR/Image technique and step two is the use of IFFT and Hermitian processing [14].
The SBR/Image method can deal with high-frequency radio wave propagation in the complex outdoor environment. It conceptually assumes that many ray tubes are short from the transmitting antenna (TX) and each ray tube bouncing and penetrating in the environment is traced. The first-order wedge diffraction is included, and the diffracted rays are attributed to the corresponding image. A frequency response is transformed to the time domain by using inverse Fourier transform with Hermitian signal processing. Using Hermitian processing, the pass-band signal is obtained with zero padding from the lowest frequency down to DC, taking the conjugate of the signal, and reflecting it to the negative frequencies. The result is then transformed to the time domain by using IFFT. Since the signal spectrum is symmetric around DC, the resultant doubled-side spectrum corresponds to a real signal in the time domain.
2.2. System block diagram
where t and σ are time and standard deviation of the Gaussian wave, respectively.
where x(t) is the transmitted signal and h_{ b }(t) is the impulse response of the UWB channel, n(t) is the white Gaussian noise with zero mean and power spectral density N_{0}/2 w/Hz.
where τ_{1} is the delay time of the first received wave.
2.3. RMS delay spread and mean excess delay
2.4. Rake receiver techniques
Signal reception in a multipath fading channel can be enhanced by a diversity technique using the rake receiver. Rake receivers combine different signal components that have propagated through different paths in the channel. This is a time diversity technique. The combination of different signal components will increase the signal-to-noise ratio (SNR), thus improving the reception performance. There are three major types of rake receivers to be considered here, i.e., I-rake, S-rake, and P-rake. First, I-rake is an ideal rake receiver that captures all of the received signal power by having numbers of fingers equal to the number of multipath components. Using the I-rake, an infinite number of correlators are required to distinguish infinite multipath components. This is difficult to carry out in reality. Second, S-rake is a selective rake receiver which selects L multipath components having largest signal amplitudes. The S-rake receiver has much less complexity than the A-rake. Third, P-rake is similar to S-rake. The principle is that the first multipath component will typically be strongest and contain the most received signal power. But, the disadvantage is that it may not correctly choose the strongest multipath components. In this article, we shall take the S-rake receiver. We will evaluate system performance using L as a parameter (L = 1, 2, 5, and 8).
3. Numerical results
Mean excess delay with and without traffic in Areas I and II
Area | Mean (ns) | Max (ns) | Min (ns) |
---|---|---|---|
Area I without traffic | 147.67 | 198.23 | 73.301 |
Area I with traffic | 216.08 | 397.08 | 80.411 |
Area II without traffic | 449.43 | 548.24 | 287.19 |
Area II with traffic | 509.93 | 691.80 | 339.27 |
RMS delay spread with and without traffic in the Area I and II
Area | RMS delay spread | |||
---|---|---|---|---|
Mean (ns) | Standard deviation (ns) | RMS max (ns) | RMS min (ns) | |
Area I without traffic | 16.72 | 12.99 | 58.89 | 1.41 |
Area I with traffic | 28.70 | 14.64 | 83.21 | 2.46 |
Area II without traffic | 41.53 | 42.20 | 173.55 | 3.78 |
Area II with traffic | 59.69 | 43.87 | 134.87 | 6.95 |
Our goal is to realize the characteristics of wireless communication system in outdoor environments. We use the ray tracing model to simulate the environments in wireless communication systems. Moreover, taking advantage of differencnt techniques to improve the performance of wireless communications systems different antenna arrays and rake receivers.
We considered the conditions with and without traffic in the UWB system in this article. We adopt the S-rake with different numbers "L" of rakes in our simulations to reduce the BER and improve outage probabilities.
4. Conclusions
UWB outdoor communication characteristics with and without traffic are presented. By using the impulse response of the multipath channel, the BER, mean excess delay, and RMS delay spread for UWB outdoor communication are evaluated. Also, outage probabilities are calculated for various different scenarios. Cases with and without traffic in the UWB outdoor environment are considered. Moreover, we use the rake receiver technique to improve the performance of outage probability.
Declarations
Authors’ Affiliations
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