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Wireless Communication Systems
Published in Keshab K. Parhi, Takao Nishitani, Digital Signal Processing for Multimedia Systems, 2018
The rake receiver is a type of equalizer employed in systems that utilize spread spectrum modulation (i.e., systems where the bandwidth of the transmitted signal is much greater than the data rate) and where the channel undergoes frequency selective fading. Let us consider the transmission of one data symbol using the pulse s(t) = δ(t) as the input to the system shown in Fig. 8.1. Let the output of the channel, as in Fig. 8.1, be y(t) and let its complex envelope (i.e., low-pass representation) be Y(t), where the real and imaginary parts of Y(t) represent the in-phase and quadrature components of the band-pass signal y(t). Then we have () y(t)=∑k=0nαkg(t−τk)
Introduction to Spread Spectrum Systems
Published in Jerry D. Gibson, Mobile Communications Handbook, 2017
One of the original applications of DS–CDMA in digital cellular systems is the IS-95 standard. The second-generation cellular technology uses a combination of a Walsh code and a long PN code to spread the data bits. The receiver uses a rake receiver to combine the signal from the various multipaths and boost signal performance. The standard also supports a soft handover process between multiple cells. Subsequently, this standard has been replaced by the CDMA2000 and W-CDMA standards.
Advanced Transmission Techniques to Support Current and Emergent Multimedia Services
Published in Mário Marques da Silva, Cable and Wireless Networks, 2018
A RAKE receiver consists of a bank of decorrelators, each receiving a different multipath signal. After despreading by the decorrelators, the signals are combined using, usually, the MRC algorithm, as described in Section 7.6.2. Since the received multipath signals present uncorrelated fading coefficients, diversity order, and thus, performance is improved. Figure 7.16 illustrates the principle of the RAKE receiver. After spreading and modulation in the transmitter, the signal is passed through a multipath channel, which can be modeled by a tapped delay line. In the multipath channel there are L multipath components with different delays (τ1 ,τ0, …, τL), attenuation factor (α1 ,α2, …, αL), and phase components (θ1 ,θ2, …, θL), each corresponding to a different propagation path. The RAKE receiver has, ideally, a different finger for each multipath component. In each finger, the received signal is correlated by the spreading sequence, which is previously time-aligned with the delay of the corresponding multipath signal. After despreading, each captured multipath signal (in each jth finger of the RAKE) is weighted, and they are then combined. Moreover, when the MRC algorithm is considered to combine the several multipath signals, each finger aligns the phase of the signal corresponding to that multipath, and weights it by the channel attenuation, in order to allow a coherent sum. Therefore, when the RAKE considers a MRC algorithm, the signal in each finger is weighted by the complex conjugate of the corresponding multipath coefficient (wj = [αjejθ]* = αje -jθ) to allow a coherent sum. Each finger of the RAKE receiver corresponds to a decorrelator chain depicted inside each box of Figure 7.16. Owing to the mobile movement, the scattering environment will change, and thus, the delays and attenuation factors will change as well. Therefore, it is necessary to measure the tapped delay line profile and to reallocate RAKE fingers whenever the delays have changed by a significant amount. Small-scale changes, less than one chip, are taken care of by a code tracking loop, which tracks the time delay of each multipath signal.
Chaos-Based transmitted-reference ultra-wideband communications
Published in International Journal of Electronics, 2019
Marijan Herceg, Denis Vranješ, Ratko Grbić, Josip Job
Due to the hardware complexity of coherent ultra-wideband (UWB) receivers, the development of non-coherent modulation schemes has gained increased popularity in the last decade. Namely, UWB pulses have a very short duration (less than 1 ns) which implies an extremely wideband power spectrum (several Gigahertz). When such pulse is transmitted over a channel, its energy is spreaded over hundreds of different multipath rays (Win & Scholtz, 1998a). In order to collect enough energy, needed for proper detection, complex methods have to be used at the receiver. One of the methods is the usage of a RAKE receiver. The RAKE receiver consists of many fingers, where each finger collects the energy of only one multipath ray. Furthermore, the multipath ray gain and its time delay should be estimated at each finger, in order to make a proper energy collection (Lottici, D’Andrea, & Mengali, 2002; Win, Chrisikos, & Sollenberger, 2000; Win & Scholtz, 1998b). Then, the collected energy of each finger is combined and used for detection and demodulation of the received signal. Some efforts toward decreasing RAKE receiver complexity have been made in (Cassioli, Win, Vatalaro, & Molisch, 2007). Another approach is to reconstruct the template signal with the exact shape as a channel-distorted pulse at the receiver, in order to make a proper correlation with a data-modulated received pulse (Zhang & Song, 2006). However, both aforementioned methods have to perform channel estimation at the receiver, which is a very difficult and hardware-consuming task.
Performance Analysis of DCSK-BDR Systems over Nakagami-m Fading Channels
Published in IETE Technical Review, 2020
Chaotic signals are having the inherent property of wideband and aperiodic and hence are suitable for spread-spectrum communication. Among various chaotic modulation techniques proposed in the literature, differential chaos shift keying (DCSK) modulation is the most popular [1, 2]. Based on non-coherent detection technique, DCSK demodulation does not require an exact replica of chaotic sequence and chaotic synchronization at the receiver. Under the assumption that the channel does not vary within a symbol period, both the reference and data samples get subjected to identical channel distortion. Thus, using a simple autocorrelation receiver (AcR), DCSK obtains the advantage of multipath diversity without requiring any channel state information (CSI). On the contrary, the conventional spread spectrum techniques (code-division multiple access, CDMA, and frequency hopping multiple access, FHMA) need to employ a RAKE receiver to get the advantage of multipath diversity. It is to be noted that the implementation of RAKE receiver requires the full CSI and hence increases the circuit complexity. Because of its simple structure, DCSK has been widely used in various wireless communication systems [3–6]. Recently, many applications of DCSK modulation to improve the system performance in terms of energy efficiency, high data rate and bit error rate (BER) are reported in the literature. In [7], a multi-carrier DCSK (MC-DCSK) system is proposed which achieves the high data rate, energy efficiency and robust system performance by using multiple orthogonal subcarriers. A novel analog space–time block code DCSK (STBC-DCSK) scheme is proposed in [8], which combines the benefits of STBC and DCSK modulation to offer better system performance and robustness against multipath fading delay spread. Phase-separated DCSK (PS-DCSK) [9], improved DCSK (I-DCSK) [10] and short reference DCSK (SR-DCSK) [11] schemes are proposed to increase the data rate and enhance spectral efficiency. Multiresolution M-ary DCSK modulation scheme [12], adaptive Multiresolution M-ary DCSK scheme [13] and orthogonal multi-level DCSK scheme [14] achieve the higher data rate, increase spectral efficiency and offer lower energy consumption and better system performance as compared to the conventional DCSK system. For multi-user scenario, a multi-user OFDM-based DCSK (MU OFDM-DCSK) system is presented in [15], which increases the data rate and uses less transmitted bit energy. Most recently, the noise reduction DCSK (NR-DCSK) [16] and continuous-mobility DCSK (CM-DCSK) [17] systems are proposed to improve the error performance of DCSK system.