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Signal Propagation Over Wireless Channel
The performance of any communication system is eventually affected by the medium which the message signal passes through, referred to as communication channel. The random and severe behavior of wireless propagation channels turns communication over such channels into a difficult task and puts fundamental limitations on the performance of wireless communication systems. Wireless channels may be distinguished by the propagation environment encountered. Many propagation environments have been identified, such as rural, mountainous, urban, sub-urban, indoor, underwater or orbital environments, which differ in various ways. There are a lot of mechanisms that influence the electromagnetic radio wave propagation generally at-tributed to the interaction between waves and material such as reflection, refraction, diffraction and scattering. The transmission path between the receiver and the transmitter can be altered from simple line-of-sight (LOS) to one that is drastically obstructed by obstacles encountered such as buildings, foliage and mountains. Even the speed of the mobile impacts how rapidly the signal level fades. The characteristics of the channel appear to change randomly with time. Due to the wireless channel limitations, radio propagation can be roughly described by three nearly independent phenomenon; path loss variation with distance, shadowing and multipath fading, see figure 1.2.1. The path loss is a deterministic effect which depends only on the dis-tance between the transmitter and the receiver. It plays an important role on larger time scales since the distance between transmitter and receiver in most situations does not change signifi-cantly on smaller time scales. While shadowing and fading both have stochastic nature causes fluctuations and significant attenuation changes of the received signal strength within smaller time scales. Shadowing occurs due to the varying terrain conditions in suburban areas and due to the obstacles such as buildings etc. The term fading in wireless communications refers to the interference caused by the reception of numerous reflected, diffracted, scattered copies of a given signal at an antenna. Fading leads to significant attenuation changes within smaller time scales [18–20]. In the communications literature, fading is roughly grouped into two categories: large-scale and small-scale fading.
Large-scale fading
Large-scale fading characterizes the attenuation of the average signal power or path loss as a function of distance and shadowing by large objects such as buildings and hills. This occurs as the receiver moves over large areas. The statistics of large-scale fading is typically frequency independent and normally described in terms of a mean-path loss and a log-normally distributed variation about the mean which is known as shadowing. Hence the term large-scale fading correspond to the combined effects of path-loss and shadowing loss.
Small-scale fading
Small-scale fading refers to the rapid fluctuations in signal amplitude and phase that occur as a result of the constructive and destructive interference of the multiple signal paths due to small changes in the spatial separation between a receiver and transmitter. Small-scale fading is referred to as Rayleigh fading if the number of multiple paths at the receiver is large and there is no line of sight signal, hence the envelope of the received signal is modeled by a Rayleigh probability density function (PDF). However, if the line-of sight path is present as a dominant non-fading signal component, the small scale fading envelope is described by a Rician PDF.
During its propagation in space, electromagnetic signal undergoes many interactions with the environment. In addition, these signals take different paths before reaching their destination, so they do not travel the same distance and interact differently with the environment. Hence, these signals may arrive out of phase at the receiver and with different power levels. Multipath is the main cause of troubles in a wireless channel. When the delay differences between the multipath components are small compared to the inter-symbol interval, these components may be interfere constructively or destructively at the receiver depending on the carrier frequency and the delay differences. This phenomena lead to instantaneous and severe drop in the signal-to-noise ratio (SNR), which significantly degrades the performance.
The most practical and effective countermeasures nowadays to combat multipath effects are diversity techniques. Diversity provides a receiver with redundant signal information. It allows the receiver to average individual channel effects. In the following sections we give a brief background of the main diversity techniques.
Diversity techniques
Diversity is one of the most important methods used in combating the detrimental effects of channel fading. It is also a technique to fight against errors [21–24]. The idea behind diversity is to obtain two or more independent paths for the same transmitted signal to increase the chance of having fewer errors at the receiver. Indeed, if multiple copies of the original signal are sent through different paths, they will exhibit distinct channel properties. Diversity can effectively combat channel fading since the probability of all paths suffering from deep fading simultaneously is low and the duplication of the transmitted signal enhances SNR and improves signal robustness. The main common forms of diversity are frequency, time and space which well be presented in the following, respectively.
Spectral diversity
The signal is transmitted simultaneously over two or more frequency bands. This form of diversity is effective when the transmission bandwidth is large enough. The principle behind this technique is that bands separated by more than the coherence bandwidth of the channel will be uncorrelated. The coherence bandwidth of a wireless channel is the range of frequencies that are allowed to pass through the channel without distortion and the channel can be considered flat fading. The separated bands therefore will experience different fades giving rise to a set of subbands with different fading rates. Examples of systems employing frequency diversity are frequency hopping spread spectrum (FH-SS) and orthogonal frequency-division multiplexing (OFDM) communication systems.
Temporal diversity
The signal is transmitted multiple times over several time slots. Signals that are received at different times are uncorrelated, since the wireless propagation channel is time variant. The time slots must exceed the coherence time of the channel which is the time duration over which the channel impulse response is considered invariant. Therefore, distinct time slots well experience independent channel fading. However, this diversity technique introduces a significant delay in the system. Temporal diversity can be employed using techniques such as interleaving, forward error correction (FEC) code, or automatic repeat request (ARQ) protocols.
Spatial diversity
The signal transmission is done using multiple transmitting and/or receiving antennas. The prin-cipal requirement is the separation among adjacent antennas which should be large enough (at least 10 carrier wavelength) to ensure that signals from different antennas undergo independent fading. This diversity technique is not, therefore, suitable for application involving small de-vices such as handsets and sensor nodes. In addition to the physical hardware requirement and complex software integration versus a single antenna system. Spatial diversity can be achieved through multiple-input multiple-output (MIMO) techniques as we well show in section 1.4.
Diversity is not only achieved by the previously presented techniques, other mechanisms exist such as angle-of-arrival diversity, polarization diversity, and pattern diversity [25–27].
Multiple-Input Multiple-Output (MIMO) system
Spatial diversity is traditionally implemented by the use of multiple antennas for transmission and/or reception, referred to as MIMO technique in contrast with a SISO (single-input single-output) system that uses one transmit antenna and one receive antenna. In the literature, there are other models of simplified MIMO systems that exploit spatial diversity. The use of a sin-gle transmit antenna and multiple receive antennas is known as single-input multiple-output (SIMO) system, whereas the reverse case of using multiple transmit antennas and a single re-ceive antenna is called multiple-input single-output (MISO) system [28, 29].
For richly scattered wireless environments, MIMO systems (i.e. separation among adjacent antennas) allow the receiver to see independent versions of the transmitted information by pro-viding independent spatial paths between each pair antennas as illustrated in Fig. 1.4.1. MIMO systems use space-time processing techniques to provide reliable and high data communication and it is necessary that fading signals at different antennas are uncorrelated or slightly correlated in order to achieve the maximum diversity. These diversity techniques have revolutionized wire-less communications over the past decade since it serves to improve reliability (bit error rate) and throughput (bit/sec) in these systems.
Table of contents :
1 Cooperative Network – Background
1.1 Introduction
1.2 Signal Propagation Over Wireless Channel
1.2.1 Large-scale fading
1.2.2 Small-scale fading
1.3 Diversity techniques
1.3.1 Spectral diversity
1.3.2 Temporal diversity
1.3.3 Spatial diversity
1.4 Multiple-Input Multiple-Output (MIMO) system
1.5 Cooperative communications
1.5.1 Ad-hoc Networks
1.5.2 Relaying protocols
1.5.3 Virtual MIMO System (Cooperative Diversity)
1.6 Antenna/relay selection
1.7 Conclusion
2 UltraWideband Communications
2.1 Introduction
2.2 Definition of UWB signals
2.3 Motivation for the use of ultra wideband (UWB)
2.4 UWB Pulse Waveforms
2.4.1 Gaussian Pulse
2.4.2 Gaussian Monocycle Pulse
2.4.3 Gaussian doublet Pulse
2.5 Pulse Modulation
2.5.1 Pulse Position Modulation (PPM)
2.5.2 Binary Phase Shift Keying (BPSK)
2.6 Multiple Access Techniques
2.6.1 Time Hopping Multiple Access
2.6.2 Direct Sequence Multiple Access
2.7 UWB Channel Model
2.7.1 General Overview of UWB channel models
2.7.2 The IEEE 802.15.4a channel Models
2.8 Conclusions
3 Proposed System Model: Description and Methodology
3.1 Introduction
3.2 System Description
3.3 Transmitter Model
3.4 Channel Models
3.4.1 Residential environments
3.4.2 Indoor office environments
3.5 Receiver Structure
3.5.1 Rake Receiver
3.5.2 Channel estimation
3.5.3 Maximum Ratio Combining (MRC)
3.6 Conclusions
4 System Performance – TH-PPM
4.1 Introduction
4.2 Cooperative TH-PPM Impulse Radio UWB system
4.3 Decision Variable Statistics
4.4 BER Performance Analysis
4.5 Simulation Results And Discussion
4.6 Conclusions
5 System Performance – DS-BPSK
5.1 Introduction
5.2 Cooperative DS-BPSK IR-UWB system
5.3 Decision Variable Statistics
5.4 BER Performance Analysis
5.5 Simulation Results And Discussion
5.6 Conclusions
General Conclusions and Future Works
Bibliography
