CHAPTER 2 WIRELESS CHANNEL

Transcription

1 CHAPTER 2 WIRELESS CHANNEL 2.1 INTRODUCTION In mobile radio channel there is certain fundamental limitation on the performance of wireless communication system. There are many obstructions between transmitter to receiver like buildings, mountains, trees etc. Radio channels are random and very difficult to analyze. When mobile terminal moves from one place to another speed of motion impacts on signal level and performance of channel. When line of sight is present between transmitter and receiver the performance get improves. In our thesis we have analyze the performance of wireless network in Rician fading channel. In this chapter large scale and small scale fading has been discussed. 2.2 LARGE SCALE FADING The electromagnetic wave propagation are generally attributed to reflection, diffraction, and scattering. Most cellular radio systems operate in urban areas where there is no direct line-of-sight path between the transmitter and the receiver, and where the presence of high rise buildings causes severe diffraction loss. Due to multiple reflections from various objects, the electromagnetic waves travel along different paths of varying lengths. The interaction between these waves causes multipath fading at a specific 11

2 location, and the strengths of the waves decrease as the distance between the transmitter and receiver increases. Propagation models have traditionally focused on predicting the average received signal strength at a given distance from the transmitter, as well as the variability of the signal strength in close spatial proximity to a particular location. Propagation models that predict the mean signal strength for an arbitrary transmitterreceiver separation distance are useful in estimating the radio coverage area of a transmitter and are called large-scale propagation models, since they characterize signal strength over large T-R separation distances (several hundreds or thousands of meters). Reflection, diffraction, and scattering are the three basic propagation mechanisms which impact propagation in a mobile communication system Reflection Reflection occurs when a propagating electromagnetic wave impinges upon an object which has very large dimensions when compared to the wavelength of the propagating wave. Reflections occur from the surface of the earth and from buildings and walls Diffraction Diffraction occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities. The secondary waves resulting from the obstructing surface are present throughout the space and even behind the obstacle, giving rise to a bending of waves around the obstacle, even when a line-of-sight path does not exist between transmitter and receiver. At high frequencies, diffraction, like reflection, depends on the geometry of the object, as well as the amplitude, phase, and polarization of the incident wave at the point of diffraction. 12

3 2.2.3 Scattering Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength, and where the number of obstacles per unit volume is large. Scattered waves are produced by rough surfaces, small objects, or by other irregularities in the channel. In practice, foliage, street signs, and lamp posts induce scattering in a mobile communications system. 2.3 SMALL SCALE FADING Small-scale fading is used to describe the rapid fluctuation of the amplitude of a radio signal over a short period of time or travel distance, so that large-scale path loss effects may be ignored. Fading is caused by interference between two or more versions of the transmitted signal which arrive at the receiver at slightly different times. These waves, called multipath waves, combine at the receiver antenna to give a resultant signal which can vary widely in amplitude and phase, depending on the distribution of the intensity and relative propagation time of the waves and the bandwidth of the transmitted signal. Multipath in the radio channel creates small-scale fading effects. The three most important effects are: Rapid changes in signal strength over a small travel distance or time interval. Random frequency modulation due to varying Doppler shifts on different multipath signals. Time dispersion (echoes) caused by multipath propagation delays. 13

4 2.4 FACTORS INFLUENCING SMALL-SCALE FADING Many physical factors in the radio propagation channel influence small scale fading. These include the following: Multipath propagation The presence of reflecting objects and scatterers in the channel creates a constantly changing environment that dissipates the signal energy in amplitude, phase, and time. These effects result in multiple versions of the transmitted signal that arrive at the receiving antenna, displaced with respect to one another in time and spatial orientation. The random phase and amplitudes of the different multipath components cause fluctuations in signal strength, thereby inducing small-scale fading, signal distortion, or both. Multipath propagation often lengthens the time required for the baseband portion of the signal to reach the receiver which can cause signal smearing due to inter symbol interference Speed of the mobile The relative motion between the base station and the mobile results in random frequency modulation due to different Doppler shifts on each of the multipath components. Doppler shift will be positive or negative depending on whether the mobile receiver is moving toward or away from the base station Speed of surrounding objects If objects in the radio channel are in motion, they induce a time varying Doppler shift on multipath components. If the surrounding objects move at a greater rate than the mobile, then this effect dominates the small-scale fading. Otherwise, motion of 14

5 surrounding objects may be ignored, and only the speed of the mobile need to be considered The transmission bandwidth of the signal If the transmitted radio signal bandwidth is greater than the "bandwidth" of the multi-path channel, the received signal will be distorted, but the received signal strength will not fade much over a local area. The bandwidth of the channel can be quantified by the coherence bandwidth which is related to the specific multipath structure of the channel. The coherence bandwidth is a measure of the maximum frequency difference for which signals are still strongly correlated in amplitude. If the transmitted signal has a narrow bandwidth as compared to the channel, the amplitude of the signal will change rapidly, but the signal will not be distorted in time. Thus, the statistics of small-scale signal strength and the likelihood of signal smearing appearing over small-scale distances are very much related to the specific amplitudes and delays of the multipath channel, as well as the bandwidth of the transmitted signal. 2.5 CLASSIFICATION OF FADING CHANNELS Depending on the parameters of the channels and the characteristics of the signal to be transmitted, time-varying fading channels can be classified as: Flat fading versus frequency selective fading This classification is made on the basis of time delay spread is shown in fig.2.1. If BW of transmitted signal (B S ) is small compared to coherence bandwidth (B C ), then all frequency component of signal would roughly undergo same amount of fading. 15

6 Then the channel is classified as flat fading. In this rms delay spread (σ τ ) is less than symbol period (T S ). On the other hand, if BW B S is large compared to B C, then different frequency component of signal would undergo different degree of fading. Then the channel is classified as frequency selective fading. In this rms delay spread (σ τ ) is greater than symbol period (T S ) Fast fading versus slow fading This classification is made on the basis of Doppler spread is shown in fig.2.2. If BW of transmitted signal (B S ) is small compared to Doppler spread (B D ), then channel impulse response changes rapidly within the symbol duration. Then the channel is classified as fast fading. In this symbol period (T S ) is greater than coherence time (T C ). On the other hand if BW of transmitted signal (B S ) is large compared to Doppler spread (B D ), then channel impulse response changes at a rate much slower than the transmitted base band signal. Then the channel is classified as slow fading. In this symbol period (T S ) is less than coherence time (T C ). The above classification of fading channels depends on the properties of transmitted signal. The two ways classification based on time delay spread and Doppler spread gives rise to four different types of channel are shown in fig.2.3.and fig Flat Slow fading Flat Fast fading Frequency Selective Slow fading 16

7 Symbol Period of Transmitting Symbol Frequency Selective Fast fading Small Scale Fading (Based on multipath time delay spread) Flat fading Frequency Selective Fading 1. BW of signal < BW of channel 1. BW of signal > BW of channel 2. Delay spread < Symbol period 2. Delay spread > Symbol period Fig. 2.1 Types of small-scale fading based on multipath time delay spread Small Scale Fading (Based on Doppler spread) Fast fading Slow Fading 1. High Doppler spread 1. Low Doppler spread 2. Coherence time < Symbol period 2. Coherence time > Symbol period 3. Channel variations faster than base- 3. Channel variations slower than band signal variations baseband signal variations Fig. 2.2 Types of small-scale fading based on Doppler spread T s Flat Slow Flat Fast Fading Fading Fading σ τ Frequency selective Frequency selective Slow Fading Fast Fading T c T s Transmitted Symbol Period Fig. 2.3 Matrix illustrating type of fading experienced by a signal as a function of Symbol Period 17

8 Transmitted Baseband Signal Bandwidth B c B s Frequency Selective Fast Fading Flat Fast Flat Slow Fading Fading Fading Frequency Selective Slow Fading B d B s Transmitted Baseband Signal Bandwidth Fig. 2.4 Matrix illustrating type of fading experienced by a signal as a function of Baseband Signal Bandwidth. 2.6 MULTIPATH DIVERSITY METHODS Diversity is a very effective communication technique that provides higher data rate, mitigates effect of fading, and brings effective improvement in wireless link at relatively low cost. In diversity, the receiver is provided with multiple copies of the same message, which helps effectively in combating channel impairment occurring due to multipath fading Space Diversity Space diversity is a method of transmission or reception, or both, in which the effects of fading are minimized by the simultaneous use of two or more physically separated antennas. Thus, space diversity requires multi-antennas which can be used at either mobile or base station, or both. Spatially separated antennas on the devices with separations of one half wavelength or more will have uncorrelated envelopes. Base station antennas must be separated far apart on the order of several tens of wavelengths to achieve de-correlation. Multi-antenna wireless communication systems are shown in Fig

9 Single Input Single Output (SISO) Systems: This is the simplest communication link between one transmit antenna and one receive antenna. Therefore, in this spatial diversity cannot be applied. Single Input Multiple Output (SIMO) Systems: This communication system has one transmit antenna and multiple antennas at receiver side. The transmitted signal from single transmit-antenna arrives at all receiver antennas through different channels and it is assumed that channels are completely decorrelated. Thus multiple independent copies of same signal arrive at receiver. With help of any of the combining technique such as selection, maximal ratio or equal gain combining at receiver, SNR at output can be maximized. Multiple Input Single Output (MISO) Systems: MISO communication systems use multiple antennas at the transmitter and a single antenna at the receiver. Multiple Input Multiple Output (MIMO) Systems: MIMO communication systems use multiple antennas at both the transmitter and receiver. In MIMO systems generally space-time diversity is employed. The Alamouti space time block code is the simplest code belonging to the family of orthogonal space time code. 19

10 Fig.2.5 Multi-antenna Wireless Communication systems Frequency Diversity In frequency diversity the same information signal is transmitted and received simultaneously on two or more independent fading carrier frequencies. The idea behind this technique is that frequencies separated by more than the coherence bandwidth of the channel will not experience the same fading. This is employed in microwave line-of-sight links which carry several channels in a frequency division multiplex mode (FDM) Time Diversity The signal representing the same information is sent over the same channel at different times. Time diversity is obtained by repeatedly transmitting same information at time spacing that is separated by at least the coherence time of the channel. Thus multiple repetitions of the signal are received with independent fading conditions, thereby providing diversity, an example is RAKE receiver. 20