The wireless channel, diversity and capacity of wireless channels
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This document discusses wireless communication channels and diversity techniques. It describes how wireless channels experience both large-scale and small-scale fading, unlike wired channels. Small-scale fading is caused by multipath interference and results in rapid channel variations. Diversity combines multiple independent copies of the signal to improve link performance over fading channels. It can be achieved through techniques like antenna, site, frequency, time, polarization and angle diversity. The capacity of wireless channels is fundamentally limited by information theory based on available channel resources.
The wireless channel, diversity and capacity of wireless channels
- 1. THE WIRELESS CHANNEL, DIVERSITY AND CAPACITY OF WIRELESS CHANNELS Presented By: Faiz.ur.Rehman MS(CS) [email protected]
- 2. CONTENTS Communication channel The wire line channel The wireless channel Fading effect Large scale fading Small scale-multipath fading Diversity Capacity of wireless channels Methods for obtaining multiple replicas
- 3. COMMUNICATION CHANNEL Transmitter Receiver The medium used to transmit the signal from the transmitter to the receiver Wireline / Wireless channel Channel
- 4. THE WIRE LINE CHANNEL Transmitter Receiver Wireline Channel, e.g. copper wire Too many noises? Shielded against electromagnetic noise Large signal attenuation? Use repeaters Data speed too low? Upgrade to coaxial cable Data speed still too low? Upgrade to optical fiber
- 5. THE WIRELESS CHANNEL Channel varies at two spatial scales: large scale fading small scale fading
- 6. FADING EFFECT Typical Indoor Wireless Environment Signal strength fluctuates significantly Wireless channel cannot be engineered. You can only improve your transmission and reception techniques.
- 7. LARGE-SCALE FADING In free space, received power attenuates like 1/r2. ( r => radius ) With reflections and obstructions, can attenuate even more rapidly with distance. Detailed modeling complicated. Time constants associated with variations are very long as the mobile moves, many seconds or minutes. More important for cell site planning, less for communication system design.
- 8. SMALL-SCALE MULTIPATH FADING Wireless communication typically happens at very high carrier frequency. (e.g. fc = 900 MHz or 1.9 GHz for cellular) Multipath fading due to constructive and destructive interference of the transmitted waves. Channel varies when mobile moves a distance of the order of the carrier wavelength. This is 0.3m for GHz cellular. For vehicular speeds, this translates to channel variation of the order of 100 Hz. Primary driver behind wireless communication system design.
- 9. What is diversity? Diversity is a technique to combine several copies of the same message received over different channels. Why diversity? To improve link performance. DIVERSITY
- 10. DIVERSITY Communication over a flat fading channel has poor performance due to significant probability that channel is in deep fading. Reliability is increased by providing more signal paths that fade independently. Diversity can be provided across time, frequency and space.
- 11. CAPACITY OF WIRELESS CHANNELS Information Theory (1948): Information theory says every channel has a capacity. Information theory provides a fundamental characterization of coded performance. It identifies the impact of channel resources on performance as well as suggests new and cool ways to communicate over the wireless channel. It provides the basis for the modern development of wireless communication.
- 12. METHODS FOR OBTAINING MULTIPLE REPLICAS • Antenna Diversity • Site Diversity • Frequency Diversity • Time Diversity • Polarization Diversity • Angle Diversity
Editor's Notes
- #8: Large-scale fading, due to path loss of signal as a function of distance and shadowing by large objects such as buildings and hills. This occurs as the mobile moves through a distance of the order of the cell size, and is typically frequency independent.
- #9: Small-scale fading, due to the constructive and destructive interference of the multiple signal paths between the transmitter and receiver. This occurs at the spatial scale of the order of the carrier wavelength, and is frequency dependent.
- #10: A signal transmitted at a particular carrier frequency and at a particular instant of time may be received in a multipath null. Diversity reception reduces the probability of occurrence of communication failures (outages) caused by fades by combining several copies of the same message received over different channels.
- #11: Fading: In wireless communications, fading is deviation of the attenuation affecting a signal over certain propagation media. Flat fading: In flat fading, the coherence bandwidth of the channel is larger than the bandwidth of the signal. Therefore, all frequency components of the signal will experience the same magnitude of fading. Deep fading: Strong destructive interference is frequently referred to as a deep fade and may result in temporary failure of communication due to a severe drop in the channel signal-to-noise ratio. The terms slow and fast fading refer to the rate at which the magnitude and phase change imposed by the channel on the signal changes. The coherence time is a measure of the minimum time required for the magnitude change of the channel to become uncorrelated from its previous value.
- #12: Information theory in 1948 Information theory says every channel has a capacity. Many recent advances based on understanding wireless channel capacity. While Wireless communication has been around since 1900’s. Ingenious but somewhat adhoc design techniques
- #13: In antenna (or micro) diversity the signal from antennas mounted at separate locations are combined. Typically these antennas are located on the vehicle or at the same base station tower and their spacing is a few wavelengths. In site (or macro) diversity the receiving antennas are located at different receiver sites. For instance, signals from within a cell may be received at the different corners of the hexagonal area. The advantage is that not only the multipath fading attenuation is independent at each branch but that the shadowing and path losses are also uncorrelated to some extent. In Frequency diversity, Each message is transmitted at different carrier frequencies simultaneously. Frequency separation >> coherence bandwidth In time diversity, the time difference between two transmissions should be large compared to the time is takes the mobile antenna to move half a wavelength. In systems with stationary antennas, such as indoor wireless communication, time diversity will be less effective as the channel characteristics do not change very much with time. Polarization diversity exploits the fact that obstacles scatter waves differently depending on their polarization. It is hoped that in (at least) one of the branches the received waves do not cancel each other, resulting in a relatively strong signal. In Angle Diversity, In angle diversity, directional antennas receive only a fraction of all scattered energy. Waves from different angles of arrival are combined optimally, rather than with random phase.