Diversity techniques for mobile wireless radio systemsDiversity @ Dictionary “the presence of a wide range of variation , Including everyone”. AJAL.A.J Assistant Professor –Dept of ECE, Federal Institute of Science And Technology (FISAT) TM MAIL: email@example.com
Cultural Dimensions NATIONAL REGIONALPROFESSIONAL / EDUCATIONAL GENDER CLASS RELIGIOUS GENERATIONAL ETHNIC CORPORATE PERSONAL UNITY IN DIVERSITY
What is Diversity?• Diversity may refer to: Multiculturalism, the ideology of including people of diverse cultural and religious backgrounds Diversity (politics), the political and social policy of encouraging tolerance for people of different backgrounds Diversity (business), the business tactic which encourages diversity to better serve a heterogeneous customer base Diversity training, the process of educating employees, students or volunteers to function in a diverse environment Biodiversity, the variation of life forms within a given ecosystem Diversity scheme, a method for improving the reliability of a message signal by using multiple communications channels
DIVERSITY asfar as mobilewireless radio systems areconcerned !
Common assumption:• Common assumption: signals that scatter off of different objects fade independently
How / when diversity is created ? • Diversity is created when these signals are separated in the receiver Examples: RAKE receiver - - separates paths by delay [PATH DIVERSITY]Multi beam antenna - - separates paths by angle [ANGLE DIVERSITY]
Another way to create diversity:change the relative phases of the multi pathsignals • Examples: • �Identical antennas, slightly different locations [SPACE DIVERSITY] • �Same signal received on different RF carriers [FREQUENCY DIVERSITY]. Required carrier separation depends inversely on delay spread
Still other ways to generate diversity:• Dual polarized antennas [POLARIZATION DIVERSITY]
Introduction• Fading Signal fluctuations caused by multipath propagation and shadowing effects.• Diversity Receiving the same information bearing signal over 2 or more fading channels.
Introduction (cont’d)• Space Transmission using multiple transmit/receive antennas.• Frequency Transmission using multiple frequency channels separated by at least the coherence bandwidth.• Time Transmission using multiple time slots separated by at least the coherence time.
The following classes of diversity schemes can be identified:1.Space diversity2.Polarization diversity3.Frequency diversity4.Time diversity5.Angle or direction diversity
1] Space diversity• The signal is transferred over several different propagation paths.• In the case of wired transmission, this can be achieved by transmitting via multiple wires.• In the case of wireless transmission, it can be achieved by antenna diversity using multiple transmitter antennas (transmit diversity) and/or multiple receiving antennas (reception diversity).• In the latter case, a diversity combining technique is applied before further signal
2] Polarization diversity• Polarization diversity: Multiple versions of a signal are transmitted and received via antennas with different polarization.• A diversity combining technique is applied on the receiver side.
3] Frequency diversity• Frequency diversity: The signal is transferred using several frequency channels or spread over a wide spectrum that is affected by frequency-selective fading.• Middle-late 20th century microwave radio relay lines often used several regular wideband radio channels, and one protection channel for automatic use by any faded channel. Later examples include: OFDM modulation in combination with subcarrier interleaving and forward error correction Spread spectrum, for example frequency hopping or DS-CDMA.
4] Time diversity• Time diversity: Multiple versions of the same signal are transmitted at different time instants.
Terrestrial microwave radio system with twoantenna arrays configured for space-diversity
Diversity Combining• Once you have created two or morediversity channels, what do you do with them?
Spatial diversity (cont’d)•Maximum ratio combining (MRC) h1* h1 x y h2 h2*
Combining schemes:•Selection combining (SC) Monitor Select SNR branch h1 x y h2
COMBINING AND SWITCHNING METHODSFOR COHERENT FOR NON - COHERENT DETECTION DETECTION
FOR COHERENT DETECTION• There is no difference whether the combining is carried out in the pre detection or in the post detection stage FOR NON-COHERENT DETECTION • There is a difference in performance exists between whether pre detection & post detection combining methods
• MRC is the best performance improvement compared with the other methods• However the trade off is :It requires 1. cophasing , 2. weighting 3.summing circuitsResulting in most complicated implementationComplexity performance
EGC• EGC is similar to MRC, except the weighting circuits are omitted• The performance improvement obtained by equal gain combiner is slightly inferior to that of a MRC , since the interference and noise corrupted signal combined with high quality signals
For VHF, UHF and microwave wireless radio applications• Both MRC and EGC methods are unsuitable• Compared with MRC, EGC methods, the selection method is more suitable for mobile radio applications, because of its simple implementation
• The diversity branch having the highest signal level (or) lowest Pe is selected• Also, in addition, stable operation is easily achieved , even in the fast multipath fading environments• But the performance improvement obtained by the selection method is still only slightly inferior to that of MRC method
Disadvantage of Selection or Switching• Combining: The continuous monitoring of the signals requires the same no. of receivers as the no. of diversity branches• This redundancy can be alleviated by the use of switching or scanning receiver
FIXED THERSHOLD• The switching from one branch to the other occurs when the signal level falls below a threshold value• The threshold value is fixed• However this threshold value is not necessarily the best over the entire service area
VARIABLE THERSHOLD• Here the threshold value is adjusted dynamically as the vehicle moves• It includes the feedback using Estimation• But it may introduce the envelope and phase transients that will reduce the performance improvement
Conclusions• The diversity is used to provide the receiver with several replicas of the same signal• Diversity techniques are used to improve the performance of the radio channel without any increase in the transmitted power• As higher as the received signal replicas are decorrelated, as much as the diversity gain