DIVERSITY

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In the field of wireless communication channel ,Macro diversity is a kind of diversity in space scheme using several receiver antennas and/or transmitter antennas for transferring the same signal to distances. The distance between the transmitters is much longer than the signal’s wavelength.

In a cellular network macro-diversity implies that the antennas are typically situated in different BS sites or AP. Receiver macro-diversity is a form of combining antenna and requires an infrastructure that mediates the signals from the local antennas or receivers to a central receiver or central decoder. Transmitter macro-diversity is  a form of simulcasting and where the same signal is sent from several nodes to the destined nodes. If the signals are sent over the same physical channel i.e.the channel frequency and spreading sequence, the transmitters are said to form a network with single frequency- a term which is used especially in the broadcasting world.

The aim is for combating fading and to increase the received signal strength and signal quality in exposed positions in between the BS or AP. Macro diversity also facilitates efficient broadcasing and multicasting services  and where the same frequency channel can be used for all transmitters sending the same information to the destination nodes. The diversity scheme can be based on transmitter (downlink) macro-diversity and/or receiver (uplink) macro-diversity.

The Forms of Macrodiversity

The baseline form of macrodiversity is called as single-user macrodiversity. In this form  a single user which may have multiple antennas can communicate with several BS. Therefore, depending on the spatial degree of freedom (DoF) of the system user may transmit or receive multiple independent data streams to/from BS in the same time and frequency resource of the signal while communication.

Single-user macrodiversity can be:-

1.Uplink macrodiversity

2. Downlink macrodiversity

The next more advanced form of macrodiversity in which multiple distributed users communicate with multiple distributed base stations in the same time and frequency resource while communication.this type of configuration has been shown to utilize available spatial DoF optimally that increasing the cellular system capacity and user capacity considerably.

Multi-user macrodiversity can be:-

1.Macrodiversity multiple access channel (MAC)

2.Macrodiversity broadcast channel

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Mathematical description of macro diversity representation

The macrodiversity multi-user MIMO uplink communication system considered here includes  distributed single antenna users with  distributed single antenna BS. Following is the well established narrow band flat fading MIMO system model whoes input-output relationship can be given as:-   

Diversity gain

In wireless communication system, diversity gain is the increase in signal-to-interference ratio due to some diverse schemes i.e. or how much the transmission power can be reduced when a diversity scheme is introduced without a performance loss factor. Diversity gain is usually expressed in dB and sometimes as a power ratio. An example is soft handoff  gain in a system. For combining selection N signals are received and the strongest signal is selected. When the N signals are independent and distributed, the expected diversity gain has been shown to be as , expressed as a power ratio.

Microdiversity

Here we focus on a MIMO system that uses nT transmitting and nR receiving antenna elements, in a micro-diversiscenario. The aim here is to calculate the system capacity. This computation is based on the propagation model, and on the signal and noise variances as given by the RFFE model. Under the classical assumptions made by Foschini , the SNR is the same on all receiver branches, and is expressed as:  SNR =P/N, where P is the average received power on each branch. This is the ”standard” model. Here, the extension of the single antenna RFFE model described in section 3 to the MIMO case puts into question these assumptions, and leads to a modified relation between the SNR at the output of the RFFE block and the received power on a given receiver antenna. This new relationship also depends on the way the multi-antenna front-end is designed, in terms of number of analog RFFE blocks and in terms of AGC strategies. In any case, the resulting SNR vs received power relation affects capacity.

Fading Signal

In a typical wireless communication environment multiple propagation paths often exist from a transmitter to a receiver because of scattering by different objects in the communication. The signal copies following different paths may undergo different attenuation, distortions, delays and phase shifts in the signal. Constructive and destructive interference can occur at the receiver end of the destination. When  the destructive interference occurs the signal power can be significantly lowered. This phenomenon is called as fading. The performance of a system (in terms of probability of error) can be severely degraded by fading phenomenon.Very often especially in mobile communication systems not only do multiple propagation paths exist and they are also time-varying. The result is a system with time-varying fading channel. Communication through these channels may be difficult. Special techniques can be required to achieve satisfactory performance. 

Parameters of fading channels:-The general time varying fading channel model is too complex for the understanding and performance analysis of wireless communication channels. Fortunately many practical wireless channels can be adequately approximated by the wide-sense stationary uncorrelated scattering (WSSUS) model i.e. model [2, 3]. In the WSSUS model the time-varying fading process is assumed to be a wide-sense stationary random process and the signal copies from the scatterings by different objects are assumed to be independent on their own.

Some related questions and answers:-

1.What is  Macrodiversity?

Ans.  Macrodiversity is a kind of diversity in space scheme using several receiver antennas and/or transmitter antennas for transferring the same signal to distances where he distance between the transmitters is much longer than the signal’s wavelength.

2. Depending on the spatial degree of freedom (DoF) of the system what theuser may transmit or receive?

Ans.  Depending on the spatial degree of freedom (DoF) of the system user may transmit or receive multiple independent data streams to/from BS in the same time and frequency resource.

3.During coping if the signal what are the distortions?

Ans.  Signal copies  may undergo different attenuation, distortions, delays and phase shifts in the signal.

4.What is the main aim of microdiversity?

Ans.  The aim of microdiversity  is to calculate the system capacity.

IES SYLLABUS:

IES Syllabus for Electronics and Telecommunication 

GATE SYLLABUS:

GATE  Syllabus for Electronics and Communication

diversityforms of macro diversity and diversity gainmacro diversitymicro diversity

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