Macrodiversity


In the field of wireless communication, macrodiversity is a kind of space diversity scheme using several receiver antennas and/or transmitter antennas for transferring the same signal. The distance between the transmitters is much longer than the wavelength, as opposed to microdiversity where the distance is in the order of or shorter than the wavelength.
In a cellular network or a wireless LAN, macro-diversity implies that the antennas are typically situated in different base station sites or access points. Receiver macro-diversity is a form of antenna combining, and requires an infrastructure that mediates the signals from the local antennas or receivers to a central receiver or decoder. Transmitter macro-diversity may be a form of simulcasting, where the same signal is sent from several nodes. If the signals are sent over the same physical channel, the transmitters are said to form a single frequency network—a term used especially in the broadcasting world.
The aim is to combat fading and to increase the received signal strength and signal quality in exposed positions in between the base stations or access points. Macro diversity may also facilitate efficient broadcasting and multicasting services, where the same frequency channel can be used for all transmitters sending the same information. The diversity scheme may be based on transmitter macro-diversity and/or receiver macro-diversity.

Examples

The baseline form of macrodiversity is called single-user macrodiversity. In this form, single user which may have multiple antennas, communicates with several base stations. Therefore, depending on the spatial degree of freedom of the system, user may transmit or receive multiple independent data streams to/from base stations in the same time and frequency resource.
In next more advanced form of macrodiversity, multiple distributed users communicate with multiple distributed base stations in the same time and frequency resource. This form of configuration has been shown to utilize available spatial DoF optimally and thus increasing the cellular system capacity and user capacity considerably.
The macrodiversity multi-user MIMO uplink communication system considered here
consists of distributed single antenna
users and distributed single antenna
base stations. Following the well established narrow band flat
fading MIMO system model, input-output relationship can be given as
where and
are the receive and transmit
vectors, respectively, and and
are the macrodiversity channel
matrix and the spatially uncorrelated AWGN noise vector,
respectively. The power spectral density of AWGN noise is assumed to
be. The th element of,
represents the fading coefficient of the th constituent link
which in this particular case, is the link between
th user and the th base station. In macrodiversity scenario,
where is called the average link
gain giving average link SNR of. The macrodiversity power profile matrix
can thus be defined as
The original input-output relationship may be rewritten in terms of
the macrodiversity power profile and so-called normalized channel
matrix, , as
where is the element-wise
square root of, and the operator,, represents Hadamard
multiplication. The th element of,, satisfies the condition given by
It has been shown that there exists a functional link between the permanent of macrodiversity power profile matrix, and the performance of multi-user macrodiversity systems in fading. Although it appears as if the macrodiversity only manifests itself in the power profile, systems that rely on macrodiversity will typically have other types of transmit power constraints and different sets of coordinating transmitters/receivers when communicating with different users. Note that the input-output relationship above can be easily extended to the case when each transmitter and/or receiver have multiple antennas.