2.3 Difference
between uplink and downlink
2.3.1 Uplink Processing
It is assumed here that a smart antenna is only employed at
the base station and not at the handset or subscriber unit. Such remote radio
terminals transmit using omni directional antennas, leaving it to the base
station to separate the desired signals from interference selectively.
Typically, the received signal from the spatially distributed
antenna elements is multiplied by a weight, a complex adjustment of amplitude
and a phase.
These signals are combined to yield the array output. An
adaptive algorithm controls the weights according to predefined objectives. For
a switched beam system, this may be primarily maximum gain; for an adaptive
array system, other factors may receive equal consideration. These dynamic
calculations enable the system to change its radiation pattern for optimized
signal reception [7].
2.3.2 Downlink Processing
The task of transmitting in a spatially selective manner is
the major basis for differentiating between switched beam and adaptive array
systems. As described below, switched beam systems communicate with users by
changing between preset directional patterns, largely on the basis of signal
strength. In comparison, adaptive arrays attempt to understand the RF
environment more comprehensively and transmit more selectively. The type of
downlink processing used depends on whether the communication system uses time
division duplex (TDD), which transmits and receives on the same frequency or
frequency division duplex (FDD), which uses separate frequencies for transmit
and receiving which is the case in GSM system. In most FDD systems, the uplink
and downlink fading and other propagation characteristics may be considered
independent, whereas in TDD systems the uplink and downlink channels can be
considered reciprocal [16]. Hence, in TDD systems uplink channel information
may be used to achieve spatially selective transmission. In FDD systems, the
uplink channel information cannot be used directly and other types of downlink
processing must be considered [7].
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