mohatta2015.pdf

184 PRACTICAL CONSIDERATIONS
from acquisition can be used to determine path delays, if needed. Depending on
the equalization approach, the path delays may be determined by the processing
delays, for example the tap locations of a linear equalizer.
8.4.5 Pilot symbol and traffic symbol powers
In certain systems, the channel is estimated from a pilot signal (known symbols)
with a different power level than the traffic signal (unknown symbols to be detected).
If the power relation between these two signals is known, then it is straightforward
to estimate the channel response for the traffic signal by scaling the channel response
estimate for the pilot signal. If the power relation is not known, then it may need to
be estimated, depending on the equalization approach. This can be done by either
estimating the ratio of the two powers or estimating the two powers separately. If
done separately, it corresponds to estimating E¡ {k) using the notation of Chapter
1. In CDM systems, this is referred to as code power estimation, as the pilot and
traffic signals are often multiplexed onto different spreading codes.
8.4.6 Pilot symbols and multi-antenna transmission
With multi-antenna transmission, pilot symbols are sent from different transmit
antennas. One approach is to send different pilot symbols from each transmit
antenna so that the channel from each transmit antenna to each receive antenna can
be estimated. If the multiple antennas are used to transmit the same traffic symbols,
then it is also possible to bundle the pilot symbols with the traffic symbols, so that
if weighted copies of the traffic signal are sent from the different transmit antennas
(e.g., beamforming), then the pilot symbols pass through the same beamforming.
8.5 AN EXAMPLE
Here we consider channel estimation for various cellular systems. For GSM/EDGE,
the channel can be estimated using a set of pilot symbols transmitted in the middle
of a burst of data (midamble). The length of the burst is short enough that the
channel can be considered static (not changing) over the burst (at least for most
scenarios).
For the US TDMA system, the burst of data is much longer. While there is a
set of pilot symbols at the beginning of the burst to get started, the channel must
be tracked as it changes over the data portion of the burst.
For CDMA systems, it is common to employ a pilot channel, a stream of pilot
symbols sent on a particular spreading code. The channel can be estimated and
tracked using this continuous stream of pilot symbols.
For OFDM systems, pilot symbols are transmitted at different times and on
different subcarriers. This provides channel measurements in time and frequency
which can be used to estimate the channel for other subcarriers and other times.