Space-Time Coding
Background
In wireless communications (electro-magnetic as well as acoustic) multipath
propagation results in a fading channel. Robustness against fading can
be achieved by introducing diversity into the system, for example using,
frequency diversity, time diversity, spatial diversity or some combination
of these. Space-time coding is a recent technique not yet used in commercial
systems where spatial diversity at the transmitter is exploited to create
very reliable communication over a fading channel. Multiple transmitters
are required and one or more receivers can be used.
Several space-time codes have been proposed in the literature. A common
space-time orthogonal block code using two transmitters is the so-called
Alamouti space-time code.
Objective
The goal of this project is to implement a real-time digital acoustic
communication system using the Alamouti space-time code. Robustness against
flat fading should be demonstrated and reliable communication established
in a multipath environment.
Specification
A system with two transmitters and one receiving microphone is referred
to as a 2x1 system. And correspondingly, a 2x2 system has two transmitters
and two receivers. The transmitting strategy is the same in both cases
but the receivers are different and both should be implemented. The system
should enable a user to transmit data from one PC to another. Each PC
is equipped with a DSP card with two inputs and two outputs. The transmit
PC has two speakers and the receive PC is equipped with two microphones.
Thus, information can be transmitted between PCs by acoustic waves generated
by the speaker and received by the microphones.
The data should be coded (Alamouti) and transmitted in packets over
the channel. BPSK modulation should be used and the receiver should
perform synchronization, frequency offset compensation, channel estimation,
and detection. The data rate may be adjusted as to make the channel
flat fading (no inter-symbol interference).
As a reference system, a 1x1 communication link should be implemented.
The performance gains (in terms of BER) of the 2x1 and 2x2 systems should
be assessed through experiments and compared against the theoretical
gains that are achievable.
For demonstration purposes, a simple user interface should be implemented
displaying system parameters and allowing online user interaction (change
transmitter and receiver algorithms). Also, the interface should display
"scatter plots" of the decision variable (for some packets) at different
stages of the receiver algorithm.
Literature
Increasing data rate over wireless channels, Naguib, A.F.; Seshadri, N.;
Calderbank, A.R., IEEE Signal Processing Magazine Volume: 17 3 , May 2000
, Page(s): 76-92.
A simple transmit diversity technique for wireless communications,
Alamouti, S.M., Selected Areas in Communications, IEEE Journal on Volume:
16 8, Oct. 1998, Page(s): 1451-1458.
Space-time block coding for wireless communications: performance results,
Tarokh, V.; Jafarkhani, H.; Calderbank, A.R., Selected Areas in Communications,
IEEE Journal on Volume: 17 3, March 1999, Page(s): 451-460.
An
Acoustic Communication System Using Structured Interference Suppression:
Performance Evaluation and Real Time Implementation, Jöngren G.,
Master of Science Thesis, S3, KTH, Dec. 1998.
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