Applying Frequency-Domain Equalization to Code-Division Multiple Access and Transform-Domain Communications Systems

This research examined the theory and application of using orthogonal frequency division multiplexing (OFDM), or discrete multi-tone (DMT), frequency domain equalization (FEQ) with two communications systems that inherently possess unused carrier frequencies, or null-tones, in their respective transmission frequencies. The fundamental DMT-FEQ property relies on null-tones to equalize a non-ideal channel and mitigate the effects of interchannel interference (ICI), intersymbol interference (ISI), and noise. The two communications systems investigated were a Hadamard encoded code division multiple access (CDMA) communications system with up to 32 synchronous users and a transform domain communications system (TDCS) with only one user. Both communications systems were simulated while operating with real channel data corrupted by noise. Simulation results showed that the Hadamard encoded CDMA system worked well with DMT-FEQ only when the Hadamard code set was used to construct a transmission signal that obeyed DMT-FEQ null-tone conditions in conjunction with a vector estimation method. Simulation results also showed that a TDCS using traditional pseudo-random phase component, and traditional spectral mask with consecutive null-tones, did not work well with DMT-FEQ. Modifications to the TDCS model revealed that a TDCS with a conjugate-symmetric phase component in conjunction with a modified spectral mask with consecutive null-tones and forced null-tones provided acceptable results when equalizing with DMT-FEQ. The DMT-FEQ may be suitable for covert applications, such as TDCS, when modifications to TDCS’ phase component and forced null-tones in its spectral mask are made

Joint transceiver design for MIMO channel shortening.

Channel shortening equalizers can be employed to shorten the effective impulse response of a long intersymbol interference (ISI) channel in order, for example, to decrease the computational complexity of a maximum-likelihood sequence estimator (MLSE) or to increase the throughput efficiency of an orthogonal frequency-division multiplexing (OFDM) transmission scheme. In this paper, the issue of joint transmitter–receiver filter design is addressed for shortening multiple-input multiple-output (MIMO) ISI channels. A frequency-domain approach is adopted for the transceiver design which is effectively equivalent to an infinite-length time-domain design. A practical space–frequency waterfilling algorithm is also provided. It is demonstrated that the channel shortening equalizer designed according to the time-domain approach suffers from an error-floor effect. However, the proposed techniques are shown to overcome this problem and outperform the time-domain channel shortening filter design. We also demonstrate that the proposed transceiver design can be considered as a MIMO broadband beamformer with constraints on the time-domain multipath length. Hence, a significant diversity gain could also be achieved by choosing strong eigenmodes of the MIMO channel. It is also found that the proposed frequency-domain methods have considerably low computational complexity as compared with their time-domain counterparts

Performance Analysis of Multicarrier Code Division Multiple Access (MC-CDMA) Systems

A thesis presented to the faculty of the College of Science and Technology at Morehead State University in partial fulfillment of the requirements for the Degree of Master of Science by Pravinkumar Patil on August 11, 2008