Abstract
This paper deals with synchronous direct-sequence code-division multiple access (CDMA) transmission using orthogonal channel codes in frequency selective multipath, motivated by the forward link in 3G CDMA systems. The chip-level minimum mean square error (MMSE) estimate of the (multiuser) synchronous sum signal transmitted by the base, followed by a correlate and sum, has been shown to perform very well in saturated systems compared to a Rake receiver. In this paper, we present the reduced-rank, chip-level MMSE estimation based on the multistage nested Wiener filter (MSNWF). We show that, for the case of a known channel, only a small number of stages of the MSNWF is needed to achieve near full-rank MSE performance over a practical single-to-noise ratio (SNR) range. This holds true even for an edge-of-cell scenario, where two base stations are contributing near equal-power signals, as well as for the single base station case. We then utilize the code-multiplexed pilot channel to train the MSNWF coefficients and show that adaptive MSNWF operating in a very low rank subspace performs slightly better than full-rank recursive least square (RLS) and significantly better than least mean square (LMS). An important advantage of the MSNWF is that it can be implemented in a lattice structure, which involves significantly less computation than RLS. We also present structured MMSE equalizers that exploit the estimate of the multipath arrival times and the underlying channel structure to project the data vector onto a much lower dimensional subspace. Specifically, due to the sparseness of high-speed CDMA multipath channels, the channel vector lies in the subspace spanned by a small number of columns of the pulse shaping filter convolution matrix. We demonstrate that the performance of these structured low-rank equalizers is much superior to unstructured equalizers in terms of convergence speed and error rates.
Highlights
Mobile units in current code-division multiple access (CDMA) cellular systems use a Rake receiver, which is a maximal-ratio combiner and can be interpreted as a bank of filters matched to the channel that combine the energy from multiple paths [1]
We presented reduced-rank chip-level minimum mean square error (MMSE) equalizers for the CDMA downlink with frequency-selective multipath based on the multistage nested Wiener filter, for known channel case and for training-based adaptation
The convergence rate for multistage nested Wiener filter (MSNWF) operating in a very low-rank subspace was significantly better than least mean square (LMS), and somewhat better than recursive least square (RLS)
Summary
Mobile units in current code-division multiple access (CDMA) cellular systems use a Rake receiver, which is a maximal-ratio combiner and can be interpreted as a bank of filters matched to the channel that combine the energy from multiple paths [1]. In urban wireless systems, the fading is often not flat and the orthogonality of the underlying WalshHadamard codes is destroyed at the receiver, resulting in multiple-access interference (MAI) at the receiver. There are major interference issues if the mobile unit is near the edge of a cell and is receiving significant out-of-cell transmission, EURASIP Journal on Applied Signal Processing regardless of whether the fading is flat or not. In such environments, the Rake receiver is suboptimal, because it inherently treats MAI as uncorrelated noise. In situations where the number of active users approaches the spreading gain, the Rake receiver does not provide adequate performance
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