In mobile communication systems, the channel state information (CSI) is severely affected by the noise effect of the receiver. The adaptive subcarrier grouping (ASG) for sample matrix inversion (SMI) based minimum mean square error (MMSE) adaptive array has been previously proposed. Although it can reduce the additive noise effect by increasing samples to derive the array weight for co-channel interference suppression, it needs to know the signal-to-noise ratio (SNR) in advance to set the threshold for subcarrier grouping. This paper newly proposes adaptive zero padding (AZP) in the time domain to improve the weight accuracy of the SMI matrix. This method does not need to estimate the SNR in advance, and even if the threshold is always constant, it can adaptively identify the position of zero-padding to eliminate the noise interference of the received signal. Simulation results reveal that the proposed method can achieve superior bit error rate (BER) performance under various Rician K factors.

The sliding discrete Fourier transform (DFT) is a well-known algorithm for obtaining a few frequency components of the DFT spectrum with a low computational cost. However, the conventional sliding DFT cannot be applied to practical conditions, e.g., using the sine window and the zero-padding DFT, with preserving the computational efficiency. This paper discusses the extension of the sliding DFT to such cases. Expressing the window function by complex sinusoids, a recursive algorithm for computing a frequency component of the DFT spectrum using an arbitrary sinusoidal window function is derived. The algorithm can be easily extended to the zero-padding DFT. Computer simulations using very long signals show the validity of our algorithm.

In time-varying frequency selective channels, to obtain high-rate joint time-frequency diversity, linear dispersion coded orthogonal frequency division multiplexing (LDC-OFDM), has recently been proposed. Compared with OFDM systems, single-carrier systems may retain the advantages of lower PAPR and lower sensitivity to carrier frequency offset (CFO) effects, which motivates this paper to investigate how to achieve joint frequency and time diversity for high-rate single-carrier block transmission systems. Two systems are proposed: linear dispersion coded cyclic-prefix single-carrier modulation (LDC-CP-SCM) and linear dispersion coded zero-padded single-carrier modulation (LDC-ZP-SCM) across either multiple CP-SCM or ZP-SCM blocks, respectively. LDC-SCM may use a layered two-stage LDC decoding with lower complexity. This paper analyzes the diversity properties of LDC-CP-SCM, and provides a sufficient condition for LDC-CP-SCM to maximize all available joint frequency and time diversity gain and coding gain. This paper shows that LDC-ZP-SCM may be effectively equipped with low-complexity minimum mean-squared error (MMSE) equalizers. A lower complexity scheme, linear transformation coded SCM (LTC-SCM), is also proposed with good diversity performance.

If ranging processes are not frequent in an uplink OFDMA system, timing synchronization between the base and mobile stations may not be maintained and the performance may be degraded. This paper proposes a random access scheme in which a short OFDMA symbol is transmitted to maintain the orthogonality with timing offsets. A short symbol is constructed by inserting zero-padding to an OFDMA symbol.

This paper presents a high performance and hardware efficient packet detection structure, which employs a cross correlator for the M-sample time delayed correlation operation and a signal power calculator using the received input samples less than or equal to a zero-padded suffix of length M. We investigate the detailed characteristics of the proposed packet detector. In this paper, the performance of a class of packet detection algorithms in the ultra-wideband (UWB) channel environments is also studied. The best packet detection algorithm for the multi-band orthogonal frequency division multiplexing (MB-OFDM) UWB transmission is determined through analysis and extensive simulations. The results of analysis show that the proposed packet detection structure has advantages in the hardware complexity as well as performance when compared with the existing packet detection structures. In order to effectively conduct the packet detection before the automatic gain control (AGC) mode, we investigate the effects of both a frequency offset and the initial gain level of a variable gain amplifier (VGA) on the performance of the packet detection. We also suggest a VGA gain control technique to enhance the performance of packet detection.

While CDMA systems are proven to be excellent solutions for cellular communications, they suffer from severe multi-path interferences and are hard to support high-data-rate transmissions over frequency-selective fading channels. This letter introduces a novel downlink transmission method for next generation mobile communication systems. The proposed method can provide significantly improved performance in a hot-spot area while maintaining the backward compatibility with the 3rd generation CDMA systems.