In this paper, two receive beamforming techniques (Method 1 and Method 2) are proposed for a mobile station (MS) with multiple antenna arrays in an OFDM-based millimeter-wave (mm-wave) cellular communication system. Since the MS in mm-wave cellular communication requires fast processing due to its frequent movement and rotation, a receive beamforming technique with reduced computation complexity and processing time is proposed in Method 2. Of particular interest, estimation techniques for 2-dimensional (2D) direction-of-arrivals (DoAs) corresponding to each cell ID are proposed for uniform circular arrays (UCAs) and uniform rectangular arrays (URAs). Also, a cell selection technique for MSs with multiple antenna arrays is described that use the candidate cell IDs and parameters estimated for all antenna arrays to provide combining gain in addition to array gain in multipath channels. The proposed beamforming techniques are evaluated by computer simulation using a simple model of amm-wave cellular communication system with 3-dimensional spatial channel model (3D SCM).

In this paper, a new technique is proposed to reduce the frequency of cell search by user equipment (UE) in the presence of femtocells. A new common signal (CS) and a separate set of primary synchronization signals (PSSs) are employed to facilitate efficient cell search in a next-geration LTE-based system. The velocity of the UE is also utilized to determine cell search mode. A slow UE recognizes the presence of femtocells using the CS, so that it can make separate searches for macrocells and femtocells. A fast UE will not search for femtocells since the coverage of femtocells is restricted to a small region. The fast UE detects the macrocell boundary using the PSSs transmitted from neighboring macrocells, so that it can search for macrocells only at the macrocell boundary. The effects of CS and UE velocity on the number of cell searches are analyzed. The performance of the proposed technique is evaluated by computer simulations.

In this letter, a two-dimensional pilot-symbol aided (2-D PSA) channel estimation technique for coherent orthogonal frequency-division multiplexing (OFDM) systems with transmitter diversity is proposed. The 2-D PSA channel estimator and the corresponding orthogonality condition between the sets of pilot symbols are derived under minimum mean-square error (MMSE) criterion for OFDM systems with transmitter diversity. The proposed estimator is shown to be accurate and effective for tracking variations of channels between multiple transmitter antennas and receiver antennas.

In this letter, we propose an efficient signal detection method for uplink multiuser systems based on collaborative spatial multiplexing (CSM). The proposed method achieves near-optimal performance and shows only 0.8 dB loss at the target frame error rate (FER) of 10-2. Moreover, the error performance of each user is almost the same in the proposed method, which is an important property in a multiuser MIMO system where each user's error performance must satisfy some fixed error rate criteria.

In this letter, a receive frequency diversity technique is proposed to improve the performance of a multiplexed STBC OFDM system. Frequency diversity in the multiplexed STBC OFDM system is obtained by introducing frequency shifter in the successive STBC symbols and applying MRRC technique to regenerated and subtracted signals of the predecoded data from multiplexed STBC decoder. It is shown by computer simulation that the performance of the proposed multiplexed STBC OFDM systems with frequency diversity is improved by 5 dB at the BER of 10-3 over the existing multiplexed STBC OFDM systems with the same data rate.

In this letter, a phase noise reduction algorithm for orthogonal frequency division multiplexing (OFDM) systems with an input of higher-order constellation is proposed to achieve high data rate transmission. The proposed algorithm estimates dominant interchannel interference (ICI) terms, caused by phase noise, using the subcarriers adjacent to pilot subcarriers, and compensates the OFDM signal using these estimates. Also, the length of dominant ICI terms is efficiently determined by estimating 3 dB bandwidth of phase noise. The proposed algorithm is shown to reduce the effect of phase noise on OFDM systems with an input of higher-order modulation and a large amount of phase noise.

In this letter, a pilot structure and an efficient algorithm for downlink synchronization and cell searching in OFDM-based cellular systems are proposed. The pilots, randomly allocated in the frequency domain, allow us to minimize inter-cell interference (ICI) as well as to increase cell searching capability, estimation range of integer carrier frequency offset (CFO), and estimation accuracy of symbol timing offset (STO). The proposed low-complexity joint algorithm for integer CFO estimation, cell searching, and downlink detection is robust to ICI, multipath channel, STO and fine CFO.

In this letter, a method estimating the intercell carrier frequency-offset (CFO) in orthogonal frequency division multiplexing (OFDM)-based cellular systems is proposed for the user's equipment (UE), especially at the cell boundary, in downlink channels. After describing a new method of deriving the intercell CFO from the signals received by adjacent base stations (BSs), we propose a cell-searching method using the estimated CFOs. It is shown by computer simulation that the proposed methods can uniquely estimate the intercell CFOs and identify the target BS with a high detection probability at the UE.

In this letter, we propose a novel signal detection method that matches maximum likelihood (ML) performance but requires much less computational complexity than ML detection. When the well-known linear decoding method is used for space-time block coded (STBC) OFDM systems in fast-fading channels, co-channel interference (CCI) as well as inter-carrier interference (ICI) occurs. A maximum likelihood (ML) method can be employed to deal with the CCI; however, its computational complexity is very high. In this letter, we propose a signal detection method for orthogonal space-time coded OFDM systems that achieves the identical error performance as the ML method, but requires much less computational complexity.

In this letter, a concept of virtual smart antenna (SA) is introduced for OFDM-based cellular systems with a frequency reuse factor equal to 1. A method of estimating intercell symbol timing offsets (STOs) from received OFDM signals impinging on virtual antenna is proposed for users at a cell boundary. Also, adaptive beamforming methods for virtual SA are proposed to reduce intercell interference (ICI) from adjacent base stations (BSs).

In this paper, an interference rejection combining (IRC) technique is proposed for SFBC-OFDM cellular systems that exhibit multiple carrier frequency offsets (CFOs). The IRC weight and the corresponding value for CFO compensation in the proposed technique are obtained by maximizing the post-SINR, i.e., minimizing both the interference signal and inter-channel interference (ICI) terms caused by multiple CFOs. The performance of the conventional IRC and proposed IRC techniques is evaluated by computer simulation for an SFBC-OFDM cellular system with multiple CFOs.

In this paper, a simple subcarrier selection combining technique is proposed for orthogonal frequency-division multiplexing (OFDM) systems with multiple receive antennas. The subcarrier-based selection algorithm is developed in the frequency domain to achieve an optimal selection combining gain for OFDM systems, instead of the antenna-based selection algorithms in the time domain or frequency domain. The proposed technique selects an optimal subcarrier with a maximum channel gain among all the receive antennas with the same subcarrier position, based on the estimated channel frequency response during the training period. Hardware complexity for the proposed technique is minimal since it requires single front-end with multiple receive antennas and single baseband demodulator. It is shown by computer simulation that a significant gain can be achieved by the proposed technique over the conventional selection combining technique for OFDM systems in practical situations.

This letter presents an adaptive beamforming algorithm for an MC-CDMA system with adaptive antenna array. The proposed adaptive beamforming algorithm for the MC-CDMA systems is derived by (1) calculating the error signals between the pilot symbols of desired user and the received pilot signals in the frequency-domain, (2) transforming the frequency-domain error signals into time-domain error signals, (3) updating the filter coefficients of the adaptive beamformer in the direction of minimizing the MSE. Convergence behavior and user-capacity improvement of the proposed approach are demonstrated through computer simulation by applying it to the MC-CDMA system in the presence of interferences from other users.

In this letter, a new channel-adaptive beamforming method is proposed for OFDMA systems with smart antennas. In the method, the size of a cluster for resource unit is determined adaptively according to a region-splitting criterion. It is shown by simulations that the proposed method shows good performance in both frequency-flat and frequency-selective channels.

In this letter, a simple peak-to-average power ratio (PAPR) reduction method using subband permutation is proposed for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. The proposed method is shown to have a 2-3 dB PAPR reduction gain for 2 transmit antennas, compared with the conventional MIMO-OFDM, and a low computational complexity at the transmitter and receiver.

This paper deals with a beamforming and cell ID detection technique for a mobile station (MS) with multiple antenna arrays in millimeter wave (mm-wave) cellular communication systems. Multiple antenna arrays, required to cover the entire space around the MS, can be used to estimate the direction of arrivals (DoAs) and cell IDs, form beams in the direction of DoAs, select a serving cell in a cooperative manner, and improve BER performance by signal combining. However, a signal may enter the overlapped region formed by two adjacent arrays in the MS, resulting in a double-counting problem during the cell searching period. In this paper, a beamforming and cell detection technique without double-counting is proposed to handle this problem, and they are evaluated by simulation in a simple scenario of an mm-wave cellular system with spatial channel model (SCM).

In this letter, we propose a new detection method for an OFDM signal distorted by IQ imbalance, and a pilot pattern to estimate the channel associated with IQ imbalance. It is shown by computer simulation that the proposed method can achieve robust detection even when severe IQ imbalance exists in OFDM systems with an input of higher-order constellation.

An efficient handover measurement technique is proposed for millimeter-wave (mm-wave) cellular systems with directional antenna beams. As the beam synchronization signal (BSS) carries the cell ID and the beam ID in a hierarchal manner, handover events (interbeam handover and intercell handover) are distinguished at the physical layer. The proposed signal metrics are shown to be effective in detecting the beam boundaries and cell boundaries in mm-wave cellular systems, which allows to distinguish interbeam handover from intercell handover. The proposed handover measurement technique is shown to reduce the processing time significantly using the proposed signal metrics produced by the BSS.

In this paper, we propose an efficient channel estimation technique for orthogonal frequency-division multiplexing (OFDM) systems with transmitter diversity. The proposed technique estimates uniquely all channel frequency responses needed in space-time coded OFDM systems using "comb-type" training symbols. The computational complexity of the proposed technique is reduced dramatically, compared with the previous minimum mean-squared error (MMSE) technique, due to the processing made all in the frequency-domain. Also, several other techniques for mitigating random noise effect and tracking channel variation are discussed to further improve the performance of the proposed approach. The performances of the proposed approach are demonstrated by computer simulation for mobile wireless channels.

The VLSI implication of the guard interval in orthogonal frequency-division multiplexing (OFDM) systems is described. A new OFDM transmission scheme using cyclic suffix is proposed to reduce the hardware complexity required for implementing the guard interval in the transmitter, and is shown to have the same performance as the conventional approach using cyclic prefix, even with a significantly lower hardware complexity (smaller buffer size and no processing delay).