Recently, in order to improve high speed data transmission and spectral efficiency in wireless communication systems, the combination of OFDM and space-time coding is being actively studied. In order to maximize the system efficiency, the problem of co-channel interference must be solved. One technique to overcome the co-channel interference and to increase the system capacity is to use adaptive antennas. Conventional beamforming techniques for single antenna cannot be applied directly to STBC-OFDM systems, because the signals transmitted from the two transmit antennas are superposed at the receive antenna and the interference between signals of the two transmit antennas occurs. In this paper, we present the MMSE beamforming technique using training sequence for STBC-OFDM systems in reverse link and evaluate the performance by using various parameters such as the number of training blocks, cluster sizes and angle spreads in Two-ray, TU and HT channels. From the simulation results, we show the best cluster sizes and the number of training blocks corresponding to these cluster sizes.

In this paper, we propose a robust timing synchronization algorithm for wireless MAN OFDM systems. The proposed algorithm is composed of frame synchronization and FFT timing synchronization. The conventional correlation scheme for estimation of timing offset has an ambiguity in the detection of accurate symbol timing. The proposed scheme, however, shows a clear peak at the correct timing by using the conjugate-symmetric characteristic of preamble and is able to control the FFT window position continuously with the 1-st order loop filter. Simulation results demonstrate the superiority of the proposed algorithm.

In transmitter diversity, the received signal is the superposition of signals transmitted from transmitter antennas. Thus, the separation of channel characteristics corresponding to each transmitter antennas from these signals is very important. The conventional channel estimation scheme tends to show higher computational complexity for larger channel delay profile. To reduce this computational complexity, significant-tap-catching method has been proposed. However, there is still a burden of complexity for data transmission mode. Reference [14] has shown how to reduce the complexity for data transmission mode in system with constant modulus modulation. However, this method can't reduce the complexity required for multi-level signals such as QAM. In this paper, we propose an efficient channel estimation scheme for OFDM systems with transmitter diversity using space-time trellis coding. The computational complexity of the proposed scheme is independent of channel delay profile. Also, compared with the conventional scheme, the complexity of the proposed scheme is not related to modulation methods including multi-level signals such as QAM. The performance of the proposed scheme is evaluated by computer simulation in various multipath fading environments.

3GPP (3rd Generation Partnership Project) has started to discuss D2D (Device-to-Device)-aided OTDOA (Observed Time Difference Of Arrival) as one of the mobile positioning enhancement techniques for LTE (Long Term Evolution) systems. It is a kind of multi-node based OTDOA which directly receives D2D signals from adjacent multiple UEs (User Equipment) to measure RSTD (Reference Signal's Time Difference). D2D signals provide valuable advantages in terms of OTDOA positioning because it can guarantee more reference nodes and high SNR (Signal-to-Noise Ratio) of PRS (Positioning Reference Signal). Two typical methods for multi-node based OTDOA can be applied to D2D-aided OTDOA. Multiple OTDOA positioning is one of the multi-node based methods that averages multiple results from OTDOA; however, it cannot always guarantee high accuracy due to the non-uniform geometry of UEs. OTDOA positioning based on TSE (Taylor Series Expansion) algorithm may be one of the solutions; however, it has the initial value problem and high computational complexity due to its iterative procedure. Therefore, in this paper, we propose a novel D2D-aided OTDOA positioning method which utilizes UEs not as reference node of OTDOA but as assisting node for RSTD error reduction. The proposed method can reduce RSTD error of eNB based hyperbola by using multiple hyperbola bands. The hyperbola band indicates the possible range in which a hyperbola can occur due to RSTD error. Then, by using principal axes of hyperbolas, we estimate a modified hyperbola from the overlap area of hyperbola bands, which has less RSTD error. We verify that the proposed method can effectively reduce RSTD error and improve positioning performance with lower computational complexity.

In this paper, we propose a modified CP-AFC (Cross-Product Automatic Frequency Control) algorithm to enhance coherent signal detection for WCDMA reverse link receiver. We introduce a moving average filter at the FDD input to decrease the noise effect by increasing the number of cross-products, since pilot symbol in WCDMA is not transmitted continuously. We also add normalization algorithm to overcome the conventional CP-FDD's sensitivity to the variance of input signal amplitude and to increase the linear range of S-curve. For rapid frequency acquisition and tracking, we adopt a multi-stage tracking mode. We applied the proposed algorithm in the implementation of WCDMA base station modem successfully.

In recent 3GPP (3rd Generation Partnership Project) standardization meetings, D2D (Device-to-Device) discovery has been a major issue to support commercial/social services and public safety in disaster environment, and TDM (Time Division Multiplexing) based discovery channel structure is mainly considered to prevent mutual interference between D2D and cellular traffic. In this structure, D2D discovery among the same cell UEs (User Equipment) has no problem because they have the same timing source. However, LTE (Long Term Evolution) assumes an asynchronous network where two adjacent eNBs (evolved Node B) have a symbol-level timing offset. For that reason, asynchronous interference among discovery signals can appear in inter-cell D2D discovery. Therefore, channel re-use scheduling was studied previously in which neighboring cells do not use the same portion of the extended discovery channel and other non-neighboring cells re-use it. However, it still shows interference problems in small cell networks which cause substantial cellular traffic loss. Therefore, in this paper, we propose a novel discovery channel scheduling in which eNBs time-align their discovery channels from each other by sample-level. In the proposed scheme, serving eNB requests cell edge UEs to estimate NTD (Network Time Difference) between serving eNB and neighboring eNB. Then, considering multiple NTDs, eNB adjusts the sample position of its discovery channel based on a novel decision rule. We verify that the proposed scheme can match the discovery performance of a synchronous network with less cellular uplink loss.

To mitigate the asynchronous ICI (Inter-Cell Interference), SCM (Spatial Covariance Matrix) of the asynchronous ICI plus background noise should be accurately estimated for MIMO-OFDMA (Multiple-input Multiple-output-Orthogonal Frequency Division Multiple Access) system. Generally, it is assumed that the SCM of the asynchronous ICI plus background noise is estimated by using training symbols. However, it is difficult to measure the interference statistics for a long time and considering that training symbols are not appropriate for OFDMA system such as LTE (3GPP Long Term Evolution). Therefore, noise reduction method is required to improve the estimation accuracy. Although the conventional time-domain low-pass type weighting method can be effective for noise reduction, it causes significant estimation error due to the spectral leakage in practical OFDM system. Therefore, we propose a time-domain sinc type weighing method which can not only reduce noise effectively minimizing estimation error caused by the spectral leakage but also can be implemented using frequency-domain weighted moving average filter easily. We also consider the iterative CFR (Channel Frequency Response) and SCM estimation method which can effectively reduce the estimation error of both CFR and SCM, and improve the performance for LTE system. By using computer simulation, we show that the proposed method can provide up to 2.5 dB SIR (Signal to Interference Ratio) gain compared with the conventional method, and verify that the proposed method is attractive and suitable for implementation with stable operation.

This letter presents a method of finding the optimal quantization stepsize that minimizes quantization loss and maximizes coded BER performance. We define 'Information Error Rate'(IER) and obtain the equation of the modified constraint length (Km) to obtain an upper bound of coded BER performance of a l bit quantized soft decision Viterbi decoder. Using IER and Km, we determine the optimal quantization stepsize of a 2 bit and 3 bit quantized soft decision decoding system in an AWGN channl with respect to SNR, and verify our strategies by simulation results.

In this paper, we propose an AGC (Automatic Gain Control) algorithm for initial cell search in 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) TDD (Time Division Duplex) system. Since the received signal has a large signal power difference between uplink and downlink subframe in wireless communication systems using a TDD scheme, conventional AGC scheme cannot sufficiently adjust the AGC gain because the AGC gain cannot converge fast enough to properly respond. Therefore, conventional AGC scheme leads to increased AGC gain variation, and the received signal will be attenuated by large AGC gain variation. To overcome this limitation, we propose an AGC scheme based on the average amplitude ratio calculation which can not only effectively increase convergence speed of the AGC gain but also maintain the stability of AGC operation in LTE TDD system. Also, it is important for AGC to converge efficiently for the accurate radio frame timing detection during the subsequent initial cell search procedure. Therefore, we also consider the proposed AGC scheme in combination with PSS (Primary Synchronization Signal) detection interface for the first step of initial cell search process in LTE TDD system to obtain both a stable AGC operation and accurate PSS detection performance. By extensive computer simulation in the presence of frequency offset and various channel environments, we verified that the proposed method can obtain a good behavior in terms of demodulation and PSS detection performance in LTE TDD system.

This letter presents a new approximation algorithm suitable for Taylor series expansion. The algorithm can effectively reduce the approximation error by using only a small number of terms in series expansion. The proposed algorithm can be more easily implemented and has better performance than the conventional Euler approximation algorithm. Also, the approximation performance of the proposed algorithm is compared with direct approaches, such as least-squares and Chebyshev approximation algorithm to show the relative advantages of the proposed algorithm. This letter also presents an application example of the proposed approximation algorithm in carrier recovery of OFDM (Orthogonal Frequency Division Multiplexing) modem.

D2D (Device-to-Device) communication has received considerable attention in recent years as one of the key technologies for future communication systems. Among the typical D2D communication systems, FlashLinQ (FLQ) adopted single-tone OFDM (Orthogonal Frequency Division Multiplexing) transmission which enables wide-sense discovery and distributed channel-aware link scheduling. Although synchronization based on a CES (Common External Source) is basically assumed in FLQ, a means to support devices when they are unable to use a CES is still necessary. In most OFDM systems, CFO (Carrier Frequency Offset) induces ICI (Inter Channel Interference) which degrades overall system performance drastically. Especially in D2D systems, ICI can be amplified due to different path losses between link and a precise estimation and correction of CFO is very important. Many CFO estimation algorithms based on DA (Data Aided) and NDA (None Data Aided) were proposed for OFDM systems, but there are several constraint conditions on frequency synchronization in D2D systems. Therefore, in this paper, we propose a new NDA-CFO estimation method for OFDM based D2D systems. The proposed method is based on the characteristics of single-tone OFDM signal, and is composed of two estimation stages: initial estimation and feed-back estimation. In initial estimation, the estimation of CFO is obtained by using two correlation results in a symbol. Also, estimation range can be adaptively defined as the distance between the two windows. In feed-back estimation, the distance between the two correlation results is gradually increased by re-using the estimated CFO and the correlation results. Therefore, more precise CFO estimation can be obtained. A numerical analysis and performance evaluation verify that the proposed method has a large estimation range and achieves precise estimation performance compared to the conventional methods.

In this paper, we propose a low-complexity frequency offset insensitive detection method for the 2.45 GHz LR-WPAN demodulator. In IEEE 802.15.4 LR-WPAN (Low-Rate Wireless Personal Area Network) specification, the frequency offset as highest 80 ppm in the 2.45 GHz band is recommended for low-complexity, low-cost, and low-power implementation. The proposed detection method is verified such that the performance is within 2 dB of the optimal coherent detection with low complexity, which is less than half in comparison with conventional detection methods.

In this paper, the forward link erlang capacity and outage probability for hierarchical cellular system based on 2 layer macrocell/microcell are derived analytically by considering the impact of imperfect power control and soft hand-off. The analysis on the outage probability is carried out using two methods: lognormal approximation and Chernoff upper bound. We assume that voice and multi-rate data service users are distributed uniformly in each cell and the same spectrum is applied in both layers. In addition, we take into account the base station transmission power ratio between tiers and the relative position of microcell having island distribution in macrocell. The forward link interference is evaluated by using Monte-Carlo simulation introduced in [2]. In this paper, we compare the forward link erlang capacity of 1x system to 3x system and show that 3x system can increase the user capacity by 3.4 times in case of macrocell and microcell, respectively, compared to 1x system.

In this paper, we propose an enhanced signal to interference ratio (SIR) measurement algorithm and evaluate the performance of the proposed algorithm in a WCDMA forward link receiver with closed-loop fast transmit power control. The proposed algorithm reduces measured SIR offset by using a pilot channel (CPICH) to compensate for the attenuated signal power in fading channel environment. The proposed SIR measurement algorithm outperforms conventional SIR measurement algorithm with regard to mean SIR values and jitter, especially in high speed mobile channel environment. Also, performance results with closed loop power control show that the proposed algorithm has better performance than the conventional algorithm.

This paper presents a BER performance derivation considering imperfect channel estimation for a pilot-aided coherent forward link of W-CDMA system under multipath Rayleigh fading channels. In the forward link of the W-CDMA system, pilot signal is usually used for coherent demodulation. In this paper, the maximum likelihood estimator, Wiener filter, and moving average filter are applied to estimate the channel effect due to mobile speed and frequency offset. Then, we concentrate on determining optimal parameter values of the estimators such as the observation length, delay parameters for causal/non-causal filter, and filter resolution. Also it is verified that these parameters are closely associated with the performance, hardware complexity, and characteristics of OVSF code. In particular, effect of data rate and filter resolution on the BER performance is analyzed in more detail. In addition, we show the performance comparison between the estimators considering various imperfections. Finally, we verify the derived BER by using an extensive Monte-Carlo computer simulation.

In LTE (Long Term Evolution)-Advanced system, a transparent MU-MIMO (Multi-User Multiple-Input Multiple Output) scheduling is basically considered, so the performance degradation in channel estimation may occur due to the unpredictable interference from co-scheduled layers. In order to detect and mitigate the interference, traditional binary hypothesis testing based interference detection method and iterative channel estimation method can be applied. However, there are two major problems. First, the binary hypothesis testing based interference detection is not suitable solution for LTE-Advanced system which has four dynamically changing interference hypotheses. Second, the conventional iterative operation does not guarantee sufficient performance gain with limited iteration time due to the estimation error in initial estimation stage. To overcome these problems, we introduce an enhanced iterative channel estimation method which considers simple matrix operation-based partial interference estimation. Based on the outcomes of the partial interference estimation, we can not only detect interference layers individually, but also partially eliminate the interference in initial channel estimation stage. Consequently, the proposed method can effectively mitigate the interference adaptively to the dynamically changing interference condition.

In 3GPP (3-rd Generation Partnership Project) LTE (Long Term Evolution) systems, D2D (Device-to-Device) communication has been selected as a next generation study item. In uplink D2D communication that underlies LTE systems, uplink interference signals generated by CUE (Cellular User Equipment) have a profound impact on the throughput of DUE (D2D User Equipment). For that reason, various resource allocation algorithms which consider interference channels have been studied; however, these algorithms assume accurate channel estimation and feedback of D2D related links. Therefore, in order to estimate uplink channels of D2D communication, SRS (Sounding Reference Signal) defined in LTE uplink channel structure can be considered. However, when the number of interferes increases, the SRS based method incurs significant overheads such as side information, operational complexity, channel estimation and feedback to UE. Therefore, in this paper, we propose an efficient channel estimation and CSI (Channel State Information) feedback method for D2D communication, and its application in LTE systems. We verify that the proposed method can achieve a similar performance to SRS based method with lower operational complexity and overhead.

In 3GPP (3-rd Generation Partnership Project) LTE (Long Term Evolution) system, the use of PRS (Positioning Reference Signal) for OTDOA (Observed Time Difference of Arrival) based positioning method has been agreed. However, PRSs can be overlapped at the receiver side in synchronous network because the frequency shift pattern of PRS is decided by cell ID (Identity). Moreover, in asynchronous network, the loss of orthogonality between received PRSs generates continuous interferences. Even though autonomous muting can be applied to solve the interference problems in synchronous and asynchronous networks, the muting scheme degrades the overall positioning efficiency and requires additional network complexity. Therefore, in this paper, we propose novel OTDOA based positioning methods at the receiver side to improve positioning efficiency: cancellation method of serving PRS for synchronous network, TDORS (Time Domain Orthogonal Reference Signal) generation and useful CIR (Channel Impulse Response) selection methods for asynchronous network. We verified that the proposed methods can achieve an accurate estimation and stable operation without PRS muting.

In this letter, we propose a new practical architecture of channel estimator that can compensate for the signal distortion due to variable mobile station velocity in WCDMA forward link. The proposed Channel Estimator consists of IIR filter for channel estimation and Velocity Estimator for selection of IIR filter coefficients matched to mobile station velocity. The combination of IIR filter and Velocity Estimator can overcome the divergence problem of IIR filter due to the mobile station velocity. The Velocity Estimator estimates the speed of mobile station velocity by observing power spectrum of the received signal and exhibits stable operation in low SNR environment. To improve the resolution of velocity estimation without additional complexity due to large FFT size, an interpolator is adopted in the velocity estimator. The proposed channel estimation architecture can not only be used for WCDMA forward link but also is applicable for CDMA-2000 system without major modifications. Also, the Velocity Estimator can be applied in the channel quality measurement for the selection of MCS (Modulation and Coding Scheme) level in HSDPA transmission.

In this paper, we propose a high speed Preamble Searcher suitable for the RACH (Random Access Channel) structure in WCDMA reverse link receivers. Unlike IS-95, WCDMA system uses the AISMA (Acquisition Indication Sense Multiple Access) scheme. Because of the time limit between RACH preamble transmission and AI (Acquisition Indicators), and the restriction on the number of RACH signatures assigned to RACH preamble, fast acquisition is required for efficient operation. The Preamble Searcher proposed in this paper is designed for 2-antenna systems; it adopts the FHT (Fast Hadamard Transform) algorithm that has the radix-2 16 point FFT (Fast Fourier Transform) structure. The acquisition speed using FHT is 32 times faster than the conventional method that correlates each signature. Based on this fast acquisition scheme, we improved the acquisition performance by calculating correlation up to 4096 chips of the total preamble length.