In this paper, we propose a transmit multi-block frequency-domain equalization (MB-FDE) for frequency-domain space-time block coded joint transmit/receive diversity (FD-STBC-JTRD). Noting that a STBC codeword consists of multiple coded blocks, the transmit MB-FDE uses the multiple transmit FDE weight matrices, each associated with each coded block. Both single-carrier (SC) transmission and orthogonal frequency-division multiplexing (OFDM) transmission are considered. For SC transmission, the transmit MB-FDE weight matrices are jointly optimized so as to minimize the mean square error (MSE) between the transmit signal before STBC encoding and the received signal after STBC decoding. For OFDM transmission, they are jointly optimized so as to maximize the received signal-to-noise power ratio (SNR) after STBC decoding. We show by theoretical analysis that the proposed transmit MB-FDE can achieve 1/RSTBC times higher received SNR than the conventional transmit single-block FDE (SB-FDE), where RSTBC represents the code rate of STBC. It is confirmed by computer simulation that, when more than 2 receive antennas are used, MB-FDE can always achieve better BER performance than SB-FDE irrespective of the number of transmit antennas, and the channel frequency-selectivity.

The deployment of small cells is one of the most effective means to cope with the traffic explosion of cellular mobile systems. However, a small cell system increases the inter-cell interference, which limits the capacity and degrades the cell-edge user throughput. Inter-cell interference coordination (ICIC), such as fractional frequency reuse (FFR), is a well-known scheme that autonomously mitigates inter-cell interference. In the Long Term Evolution (LTE)-Advanced, the three-dimensional (3D) beamforming, which combines conventional horizontal beamforming and vertical beamforming, has been gaining increasing attention. This paper proposes a novel centralized ICIC scheme that controls the direction of narrow 3D beam for each frequency band of each base station. The centralized controller collects information from the base stations and calculates sub-optimum combinations of narrow beams so as to maximize the proportional fair (PF) utility of all users. This paper describes the throughput of the new centralized ICIC scheme as evaluated by computer simulations and shows it has a significant gain in both average user throughput and cell-edge user throughput compared with the conventional ICIC scheme. This paper also investigates the feasibility of the scheme by assessing its throughput performance in a realistic deployment scenario.

In this letter, pilot-assisted adaptive prediction iterative channel estimation in frequency-domain is presented for the antenna diversity reception of orthogonal frequency division multiplexing (OFDM) signals. A frequency-domain adaptive prediction filtering is applied to iterative channel estimation for improving the tracking capability against frequency-domain variations in a severe frequency-selective fading channel. Also, in order to track the changing fading environment, the tap weights of frequency-domain prediction filter are updated using the simple NLMS algorithm. Updating of tap weights is incorporated into the iterative channel estimation loop to achieve faster convergence rate. The average bit error rate (BER) performance in a frequency-selective Rayleigh fading channel is evaluated by computer simulation. It is confirmed that the frequency-domain adaptive prediction iterative channel estimation provides better BER performance than the conventional iterative channel estimation schemes.

In this paper, the channel segregation dynamic channel allocation (CS-DCA) scheme is applied to a multi-hop DS-CDMA virtual cellular network (VCN). After all multi-hop routes are constructed over distributed wireless ports in a virtual cell, the CS-DCA is carried out to allocate the channels to multi-hop up and down links. Each wireless port is equipped with a channel priority table. The transmit wireless port of each link initiates the CS-DCA procedure and selects a channel among available ones using its channel priority table to check. In this paper, the channel allocation failure rate is evaluated by computer simulation. It is shown that CS-DCA reduces remarkably the failure rate compared to FCA. The impact of propagation parameters on the failure rate is discussed.

In mobile communication systems, high speed packet data services are demanded. In the high speed data transmission, throughput degrades severely due to severe inter-path interference (IPI). Recently, we proposed a random transmit power control (TPC) to increase the uplink throughput of DS-CDMA packet mobile communications. In this paper, we apply IPI cancellation in addition to the random TPC. We derive the numerical expression of the received signal-to-interference plus noise power ratio (SINR) and introduce IPI cancellation factor. We also derive the numerical expression of system throughput when IPI is cancelled ideally to compare with the Monte Carlo numerically evaluated system throughput. Then we evaluate, by Monte-Carlo numerical computation method, the combined effect of random TPC and IPI cancellation on the uplink throughput of DS-CDMA packet mobile communications.

Pilot-aided adaptive prediction channel estimation is proposed for coherent detection in a frequency-nonselective fading channel. It is an extension of the conventional weighted multi-slot averaging (WMSA) channel estimation and consists of 3 steps. A block of Np pilot symbols is periodically transmitted, each pilot block being followed by Nd data symbols to form a data slot. In the first step, the instantaneous channel gain is estimated by coherent addition of Np pilot symbols. Using the K past and K future estimated instantaneous channel gains, the second step predicts the instantaneous channel gains at the end and beginning of data slot of interest by a forward predictor and a backward predictor, respectively. The tap-weights of forward prediction and backward prediction are adaptively updated using the normalized least mean square (NLMS) algorithm. Finally, in the third step, the instantaneous channel gain at each data symbol position within the data slot of interest is estimated by simple averaging or linear interpolation using the two adaptively predicted instantaneous channel gains. The computer simulation confirms that the proposed adaptive prediction channel estimation achieves better bit error rate (BER) performance than the conventional WMSA channel estimation in a fast fading channel and/or in the presence of frequency offset between a transmitter and a receiver.

In direct sequence code division multiple access (DS-CDMA), variable rate transmission can be realized by simply changing the spreading factor SF for the given chip rate. In a frequency-selective fading channel, the transmission performance can be improved by using rake combining. However, when a very low SF is used for achieving a high transmission rate, error floor is produced due to insufficient suppression of inter-chip interference (ICI). In this paper, decision feedback chip-level maximum likelihood detection (DF-CMLD) is proposed that can suppress the ICI. An upper-bound for the conditional bit error rate (BER) is theoretically derived for the given spreading sequence and path gains. The theoretical average BER performance is numerically evaluated by Monte-Carlo numerical computation using the derived conditional BER. The numerical computation results are confirmed by computer simulation of DS-CDMA signal transmission with DF-CMLD.

In this letter, we introduce frequency-domain space-time transmit diversity (STTD) encoding/decoding to orthogonal frequency division multiplexing combined with time division multiplexing (OFDM/TDM) on a frame-by-frame basis (i.e., over several concatenated OFDM signals in the frequency-domain) to achieve both spatial and frequency diversity gains and improve the bit error rate (BER) performance. The theoretical BER performance is evaluated by numerical computation using the derived conditional BER and confirmed by computer simulation.

Frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion can replace the conventional rake combining to significantly improve the bit error rate (BER) performance in a frequency-selective fading channel. MMSE-FDE requires an accurate estimate of the channel transfer function and the signal-to-noise power ratio (SNR). Direct application of pilot-assisted channel estimation (CE) degrades the BER performance, since the frequency spectrum of the pilot chip sequence is not constant over the spreading bandwidth. In this paper, we propose a pilot-assisted decision feedback frequency-domain MMSE-CE. The BER performance with the proposed pilot-assisted MMSE-CE in a frequency-selective Rayleigh fading channel is evaluated by computer simulation. It is shown that MMSE-CE always gives a good BER performance irrespective of the choice of the pilot chip sequence and shows a high tracking ability against fading. For a spreading factor SF of 16, the Eb/N0 degradation for BER=10-4 with MMSE-CE from the ideal CE case is as small as 0.9 dB (including an Eb/N0 loss of 0.28 dB due to the pilot insertion).

Recently, overlap frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion which requires no guard interval (GI) insertion was proposed for signal transmission using multi-code direct sequence code division multiple access (DS-CDMA) in a frequency-selective fading channel. Another promising equalization technique is time-domain sliding-window chip equalization (SWCE). In this paper, the bit error rate (BER) performances achievable with overlap FDE and SWCE are compared. It is shown that, by extending the fast Fourier transform (FFT) block size, overlap FDE can achieve almost the same BER performance as SWCE with much less computational complexity than SWCE.

Packet-based and stream-based traffic will be widely accommodated in third generation mobile systems. In direct sequence code division multiple access (DS-CDMA) systems, the impact of packet-based traffic is different from stream-based traffic because of different power control schemes adopted in a multipath fading environment. In this paper, a closed-loop like power control scheme is considered for packet-based traffic on the reverse link. The concept of packet cost is introduced that represents how packet traffic consumes the link capacity of stream-based traffic. The effects of the response delay, the fading maximum Doppler frequency, and the number of resolvable paths on the packet cost for a single cell system are investigated by using Markov modeling for a multipath fading channel with a uniform power delay profile.

As the channel frequency selectivity becomes severer, the bit error rate (BER) performance of direct sequence spread spectrum (DSSS) signal transmission with rake combining degrades due to an increasing inter-path interference (IPI). Frequency-domain equalization (FDE) can replace rake combining with much improved BER performance in a severe frequency-selective fading channel. For FDE, accurate estimation of the channel transfer function is required. In this paper, we propose an iterative channel estimation that uses pilot chips which are time-multiplexed within each chip block for fast Fourier transform (FFT). The pilot acts as a cyclic-prefix of FFT block as well. The achievable BER performance is evaluated by computer simulation. It is shown that the proposed channel estimation has a very good tracking ability against fast fading.

Ultra-densification of radio access network (RAN) is essential to efficiently handle the ever-increasing mobile data traffic. In this paper, a joint multi-layered user clustering and scheduling is proposed as an inter-cluster interference coordination scheme for ultra-dense RAN using cluster-wise distributed MIMO transmission/reception. The proposed joint multi-layered user clustering and scheduling consists of user clustering using the K-means algorithm, user-cluster layering (called multi-layering) based on the interference-offset-distance (IOD), cluster-antenna association on each layer, and layer-wise round-robin-type scheduling. The user capacity, the sum capacity, and the fairness are evaluated by computer simulations to show the effectiveness of the proposed joint multi-layered user clustering and scheduling. Also shown are uplink and downlink capacity comparisons and optimal IOD setting considering the trade-off between inter-cluster interference mitigation and transmission opportunity.

In a severe frequency-selective fading channel, the bit error rate (BER) performance of orthogonal multicode DS-CDMA is severely degraded since the orthogonality property of spreading codes is partially lost. The frequency-selectivity of a fading channel can be exploited by using frequency-domain equalization to improve the BER performance. Further performance improvement can be obtained by using transmit diversity. In this paper, joint transmit diversity and frequency-domain equalization is presented for the reception of orthogonal multicode DS-CDMA signals in a frequency-selective fading channel. As for transmit diversity, delay transmit diversity (DTD) and frequency-domain space-time transmit diversity (STTD) are considered. The achievable BER performance of multicode DS-CDMA in a frequency-selective Rayleigh fading channel is evaluated by computer simulation. It is shown that the frequency-domain STTD significantly improves the BER performance irrespective of the degree of the channel frequency-selectivity while DTD is useful only for a weak frequency-selective channel.

As a flexible way to accommodate a variety of services, a number of spreading bands are now considered in International Mobile Telecommunications-2000 (IMT-2000) systems and more than two (overlaid) bands can be operated simultaneously in CDMA systems. Capacity estimation in CDMA systems is an important issue in performance analysis and call admission control (CAC), which is closely related to power control. This study derives the reverse link capacity of signal-to-interference ratio (SIR)-based power-controlled overlaid multiband CDMA systems in single and multiple cell environments. The weighted-aggregated data rate is introduced as the link capacity, which can reflect different spreading bandwidths and different QoS requirements. Various combinations of 5, 10, and 20 MHz subsystems are compared to one another in view of the maximum weighted-aggregated data rate. The impact of pulse shaping on CAC and the effect of multiple traffic accommodation on link capacity are also investigated.

Multiple-input multiple-output (MIMO) multiplexing is an attractive technique to achieve very high-speed transmission with a limited bandwidth. Recently, we proposed an iterative frequency-domain interference cancellation (FDIC) for single-carrier MIMO (SC-MIMO) multiplexing. In our previous work, assuming that the interference from the other antennas can be perfectly cancelled in FDIC, one-dimensional minimum mean square error (1D-MMSE) frequency-domain equalization (FDE) was used. However, the residual interference remains after performing FDIC. In this paper, to improve the transmission performance with iterative FDIC, we replace 1D-MMSE FDE by 2D-MMSE FDE, which takes the residual interference from the other antennas after FDIC into account. We investigate, by computer simulation, the throughput performance of rate compatible punctured turbo coded hybrid ARQ (RCPT-HARQ) with MIMO multiplexing in a frequency-selective Rayleigh fading channel.

OFDM combined with TDM (OFDM/TDM) can be used to reduce a high peak-to-average power ratio (PAPR) of OFDM, but the PAPR reduction is not sufficient. To further reduce the PAPR, an amplitude clipping can be applied. In this letter, we investigate the effect of clipping on OFDM/TDM with and without channel coding. It is shown that amplitude clipped OFDM/TDM has an advantage over clipped OFDM with respect to the PAPR.

Diversity combining and error correction coding are powerful means to combat the multipath fading encountered in mobile radio communications. In this paper, we theoretically analyze the joint effects of postdetection diversity combining and convolutional coding with soft decision Viterbi decoding for QDPSK signal transmissions. The union bounding formula is used for average BER performance calculation taking into account the additive white Gaussian noise (AWGN), cochannel interference, and multipath channel delay spread. Symbol puncturing is applied to produce higher rate codes from original 1/2-rate convolutional codes. Numerical calculations are presented for the required average signal energy per bit-to-noise power spectrum density ratio (Eb/No), required average signal-to-interference power ratio (SIR) and tolerable rms delay spread needed to achieve a certain average BER. Spectrum efficiency of cellular systems is also calculated.

Single cell reuse of the same frequency, which is possible in DS-CDMA cellular systems, yields the option of site diversity to increase link capacity. In this letter, a generalized case of site diversity transmission is considered where multiple base stations (BS's) are involved in weighted transmissions with constant total transmit power to a target mobile station (MS). A general equation of conditional bit error rate (BER) is derived based on the model of weighted transmissions combined with antenna diversity reception and rake combining. It turns out theoretically that the optimum set of weights to maximize forward link capacity makes site selection diversity transmission (SSDT) the best performer. This theoretical analysis is confirmed by performance evaluation based on the Monte-Carlo simulation.

This paper presents a generation method of orthogonal spreading codes with different spreading factors (SFs), which are called orthogonal multi-SF spreading codes in this paper, for DS-CDMA mobile radio. The generated orthogonal multi-SF spreading codes have a tree structure and the codes are applied to the forward link such that all users, who transmit data at different rates, are orthogonalized. A group spreading modulator that simplifies the base station transmitter structure is also described. The transmission performance of the orthogonal multi-SF forward link under multi-user and frequency selective Rayleigh fading environments is evaluated by computer simulation to show that its performance is identical to that achieved by using multiple orthogonal spreading codes in parallel (orthogonal multicode forward link). Unlike the orthogonal multicode forward link, only a single Rake combiner is required at a mobile receiver which significantly simplifies the mobile receiver structure.