In this paper, we propose pilot-assisted decision feedback channel estimation (PA-DFCE) for space-time coded transmit diversity (STTD) in orthogonal frequency division multiplexing (OFDM). Two transmit channels are simultaneously estimated by transmitting the STTD encoded pilot. To improve the tracking ability of the channel estimation against fast fading, decision feedback is also used in addition to pilot. For noise reduction and preventing the error propagation, windowing of the estimated channel impulse response in the time-delay domain is applied. The average bit error rate (BER) performance of OFDM with STTD is evaluated by computer simulation. It is found that the use of PA-DFCE can achieve a degradation in the required Eb/N0 from ideal CE of as small as 0.6 dB for an average BER = 10-3 and requires about 2.4 dB less Eb/N0 compared to differential STTD that requires no CE.

A wireless multi-hop virtual cellular network (VCN) was recently proposed to avoid the large peak transmit power, resulting from the high transmission rates expected for future mobile communication systems. In VCN, calls hop through several links to reach the central port, which is the gateway to the network. With the use of a routing algorithm based on the total uplink transmit power minimization criterion, the total transmit power of all the multi-hop links between the mobile terminal and the central port can be significantly reduced, in comparison with the present (single-hop) cellular network. In this paper, an "on-demand" channel assignment strategy, using the channel segregation dynamic channel allocation (CS-DCA) algorithm, is proposed for multi-hop DS-CDMA VCN. Computer simulation is conducted to evaluate the blocking probability performance and make a comparison between the VCN and the present cellular network.

Recently, frequency-domain equalization (FDE) has been attracting much attention as a way to improve single-carrier (SC) signal transmission in a frequency-selective wireless channel. Since the SC signal spectrum is spread over the entire signal bandwidth, FDE can take advantage of channel frequency-selectivity and achieve the frequency diversity gain. SC with FDE is a promising wireless signal transmission technique. In this article, we review the pioneering research done on SC with FDE. The principles of simple one-tap FDE, channel estimation, and residual inter-symbol interference (ISI) cancellation are presented. Multi-input/multi-output (MIMO) is an important technique to improve the transmission performance. Some of the studies on MIMO/SC with FDE are introduced.

In virtual cellular network (VCN), proposed for high-speed mobile communications, the signal transmitted from a mobile terminal is received by some wireless ports distributed in each virtual cell and relayed to the central port that acts as a gateway to the core network. In this paper, we apply the multi-route MHMRC diversity in order to decrease the transmit power and increase the multi-hop link capacity. The transmit power, the interference power and the link capacity are evaluated for DS-CDMA multi-hop VCN by computer simulation. The multi-route MHMRC diversity can be applied to not only DS-CDMA but also other access schemes (i.e. MC-CDMA, OFDM, etc.).

To improve the DS-CDMA signal transmission performance in a frequency-selective fading channel, the frequency-domain equalization (FDE) can be applied, in which simple one-tap equalization is carried out on each subcarrier component obtained by fast Fourier transform (FFT). Equalization weights for joint FDE and antenna diversity combining based on maximal ratio combining (MRC), zero-forcing (ZF), and minimum mean square error (MMSE) are derived. The conditional bit error rate (BER) is derived for the given set of channel gains in a frequency-selective multipath fading channel. The theoretical average BER performance is evaluated by Monte-Carlo numerical computation method using the derived conditional BER and is confirmed by computer simulation. Performance comparison between DS- and multi-carrier (MC)-CDMA both using FDE is also presented.

Field experiments using the 2 GHz carrier frequency band were conducted nearby Tokyo to evaluate the effect of joint use of Rake combining and antenna diversity and also the effect of spreading chip rate (or bandwidth) on the achievable bit error rate (BER) performance and the mobile station transmit power distribution of power controlled coherent DS-CDMA reverse-link (mobile-to-base). Four chip rates, 0. 96, 1. 92, 3. 84, and 7. 68 Mcps, were used. The command interval and power step size of the fast transmission power control (TPC) used in the experiments, 1. 25 ms and 1 dB, respectively, were based on measurements of signal-to-interference plus background noise power ratio (SIR) after Rake combining. The field experiments demonstrate that the joint use of antenna diversity and Rake combining significantly improves the BER performance and, furthermore, that increasing the chip rate improves the BER performance and decreases the transmit power because of enhanced Rake combining through an increase in the number of resolved paths.

Inter-cell asynchronous DS-CDMA cellular mobile radio allows continuous system deployment from outdoors to indoors since no outer timing source is required. All the forward link channels(control and traffic channels)of each cell site are first spread by orthogonal short spreading codes and then randomized by a long random code uniquely assigned to each cell site. However, inter-cell asynchronous systems generally require much longer cell search time than inter-cell synchronous systems. This paper proposes a fast cell search algorithm based on the periodic masking of the long random code when transmitting the control channel(CCH)signal. The same short spreading code is used for the CCHs of all cell sites. The same short spreading code periodically appears in the signals transmitted from all cell sites so the mobile station can detect the long random code timing(or more precisely the masking timing)by using a matched filter. By grouping the long random codes used in the system and transmitting a group identification(GI)code from each cell site during the masking period, we can avoid searching all long random codes. This significantly reduces the cell search time. Simulation results demonstrate that cell search can be accomplished in less than 500 ms at 90% of the locations when the number of long random codes(having a repetition period of 10 ms)is 512 and the number of those per group is 32.

In this paper, joint transmit/receive frequency-domain equalization (FDE) is proposed for analog network coded (ANC) single-carrier (SC) bi-directional multi-antenna relay. In the proposed scheme, diversity transmission using transmit FDE is performed at relay station (RS) equipped with multiple antennas while receive FDE is carried out at base station (BS) and mobile terminal (MT) both equipped with single antenna. The transmit and receive FDE weights are jointly optimized so as to minimize the end-to-end mean square error (MSE). We evaluate, by computer simulation, the throughput performance and show that the joint transmit/receive FDE obtains the spatial and frequency diversity gains and accordingly achieve better throughput performance compared to either the transmit FDE only or the receive FDE only. It is also shown that ANC SC bi-directional multi-antenna relay can extend the communication coverage area for the given required throughput compared to conventional direct transmission.

The Communications Research Laboratory (CRL) started a new project named the New Generation Mobile Network Project in April 2002. The target of this project is the development of new technologies to enable seamless and secure integration of various wireless access networks such as 3rd and 4th generation cellular, wireless LAN, high-speed mobile wireless, wired communications, and broadcasting networks. This paper presents an overview of CRL's new generation mobile communication system that is called The Multimedia Integrated Network by Radio Access Innovation Plus (MIRAI+), as well as details the role of Software Radio Technology (SDR) in MIRAI+.

The improvement made by the scanning diversity reception on the cochannel interference performance of a digital FM land mobile radio is analyzed in the Rayleigh fading signal environment. The results are verified by the laboratory simulation test.

Cell-free massive MIMO (CF-mMIMO), which cooperatively utilizes a large number of antennas deployed over a communication area, has been attracting great attention as an important technology for realizing 5G-advanced and 6G systems. Recently, to ensure system scalability and mitigate inter-user interference in CF-mMIMO, a user-centric (UC) approach was investigated. In this UC approach, user-centric antenna-sets are formed by selecting appropriate antennas for each user, and postcoding is applied to reduce the strong interference from users whose antenna-sets overlap. However, in very high user density environments, since the number of interfering users increases due to increased overlapping of antenna-sets, the achievable link capacity may degrade. In this paper, we propose a user-cluster-centric (UCC) approach, which groups neighborhood users into a user-cluster and associates the predetermined number of antennas to this user-cluster for spatial multiplexing. We derive the uplink postcoding weights and explain the effectiveness of the proposed UCC approach in terms of the computational complexity of the weight computation. We also compare the uplink user capacities achievable with UC and UCC approaches by computer simulation and clarify situations where the UCC approach is effective. Furthermore, we discuss the impact of the number of interfering users considered in the zero-forcing and minimum mean square error postcoding weight computation on the user capacity.

To cope with ever growing mobile data traffic, we recently proposed a concept of cellular ultra-dense radio access network (RAN). In the cellular ultra-dense RAN, a number of distributed antennas are deployed in the base station (BS) coverage area (cell) and user-clusters are formed to perform small-scale distributed multiuser multi-input multi-output (MU-MIMO) transmission and reception in each user-cluster in parallel using the same frequency resource. We also proposed a decentralized interference coordination (IC) framework to effectively mitigate both intra-cell and inter-cell interferences caused in the cellular ultra-dense RAN. The inter-cell IC adopted in this framework is the fractional frequency reuse (FFR), realized by applying the channel segregation (CS) algorithm, and is called CS-FFR in this paper. CS-FFR divides the available bandwidth into several sub-bands and allocates multiple sub-bands to different cells. In this paper, focusing on the optimization of the CS-FFR, we find by computer simulation the optimum bandwidth division number and the sub-band allocation ratio to maximize the link capacity. We also discuss the convergence speed of CS-FFR in a cellular ultra-dense RAN.

In this study, an overloaded large-scale distributed antenna network is considered, for which the number of active users is larger than that of antennas distributed in a base station coverage area (called a cell). To avoid overload, users in each cell are divided into multiple user groups, and, to reduce the computational complexity required for multi-user multiple-input and multiple-output (MU-MIMO), users in each user group are grouped into multiple user clusters so that cluster-wise distributed MU-MIMO can be performed in parallel in each user group. However, as the network size increases, conventional computational methods may not be able to solve combinatorial optimization problems, such as user scheduling and user clustering, which are required for performing cluster-wise distributed MU-MIMO in a finite amount of time. In this study, we apply quantum computing to solve the combinatorial optimization problems of user scheduling and clustering for an overloaded distributed antenna network and propose a quantum computing-based user scheduling and clustering method. The results of computer simulations indicate that as the technology of quantum computers and their related algorithms evolves in the future, the proposed method can realize large-scale dense wireless systems and realize real-time optimization with a short optimization execution cycle.

Turbo coded hybrid ARQ (HARQ) is known as one of the promising error control techniques for high speed wireless packet access. However, in a severe frequency-selective fading channel, the HARQ throughput performance significantly degrades for direct sequence code division multiple access (DS-CDMA) system using rake combining. This problem can be overcome by replacing the rake combining by the frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion. In a system with the conventional FDE, the guard interval (GI) is inserted to avoid the inter-block interference (IBI). The insertion of GI reduces the throughput. Recently, overlap FDE that requires no GI insertion was proposed. In this paper, we apply overlap FDE to HARQ and derive the MMSE-FDE weight for packet combining. Then, we evaluate the throughput performance of DS-CDMA HARQ with overlap FDE. We show that overlap FDE provides better throughput performance than both the rake combining and conventional FDE regardless of the degree of the channel frequency-selectivity.

DS-CDMA provides a flexible support for the low-to-high bit rate of multimedia services upon a specific user's request. A simple capacity expression is derived for a power-controlled reverse link of orthogonal multi-code DS-CDMA with multiple connections. It is found that an orthogonal multi-code user having multiple connections is equivalent to a single connection user, but with a spreading factor reduced by a factor of the total number of parallel codes and a required signal energy per symbol-to-interference plus noise power spectrum density ratio which is the average taken over multiple connections. Furthermore, the use of antenna diversity is found equivalent to the use of higher spreading factor increased by a factor of the number of antennas.

A DS-CDMA mobile communication system accommodating multi-class users is considered. The number of supportable users depends on the distributions of data rate and required communication quality among users. Simple expressions for the reverse link capacity with transmit power control, antenna diversity, and rake combining, are derived for a single-cell system and a multi-cell system.

One of the promising wireless access techniques for the next generation mobile communications systems is multi-carrier code division multiple access (MC-CDMA). MC-CDMA can provide good transmission performance owing to the frequency diversity effect in a severe frequency-selective fading channel. However, the bit error rate (BER) performance of coded MC-CDMA is inferior to that of orthogonal frequency division multiplexing (OFDM) due to the residual inter-code interference (ICI) after frequency-domain equalization (FDE). Recently, we proposed a frequency-domain soft interference cancellation (FDSIC) to reduce the residual ICI and confirmed by computer simulation that the MC-CDMA with FDSIC provides better BER performance than OFDM. However, ideal channel estimation was assumed. In this paper, we propose adaptive decision-feedback channel estimation (ADFCE) and evaluate by computer simulation the average BER and throughput performances of turbo-coded MC-CDMA with FDSIC. We show that even if a practical channel estimation is used, MC-CDMA with FDSIC can still provide better performance than OFDM.

Independent shadowing losses are often assumed for evaluating the link capacity of direct sequence code division multiple access (DS-CDMA) cellular system. However, shadowing losses may be partially correlated since the obstacles surrounding a mobile station block similarly the desired signal and the interfering signals. In this letter, we discuss how the shadowing correlation impacts the reverse link capacity of a power-controlled DS-CDMA cellular system, by numerical analysis.

It is well known that some of urban man-made noises can be characterized by a wideband impulsive noise (pure impulsive noise). The presence of this pure impulsive noise may significantly degrade the wireless digital transmission performance. As the data rate becomes higher and the radio bandwidth becomes wider, the performance degradation due to pure impulsive interference may become larger. In this paper, the DS-CDMA transmission performance in the presence of pure impulsive interference is theoretically analyzed. First, the BER expressions are derived for DS-CDMA with antenna diversity and Rake combining in a frequency selective fading channel. Then, the numerical computation based on Monte-Carlo method is performed to evaluate the BER performance. Two types of error floor are observed: one is due to impulsive interference and the other due to the multi access interference (MAI). It is found that the error floor due to impulsive interference becomes larger as the area of impulse and the error floor is linearly proportional to the impulse occurrence rate. Furthermore, it is found that the antenna diversity and Rake combining do not help to reduce the error floor caused by impulsive interference and that the influence of impulsive interference can be negligible when the channel is limited by the MAI (i.e., large number of users are in communication).