Hiromasa Fujii

Theoretical Capacity and Outage Rate of OFDMA Cellular System with Fractional Frequency Reuse

Employing fractional frequency reuse (FFR) in OFDMA cellular systems is attracting a lot of attention since it offers large capacity and one-cell frequency reuse. However its performance under practical environments has not been revealed. This paper analyzes the theoretical capacity and outage rate of an OFDMA cellular system employing FFR. Numerical examples show that FFR achieves higher capacity than the non-FFR equivalent when the outage rate is low.

A hybrid MIMO system using spatial correlation

Fading correlation is an obstacle that degrades the potential capacity of multiple input and multiple output (MIMO) channels. This paper proposes a hybrid MIMO system that mitigates the throughput loss in highly correlated propagation channels. In the proposed system, the transmitter uses the spatial multiplexing scheme or the selection transmit diversity scheme according to information fed back from the receiver. The receiver determines which of the two transmission schemes provides higher throughput from the metrics of the spatial correlation and signal to noise ratio (SNR) of the channel. We define the spatial correlation for MIMO channels, and by computer simulations obtain throughput loss as a function of spatial correlation for the spatial multiplexing scheme. The obtained data is used for transmission scheme selection. Since the feedback information contains only commands of the selected transmission scheme and symbol constellation size, bandwidth loss due to the feedback is acceptable. Simulation results show that by using spatial correlation the proposed system can switch from the spatial multiplexing scheme to the transmit diversity scheme in highly correlated propagation environments, and so mitigate the severe throughput loss which occurs in conventional nonhybrid spatial multiplexing MIMO systems.

Spectrum Sharing by Adaptive Transmit Power Control for Low Priority Systems and its Achievable Capacity

A spectrum sharing method is proposed for the systems which share the same frequency band or adjacent bands with different priorities. The proposed method adaptively controls transmission power according to information offered by high-priority system receivers. We give theoretical capacities achieved by low-priority systems when the proposed method and a conventional method (constant transmit power) are applied. Numerical results confirm that the proposed method attains 1.5-2 times larger capacity than the conventional method.

Turbo receiver with SC/simplified-MMSE (S-MMSE) type equalizer for MIMO channel signal transmission

The paper proposes a novel turbo equalization method, called SC/simplified-MMSE (S-MMSE), for multiple-input multiple-output (MIMO) channel signal transmission. Turbo equalization is an iterative equalization and decoding process. Our method, which deals with residual inter sub stream interference as their average over a frame, achieves excellent error performance at small computational cost. We evaluate our method by link-level and system-level simulations. A complexity comparison is also presented.

Iterative Cyclostationarity-Based Feature Detection of Multiple Primary Signals for Spectrum Sharing Scenarios

One of the important and widely used detection techniques is cyclostationarity-based feature detection, because the method does not need prior information such as signal bandwidth or frame format, and time and frequency synchronization are likewise not required. The original cyclostationarity cannot distinguish signals if several signals have the same signal format and parameters, but the cyclostationarity-inducing transmission method can overcome this problem by inducing different features in the OFDM signals that have the same parameters. Another problem of conventional cyclostationarity-based feature detection is that the detection probability of weak signals worsens if multiple signals with different received-power levels are captured simultaneously. This paper proposes iterative cyclostationarity-based feature detection to detect such weak signals. The proposed detection method suppresses the effects of previously-detected signals in the cyclic auto-correlation domain, and so improves the detection probability of the weak signals. The detection performances of the conventional and proposed detection methods are evaluated by computer simulations. The results reveal the effectiveness of the proposed detection in spectrum sharing scenarios.

Peak Reduction Improvement in Iterative Clipping and Filtering with a Graded Band-Limiting Filter for OFDM Transmission

The large PAPR of orthogonal frequency division multiplexing (OFDM) transmission is one of the serious problems for mobile communications that require severe power saving. Iterative clipping and filtering is an effective method for the PAPR reduction of OFDM signals. This paper evaluates PAPR reduction effect with a graded band-limiting filter in the iterative clipping and filtering method. The evaluation result by computer simulation shows that the excellent peak reduction effect can be obtained in the fewer iteration numbers by using a roll-off filter instead of the conventional rectangular filter, and the iteration number with the roll-off filter achieving the same PAPR is fewer by twice. The result confirms that the clipping and filtering method by using a graded band-limiting filter can achieve low peak OFDM transmission with less computational complexity.

Performance Enhancement of SC/S-MMSE Turbo Receiver for MIMO-SDM/OFDM Transmission

This paper aims to improve the performance of the soft canceller followed by simplified minimum mean-square error (SC/S-MMSE) turbo receiver [11] for multiple-input and multiple-output space-division multiplexing/orthogonal frequency division multiplexing (MIMO-SDM/OFDM) transmission; it performs iterative parallel soft interference cancellation and MMSE filtering, and stream-wise soft-input and soft-output decoding. For this aim, we newly introduce two detection techniques: 1) serial interference cancellation, and 2) cyclic redundancy check (CRC)-assisted interference cancellation and MMSE filter tap computation. Various computer simulations are conducted to evaluate the performance enhancement obtained via the use of the two detection techniques. The computer simulation results show that this papers proposed serial SC/S-MMSE turbo receiver with CRC achieves frame error rate (FER) performance gain over existing MIMO receivers (MMSE receiver, V-BLAST receiver, parallel SC/MMSE-matched filter (MF) turbo receiver, and parallel SC/S-MMSE turbo receiver) for QPSK, 16QAM and 64QAM modulation while keeping the comparable complexity order.

Novel cognitive radio technique for using white space in public cellular networks

In this study, we investigate the potential of cognitive radio systems (CRSs) with the aim of using the white space (WS) in Public Cellular Networks (PCNs). We first clarify the difficulties specific to PCN WS usage and propose a CRS technique with Cognitive-radio Supportive Accommodation System (CSAS) as a scheme to overcome the difficulties. The main concept of the scheme is that CSAS, which is the primary system that owns the frequency band, agrees to support CRS functions required for spectrum sharing. The spectrum sharing scheme is designed on the PCN signal formats so as to suppress inter-system control signal overheads. We outline the prototype devices equipped with the proposed scheme, and some initial evaluation results obtained on an indoor testbed composed of the prototype devices are presented.

AM-AM characteristics of low noise block converters

The low noise block converter is one of the key nonlinear devices for spectrum sharing between IMT-A and FSS in the 3GHz band. We measure the AM-AM characteristics of several off-the-shelf LNBs and compare the characteristics against those of conventional nonlinear device models. The comparison shows that the conventional models work well on the whole, however their accuracy has room for improvement. This paper also proposes novel AM-AM models based on Rapps and Salehs AM-AM model.

Experimental evaluation of multiple signal identification based on cyclostationarity in spectrum sharing scenarios

An important and widely used detection technique is cyclostationarity-based feature detection because the method does not require prior information such as the signal bandwidth or frame format, and time and frequency synchronization are likewise not required. The problem with conventional cyclostationarity-based feature detection is that the detection probability of weak signals degrades if multiple signals with different received-power levels are captured simultaneously. Multiple signal identification has been studied in order to solve such a problem. The iterative detection method suppresses the effects of previously-detected signals in the cyclic auto-correlation domain, and so improves the detection probability of weak signals. In this paper, the multiple signal identification method is evaluated based on testbed experiments. In addition, the modification of the multiple signal identification method in which the effects of other signals are previously-relieved is proposed and applied to the testbed instead of the iterative detection method. The detection performance levels of the conventional and proposed detection methods are evaluated and compared to the results of computer simulations. The results reveal the effectiveness of the proposed detection method in spectrum sharing scenarios.