Ren Sakata

A/D Converter Clipping Noise Suppression for High-Sensitivity Carrier-Sensing of Cognitive Radio Transceiver

A technique for suppressing the clipping noise of an analogue to digital converter (ADC) is proposed to achieve the high-sensitivity carrier-sensing of a cognitive radio transceiver. When a large bandwidth cognitive radio transceiver performs carrier-sensing, it must receive a radio wave that includes many primary user transmissions. The radio wave may have high peak to average power ratio and clipping noise may be generated. Clipping noise becomes an obstacle to the achievement of high- sensitivity carrier-sensing In the proposed technique, the original values of the samples clipped by an ADC are estimated by interpolation. Polynomial spline interpolation to the clipped signal is performed in the first step, and then SINC function interpolation is applied to the spline interpolated signal. The performance was evaluated and about 20 dB suppression of clipping noise was achieved with the medium degree of clipping.

A phase compensation scheme using feedback control for IEEE 802.11a receiver

In orthogonal frequency division multiplexing (OFDM) receivers, a phase compensator plays an important role in the provision of good packet error rate (PER) performance in channels with carrier frequency offset. A feedback type phase compensator realizes a less complex OFDM receiver. However, the conventional method has two disadvantages. First, a residual phase distortion proportional to a residual carrier frequency offset remains. Second, the estimated phase distortion may oscillate. Both of them degrade phase compensation accuracy, causing PER performance to deteriorate. We propose a new feedback type compensator which calculates a phase compensation value for pilot subcarriers in order to avoid the oscillation of the estimated phase distortion and another phase compensation value for data subcarriers in order to avoid the residual phase distortion. The proposed method is not subject to the conventional methods disadvantages and improves the PER performance.

Group-Wise Scrambling Diversity for Broadcast and Multicast Services in OFDM Cellular System

Broadcast and multicast services in OFDM cellular system provide the ability to transmit the same information from multiple base stations. For the services, cyclic delay diversity (CDD) is one of the promising diversity schemes, since it can be easily applied to any number of base stations. However, there is a problem in that CDD increases frequency selectivity and makes channel estimation more difficult. In this paper, we propose a new diversity scheme that can overcome the problem of CDD. In the proposed scheme, subcarriers are divided into several subcarrier-groups. By rotating phase of each subcarrier-group independently among base stations, the frequency selectivity of the whole band is increased as in the case of CDD. On the other hand, unlike in the case of CDD, the frequency selectivity within each subcarrier-group is not changed. Utilizing this property, channel estimation carried out within each subcarrier-group can avoid the problem of CDD.

A Proposal of High-Flexibility Short-Range Cognitive Radio System

A new cognitive radio (CR) system for short-range high-capacity wireless communication is proposed. A transceiver selects 1 GHz-bandwidth main bands from the frequency range of 3-12 GHz, and places single-carrier modulated sub-bands at unused frequencies. The frequencies of main bands and sub-bands are flexibly changed according to the occupancy condition. A transceiver performs two kinds of carrier sensing. Longer duration carrier sensing is performed to recognize frequency occupancy conditions and determine the frequencies of main bands and sub-bands. Shorter duration carrier sensing is performed, which checks the sub-band frequencies just before transmitting to detect whether any primary system signal is included or not. The transceiver estimates the type of detected interference and transmission will be stopped if it is from a primary user. This system realizes the reduction of interference to other systems, and improves the throughput of the CR system itself. Furthermore, some of the key technologies to realize the proposed system are introduced.

Real-time Phase Tracking Method for IEEE 802.11a/g/n Receiver under Phase Noise Condition

In the IEEE 802.11a/g/n system, frequency offset between a transmitter and a receiver is compensated using the preamble of the frame. Moreover, continuous frequency compensation is possible using pilot subcarriers in cases where the frequency offset varies in following data symbols. This continuous compensation is important since serious phase noise occurs in the frequency synthesizer of 5/2.4 GHz, distorts OFDM symbols, and makes demodulation of 16QAM or 64QAM signals difficult. Recently the problem regarding phase noise has come one of the most prominent issues in development of radio communication systems. Conventionally, a method that measures the phase rotation of the signal using a pilot subcarrier and compensates the rotation by extrapolation is known. However, by the conventional method, a fast phase fluctuation cannot be followed since extrapolation mainly follows the constant phase rotation. Therefore, we propose a phase compensation method which can acquire good receiving characteristics also under phase noise environment. The proposed method applies both measured phase rotation of each OFDM symbol itself and the average of them with constant ratio. Then, tracking for both the steady and the fluctuated frequency offset becomes possible

Proposal of an A/D Converter Clipping Noise Suppression Technique for High-Sensitivity Carrier-Sensing of Cognitive Radio Transceiver

A technique for suppressing the clipping noise of an analogue-to-digital converter (ADC) is proposed to realize a cognitive radio transceiver that offers high sensitivity carrier-sensing. When a large bandwidth cognitive radio transceiver performs carrier-sensing, it must receive a radio wave that includes many primary user transmissions. The radio wave may have high peak-to-average power ratio (PAPR) and clipping noise may be generated. Clipping noise becomes an obstacle to the achievement of high-sensitivity carrier-sensing. In the proposed technique, the original values of the samples clipped by an ADC are estimated by interpolation. Polynomial spline interpolation to the clipped signal is performed in the first step, and then SINC function interpolation is applied to the spline interpolated signal. The performance was evaluated using the signals with various PAPR. It has been found that suppression performance has a dependency on the number of samples clipped at once rather than on PAPR. Although there is an upper limit for the number of samples clipped at once that can be compensated with high accuracy, about 20dB suppression of clipping noise was achieved with the medium degree of clipping.