In multiple antenna systems employing spatial multiplexing to raise data rates, it is preferable to use maximum likelihood (ML) detection to fully benefit from multiplexing and diversity gain. In this paper, we present a new tree search based detector that uses layer ordering in conjunction with a partial lattice basis reduction. Compared with conventional tree detectors, the error performance of the new detector closely approximates that of the optimal ML detector while achieving a dramatic reduction in complexity.

This paper proposes a novel interference cancellation technique using the concept of a distributed array, for the secondary use of the frequency resources in frequency division duplexing (FDD)-based cellular systems. In order to achieve efficient distributed interference cancellation, the secondary system exploit intervals for periodic training signals transmitted by FDD-time division multiple access (TDMA)-based cellular systems. We show that an efficient interference cancellation can be achieved by introducing helpers that decode the interference and transfer it to the secondary nodes. We take advantage of the high-speed links within the secondary system and devise a data compressing scheme that reduces time for transferring the decoded interference. We clarify the effectiveness of the proposed scheme by using computer simulation. As a parameter, the transmission quality between the helper and secondary nodes is varied. The results indicate that the proposed distributed interference cancellation is effective compared to a conventional adaptive array using a zero forcing algorithm, when the signal to noise ratio between the helper and the target secondary node is greater than 20 dB.

In multiple antenna systems using spatial multiplexing to raise data rates, it is preferable to use maximum likelihood (ML) detection to fully benefit from multiplexing and diversity gain. In this paper, we present two tree based near ML detectors that use new ordering criteria in conjunction with an efficient tree search strategy. Compared with conventional tree detectors, the error performance of the new detectors closely approximates that of the exact ML detector while achieving a dramatic reduction in complexity. Moreover, the new schemes ensure a fixed detection delay and parallelization in tree search.

Multiuser (MU) - multiple input multiple output (MIMO) techniques were proposed to increase the spectrum efficiency because only a few antenna branches are allowed at a simple mobile terminal (MT). Among the MU-MIMO techniques, the block diagonalization (BD) algorithm is well known as a practical linear precoding technique for a downlink. In this paper, we analyze an achievable bit rate in MU-MIMO using BD algorithm and derive the simplified expression using the signal spaces of users. Based on the derived expression, two kinds of user selection methods for the BD algorithm (USBD) in MU-MIMO systems are proposed. The proposed methods estimate the improvement in the achievable bit rate compared to that in a single user (SU)-MIMO system using the signal spaces of the supported users. The first proposed method is USBD with a full search (USBD-FS) and it selects the combination of users to achieve the highest total throughput by considering all the combinations. The second proposed method is USBD with a tree structure search (USBD-TS), which drastically reduces the calculation complexity. Although all combinations of users are not considered in USBD-TS, a sufficient achievable bit rate is obtained while the calculation load is much less than that for the conventional method. Computer simulations confirm that USBD- FS and USBD-TS obtain the achievable bit rate of 100.0% and 91.1% compared to that for the best combination of users when the angle spread at the transmitter is 75deg.

In a spatial multiplexing system with multiple antennas on both sides of a wireless link, it is preferable to adopt the maximum likelihood (ML) detection to fully extract both multiplexing and diversity gain. In this paper, we present a new tree search based near ML detector that applies a new ordering in conjunction with efficient node exploration strategy. The error performance of the new detector closely approaches that of the ML detector. However, it is far less complex than conventional suboptimal tree detectors and maintains a fixed detection delay.

Single-carrier (SC) transmission with frequency- domain equalization (FDE) is expected to expand the service area of wireless communication systems by taking advantage of its lower peak-to-average power ratio (PAPR) compared to when using orthogonal frequency division multiplexing (OFDM). In the uplink, the transmit timing difference among the signals from multiple mobile terminals (MTs) becomes significant in a large service area. If the difference in arrival timing among MTs exceeds the cyclic prefix (CP) length, it is difficult to separate the signal of each MT and the transmission quality is severely degraded. In this paper, we propose a novel multiuser detection (MUD) technique that can separate the signals regardless of the difference in the arrival timing between MTs with an acceptable level of calculation complexity. The achievable bit error rate (BER) performance is evaluated by computer simulation in a frequency selective fading channel and the results confirm the effectiveness of the proposed method.

The multiple input multiple output (MIMO) technique enables the application of spatial multiplexing as a way of increasing the spectrum efficiency. However, only a few antenna branches are allowed at a simple mobile terminal (MT) and the MIMO effect is limited for those MTs. Multiuser (MU)-MIMO techniques were proposed to support such simple terminals to overcome such problems. Among them, the block diagonalization algorithm is well known as a practical linear precoding technique for downlink in the MU-MIMO systems. Because the number of users supported by the access point is limited and the improvement in the spectrum efficiency greatly depends on the combination of spatially multiplexed user, a simple transmission performance estimation method is needed. In this paper, we propose a new capacity estimation method for MU-MIMO systems. The proposed method estimates the achievable bit rate in MU-MIMO while significantly reducing the calculation load compared to that for direct estimation in the block diagonalization algorithm. Theoretical analysis and the simulation results confirm the validity of the proposed method.

In this paper, the possibility for a short-range cognitive radio (secondary communication system) to be located within the service area of the primary system is discussed. Although the secondary system interferes with the primary system, there can be certain locations in the service area of the primary system where the cognitive radio can reuse the frequency of the primary system without disturbing it or being disturbed by the primary system. We say that in those locations there is a spatial opportunity for communication in the secondary system. The primary and secondary systems have different features and usage scenarios, which result in different deployment conditions. This gives rise to differences between the path loss for a link in the primary system and the path loss for the interference from the primary to secondary systems. This paper investigates the impact of the heterogeneous path loss conditions on the spatial opportunity for the secondary system. Our approach can be applied as a general way to investigate the coexistence among multiple systems. We clarify that the change of the path loss coefficient due to the different antenna heights of the primary and secondary systems can largely affect the spatial opportunity for frequency reuse by the cognitive radios.

We propose a wireless download agent which effectively controls the point at which a download starts for providing a comfortable mobile computing environment. A user can get the desired data with the wireless download agent while walking through a service area without stopping. We conducted simulations to evaluate its performance in terms of throughput, download period, and probability of successful download. Our results show that the proposed scheme very well suits the wireless download agent in high-speed wireless access systems with many users. Furthermore, we describe the use of the proposed scheme considering the randomness of the walking directions of users.

Multi-hop communication systems have been widely studied and many of them are based on the IEEE 802.11 WLAN system. The paper explains the problem of throughput degradation of a relay station when IEEE 802.11 DCF is adopted in multi-hop communications. The relay stations lose some of their capacity compared to stations that are not relaying the data of other stations. Because all stations, under the CSMA/CA protocol, have equal transmission opportunity, the relay stations have to send packets from other terminals. We propose an autonomous priority control method that changes the contention window size depending on the number of relay stations and the number of hops to ensure that relay stations are offered the same transmission capacity as regular stations in the service area. Then its performance is evaluated by computer simulations.

This paper proposes the concept of system diversity on software defined radio (SDR) and investigates the effectiveness of system diversity by using a concrete simulation model. System diversity allows the wireless communication system to be dynamically changed in addition to the signal processing algorithm or modulation/coding scheme being used. To clarify the validity of system diversity, we examine a system simulation model consisting of three wireless communication systems; algorithms are introduced to show how system diversity can be controlled using the QoS parameters of the received signal level, the data transmission rate and the channel capacity. The process by which system diversity switching is triggered is elucidated and a practical example is introduced. Simulation results confirm that system diversity offers higher performance in terms of data throughput and system channel capacity than existing wireless communication systems.

A software defined radio (SDR) prototype based on a multiprocessor architecture (MPA) is developed. Software for Japanese personal handy phone system (PHS) of a 2G mobile system, and IEEE 802.11 wireless LAN, which has a much wider bandwidth than the 2G systems, is successfully implemented. A newly developed flexible-rate pre-/post-processor (FR-PPP) achieves the flexibility and wideband performance that the platform needs. This paper shows the design of the SDR prototype and evaluates its performance by experiments that include PHS processor load and processing delay characteristics and wireless LAN processor load and throughput characteristics.

For multiprocessor architecture (MPA) software radios, allocating the functions to the processors appropriately and controlling multiple interruptions between the processors are important issues in achieving real-time processing ability. This paper describes the design and evaluation of Japanese Personal Handy-phone System (PHS) software implemented in an MPA software radio platform for the realization of full-duplex real-time communication. A novel API between the CPU and DSP programs is shown that achieves real-time performance. Average DSP load and processing delay characteristics of the PHS software are measured; the results show that real-time communication was successfully achieved.

Over-the-air (OTA) download technology will play an important role in realizing software radio. We have already constructed software radio prototypes based on multiprocessor architecture. This paper reports on the implementation of the OTA download function in the software radio prototypes. OTA download time and reconfiguration time were evaluated and analyzed. The results show that PHS mode software could be downloaded within two minutes while prototype reconfiguration took about ten seconds. The current version of this OTA download function suits services that do not have severe time limits, such as updating programs held in memory.

Software radio base and personal station prototypes for cellular communication systems are designed and implemented. The prototypes are composed of commercial multipurpose DSPs and CPU, pre- and post-processors, A/D/A converters, and RF/IF units. In order to use processor resources effectively, the DSP program handles signal processing in physical and data-link protocol-layers, while the CPU program takes charge of high protocol-layers including call control and system control. The prototypes support various air interfaces, some of which are equivalent to the 384 kbit/s transmission rate PHS (personal handy phone system) and a 96 kbit/s transmission rate FDMA/TDD system. Excellent transmitting and receiving performance, compared to conventional hardware radios, is achieved. In addition, expanding the prototypes to support IEEE 802.11 wireless LANs is examined. This paper describes the design and evaluation of the prototypes for cellular systems, and discusses issues and strategies for supporting IEEE 802.11 wireless LANs.

This paper presents a very low power consumption one-chip baseband large-scale integrated circuit (LSIC) for personal communication terminals. It comprises a /spl pi//4-shift QPSK modem, an adaptive differential pulse code modulation (ADPCM) codec, a time division multiple access time division duplex (TDMA-TDD) controller and a link access procedure for a digital cordless (LAPDC: Layer-2 protocol) controller. The developed LSIC meets all the specifications of the personal handy-phone system (PHS) standard. By employing a novel coherent demodulator and an ADPCM codec with a click noise suppressor, a higher quality voice transmission has been achieved in a fading environment. In addition, it has 61-kb/s data transmission capability to achieve wireless multimedia services based on PHS. Moreover, the novel circuit configurations of the modem, the ADPCM codec, the TDMA-TDD controller, and the LAPDC controller achieve significant power reduction of the baseband circuits (57.4 mW) of personal communication terminals. It enables very low power consumption wireless multimedia terminals to be achieved based on the PHS common air interface.

Diversity combining methods for mobile satellite communication systems employing convolutional encoding and soft-decision Viterbi decoding are evaluated. Computer simulation clarifies that the pre-Viterbi-decoding maximal ratio combining method has better performance than other methods in Rician fading channels. The simulation results agree with the Pe (bit error probability) performance derived from the numerical analysis for slow fading using the approximate Pe performance of Viterbi decoding in AWGN (additive white Gaussian noise) channels and the probability density function of Rician fading. Applying this diversity method to the multi-beam mobile satellite communication systems, a satellite beam diversity reception scheme is proposed. A computer simulation result shows that the proposed scheme decreases the packet error rate of the control signals to less than 1/100 around the satellite beam boundary.>

This paper proposes a very low power consumption ADPCM voice codec LSIC. The developed LSIC satisfies CCITT recommendation G.721 completely and is suitable for realizing the high-quality voice signal transmission expected from personal communication systems in fading environments. By employing a time sharing signal processing, a very low power consumption (only 500 /spl mu/W) for ADPCM codec core has been achieved. Moreover, the LSIC has a novel click noise detection function and erroneous received signals are suppressed digitally according to the click noise detection information. This makes it possible to improve the voice transmission quality significantly in fading environments, for instance, the MOS (mean opinion score) evaluation is improved by 0.5 points compared with that of the conventional scheme.

This paper presents a low power coherent /spl pi//4-shift QPSK demodulator LSIC for personal communication systems in Japan. The developed LSIC, implemented on a CMOS 0.8 /spl mu/m standard cell with 2-V operation, achieves better frame error rate performance and lower power consumption than conventional demodulators. It uniquely employs (1) a reverse-modulation carrier recovery scheme with a -/spl pi//4 signal-phase rotator and a band-width-varied carrier filter, (2) a clock recovery scheme using initial clock phase estimation, (3) a fully digital orthogonal detection scheme suitable for low power consumption, and (4) a burst detection scheme using phase information in preamble. Performance evaluation confirms that the demodulator LSIC reduces the irreducible frame error rate by 40%, and achieves an Eb/No improvement of 3 dB at the frame error rate of 10% compared with differential detection under the Rayleigh fading channel common in personal communication.

A /spl pi//4-shift QPSK digital modulator LSIC consisting of a digital pulse shaping filter and a baseband quadrature modulator is newly developed. By employing a novel digital filter configuration, the required filter memory is reduced to just 1/2048th that of the conventional modulator. Moreover, since the proposed baseband modulation scheme does not employ a complex multiplier or a quadrature modulator with multipliers, a compact and low power consumption modulator is realized.

The paper proposes a half chip offset QPSK (quadrature phase shift keying) modulation CDMA (code division multiple access) scheme coupled with time diversity and high-coding-gain FEC (forward error correction) for wireless LAN (local area network) systems. It is clarified that the proposed scheme realizes compact spectrum characteristics in hard limited channels and excellent Pe performance for high-speed data transmission with simple hardware configuration. The experimental results show that by employing time diversity with FEC, the half chip offset QPSK CDMA scheme realizes a required Eb/No of 14.2 dB at a Pe of 10/sup -5/ in Rician fading environments with fD of 10 Hz and delay spread of 40 nsec.

An advanced, high-speed, and universal-coding-rate Viterbi decoder VLSI implementation is presented. Two novel circuit design schemes have been proposed: scarce state transition (SST) decoding and direct high-coding-rate convolutional code generation and variable-rate decoding. SST makes it possible to omit the final decision circuit and to reduce the required path memory length without degrading error probability performance. Moreover, the power consumption of the SST Viterbi decoder is significantly reduced when implemented as a CMOS device. These features overcome the speed limits of high-speed and high-coding-gain Viterbi decoder VLSIs in the rate one-half mode imposed by the thermal limitation. The other Viterbi decoding scheme makes it possible to realize a simple and variable coding-rate forward-error-correction circuit by changing only the branch metric calculation ROM tables. By employing these schemes, high-speed (25-Mb/s) and universal-coding-rate Viterbi decoder VLSIs have been developed.>

In order to increase the channel capacity of CDMA (code division multiple access) mobile/personal communication systems, an inter-channel interference cancellation (IIC) technique is proposed. By employing one or few additional interference cancellation circuits, it makes it possible to increase allowable error of the TXPC (transmission power control) at the mobile/personal terminals. The experimental results show that by employing the proposed IIC scheme, Pe performance degradation due to a desired and an undesired signal power ratio of -20 dB (20 dB of TXPC control error) can be improved to only 1.5 dB at Pe=10/sup -3/.>

In order to realize a higher-code-gain forward error correction scheme in mobile satellite communication systems, a novel concatenated coding scheme employing soft decision decoding for not only inner codes but also outer codes (double soft decision, or DSD, concatenated forward error correction scheme) is proposed. Soft-decision outer decoding can improve the bit error probability of inner decoded data. In this scheme, likelihood information from an inner Viterbi decoder is used in the decoding of outer codes. A technique using the path memory circuit status 1.0 ratio for likelihood information is proposed, and is shown to be the most reliable even though it requires the simplest hardware among the alternative methods. A computer simulation clarifies that the DSD scheme improves Pe performance to one-third of that of the conventional hard-decision outer decoding. Moreover, to reduce the interleaving delay time in fading channels or inner decoded data of concatenated codes, a parallel forward error correction scheme is proposed.>

The authors propose a satellite video signal transmission system that uses a time division multiple access (TDMA) scheme for digital video signal transmission and a code division multiple access (CDMA) scheme for voice signal transmission from a video signal reception-only earth station (backward voice signal transmission). The adoption of a TDMA scheme makes it possible to transmit video signals from various places and to communicate in multipoint-to-(multi)point mode. For backward voice channel transmission from video signal reception-only earth stations, a superposed CDMA voice signal transmission over TDMA video signals by using the same transponder is proposed. The employment of high coding gain forward error correction and development of a cochannel interference cancellation technique have made it possible for the proposed system to transmit a practical number of voice channels. The performance of the proposed system has been experimentally evaluated and the results show the proposed scheme can transmit about 20 CDMA voice channels simultaneously.>

The authors propose a TDMA (time-division multiple-access) satellite communication system suitable for both basic and primary ISDN services in addition to B-channel transmission lines among ISDN switches. To provide these services economically and efficiently, the proposed system uses an offset quadrature phase-shift keying (QPSK) burst model coupled with high-coding gain and rate-forward-error correction, demand assignment technology for each data burst, and a transmission and reception transponder (carrier) hopping scheme. To realize cost-effective transmission and subscriber lines and to increase the reliability of TDMA terminals, the major functions of TDMA terminals have been implemented in large-scale integrated circuits (LSICs), monolithic ICs (MICs) and hybrid ICs (HICs). The experimental results show narrow spectrum transmission with little bit error probability degradation for the offset QPSK burst modem and perfect operation of demand assignment and carrier hopping capability of the system.>

A DCF (dual carrier filter) reverse-modulation-type carrier recovery circuit is proposed to achieve a low carrier skipping rate and satisfactory phase tracking performance for coherent detection of PSK (phase shift keying) signals in fast Rician fading channels. The proposed scheme employs both narrow and wide bandwidth carrier filters simultaneously for the reverse-modulation-type carrier recovery circuit. It is clarified by computer simulation that the Pe performance of a QPSK (quadriphase shift keying) modem employing the proposed scheme shows an improvement of 1.5 dB in required E/sub s//N/sub O/ at Pe=10/sup 4/ (after Viterbi decoding (R=7/8, K=7), C/M (direct-to-multipath signal power ratio)=10 dB, interleaving size=64*64), compared with conventional coherent detection employing the reverse modulation tank-limiter scheme or the Costas loop scheme.>

A DCF (dual carrier filter) reverse-modulation-type carrier recovery circuit is proposed to achieve a low carrier skipping rate and satisfactory phase tracking performance for coherent detection of PSK (phase shift keying) signals in fast Rician fading channels. The proposed scheme employs both narrow and wide bandwidth carrier filters simultaneously for the reverse-modulation-type carrier recovery circuit. It is clarified by computer simulation that the Pe performance of a QPSK (quadriphase shift keying) modem employing the proposed scheme shows an improvement of 1.5 dB in required E/sub s//N/sub O/ at Pe=10/sup 4/ (after Viterbi decoding (R=7/8, K=7), C/M (direct-to-multipath signal power ratio)=10 dB, interleaving size=64*64), compared with conventional coherent detection employing the reverse modulation tank-limiter scheme or the Costas loop scheme. >

An architecture is proposed for TDMA (time division multiple access) equipment and functional-module realization in microelectronics to increase reliability and to reduce hardware size and development time. The approach described basically involves digitization of analog circuits (allowing their realization using digital LSI circuits) and analog IC implementation for high-speed circuits. In order to use general-purpose LSI circuits and ICs, TDMA equipment is reconfigured into a hardware-oriented and simplified architecture. Using this architecture and optimal function assigning to each module, six types of general-purpose synchronization unit LSI circuits have been developed in addition to eleven types of LSI circuits and ICs, i.e. three types of digital LSI circuits, four types of MAICs (monolithic analog ICs), and four types of HICs (hybrid ICs) for a burst modem. As a result of this LSI circuit and IC implementation, the hardware size of TDMA equipment has been reduced to one-fifth of the conventional size, and maintenance-free capability has been achieved.>