A simple exact error rate analysis is presented for random binary direct sequence code division multiple access (DS-CDMA) considering a general pulse shape and flat Nakagami fading channel. First of all, a simple model is developed for the multiple access interference (MAI). Based on this, a simple exact expression of the characteristic function (CF) of MAI is developed in a straight forward manner. Finally, an exact expression of error rate is obtained following the CF method of error rate analysis. The exact error rate so obtained can be much easily evaluated as compared to the only reliable approximate error rate expression currently available, which is based on the Improved Gaussian Approximation (IGA).

This paper introduces a unified method of spectrum sensing for all existing analog television (TV) signals including NTSC, PAL and SECAM. We propose a correlation based method (CBM) with a single reference signal for sensing any analog TV signals. In addition we also propose an improved energy detection method. The CBM approach has been implemented in a hardware prototype specially designed for participating in Singapore TV white space (WS) test trial conducted by Infocomm Development Authority (IDA) of the Singapore government. Analytical and simulation results of the CBM method will be presented in the paper, as well as hardware testing results for sensing various analog TV signals. Both AWGN and fading channels will be considered. It is shown that the theoretical results closely match with those from simulations. Sensing performance of the hardware prototype will also be presented in fading environment by using a fading simulator. We present performance of the proposed techniques in terms of probability of false alarm, probability of detection, sensing time etc. We also present a comparative study of the various techniques.

In this paper, we propose two schemes that improve the delay and the current consumption for efficient polling communications in multi-hop networks based on the receiver-initiated media access control (MAC) protocol. Polling communications can offer reliable data collection by avoiding communication collisions, but the larger delay and current consumption for the round-trip operation should be improved. The first proposal is an enhanced source routing scheme for downlink communications. In the proposed scheme, multiple candidates of relay terminals can be loaded in the routing information, so the route is not specified uniquely. The improvement of the delay and the current consumption is achieved by shortening the waiting time for communication timings based on the flexible routing. The second proposal is a round-trip delay reduction scheme which focuses on the bi-directionality of polling communications. The proposed scheme reduces the round-trip delay by offering frequent communication timings for uplink communications. Also, this paper proposes the joint application of the enhanced source routing scheme and the round-trip delay reduction scheme in polling communications. Computer simulations that suppose a multi-hop network based on the feathery receiver-initiated transmission (F-RIT) protocol in stable channel conditions are carried out. The results show the effectiveness of the proposed schemes in improving the delay and the current consumption. When the polling interval is 900s, the combination of the two proposed schemes improves the round-trip delay by up to 44.1% and the current consumption by up to 38.7% in average.

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+.

We have investigated the feasibility of the 16QAM-OFDM transmission scheme by calculating its transmission performance by means of some evaluation models. These models have been proposed by standardization projects such as ETSI-BRAN or ARIB-MMAC project. We evaluated the effects of the following four factors on transmission performance: (1) modulation scheme of each OFDM sub-carrier-channel; (2) FEC scheme, especially coding rate and coding scheme; (3) multipath fading environments; and (4) phase noise and nonlinear amplifier. By these evaluations, we have compiled a fundamental data in order to realize multimedia mobile access communication system based on 16QAM-OFDM transmission scheme.

This paper proposes a new parallel high speed mobile radio transmission scheme using cyclic-shifted-codes generated from a modified M-sequence. The modified M-sequence is biased with constant direct current (dc) on an M-sequence and is inserted the guard chips before and after this biased M-sequence. The proposed system has the following features: i) Orthogonality of the codes is kept not only between direct waves of each parallel channels but also between direct and delayed waves within the guard chips; ii) It is possible to reduce the number of kinds of codes allocated to one user; and iii) It is easy to recover both code and chip timings. In this paper, moreover, the performance of the proposed system was evaluated in terms of bit error rate (BER) under additive white Gaussian noise (AWGN), non-selective one path Rayleigh fading and double-spike Rayleigh fading channels. As a result, the proposed transmission scheme can transmit several Mbps in a high-speed double-spike Rayleigh fading channel with better quality in comparison with a conventional multicode CDM transmission scheme based on M-sequences.

In the future satellite broadcasting system in 21GHz band, the rainfall attenuation is a most significant problem. To solve this problem, the hierarchical transmission systems have been studied. This paper analyzes the performance of the hierarchical modulation scheme from the view point of power assignment in the presence of the rainfall attenuation. This paper shows an optimum power assignment ratio to maximize the spectral efficiency and the signal-to-noise ratio of received image, and these optimum ratio is varied with the measure of system performance.

This paper proposes an OFDM based adaptive modulation scheme employing variable coding rate (VCR OFDM AMS), which selects optimum modulation and coding scheme (MCS) realized by combination of several modulation schemes and coding rates. The OFDM AMS with multilevel transmit power control (OFDM AMS/MTPC) can realize high data rate transmission in the dynamic parameter controlled-orthogonal frequency and time division multiple access (DPC-OF/TDMA). The employment of OFDM AMS/MTPC, however, makes transceiver design rather complex. To solve this problem, we propose to improve throughput performances of the OFDM AMS without employment of the MTPC. The simple OFDM AMS, however, does not fully utilize transmit power for throughput improvement because there is surplus transmit power which corresponds to power margin over required signal to interference plus noise power ratio (SINR). Thus, in order to improve transmit power efficiency for throughput increase, we reduce the required SINR gaps between adjacent MCSs by introducing many coding rates. Furthermore, this paper presents an effective bit loading algorithm when multiple coding rates as well as modulation schemes are used. Computer simulation confirms that the proposed VCR OFDM AMS gives sufficient throughput performances as an alternative to the OFDM AMS/MTPC.

Some key challenges remain to be overcome before spectrum sensing can be widely used to identify spectrum opportunities in the TV bands. To fulfill the strict sensing requirement specified by FCC, a comprehensive sensing algorithm, which produces high SNR gain and maintains sensing robustness under complex noise conditions, needs to be implemented. In addition, carefully designed physical features and improvement on cost performance ratio are also essential if a prototype is to be commercialized. To the best of our knowledge, no success has ever been announced in developing a sensing prototype that fulfills both FCC sensing requirement and the above mentioned features. In this paper, we introduce a recently developed sensing prototype for Japanese digital TV signals of ISDB-T. The prototype operates in the Japanese UHF TV band of 470-770MHz and can reliably identify presence/absence of an ISDB-T signal at the level of -114dBm in a 6MHz channel. Moreover, it has constrained size and weight, and is capable of accurately measuring power of an ISDB-T signal at an extremely low level. Efforts on reducing cost have also been made by avoiding the use of electronic components/devices of high price. Both laboratory and field tests are performed to evaluate its sensing performance and power measurement capability. In the laboratory test, sensing performance under conditions of adjacent channel interference and frequency offset, and power measurement accuracy, are checked. In field tests, the prototype is attached in a vehicle and is checked for its capability to identify the presence of purposely broadcasted ISDB-T signals at some fixed locations and also during movement of the vehicle.

In this letter, we investigate the performance of using subband adaptive loading for the combination of orthogonal frequency division multiplexing (OFDM) and adaptive antenna array. The frequency-domain adaptive loading is very effective to deal with the frequency-selective fading which is inevitable in broadband wireless communications. However, almost all of the existing adaptive loading algorithms are based on "subcarrier-to-subcarrier" mode which may results in awfully large signaling overhead, since every subcarrier needs its own signaling loop between the transmitter and receiver. We investigate the performance of the combination of OFDM and adaptive antenna array when a subband adaptive loading algorithm is used to decrease the signaling overhead. It is shown by simulation results that at the cost of some tolerable performance loss, the signaling overhead of adaptive loading can be greatly reduced.

We propose a wideband antenna that has both vertical and horizontal polarization to create access points with enhanced connectivity. The antenna is composed of a rectangular plate and a ground plate, and the rectangular plate is fed sideways from the ground plate. Its -10dB fractional bandwidth is approximately 162%. It is shown that the offset feed of the rectangular plate is important to attain wideband impedance matching and vertical polarized wave. The results of a parametric study to characterize the first- and second-lowest resonant frequencies are presented. Moreover, the behavior of the impedance matching and polarization is interpreted by dividing the current distribution around the feed port on the rectangular plate into the same direction current mode and the opposite direction current mode. The measured results for the return loss and the radiation pattern of a prototype antenna agree well with the simulation results, therefore the wideband property was experimentally confirmed.

This paper presents the summarized achievements of "Study Group on Software Technology for Radio Equipment" held at TELEC from April 2000 to March 2003. The Study Group specified the essential issues on Software Defined Radio (SDR), and discussed desirable methods to evaluate conformity to technical regulations in radios that can change RF characteristics only by changing software. The biggest objective in SDR is to build the architecture to allow users to install software exclusively in the combination of hardware and software that have passed the certification test. The Study Group has reached a solution by introducing the idea of "tally." This paper explains the concept of tally, and proposes two types of systems to use tallies in checking adaptability in combinations of hardware and software.

Pilot-symbol-aided (PSA) channel estimation for OFDM wireless access systems enables the periodic estimation of channel frequency response by generating reference data from the received OFDM signals. The accuracy of this channel estimation can be improved through the average over a certain time period in each subcarrier-channel. However, the accuracy of the channel estimates by the average degrades as the Doppler shift is large due to a decrease in the average section size according to the Doppler shift for the tracking of the time-varying channel. This paper proposes a novel PSA channel estimation method to mitigate the influence of the noises and interferences. This method detects the channel estimates affected by the noises and interferences, and then removes them before the arithmetic or harmonic averaging to avoid propagating the influence of the noises and interferences. This paper also evaluated the proposed channel estimation method by clipping log-likelihood ratio (LLR) data to inspect the influence of the channel estimation on the LLR calculation by computer simulation.

We have developed a code-division-multiplexing (CDM) transmission scheme for future road-vehicle communication systems, which uses cyclic shifted-and-extended (CSE) codes generated from a basic code with superior auto-correlation characteristics. This paper proposes to use a Quasi-Orthogonal (QO) sequence as the basic code. Its auto-correlation values are zero except at zero and middle shifts. When the CDM transmission is performed by the CSE codes based on the QO sequence, a desired correlation value is, at a receiver, interfered by the auto-correlation value at middle shift. Therefore, an elimination technique for the interfered correlation value is proposed and realizes zero cross-correlation characteristics within the cyclical shift interval. The new CDM transmission scheme based on the proposed scheme is evaluated by computer simulations in terms of the bit-error-rate performance.

This paper investigates primary signal detection by using a quiet period (QP) in cognitive wireless communications. In particular, we provide an analytical model for studying the impact of QPs on the system performance. Our analysis shows that two successive QPs have a significant impact on system performance. Moreover, the analytical results obtained reveal an optimum period of two successive QPs that maximize system performance.

This paper addresses the coexistence issue of distributed heterogeneous networks where the network nodes are cognitive radio terminals. These nodes, operating as secondary users (SUs), might interfere with primary users (PUs) who are licensed to use a given frequency band. Further, due to the lack of coordination and the dissimilarity of the radio access technologies (RATs) among these wireless nodes, they might interfere with each other. To solve this coexistence problem, we propose an architecture that enables coordination among the distributed nodes. The architecture provides coexistence solutions and sends reconfiguration commands to SU networks. As an example, time sharing is considered as a solution. Further, the time slot allocation ratios and transmit powers are parameters encapsulated in the reconfiguration commands. The performance of the proposed scheme is evaluated in terms of the coexistence between PUs and SUs, as well as the coexistence among SUs. The former addresses the interference from SUs to PUs, whereas the latter addresses the sharing of an identified spectrum opportunity among heterogeneous SU networks for achieving an efficient spectrum usage. In this study, we first introduce a new parameter named as quality of coexistence (QoC), which is defined as the ratio between the quality of SU transmissions and the negative interference to PUs. In this study we assume that the SUs have multiple antennas and employ fixed transmit power control (fixed-TPC). By using the approximation to the distribution of a weighted sum of chi-square random variables (RVs), we develop an analytical model for the time slot allocation among SU networks. Using this analytical model, we obtain the optimal time slot allocation ratios as well as transmit powers of the SU networks by maximizing the QoC. This leads to an efficient spectrum usage among SUs and a minimized negative influence to the PUs. Results show that in a particular scenario the QoC can be increased by 30%.

We have been investigating an orthogonal frequency division multiple access (OFDMA) based cellular system that is called "dynamic parameter controlled orthogonal frequency and time division multiple access (DPC-OF/TDMA)" for the development of beyond third generation (B3G) mobile communication systems. Moreover, we have already proposed a time alignment control (TAC) to compensate propagation delays that induce a multiple-access interference (MAI) in the uplink OFDMA. However, that TAC includes a large amount of computations. This means that it is quite difficult for the OFDMA systems to implement TAC into volume-limited hardware devices such as field programmable gate array (FPGA). Thus, we propose a new complexity-reduced TAC (CRTAC) in this paper. CRTAC can be implemented into such devices easily. In this paper, we show some computer simulation results, and then evaluate the error rate performances of DPC-OF/TDMA employing CRTAC. Moreover, we also show the benefit of the reasonable level of the implementation complexity made by CRTAC.