Temporal and spatial (geographical) fluctuations, which are present in the traffic of wireless sensor networks (WSNs), have a significant affect on the transmission performance and power consumption of WSNs. Several medium access control (MAC) mechanisms have been proposed for IEEE802.15.4 cluster-based WSNs to counter the temporal and spatial traffic fluctuations. However, these mechanisms cannot always achieve simultaneous improvement in both transmission performance and power consumption. In this paper, we propose two enhanced 2-level active period control mechanisms, BI&CAP control and BI&SD&CAP control, to achieve higher system performance than conventional control mechanisms. Various computer simulation results demonstrate the effectiveness of the proposed mechanisms for WSNs with various traffic fluctuations.

Traffic adaptive 2-level active period control has been proposed to enhance system performance in cluster-based wireless sensor networks (WSNs) employing IEEE 802.15.4 medium access control (MAC) under temporal and spatial (geographical) non-uniform traffic environments. This paper proposes an adaptive method of controlling the backoff window for traffic adaptive 2-level active period control. The proposed method adjusts the size of the backoff window according to the length of the current active period, which is determined by 2-level active period control, and the time position for channel access in the active period. The results evaluated through computer simulations reveal that the proposed method can improve throughput as well as achieve high energy efficiency in cluster-based WSNs with non-uniform traffic distributions.

The traffic adaptive 2-level active period control has been proposed as a traffic adaptation mechanism to handle temporal and spatial (geographical) traffic fluctuations in cluster-based wireless sensor networks (WSNs) employing IEEE802.15.4 medium access control (MAC). This paper proposes a traffic adaptive distributed backoff control mechanism for cluster-based WSNs with the traffic adaptive 2-level active period control to enhance the system performance, especially transmission performance. The proposed mechanism autonomously adjusts the starting time of the backoff procedure for channel accesses in the contention access period (CAP) specified by the IEEE802.15.4 MAC, and then distributes the channel access timing over a wide range within the CAP, which can mitigate channel access congestion. The results of computer simulations show that the proposed mechanism can improve the transmission delay performance while keeping the enhancement in throughput and energy consumption at the cluster-based WSNs under non-uniform traffic environments.

The main disadvantage of orthogonal frequency division multiplexing (OFDM) is the high time domain PAPR. The larger PAPR signal would fatally degrade BER performance in non-linear channels. This paper proposes an improved DSI method, which can achieve better PAPR and BER performances in the non-linear channel with less computation complexity than the conventional DSI method. The feature of proposed method is to employ the time-frequency domain swapping algorithm in the determination of frequency data for dummy sub-carriers. This paper presents various computer simulation results to verify the effectiveness of proposed DSI method.

This paper proposes a medium access control (MAC) mechanism for the recently developed IEEE 802.15.4 standard, a promising candidate to become the physical (PHY) and MAC layer standard for Wireless Sensor Networks (WSNs). The main concern in WSNs is the energy consumption, and this paper presents a mechanism that adapts properly the duty cycle operation according to the traffic conditions. Various traffic adaption mechanisms have been presented for the MAC layer of the IEEE 802.15.4. However these conventional mechanisms only consider the temporal traffic fluctuations. The proposed mechanism outperforms the conventional mechanism when applied to cluster-tree based WSNs, because it considers not only the temporal fluctuations but also the spatial (geographical) fluctuations, which are intrinsic characteristics of traffic in WSNs with the cluster tree topology. Evaluations showed that the proposed mechanism achieves less energy consumption than the conventional traffic adaptation mechanism, with maintaining almost the same transmission performance.

This paper proposes a multicast delivery system using base station diversity for cellular systems. Conventional works utilize single wireless link communication to achieve reliable multicast. In cellular systems, received signal intensity declines in cell edge areas. Therefore, wireless terminals in cell edge areas suffer from many transmission errors due to low received signal intensity. Additionally, multi-path fading also causes dynamic fluctuation of received signal intensity. Wireless terminals also suffer from transmission errors due to the multi-path fading. The proposed system utilizes multiple wireless link communication to improve transmission performance. Each wireless terminal communicates with some neighbor base stations, and combines frame information which arrives from different base stations. Numerical results demonstrate that the proposed system can achieve multicast data delivery with a short transmission period and can reduce consumed wireless resource due to retransmission.

We propose a new analytical model of Transmission Control Protocol (TCP) in wireless environments where transmission errors occur frequently. In our proposed model, we consider the exponential increase of a congestion window and the exponential increase of a timeout back-off. Finally, we have clarified the behavior of TCP mechanisms of different versions and TCP throughput characteristics analytically. From our result, the behavior of TCP mechanisms is different in each implementation version. These differences mean that the required characteristics of wireless links are different in each implementation version. Therefore, our proposed model is a base analysis of designing wireless link mechanisms.

This paper proposes adaptive transmit window control based on both location of mobile stations and traffic load for channel state based packet transmissions in CDMA cellular downlink communications. The proposed scheme constrains downlink packet transmissions by employing a transmit window individually given to each mobile station. The transmit window size is adjusted by using the optimum threshold value, which is selected with regard to both the mobile locations and the traffic load. The simulation results show that the proposed scheme improved the transmission delay and fairness of service compared with the conventional scheme.

Software defined radio, which uses reconfigurable signal processing devices, requires the determination of multiple unknown parameters to realize the potential capabilities of adaptive communication. Evolutional algorithms are optimal multi dimensional search techniques, and are well known to be effective for parameter determination. This letter proposes an evolutional algorithm for learning the mobile time-varying channel parameters without any specific assumption of scattering distribution. The proposed method is very simple to realize, but can provide precise channel estimation results. Simulations of an OFDM system show that for an example of OFDM communication under the time-varying fading channel, the proposed learning method can achieve the better BER performance.

Multi-carrier code division multiple access (MC-CDMA) has been considered as one of the promising techniques for the next generation of mobile communication systems because of its efficient bandwidth usage, robustness to the multi-path fading and simple channel-sharing scheme. However, MC-CDMA cannot be employed in the uplink communication where the transmitted signal from each user propagates through the different multi-path fading channel, and the received signals are no longer orthogonal at the base station. As a result, bit error rate (BER) performance in the uplink MC-CDMA communication would be strongly degraded due to the occurrence of multi-user interference (MUI). To solve the MUI problem in the uplink MC-CDMA, the pre-equalization method was proposed in which the uplink signal is pre-equalized at the user terminal by using the channel response estimated from the downlink. Although the pre-equalization method is very effective for the stationary uplink channel with fixed users, it is hard to be employed in the time varying fading channel with mobile users, because there is a big difference in the channel responses between downlink and uplink. For the efficient MUI compensation, each user terminal would be required to predict the future channel conditions based on the current observation. This paper proposes a method for model based uplink channel response prediction by employing the spectral decomposition of the downlink channel impulse response. Computer simulation results show that the proposed method can achieve the accurate prediction of channel response for mobile users during the uplink transmission and allows the effective MUI compensation.

In this letter we investigated the packet transmission control in downlink CDMA cellular systems. The downlink packet transmission control scheme based on the soft handoff status was proposed to enhance the system performance. The proposed scheme controls the downlink packet transmissions by employing a transmission window which is individually resolved to each mobile station according to its propagation condition and soft handoff status. Computer simulation shows that compared with the conventional scheme the proposed scheme improved the delay performance and fairness of service in packet reception.

This paper proposes a resource allocation method based on TCP (Transmission Control Protocol) throughput for base station diversity systems. A goal of this study is to achieve high throughput wireless Internet access by utilizing multiple wireless links effectively. The conventional work showed that base station diversity techniques can improve TCP performance. However, the improvement depends on the wireless environment of the wireless terminal. The proposed resource allocation method allocates wireless links to a wireless terminal based on its estimated TCP throughput and current traffic of each base station. Our method can take account of some network protocols such as TCP and UDP (User Datagram Protocol) by measuring the current traffic of each base station. In addition, wireless links are preferentially assigned to the wireless terminal that has the largest performance improvement per wireless link. Therefore, the proposal provides better overall system performance than the previous technique.

This paper proposes an asymmetric traffic accommodation scheme using a multihop transmission technique for CDMA/FDD cellular communication systems. The proposed scheme exploits the multihop transmission to downlink packet transmissions, which require the large transmission power at their single-hop transmissions, in order to increase the downlink capacity. In these multihop transmissions, vacant uplink band is used for the transmissions from relay stations to destination mobile stations, and this leads more capacity enhancement in the downlink communications. The relay route selection method and power control method for the multihop transmissions are also investigated in the proposed scheme. The proposed scheme is evaluated by computer simulation and the results show that the proposed scheme can achieve better system performance.

The traffic with asymmetry between uplink and downlink has recently been getting remarkable on mobile communication systems providing multimedia communication services. In the future mobile communications, the accommodation of asymmetric traffic is essential to realize efficient multimedia mobile communication systems. This paper discusses asymmetric traffic accommodation in CDMA/FDD cellular packet communication systems and proposes its efficient scheme using an adaptive cell sizing technique. In the proposed scheme, each base station autonomously controls its coverage area so that almost the same communication quality can be achieved across the service area under the asymmetric traffic conditions. We present some numerical examples to demonstrate the effectiveness of the proposed scheme by using computer simulation. The simulation results show that, under asymmetric traffic conditions, the proposed scheme can provide fair communication quality across the service area in both links and can improve total transmission capacity in the uplink.

This paper proposes two complexity reduced Maximum Likelihood Detection (MLD) methods for Space Division Multiplexing--OFDM (SDM-OFDM) system to exploit the spatial diversity so as to achieve the improved transmission quality. The proposed methods enable to outperform the other suboptimal detection methods and achieve near MLD performance with a significant reduction in calculation complexity. The various computer simulation results confirm that the proposed methods could realize the above targets and might be promising solution in practical systems.

This letter proposes to employ the Discrete Cosine Transform (DCT) interpolation-based channel estimation (DCTI-CE) method for practical MIMO-OFDM (Multiple Input Multiple Output--Orthogonal Frequency Division Multiplexing) system so as to achieve its potential ability for enabling high transmission data rate. The various computer simulations are conducted to evaluate the DCTI-CE method in MIMO-OFDM system under the multipath fading channel for wireless LANs.

Many applications of OFDM systems require Doppler spread estimation. This is quite difficult in multi-path fading channels with no strong direct path. This letter proposes a novel Doppler spread estimation method, which uses the mean square (MS) value of channel impulse response's time derivative. The proposed method is very simple compared with the previously proposed methods. Simulation results show that it allows easy and precise Doppler spread calculation for OFDM by using the channel estimation based on either pilot tones or pilot symbols.