Tsuyoshi Kashima

Adaptive MAP receiver via the EM algorithm and message passings for MIMO-OFDM mobile communications

This paper proposes two new types of maximum a posteriori probability (MAP) receivers for multiple-input-multiple-output and orthogonal frequency-division multiplexing mobile communications with a channel coding such as the low-density parity-check code. One proposed receiver employs the expectation-maximization algorithm so as to improve performance of approximated MAP detection. Differently from a conventional receiver employing the minimum mean-square estimation (MMSE) algorithm, it applies the recursive least squares (RLS) algorithm to the channel estimation in order to track a fast fading channel. For the purpose of further improvement, the other proposed receiver applies a new adaptive algorithm that can be derived from the message passing on factor graphs. The algorithm exploits all detected signals but one of targeted time, and can gain a considerable advantage over the MMSE and RLS. Computer simulations show that the first proposed receiver is superior in channel-tracking ability to the conventional receiver employing the MMSE. Furthermore, it is demonstrated that the second proposed receiver remarkably outperforms both the conventional and the first proposed ones.

MIMO System with Relative Phase Difference Time-Shift Modulation in Rician Fading Environments

In line-of sight (LOS) environments, the performance of Multiple-Input Multiple-Output (MIMO) systems depends largely on the difference of the phase difference of direct paths from transmit antennas to each receive antenna. When the phase difference of direct paths are close to each other, the spatial division multiplexing (SDM) channel is not orthogonal to each other so that the signal detection becomes difficult. In this paper, we propose a MIMO system with relative phase difference time-shift modulation (RPDTM) in Rician fading environments. The proposed scheme transmits independent signals from each antenna at each time slot where relative phase difference between signal constellations used by transmit antennas varies with a pre-determined pattern. This transmission virtually changes the phase difference of direct paths from transmit antennas to each receive antenna without lowering data rate and without knowledge of the channels. In addition, forward error correction coding (ECC) is applied to exploit the time slots where the receiver can detect the signals easily to improve the detection performance. From the results of computer simulation, we show that MIMO system with RPDTM can achieve the better bit error rate (BER) than the conventional MIMO system. We also show that the MEMO system with RPDTM is effective by about Rician factor K = 10 dB.

Size Compatible (SC)-Array LDPC Codes

Array low density parity check (LDPC) codes are high-rate codes that can achieve good error rate performance in additive white Gaussian noise (AWGN) channels. However, array LDPC codes do not support arbitrary code lengths, because the code length of an array LDPC code with good error rate performance is limited to a multiple of a prime number. This paper proposes Size Compatible (SC)-array LDPC codes; they achieve good error rate performance while supporting arbitrary code lengths. We conduct computer simulations to evaluate the block error rate (BLER) performance of SC-array LDPC codes in AWGN channels. We also evaluate the corresponding performance of SC-array LDPC coded multiband OFDM systems for data transmission rates over 1 Gbps in the UWB multipath channel model CM 3. We show that if the submatrix size is a non- prime number, SC-array LDPC codes achieve better error rate performance than the conventional array LDPC codes in AWGN channels. We also show that the SC-array LDPC codes achieve better error rate performance than the punctured array LDPC codes in AWGN channels. We also show that if the submatrix size is a prime number, SC-array LDPC codes achieve the same error rate performance as conventional array LDPC codes in AWGN channels. Moreover, we show that for multiband OFDM systems with data transmission rates of over 1 Gbps, SC-array LDPC codes achieve better error rate performance than conventional array LDPC codes.

MIMO System with Relative Phase Difference Time-Shift Modulation for Rician Fading Environment

Multiple-Input Multiple-Output (MIMO) systems that realize high-speed data transmission with multiple antennas at both transmitter and receiver are drawing much attention. In line-of sight (LOS) environments, the performance of MIMO systems depends largely on the difference of the phase difference of direct paths from transmit antennas to each receive antenna. When the phase difference of direct paths are close to each other, the spatial division multiplexing (SDM) channels are not orthogonal to each other so signal detection becomes difficult. In this paper, we propose a MIMO system with relative phase difference time-shift modulation (RPDTM) in Rician fading environments. The proposed scheme transmits independent signals from each antenna at each time slot where the relative phase difference between signal constellations used by transmit antennas varies in a pre-determined pattern. This transmission virtually changes the phase difference of direct paths from transmit antennas to each receive antenna without lowering data rate and without knowledge of the channels. In addition, forward error correction coding (ECC) is applied to exploit the time slots where the receiver can detect the signals easily to improve the detection performance. If there are time slots where the receiver can separate the received signal, the receiver can decode the data by using the time slots and the correlation between data. From the results of computer simulation, we show that MIMO system with RPDTM can achieve the better bit error rate (BER) than the conventional MIMO system. We also show that the MIMO system with RPDTM is effective by about Rician factor K=10dB.

Controlling Network Topology in Forming Bluetooth Scatternet

Topology of a network greatly affects the network performance. Depending on the purpose of a network, a specific topology may perform much better than any other topologies. Since the ad hoc networks are formed for a specific purpose, determining, and constructing the network topology based on the application requirements will enhance system performance. This paper proposes Bluetooth scatternet forming protocol in which the network topology is determined by three parameters. The parameters affecting the topology are the number of maximum slaves in a piconet, the number of maximum piconets that a gateway Bluetooth device can service, and the number of loops needed in the formed scatternet. These parameters can be read from a script file prior to the network formation. This process of reading the important parameters from the file would give users freedom in determining the network topology. The proposed protocol also includes a role negotiation process to accommodate different capabilities of the participating devices. The negotiation process of the protocol allows the resource-limited nodes to participate in the network. Different types of scatternet topologies like star, mesh, ring and line can be formed by specifying the parameters. This paper also discusses theoretical information necessary for calculating network topologies in detail. The protocol is verified with help of simulations, and implementations using commercially available Bluetooth devices. The detailed results are also presented in this paper.

Load balancing effect of inter-frequency handover with pilot power tuning in UTRAN

Load balancing is an important technique in radio resource management (RRM). In future, when several frequencies are required to cover an increasing number of users and an increasing amount of traffic, inter-frequency handover (IFHO) will perform an important role in load balancing between frequencies. Meanwhile, there is usually a geographic load imbalance in real networks and changing coverage by tuning the common pilot channel (CPICH) power can mitigate this geographic load imbalance. Using a dynamic WCDMA simulator, we evaluate RRM techniques combining IFHO and CPICH power tuning for load balancing and for achieving better performance. First, we compare the IFHO performances in terms of the IFHO trigger types. We show that the CPICH E/sub c//N/sub o/ (code energy divided by noise energy)-triggered IFHO can achieve 30-40 % downlink (DL) load decreases in a hotspot area. Next, we analyze the relation between the IFHO and CPICH power tuning, by comparing the effect of CPICH power tuning with and without the IFHO function and show its performance by using the simulator. Then, finally, we propose an example procedure to utilize the combination of IFHO and CPICH power tuning based on the revealed relation between them.

Impact of topology on Bluetooth Scatternet

To utilize Bluetooth as a layer 2 multi‐hop subnet for global IP network, there are two missing protocols in current Bluetooth stack: network formation and routing. In this paper, we propose our network formation and routing protocols optimized for such Bluetooth subnset usage scenarios to fill this gap. In this paper, we first present some observations on performance degradations of Bluetooth PAN due to network topologies and topology change when radio independant protocols are implemented. And then we analyze the reason of performance degradation. Based on our analysis, we first propose a flexible scatternet formation algorithm under conference scenario for multi‐hop communication. By using proposed method, scatternet can be formed flexibly with star, mesh, or linked line based on several parameters like number of maximum piconets that a gateway Bluetooth device can participate, and whether loops are needed in the resulting scatternet to achieve better network performance. To utilize topology information in multi‐hop communication, we propose a Cross‐layer Optimized Routing for Bluetooth (CORB) CORB is a QoS‐extended AODV routing protocol with mainly two optimizations between networking layer and underlying Bluetooth MAC layer. The first optimization is to use a new load metric. (LM) in QoS routing protocol instead of number of hops in conventional best effort routing. LM is derived from estimation of nodes’ link bandwidth, which reflects the different roles of nodes in Bluetooth scatternet. This proposal helps routing protocol to bypass heavily loaded nodes, and find route with larger bandwidth. The second optimization is to adjust LM and some MAC layer parameters in response to the unstable network topology caused by movement and change of indoor radio condition. Finally, We present some simulation and experiment results based on implementation, which prove the effectiveness of our protocols.

Iterative-MAP adaptive detection via the EM algorithm for LDPC-coded MIMO-OFDM mobile communications in fast fading channels

This paper proposes an iterative maximum a posteriori probability (MAP) receiver for multiple-input-multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM) mobile communications. The receiver employs the expectation maximization (EM) algorithm so as to improve the performance of the approximated MAP detection, and applies adaptive algorithms such as the recursive least squares (RLS) and least mean square (LMS) algorithms to the channel estimation used in the EM algorithm in order to track a fast fading channel. For exploiting the space, time and frequency diversity, the low-density parity-check code (LDPC) is used as a channel coding with a built-in interleaver. Computer simulations under fast fading conditions show that the proposed receiver using the LMS algorithm can achieve better performance than the conventional minimum-mean-square-error (MMSE) channel estimation with much less computational complexity, and that the receiver using the RLS algorithm outperforms the ones with either the MMSE or the LMS algorithm.

Topology-controllable scatternet formation method and its implementation

This paper proposes a method to form a Bluetooth scatternet under the conference scenario. The principles and formulas for calculating network topologies are discussed in detail. The method is proved by implementation using commercially available Bluetooth devices. The scatternet topology can be star, mesh, ring and linked line, which is controlled by changing parameters like the number of maximum slaves in a piconet, the number of maximum piconets that a gateway Bluetooth device can participate, and whether loops are needed in the resulting scatternet. In this paper, we also present a lot of testing results from forming the real scatternet.

A Low Complexity Tree-Structure Based User Scheduling Algorithm for Up-Link Multi-User MIMO Systems

The paper describes a low complexity tree-structure based user scheduling algorithm in an up-link transmission of MLD-based multi-user multiple-input multiple-output (MIMO) wireless systems. An M-branch selection algorithm, which selects M most-possible best branches at each step, is proposed to maximize the whole system sum-rate capacity. To achieve the maximum capacity in multi-user MIMO systems, antennas configuration and user selection are preformed simultaneously. Then according to the selected number of antennas for each user, different transmission schemes are also adopted. Both the theoretical analysis and simulation results show that the proposed algorithms obtain near optimal performance with far low complexity than the full search procedure.