In this paper, two receive diversity combining techniques are proposed for cooperative relay systems based on single-carrier frequency division multiple access (SC-FDMA) when DFT spreading sizes for a mobile station (MS) and relay station (RS) are different. A simplifled-MRC (S-MRC) technique performs diversity combining in the time-domain by using the estimated channel weights and initial estimates obtained by SC-FDMA signal detector. An interference rejection-MRC (IRMRC) technique performs diversity combining in the frequencydomain by adjusting DFT spreading size at the receiver. It is shown by computer simulation that the proposed receive combining techniques achieve a significant diversity gain over the conventional ones.

In this paper, a new handover procedure for OFDM-based multi-hop relay systems is proposed to reduce handover overhead by distinguishing an inter-cell handover event from an intra-cell handover event at the level of the physical layer using a preamble with a hierarchical design. A subcell ID concept that identifies RS in a cell is proposed in the hierarchical design that works in conjunction with the existing cell ID that identifies BS. The proposed handover procedure can simplify the scanning procedure and skip/simplify the network re-entry procedure, resulting in a significant reduction of handover overhead.

In spatially multiplexed MIMO systems that enable high-data-rate transmission over wireless communication channels, spatial demultiplexing at the receiver is a challenging task. Thus, various demultiplexing methods have been developed by many researchers. In this paper, a novel signal detection method that achieves near-ML performance in a computationally efficient manner is proposed. The proposed method is based on the well-known minimum mean squared error (MMSE) ordered successive interference cancellation (OSIC) method. However, unlike the conventional MMSE-OSIC, a number of candidates are selected at each layer, hence, it has been termed MMSE-OSIC with candidates (MMSE-OSIC2). In this study, it is demonstrated via a series of computer simulations that the MMSE-OSIC2achieves near-ML performance while requiring a complexity comparable to that of the conventional MMSE- OSIC. Also shown here is that the log likelihood ratio (LLR) values are successfully generated via the proposed method.

In this paper, a decision-directed expectation maximization (DEM) algorithm is proposed to improve the performance of channel estimation in OFDM-based cellular systems. The DEM algorithm enables a mobile station (MS) with multiple antennas, located at the cell boundary, to increase the performance of channel estimation using transmit data, without decreasing spectral efficiency. Compared with the expectation maximization (EM) algorithm, the DEM algorithm can greatly reduce computational complexity while improving the performance of channel estimation in fast fading channels.

A novel multiuser scheduling and feedback strategy for the multiple-input multiple-output (MIMO) downlink is proposed in this paper. It achieves multiuser diversity gain without substantial feedback requirements. The proposed strategy uses per-antenna scheduling at the base station, which maps each transmit antenna at the base station (equivalently, a spatial channel) to a user. Each user has a number of receive antennas that is greater than or equal to the number of transmit antennas at the base station. Zero-forcing receivers are deployed by each user to decode the transmitted data streams. In this system, the base station requires users' channel quality on each spatial channel for scheduling. An opportunistic feedback protocol is proposed to reduce the feedback requirements. The proposed protocol uses a contention channel that consists of a fixed number of feedback minislots to convey channel state information. Feedback control parameters including the channel quality threshold and the random access feedback probability are jointly adjusted to maximize the average throughput performance of this system. Multiple receive antennas at the base station are used on the feedback channel to allow decoding multiple feedback messages sent simultaneously by different users. This further reduces the bandwidth of the feedback channel. Iterative search algorithms are proposed to solve the optimization for selection of these parameters under both scenarios that the cumulative distribution functions of users are known or unknown to the base station

TDD mode is considered as a key solution to cope with the asymmetric traffic services. With the TDD mode, however, the inter-BS/inter-MS interference becomes serious in the cross time slot (CTS) region and this interference seriously degrades the system performance. In this paper, we propose a region and time partitioning dynamic TDD (RTP D-TDD) for OFDM systems. To mitigate the inter-BS/inter-MS interference in the CTS region, a cell is divided into several regions and each time slot in the CTS region is split into minislots by the number of regions in the cell. Each minislot is assigned to the users in its corresponding region. We also propose a time slot allocation based on region and time partitioning (TRTP) under the proposed RTP D-TDD system. In the TRTP, the influence of the CCI in the CTS region is further reduced by assigning the minislots closer to the switching point to the users having higher link gains. By the computer simulation, it has been verified that the proposed TRTP outperforms previous time slot allocation schemes in both the outage probability and the bandwidth efficiency.

At present, mobile multimedia services such as sports information and mobile TV services have been delivered over point-to-point connections. But it is clear that the point-to-multipoint connections can support these mobile multimedia services more efficiently because they can simultaneously transmit data packets from a single source to multiple destinations in a multicast group. Recently, the simulcast scheme for multimedia broadcast and multicast service (MBMS) in which the same signal is transmitted from all the cells within the system, has been proposed, but, in order to adopt this technology, the same channel bands or time slots for only broadcast/multicast service should be reserved at all cells. The reservation of these resources may not be suitable for services in a local region, and some operators may also not be easy to reserve these resources. So, in this paper, we propose multicast strategies which make multicast resources easily multiplex with unicast resources and enhance cell-capacity by improving the performance of cell-boundary users. In addition, we mathematically analyze the link-level performance and cell throughput of conventional and proposed schemes, and discuss their numerical results

The TDD mode is considered as a key solution to support the asymmetric traffic for multimedia services. With the TDD mode however, the intercell interference becomes serious in the cross time slot (CTS) period and this interference seriously degrades the system performance. In this paper, we propose a minislot borrowing scheme on top of the region and time partitioning dynamic TDD (RTP D-TDD) for OFDM systems. To mitigate the inter-BS/inter-MS interference in the CTS period, each time slot in the CTS period is split into minislots, and a cell is divided into several regions by the number of minislots in a time slot in the CTS period. In the MBS systems users in the overloaded region borrow minislots which are estimated with the lowest CCI in addition to the corresponding minislots. By the computer simulation, it has been verified that the proposed MBS with TRTP outperforms conventional time slot allocation schemes in both the outage probability and the bandwidth efficiency.

This paper discusses a spectrum sharing problem in multiple network operators (NOs). Most of the previous works have illustrated the dynamic spectrum allocation (DSA) gain achieved by utilizing the time or regional variations in traffic demand between NOs. In this paper, we introduce the functionalities required for the entities related with the spectrum allocation and propose a spectrum allocation algorithm. In the proposed spectrum allocation algorithm, the priority between NOs and the priorities of multiple class services are incorporated into the spectrum sharing metric (SSM), while also accommodating the urgent bandwidth request by proposing a negotiation procedure. It is shown that the proposed scheme allocates the spectrum dynamically, reflecting the long-term occupation ratio between NOs and the priorities of multiple class services

Interchannel interference (ICI) is the key problem in wireless MIMO (multiple-input multiple-output) transmission. Numerous ICI reduction algorithms are reported in literature. It is common to these ICI reduction algorithms that they require considerable signal processing power. In this paper, a new approach is presented which avoids ICI and, thus, results in low complexity receivers whilst maintaining high spectral efficiency. It is demonstrated that this can be achieved for BPSK (binary phase shift keying) and QPSK (quadrature phase shift keying) modulation by compressing the symbols prior to transmission and by exploiting spatial information at the receiver. The V-BLAST (Vertical Bell Labs Layered Space-Time) algorithm and simple, low complexity MMSE (minimum mean squared error) and zero-forcing (ZF) detection without ICI reduction was used for comparison. It is shown that the new MIMO approach outperforms V-BLAST by about 6.5 dB at a signal-to-noise-ratio (SNR) of 15 dB while it outperforms direct MMSE detection by about 12 dB. In addition, it was found that the spectral efficiency compared to SISO (single-input single-output) is improved by a factor of 4 at about the same bit-error performance

A hybrid cellular and ad hoc network for high data rate wireless access is considered. An air-interface which uses TDD in combination with time division multiple access (TDMA) is assumed. Interference and resource reuse are important issues in hybrid networks. For this purpose analytical models for the reuse efficiency and a capacity bound of such a network are developed. These models are based on an exclusion range concept which is introduced to keep interference at an acceptable level. It is found that with this interference limiting measure, multi hop networks can benefit from a multi hop reuse gain compared to single hop networks. However, it is further found that this gain does not significantly increase for more than 5 hops. Furthermore, the pre-requisite for being able to exploit the multi hop reuse gain is a certain TS granularity. It was observed that with an increasing number of TS per frame, the mean and the variance of the number of supported hops increased almost linearly