Utilization of distributed antennas provides an additional dimension for performance enhancement in 4G and beyond cellular networks. Intelligent remote antenna allocation and practical transmission scheme are two most important aspects for the design of DAS. In this study, a sectorized distributed antenna structure is introduced and correspondingly, power adjusted beam switching as a practical transmission scheme is proposed with periodic beam switching and dynamic power allocation. Simulation results show that the proposed DAS structure and transmission scheme improve the capacity in all SINR regions especially with optimal power allocation.

We present a novel solution to the problem of handover between two different wireless technologies. We illustrate this solution with the example of LTE (Long Term Evolution) to CDMA (Code Division Multiple Access) handover. Current inter-technology handover can take approximately 6 to 7 seconds and hence can heavily impact the network performance. We assume that two receivers (primary and secondary) are used in LTE, and that the UE performs an inter-technology handover between LTE and CDMA. The basic concept is that the UE uses two receivers (which are needed in MIMO mode) to enhance the network capacity when at the cell center but switches the secondary receiver, to the other network, when at the network border to obtain system information, signal strength, and perform CDMA synchronization. This switching function can be controlled by either the network or the UE. In this paper, the switching function is evaluated for the network controlled scenarios and is compared with non-switched scenarios. This study focuses on the effects on diversity gain and network coverage. Simulation results show that the loss in diversity gain has minimal impact on the network performance and the inter-technology handover delay can be reduced to less than 1 second.

This letter proposes a probability-based scheme for combination of spectrum sensing information collected from cooperative cognitive radio users. The proposed scheme enables combination of both synchronous and asynchronous sensing information by utilizing the statistics of licensed band occupancy and is superior to conventional schemes in terms of detection performance.

To take advantage of time-varying spectrum opportunities, acognitiveradio(CR) network monitors the dynamic usage of the licensed band through cooperative spectrum sensing. In this paper, we propose a probability-based scheme for combination of spectrum sensing information collected from several CR users. Different from conventional cooperative spectrum sensing schemes that assume synchronous local sensing information, our scheme enables combination of both synchronous and asynchronous sensing information by utilizing the statistics of licensed band occupancy. In our scheme, the amount of information from each CR user is flexible and a simplified implementation is also feasible under a symmetrical case. Simulation results demonstrate that our scheme is robust and superior in terms of detection performance.

Spectrum hole (SH) is defined as a spectrum band that can be utilized by unlicensed users, which is a basic resource for cognitive radio (CR) systems. Most of existing contributions detect SHs by sensing whether a primary signal is present or absent and then try to access them so that the CR and primary users use the spectrum band either at different time slots or in different geographic regions. In this paper, we propose a novel scheme with relays or directional relays for CR users to exploit new spectrum opportunity, called spatial SH. It can provide higher spectrum efficiency by coexistence of primary and CR users at the same region, time, and spectrum band. In particular, when the spectrum opportunity of a direct link from a CR transmitter to a CR receiver does not appear, our scheme may still establish the communication through indirect links, i.e., other CR users act as relay stations to assist the communication by using other spatial domains. Furthermore, we analyze the successful communication probabilities of CR users and demonstrate that the spectrum efficiency can be considerably improved by our scheme.

Probability-based strategies are proposed in this letter to determine the optimal inter-sensing duration and power control for cognitive radio (CR). With utilization of the statistics of licensed band occupancy, appropriate inter-sensing duration is determined to capture the recurrence of spectrum opportunity in time when the licensed signal is detected, or to achieve the maximum spectrum efficiency under a certain level of interference with licensed communication when the licensed signal is declared absent. Transmit power is varied dynamically according to the non-interfering probability at each sample so as to increase the transmission rate and decrease the interference power.

In this paper, we propose a cognitive radio (CR) transmission scheme, which enables secondary users to coexist with primary users by exploiting spatial spectrum holes (SSHs) through directional antennas or antenna arrays with beamforming. To ensure reliable CR links and avoid interference to primary users, some CR users may act as relays . We investigate successful communication probability of CR users when this scheme is applied. We further demonstrate that the spectrum efficiency can be greatly improved by multiplexing CR links with directional transmission.

In this paper, detection timing and channel selection for periodic spectrum sensing are investigated to improve the performance of cognitive radio (CR) users. Designed to maximize the channel efficiency of CR users under a given level of interference with licensed users, the detection timing scheme utilizes the statistics of the licensed channel occupancy to determine the optimal starting point of each sensing action. A channel selection scheme in the multichannel multiuser environment is also proposed to specify which channel to detect for the upcoming sensing action based on detection timing. Numerical results demonstrate that our schemes considerably improve the overall channel efficiency while protecting the communication among licensed users.

Relaying can replace one transmission over a long distance with several transmissions over shorter distances, and the total transmission power of mobile stations thus can be lower in the case of relaying. As such, the relaying concept and protocols in a cellular environment have been much discussed in the literature recently. We mathematically analyze the idea of uplink relaying schemes, using a simple model that represents their essential functions. In order to simplify our performance analysis, we assume that the originator of a packet sets the maximum number of hops to the base station to be 2. Our results indicate that the performance benefits of relaying are significant

In this paper, we analyze the capacity of the forward link for CDMA systems with adaptive antenna arrays, considering soft handoffs. We take it into account that a mobile near a cell boundary can receive from two or more base stations and use MRC. For the capacity analysis we divide a cell into two regions: the region in which the soft handoff occurs and the one in which the handoff does not occur. Then, we compute the maximum number of users a base station can support under the constraint of acceptable outage probability. We assume that there is no limitation on the base station's transmission power, so a cell's capacity is limited by the interference between signals intended for different users in the same cell and in different cells.

Code collisions and packet corruption due to multiple access interference (MAI) are important problems in a receiver-oriented slotted code division multiple access (CDMA) system. Herein, we propose an enhanced distributed-queueing request update multiple access (DQRUMA)/multi-code(MC)-CDMA protocol with a variable request pool (VRP) as a new slotted MC-CDMA protocol designed to reduce request collisions and average total delays, and to improve the efficiency of receiver utilization in base stations.