Multiple Co-channel Users Research Articles

Analytical Modeling of NOMA-Based Mobile Edge Computing Systems With Randomly Located Users

Non-orthogonal multiple access (NOMA) has been treated as a promising technology to improve the computation offloading performance of mobile edge computing (MEC). A complete understanding of the impact of applying NOMA on MEC is essential for network operators to deploy MEC systems. In this letter, we develop a mathematical framework to analyze the impact of NOMA on offloading decisions in MEC, which captures the interaction among the offloading decisions of multiple co-channel users. Using stochastic geometry and order statistics, we derive the expression for the computation offloading probability of the $n$ -th ranked user in terms of serving distances in a NOMA-based MEC system. Numerical simulations are performed to verify the accuracy of our analysis and demonstrate the performance gain of applying NOMA on MEC.

Performance Analysis of Cooperative Underlay Spectrum Sharing System in Co-channel Interference Limited Environment

In this paper, performance analysis of an asynchronous distributed space time block coded system with optimum combining at the cognitive radio receiver (CR-Rx) has been done in a cooperative underlay cognitive Internet of Things network under the influence of multiple co-channel primary users (PUs) interferers. The transmit power strategy being employed at the CR-Tx considers the PIP (peak interference power) at the PU-Rx and peak transmit power constraints at the CR-Tx, respectively. We have derived the probability density function of the signal-to-interference ratio (SIR) at the destination for the proposed system. The system is numerically and mathematically analyzed in terms of average post processed SIR (APPSIR), ergodic capacity ($${\text{C}}_{\text{erg}}$$), average bit error rate (ABER) and outage probability. All the numerical outcomes are in corroboration with its simulation counterpart.

Cooperative Spatial Modulation for Cellular Networks

Spatial modulation (SM) is a unique single-stream, multiple-input multiple-output (MIMO) transmission technique. One key property of the SM is that a single transmit antenna is activated at any given time, which completely avoids inter-channel interference in a single-user scenario. In the context of multi-user cellular networks, inter-cell interference (ICI) mitigation becomes the main challenge for the SM. Network MIMO is a technique that employs precoding to enable ICI cancellation among neighbouring cells. However, the cell-edge users might still experience a high level of the ICI due to the significant channel attenuation. In this paper, we propose a novel cooperative scheme based on the SM, named cooperative SM (CoSM). The basic concept is to reschedule the transmit antennas of multiple base stations for multiple co-channel users, so as to maximize the antenna diversity gain. Unlike the traditional antenna selection for a single user, the involvement of multiple users might cause conflict due to the selection of the same antenna. An antenna rescheduling scheme (ARS) is thus proposed to address this issue. The performance of the ARS is theoretically analyzed, and closed-form expressions are derived for the probability distribution of the received SNR. Also, a novel three-tier cellular architecture is proposed to accommodate the CoSM within the context of the network MIMO. Results show that the CoSM can improve signal-to-interference-plus-noise ratio by up to 4 dB over the network MIMO. In addition, compared with spatial multiplexing using the same ARS, the CoSM can halve the energy consumption while achieving the same bit error rate.

Closed-form space-time channel blind estimation for space-time coded MC-CDMA sytems with uniform linear array

This paper proposes a closed-form joint space-time channel and Direction Of Arrival (DOA) blind estimation algorithm for space-time coded Multi-Carrier Code Division Multiple Access (MC-CDMA) systems equipped with a Uniform Linear Array (ULA) at the base station in frequency-selective fading environments. The algorithm uses an ESPRIT-like method to separate multiple co-channel users with different impinging DOAs. As a result, the DOAs for multiple users are obtained. In particular, a set of signal subspaces, every one of which is spanned by the space-time vector channels of an individual user, are also obtained. From these signal subspaces, the space-time channels of multiple users are estimated using the subspace method. Computer simulations illustrate both the validity and the overall performance of the proposed scheme.

Direct Decoder of Uplink Space-Time Block Coded MC-CDMA Systems

The uplink of a space-time block coded multicarrier modulation code division multiple access (MC-CDMA) system equipped with a uniform linear array (ULA) at the base station is studied. A blind decoder that provides closed-form solutions of both transmitted symbol sequences and directions of arrival (DOAs) for all active users in one macrocell is derived without the uplink space-time vector channel estimation. The decoder uses an ESPRIT-like method to separate multiple co-channel users with different impinging DOAs. As a result, the DOAs of multiple users are obtained. In particular, a set of signal spaces, every one of which is spanned by the coded symbol sequences of an individual user, are also obtained. From these signal spaces, the original symbol sequences of multiple users are estimated by exploiting the special structure of space-time block coding (STBC) in combination with the finite alphabet property of transmitted symbols. Performance of the proposed scheme is evaluated by extensive computer simulations.

Same cell co-channel interference reduction using multiple spatially distributed adaptive array systems

In this paper, a method is proposed whereby the interference between multiple co-channel users can substantially be reduced, compared to conventional arrays located in the cell center. The proposed technique employs planar multi-angle user discrimination. This is achieved by means of a distributed antenna array system, consisting of three antenna sub-arrays whose centers are spaced far apart compared to the operating wavelength. The sub-arrays are located at the edges of a regular hexagonal cell. Although the proposed configuration can be used in both the up- and down-link, this paper deals with the up-link only. Utilizing the least mean squares technique, the steering vector is optimized for the complete array to yield the best signal to interference ratio for all the co-channel users in the same cell. The signal to interference performance of the proposed system is compared to conventional antenna array systems, located at the center of a cell.

Multiuser channel estimation for detection of cochannel signals

Estimation of the channel impulse responses of multiple cochannel users is a key requirement of all multiuser detection and interference cancellation techniques, though little attention has been paid to the subject in the context of time-division multiple-access (TDMA) systems. This paper addresses a pilot-based MMSE technique for multiuser channel estimation in a TDMA system, and makes two key contributions. Firstly, it allows for time variation of the channels within and between training sequences, an essential feature in a multiuser environment even at moderate fading rates. Secondly, it addresses the design of training sequences for the multiple cochannel users.

Separation of cochannel signals under imperfect timing and carrier synchronization

We propose a complete receiver for separating cochannel signals in a time-division multiple-access type system when the timing and carrier synchronization are imperfect or only nominally accomplished. The receiver is all digital, data aided, assumes the presence of multiple antennas at the receiver, and has three functional components. The first part computes the timing offsets of the different cochannel users through a maximum likelihood search. The second part uses a bank of filters on an oversampled version of the baseband signals to obtain a new symbol rate sampled signal in which the different users are all synchronous but are still spatially mixed and rotated with unknown carrier offsets. The third part uses an extended Kalman-filter-based algorithm to estimate the parameters of this model and to separate and decode the transmitted signals of the different users. The performance of this receiver is demonstrated with simulations. This work is significant in the context of smart antenna technology, considered as a potentially significant improvement to wireless systems, since it is a complete solution encompassing flat fading, timing error, and frequency offsets for multiple cochannel users.

Experimental evaluation of smart antenna system performance for wireless communications

In wireless communications, smart antenna systems (or antenna arrays) can be used to suppress multipath fading with antenna diversity and to increase the system capacity by supporting multiple co-channel users in reception and transmission. This paper presents experimental results of diversity gain, interference cancellation, and mitigation of multipath fading obtained by using a smart antenna system in typical wireless scenarios. Also given are experimental results for the signal-to-interference ratio (SIR) of two moving users, comparing different beamforming algorithms in typical wireless scenarios. All of the experiments were performed using the 900-MHz smart antenna testbed at The University of Texas at Austin.

Multiuser detection in a horizontal underwater acoustic channel using array observations

We present a multisensor, multiuser receiver that is capable of operating in an underwater acoustic channel with severe multipath. For each active user, the receiver consists of a multi-input, single-output array processing filter followed by a single-channel adaptive equalizer. The array processing filter is chosen to maximize an averaged performance metric which measures reduction in the interference from multiple asynchronous cochannel users and the reduction in intersymbol interference caused by time spreading of the transmitted signal. The single-channel adaptive equalizer that follows the array processing filter eliminates the remaining intersymbol interference prior to hard symbol decisions. The division of labor between the array processing filter and single-channel equalizer reduces receiver complexity by allowing the array processing filter weights to be based on the fixed deterministic channel component and the single-channel equalizer to be based on the stochastic channel component. Receiver performance is demonstrated using data obtained from two shallow-water acoustic channels where two cochannel users are transmitting in shallow water at 18 and 30 nautical miles from the receiver array.