High Rate Users Research Articles

Resource Allocation-Based PAPR Analysis in Uplink SCMA-OFDM Systems

Sparse code multiple access (SCMA) is a non-orthogonal multiple access (NOMA) uplink solution that overloads resource elements (RE's) with more than one user. Given the success of orthogonal frequency division multiplexing (OFDM) systems, SCMA will likely be deployed as a multiple access scheme over OFDM, called an SCMA-OFDM system. One of the major challenges with OFDM systems is the high peak-to-average power ratio (PAPR) problem, which is typically studied through the PAPR statistics for a system with a large number of independently modulated sub-carriers (SCs). In the context of SCMA systems, the PAPR problem has been studied before through the SCMA codebook design for certain narrowband scenarios, applicable more for low-rate users. However, we show that for high-rate users in wideband systems, it is more meaningful to study the PAPR statistics. In this paper, we highlight some novel aspects to the PAPR statistics for SCMA-OFDM systems that is different from the vast body of existing PAPR literature in the context of traditional OFDM systems. The main difference lies in the fact that the SCs are not independently modulated in SCMA-OFDM systems. Instead, the SCMA codebook uses multi-dimensional constellations, leading to a statistical dependency between the data carrying SCs. Further, the SCMA codebook dictates that an UL user can only transmit on a subset of the available SCs. We highlight the joint effect of the two major factors that influence the PAPR statistics - the phase bias in the multi-dimensional constellation design along with the resource allocation strategy. The choice of modulation scheme and SC allocation strategy are static configuration options, thus allowing for PAPR reduction opportunities in SCMA-OFDM systems through the setting of static configuration parameters. Compared to the class of PAPR reduction techniques in the OFDM literature that rely on multiple signalling and probabilistic techniques, these gains come with no computational overhead. In this paper, we also examine these PAPR reduction techniques and their applicability to SCMA-OFDM systems.

Liberal method of access point selection in wireless mesh networks

IEEE 802.11-based wireless mesh networks (WMNs) are emerging as the promising technology to provide wireless broadband internet access. In order to access the internet via a pre-deployed WMN, a user has to associate with one of the access points (APs) present in the network. Received signal strength (RSS)-based association method defined by the IEEE 802.11 standard does not consider the channel conditions and the AP load into association process. This paper proposes a novel method of AP selection which considers the channel conditions, AP load and wireless mesh backhaul conditions. As lower-rate user association drops down the performance of higher-rate users already associated with the same AP, our method tries to minimise this negative impact. The proposed AP selection method is scalable and also independent of routing protocol and routing metric used in WMN. Through the simulations, we show that our method performs better than RSS-based AP selection method in terms of improving system throughput in WMNs.

An Adaptive Combiner Model for Multi-Rate Relaying Transmissions in Cognitive Systems

This paper proposes an adaptive combiner model that maximizes the performance of multi-rate cooperative relaying transmission in resources aware systems. Users’ cooperation is applied to smartly profit from the network resources to reach for efficient multi-rate transmissions. The low rate terminal uses a part of his bandwidth and power to relay the high rate user data side by side with his own data. This results in small performance degradation to the low rate user performance. The combiner model at the high rate terminal is carefully investigated for an optimized system performance and an adaptive combiner is proposed. The model calculates the weights adaptively for each bit based on a dynamic-blind method. The adaptive combiner is evaluated against the maximal-ratio combiner MRC and the equal gain combiner EGC to reach for the best model for the multi-rate relaying framework. Based on half-duplex mode, the probability of error performance is studied and compared for each combiner model under different channel conditions. The results show that the proposed adaptive combiner, characterized by its reliability and simplicity, leads to the best performance compared to the other models.

Performance Analysis of Single and Multirate FFH-OCDMA Networks Based on PSK Modulation Formats

In this paper, the performance of single and multirate fast frequency hopping optical code-division multiple-access (FFH-OCDMA) networks based on coherent modulation formats is addressed. The following modulation formats are assumed: binary and quadrature phase shift keying (BPSK and QPSK). For single rate networks, BPSK and QPSK are used as the modulation formats for all users. On the other hand, for multirate networks, BPSK and QPSK modulation formats are employed for low- and high-rate transmission users, respectively. For both networks, new bit error rate (BER) formalisms regarding the main deleterious source in OCDMA are derived, namely multiple-access interference. The mathematical formalism developed here accounts for any kind of users’ signal waveforms (which are accurately accounted for at the desired user’s receiver). As a result, it can be successfully used to carry out performance analysis of any OCDMA network. In this paper, it is shown that both low- and high-rate users can surprisingly achieve similar BER levels. In addition, it is demonstrated that incoherent on–off-keying-based networks are not good candidates for future single and multirate FFH-OCDMA implementations in this interference limited scenario. On the other hand, our results pave the way for BPSK- and QPSK-based networks, which are seen as potential candidates for next generation passive optical networks (NG-PON2), where higher data rates and a vast amount of users will be major network requirements.

An efficient bandwidth sharing strategy through users' cooperation for multirate networks

This paper adopts an efficient strategy that allocates the network resources to the users according to the transmission rate requirement. Based on cooperation and the use of variable length spreading codes, the terminals requiring high transmission rate are supported by larger bandwidth and power in order to enhance their quality of service. The proposed resource allocation strategy assigns the terminals with low transmission rate to cooperate with those with high transmission rate by dedicating a part of their bandwidth and power to either relay their data or send a part of it. This will result in a slight limitation on the performance of the cooperating terminals (low rate). Two proposed schemes are presented namely the relaying strategy and re‐allocation strategy. These strategies are evaluated against the commonly used rate‐matching algorithm which constrains the terminals to fairly share the bandwidth regardless of the transmission rate. The proposed receiver is simple and efficient and the utilized spreading codes provide signal separation and limit multiple access interference MAI. In a CDMA‐based framework supporting multirate transmissions, a thorough performance assessment in terms of the error probability and the probability of outage is discussed for the considered strategies under different transmission conditions. The proposed strategies enhance the performance of the high rate user compared to the rate‐matching algorithm and slightly hold back the low rate user performance. Moreover, the re‐allocation strategy enhances the system capability to support higher transmission rate at no additional cost.

Handover Incentives for Self-Interested WLANs with Overlapping Coverage

We consider an environment where self-interested IEEE 802.11 Wireless Local Area Networks (WLANs) have overlapping coverage, and investigate the incentives that can trigger handovers between the WLANs. Our focus is on the incentives for supporting handovers due solely to the improved performance for all wireless networks. Such incentives arise due to a well-known property of 802.11 networks, where low rate users that send traffic significantly degrade the performance of high rate users that are associated with the same access point. A key difference of this paper compared to other works is that WLANs are self-interested, seeking to improve the performance of their own clients. We develop a comprehensive analytical model for accurately identifying and quantifying the handover gains. The model captures cases such as uplink and downlink traffic, wired link capacity constraints, and nonsaturated traffic conditions, and yields a practical handover decision policy. Simulations and experiments on a real testbed verify the accuracy of the model, and indicate that significant gains can be achieved through performance-induced handover incentives.

Reduction of Feedback Based-on User Traffic Rate in OFDM Relay System

In order to reduce feedback while maintaining user performance in multiuser orthogonal frequency division multiplexing (OFDM) relay system, feedback mechanism based on user traffic rate is proposed. From the aspects of feedback outage probability, user weighted channel gain related to user traffic rate is compared with the system threshold. The higher user traffic rate is, the bigger the user traffic weight becomes. So the high traffic rate user has greater probability to be fed back, which can support different traffic rate. Simulation results show that the proposed feedback mechanism can bring more feedback sub-carriers to the user with high traffic rate.

Adaptive Code Assignment Algorithm for a Multi-User/Multi-Rate CDMA System

Code division multiple access (CDMA) technique is used widely since it can flexibly support multi-rate multi-media services by changing the number of orthogonal spreading codes. In this paper, we present a new adaptive code assignment algorithm, which consists of three steps: reserved-space, improved-crowded-first-space, and multi-code combination to fully use the code space. Compared with the existing algorithms, the proposed algorithm can avoid the code blocking problem and lower its total blocking probability while keeping its computational complexity relatively low. Simulation results show that increasing the free space reduces the average total blocking probability while increasing the blocking probability of high rate users.

A Linear Programming Approach to Cross-Layer Optimization in Multiclass VSG-CDMA Over Rayleigh Fading

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents a new linear programming approach for throughput maximization on the uplink of a multiclass variable spreading gain code-division multiple-access (CDMA) multicellular system in Rayleigh fading for both binary phase-shift keying (BPSK) and quaternary phase-shift keying (QPSK) modulations. Based on the improved Gaussian approximation technique, we derive exact closed-form expressions of the outage probability, which are used as a physical (PHY) layer constraint of the maximization problem. We show that it is possible to transform the nonlinear constraint into a set of equivalent linear expressions. This facilitates the formulation of a new linear throughput maximization that noticeably requires less computational complexity than the known nonlinear approaches. Furthermore, due to the simplicity of the linear approach, we can include the constraints of such higher layers, i.e., media access control (MAC) and call admission control (CAC) layers, into the proposed formula. Accordingly, we introduce two linear programming optimization formulas. One is for MAC-PHY optimization, and another is for cross-layer optimal CAC policy. In the case of MAC-PHY optimization, the throughput is maximized, at low background noise level, when every user retains the same bit energy, particularly in the case where the difference in data rates is high. Nonetheless, at high levels of background noise, the throughput would be maximized when a larger amount of bit energy is allocated to the high-rate users. For the joint optimal policy, the throughput and the blocking probability are optimized and improved up to 50% in comparison to those of the conventional CAC policy, which is known as the <emphasis emphasistype="boldital">complete sharing</emphasis> policy. </para>

Multirate Spectral Phase-Encoded Time-Spreading O-CDMA System Using Orthogonal Variable Spreading Factor Code Sequences

We propose a multirate spectral phase-encoded time- spreading optical code-division multiple-access system using orthogonal variable spreading factor code sequences. The proposed multirate access networking method can support differentiation on more bit-rate/bit-error-rate (BER) requirements. The proposed method can also improve the BER performances of higher rate users compared to the conventional single-rate system due to mitigation of the effect of optical beat interference.

Spreading Code Selection for Multiple Chip Rate DS-CDMA Systems

In this paper, we analyze the interuser interference (IUI) in multiple chip rate (MCR) direct-sequence code-division multiple-access (DS-CDMA) systems employing realistic chip waveforms. In MCR DS-CDMA systems, different users may have different chip rates, which makes IUI unavoidable even if the employed spreading codes are orthogonal. Here, we consider orthogonal variable spreading factor (OVSF) codes and random spreading codes. We find that the spreading code of the low chip rate user does not affect the IUI at all. The IUI depends only on the bit patterns of the high chip rate user corresponding to one chip duration of the low chip rate user and on the carrier frequency separation of the users. We provide a closed-form equation for the IUI power of random codes, and we show that a judicious choice of the spreading codes can reduce the IUI significantly for OVSF codes.

Performance of Multicode DS/CDMA With Noncoherent&amp;gt;tex&amp;lt;$M$&amp;gt;/tex&amp;lt;-ary Orthogonal Modulation in Multipath Fading Channels

This paper presents the performance analysis and simulation of a multicode direct-sequence code-division multiple-access system with noncoherent M-ary modulation, in a multipath fading environment. This type of transceiver is specified for the reverse link of the IS-95B and cdma2000 (radio configurations 1 and 2) systems and is intended to serve high-rate applications such as data transfer and video communications. While previous studies considered the analytical error performance of coherent multicode systems, little attention has been devoted in the literature to the noncoherent case. We provide concise and useful expressions for the interference terms as a function of the commonly used aperiodic cross correlation functions. After a statistical characterization of these terms, we make use of the Gaussian approximation (GA) in order to obtain the bit-error rate (BER). However, unlike some other analyses (for coherent detection) relying on the GA, in our derivation, we take into account the fact that all the codes transmitted by a mobile user fade in unison. As demonstrated via computer simulations, this fact is crucial to obtain a reliable estimate of the BER, especially when equal-gain combining (EGC) is used at the receiver. The analysis is also extended to include a simple closed-loop power control algorithm and hard handoff between multiple cells. In particular, we verify-for the multicode case-previous observations that the use of EGC allows improvement only for a certain range of values of the total interference seen at the receiver: When either the number of interfering users is too large, or too many codes are assigned to the high-rate user, the noncoherent combining loss becomes such that the use of many diversity branches can decrease the performance as compared to a system with little or no diversity.

Optimal admission control in cellular DS-CDMA systems with multimedia traffic

The problem of jointly controlling the data rates and transmit powers of users, so as to maximize throughput in cellular direct-sequence code-division multiple-access (CDMA) networks is addressed. The multicode (MC)-CDMA system, where the processing gain of each code is fixed and high-rate users use multiple codes for transmission, and the variable gain (VG)-CDMA architectures are considered. The throughout maximization problem is formulated as a classical optimization problem, modeling the constraints arising from the data rate requirements and power budgets. Optimal strategies to maximize throughput in both systems are derived. While the MC-CDMA system has a simple optimal rate-power allocation algorithm, the VG-CDMA system has a more complex solution.

Directional power-based admission control for WCDMA systems using beamforming antenna array systems

A directional power-based admission control (AC) scheme is proposed for base stations deploying beamforming antenna arrays. The proposed AC algorithm is a natural extension of the conventional power based AC scheme for cells using standard sector antennas. The directional scheme takes the load per beam/direction into account in the AC decision, so the cell load is controlled in accordance with the spatial filtering gain provided by using beamforming antenna arrays. The scheme is tested by means of Monte Carlo simulations in a multicell wideband code division multiple access (WCDMA) network, where users with different bit rates are present. The obtained results demonstrate that the directional power based AC scheme allows a higher traffic load in cells using beamforming antenna arrays without jeopardizing the stability of the network. Application of a four element antenna array is found to provide a capacity gain of a factor 2.7 for a network with many low bit rate users, while the gain is reduced to a factor 2.3 for traffic scenarios with a mixture of high and low bit rate users.

Blind detection of multirate asynchronous CDMA signals in multipath channels

Blind detection of a desired user's signal in a multirate direct sequence code division multiple access (DS-CDMA) system [using either variable sequence length (VSL) or multicode (MC) access] is considered. A code-constrained inverse filter criterion (IFC)-based blind detector for equal-rate CDMA signals to detect a desired user's signal was presented by Tugnait and Li (2001). The IFC method exploits the higher order statistics of the data. In multirate CDMA systems, a high-rate user signal may be treated as the superposition of several virtual basic-rate signals. The code-constrained IFC-based detector may be used to detect a given basic-rate virtual signal. This, however, does not solve the problem of combining the detected virtual basic-rate signals to yield the original high-rate signal since the former may be delayed by different equalization delays, may be multiply extracted, and may be in different "order." In this paper, novel approaches combining the code-constrained IFC and a penalty function are developed to cope with this problem for VSL and MC multirate access methods. Global minima of the proposed cost functions are analyzed. Three illustrative simulation examples are presented, including an example where the proposed algorithms are compared with an existing subspace approach (and its modifications), a clairvoyant matched filter receiver, and a known channel linear minimum mean-square error (MMSE) receiver.

Coded performance of dual-rate decorrelators combined with an array of antennas for synchronous multiuser DS/CDMA systems

Two types of decorrelating detectors proposed for a dual rate synchronous DS/CDMA channel are extended to incorporate an array of antennas. The resulting detectors are termed as the array low-rate decorrelator (ALRD) and the array high-rate decorrelator (AHRD). In AHRD, maximal ratio combining on N consecutive decorrelated outputs is applied for the low-rate users. Asymptotic performance is compared numerically in the context of channel coding with two different types of spreading code assignments. In the worst-case random assignment, both detectors are sensitive to the interferers' directions of arrivals. Especially, performance degradation of the low-rate user is noticeable. In the cyclic assignment of the maximal-length shift register code for the low-rate users, ALRD is less sensitive to the spatial distributions of the interferers, whereas AHRD is sensitive to the distributions of the low-rate users and less sensitive to the distributions of the high-rate users. We confirm that both detectors show the near-far resistant property and good asymptotic performance depending on the spatial distributions of the interferers and spreading code assignments.

Blind adaptive multiuser detection for asynchronous dual-rate DS/CDMA systems

In this paper, the authors consider an asynchronous direct-sequence code division multiple access (DS/CDMA) system wherein users are allowed to transmit their symbols at one out of two available data rates. Three possible access schemes are considered, namely, the variable spreading length (VSL), the variable chip rate (VCR), and the variable chip rate with frequency shift (VCRFS) formats. Their performance is compared for the case that a linear one-shot multiuser receiver is employed. It is also shown that detection of the users transmitting at the higher rate requires a periodically time-varying processing of the observables. Moreover, the problem of blind adaptive receiver implementation is studied, and a cyclic blind recursive-least-squares (RLS) algorithm is provided which is capable of converging to the periodically time-varying high-rate users detection structure. Numerical results show that the proposed receivers are near-far resistant, and that the VCRFS access technique achieves the best performance. Finally as to the adaptive blind receiver implementation, computer simulations have revealed that the cyclic RLS algorithm for blind adaptive high-rate users demodulation outperforms the conventional RLS algorithm in most cases of primary importance.

Channel estimation and multiuser detection in long-code DS/CDMA systems

The problems of channel estimation and multiuser detection for direct sequence code division multiple access (DS/CDMA) systems employing long spreading codes are considered. With regard to channel estimation, several procedures are proposed based on the least-squares approach, relying on the transmission of known training symbols but not requiring any timing synchronization. In particular, algorithms suited for the forward and reverse links of a single-rate DS/CDMA cellular system are developed, and the case of a multirate/multicode system, wherein high-rate users are split into multiple virtual low-rate users, is also considered. All of the proposed procedures are recursively implementable with a computational complexity that is quadratic in the processing gain, with regard to the issue of multiuser detection, an adaptive serial interference cancellation (SIC) receiver is considered, where the adaptivity stems from the fact that it is built upon the channel estimates provided by the estimation algorithm. Simulation results show that coupling the proposed estimation algorithms with a SIC receiver may yield, with a much lower computational complexity, performance levels close to those of the ideal linear minimum mean square error (MMSE) receiver, which assumes perfect knowledge of the channels for all of the users and which (in a long-code scenario) has a computational complexity per symbol interval proportional to the third power of the processing gain.

Adaptive Detection and Channel Estimation for Dual-Rate DS/CDMA Networksin Frequency-Selective Fading

In a Dual-Rate DS/CDMA system, a primary, ``high rate'' network coexists with a secondary one with a slower rate. Demultiplexing high-rate users typically entails, at the receiver end, a time-varying processing if Variable Spreading Length (VSL), Variable Chip Rate (VCR), or Variable Chip Rate with Frequency Shift (VCRFS) modulation formats are used, and Rake-Type demodulation schemes if the chip rate exceeds the channel coherence bandwidth. The realization of fully adaptive receivers thus requires the joint estimation of (a) the interference to be suppressed, whether from other users of the high-rate network or from the secondary network, and (b) the channel tap weights. The solution that we consider here assumes that, for each user, a pilot signal is transmitted along with the information-bearing signal, whether in a time-multiplexed or in a parallel-channel format. We consider both the uplink and the downlink channel and, based on a Minimum-Mean-Square-Error (MMSE) optimality criterion, we have come up with a fully adaptive system, capable on the one hand of tracking, through a cyclic Recursive-Least-Squares type algorithm, the periodically time-varying variations of the interference subspace and on the other of updating the channel tap weights estimates. A thorough performance assessment is presented, and the convergence dynamics of the system are derived through computer simulations in order to assess the impact of the prior uncertainty with regard to the other-users signatures and the channel tap weights realizations.

Experiments on coherent adaptive antenna array diversity for wideband DS-CDMA mobile radio

In wideband direct sequence code division multiple access (W-CDMA), employing an adaptive antenna array is a very promising technique to reduce severe multiple access interference (MAI) from high rate users. A four-antenna pilot symbol-assisted coherent adaptive antenna array diversity (PSA-CAAAD) receiver comprising an adaptive antenna array based on a minimum mean squared error (MMSE) criterion and a RAKE combiner is implemented in preliminary laboratory and field experiments. There are two important design concepts of the PSA-CAAAD receiver. The first is that the adaptive antenna array forms an antenna beam for each resolved propagation path and tracks only slow changes in the directions of arrival (DOAs) and average powers of the desired and interfering user signals. The second is that the RAKE combiner tracks the instantaneous changes in channel conditions and coherently combines the signals of the desired user propagating along the resolved paths to maximize the instantaneous signal-to-interference plus background noise power ratio (SINR). This paper presents, both by laboratory and field experiments, the effectiveness of PSA-CAAAD receiver as a powerful means to reduce severe MAI from high rate users, and that it is more effective than using a space diversity receiver with the same number of antennas in the W-CDMA reverse link.