Uplink System Research Articles

Frame structure for DTTB uplink systems using novel training sequences

A new frame structure based on time-domain synchronous orthogonal frequency division multiplexing for digital terrestrial television broadcasting (DTTB) uplink systems is proposed, whereby multicarrier pseudonoise (PNMC) training sequences are used as the frame header (FH). Sub-sequences of FH are inserted between signal frames as guard intervals. Simple multi-user channel estimation and equalisation can be applied and good performance is illustrated by simulations.

Time-Frequency Cyclic Shift Keying Transceiver for Low PAPR MC-CDMA Uplink System over Multipath Fading Channels

A low peak-to-average-power-ratio (PAPR) transceiver with a time-frequency cyclic shift orthogonal keying (TF-CSOK) technique is proposed for the uplink multi-carrier CDMA (MC-CDMA) system over multiple access interference (MAI) and multipath interference (MPI) channels. The low complexity structure of the TF-CSOK MC-CDMA system is designed to involve the FCSOK and TCSOK techniques to combat MPI and MAI effects, respectively. In particular, at the besestation, the multiuser detector employs the maximum likelihood (ML) rule and the TFSOK despreading and demapping techniques to acquire the M-ary modulation gain and diversity gain. Simulation results show that the multuser receiver has the robustness against strong MAI. Moreover, it outperforms the conventional single-carrier frequency division multiple access (SC-FDMA) system and the conventional MC-CDMA system under MAI and MPI environments.

Asymptotic BER Analysis of a SIMO Multiuser Diversity System

Multiuser diversity is a form of diversity that is inherent in wireless systems with more than one user. For single-best user selection, it becomes a scheme whereby the base station schedules transmission to and from the user with the best channel. Traditionally, this form of diversity is considered in conjunction with adaptive modulation, where the rate of transmission is adapted to the channel quality of the best user. In this paper, we consider a fixed modulation and analyze the asymptotic bit-error-rate performance for uplink systems for a large number of users using the theory of asymptotic order statistics. Moreover, we consider multiple antennas at the base station and show that antenna correlation, perhaps surprisingly, improves the bit error rate (BER) for a large number of users. Simulation results corroborate our analytical results.

The Resource-Optimized Differentially Modulated Hybrid AF/DF Cooperative Cellular Uplink Using Multiple-Symbol Differential Sphere Detection

In multiuser cellular uplinks, cooperating mobiles may share their antennas in order to achieve transmit diversity by forming a virtual antenna array (VAA) in a distributed fashion. In this paper, based on the minimum BER criterion, we investigate cooperating-user-selection (CUS) and adaptive-power-allocation (APA) for two types of differentially modulated cooperative cellular uplinks requiring no channel state information (CSI) at the receiver, namely, for the differential-amplify-and-forward (DAF) and the selective differential-decode-and-forward (DDF) assisted systems. They both employ multiple-symbol differential sphere detection (MSDSD) to combat rapid-fading-induced performance degradation. More specifically, we investigate the cooperative-protocol-selection (CPS) of the uplink system in conjunction with a beneficial CUS as well as the APA scheme in order to further improve the achievable end-to-end performance, leading to a resource-optimized hybrid AF/DF cooperative system.

Power-Minimizing Rate Allocation in Cooperative Uplink Systems

The rate-allocation problem, which has been aimed at minimizing the total transmit power in cooperative uplink systems, is investigated. Each user transmits over an orthogonal frequency band using cooperative broadcasting. The broadcasting nature of the wireless channel is exploited by allowing users to act as relays for one another. All users operate in the decode-and-forward mode. Depending on the number of relays that was selected by a user, we suggest two schemes: 1) the flow-optimized cooperative scheme (FCS) and 2) the single-relay cooperative scheme (SCS). We develop rate-allocation algorithms for them. In our simulation, we compare the outage performance of our schemes with two other schemes: 1) the orthogonal cooperative scheme and 2) direct transmission. Results indicate that our schemes achieve full diversity order and outperform other compared schemes. The algorithm for FCS achieves the optimality, whereas the algorithm for SCS is near optimal. In addition, our algorithms have fast convergence performance. SCS has a lower complexity than FCS, but it requires a higher total transmit power. However, the difference in total transmit power between FCS and SCS is not large under practical rate requirements. In addition to the total transmit power, we consider the improvement in the individual transmit powers of the users.

Variance-reduced partial parallel interference cancellation for MC-CDMA uplink systems

In this paper, we present an improved version of partial parallel interference cancellation (PPIC), called variancereduced PPIC (VRPPIC), for multicarrier code division multiple access uplink systems. The VRPPIC detector is a combination of the PPIC receiver and a new bit estimator, where a simplified realization is derived for the main PPIC operations and soft decisions from PPIC are linearly combined by the bit estimator using appropriate weighting factors (derived from the PPIC weighting factors) to reduce the conditional variance of the final signal estimate. Also, we derive an optimal weighting factor (OWF) selection algorithm for VRPPIC based on minimizing a monotonically increasing conditional variance function. The derived OWFs for all the interference cancellation stages are equal and can easily be obtained from a linear function of the number of active users. Simulation results show that VRPPIC with the proposed OWFs significantly outperforms VRPPIC (or PPIC) with randomly selected weighting factors in multipath Rayleigh fading channels.

Distributed Detection for Macrodiversity Enhanced Cellular Communication Systems

This paper considers the design of soft decision distributed detection schemes for achieving macrodiversity in uplink cellular communication systems with and without channel knowledge at the fusion center. Distributed detection macrodiversity (DDM) techniques are proposed where the quatizer thresholds of local detectors at base stations (radio access points) are optimized using a numerical global optimization algorithm called adaptive simulated annealing (ASA). The performance of such DDM schemes is studied for systems with and without channel coding, where the base stations employ multiple receive antennas over quasi-static Rayleigh fading channels. It is shown that properly designed quantizers for the local detectors, with a few resolution bits can make the performance very close to optimal macrodiversity that requires a much larger bandwidth from the connection links to the fusion center.

An Adaptive Channel Interpolator Based on Kalman Filter for LTE Uplink in High Doppler Spread Environments

Long-Term Evolution (LTE) systems will employ single carrier frequency division multiple access (SC-FDMA) for the uplink. Similar to the Orthogonal frequency-division multiple access (OFDMA) technology, SC-FDMA is sensitive to frequency offsets leading to intercarrier interference (ICI). In this paper, we propose a Kalman filter-based approach in order to mitigate ICI under high Doppler spread scenarios by tracking the variation of channel taps jointly in time domain for LTE uplink systems. Upon acquiring the estimates of channel taps from the Kalman tracker, we employ an interpolation algorithm based on polynomial fitting whose order is changed adaptively. The proposed method is evaluated under four different scenarios with different settings in order to reflect the impact of various critical parameters on the performance such as propagation environment, speed, and size of resource block (RB) assignments. Results are given along with discussions.

Low-Complexity Realization of a Concatenated DMT–FMT Multiuser Uplink System With Transmit Diversity

We present an efficient realization of a novel wireless uplink transmission scheme. The scheme concatenates an inner filtered multitone (FMT) modulator with transmission over multiple antennas and an outer space-time cyclically prefixed discrete multitone (DMT) modulator. The inner modulator is used to realize frequency-division multiplexing of the users by partitioning the FMT subchannels among them. The outer modulator copes with the residual subchannel intersymbol interference, and it further implements a form of transmit diversity. Frequency and space diversity is exploited via direct-sequence data spreading across the DMT tones. We also propose a multiuser analysis filter bank that significantly lowers the receiver complexity.

Iterative approach for multiuser carrier frequency offset estimation in interleaved OFDMA uplink

In this paper, the issue of carrier frequency offset (CFO) estimation in interleaved orthogonal frequency division multiple access (OFDMA) uplink systems is investigated. The problem of peaks ambiguity in MUSIC estimator proposed by Cao and Yao is analyzed firstly, and then a searching solution is presented. Furthermore, a new iterative algorithm based on this MUSIC estimator is proposed. The proposed approach iteratively searches for the correct CFO vector by minimizing the objective function using a first-order Taylor series approximation of the CFO vector. It can conquer the effect of ambiguous peaks effectively, performs better than Cao and Yao's approach in relatively low SNR region and large CFO, and at the same time has much lower computational complexity. Simulations illustrate the efficiency of the proposed algorithm.

On the Feasibility of Precoded Single User MIMO for LTE-A Uplink

The 3rd Generation Partnership Project (3GPP) is currently specifying the system requirements for Long Term Evolution - Advanced (LTE-A), having as a target peak data rates of 1 Gbit/s in local areas and 100 Mbit/s in wide areas. To meet these ambitious requirements for the uplink, multiple-input-multiple-output (MIMO) antenna techniques are expected to be deployed. In this paper, several channel-aware MIMO precoding techniques are presented. Specifically, precoded single user spatial multiplexing for both Orthogonal Frequency Division Multiplexing (OFDM) and Single Carrier Frequency Division Multiplexing (SCFDM) is studied, and its feasibility in a LTE-A uplink system is discussed. Particular emphasis is given to the limited feedback precoding, where a codebook generation method based on the Lloyd algorithm is proposed. Results show that, when full channel knowledge is available at both the transmitter and the receiver, precoding leads to a spectral efficiency gain up to 4 dB over open loop transmission; furthermore, OFDM slightly outperforms SC-FDM because of its higher robustness to the noise. Limited feedback precoding has been shown to be effective and consistently robust to the subcarrier grouping in a urban micro scenario. However, the performance is severely degraded in the blind precoding case, where transmitter and receiver compute the precoding matrix independently, due to the high sensitivity to the delay. Finally, the precoding operation over the SCFDM signal is shown to increase its Peak to Average Power Ratio (PAPR), thus reducing its advantage with respect to OFDM.

Joint Throughput Maximization and Fair Uplink Transmission Scheduling in CDMA Systems

We study the fundamental problem of optimal transmission scheduling in a code-division multiple-access wireless system in order to maximize the uplink system throughput, while satisfying the users quality-of-service (QoS) requirements and maintaining fairness among them. The corresponding problem is expressed as a weighted throughput maximization problem, under certain power and QoS constraints, where the weights are the control parameters reflecting the fairness constraints. With the introduction of the power index capacity, it is shown that this optimization problem can be converted into a binary knapsack problem, where all the corresponding constraints are replaced by the power index capacities at some certain system power index. A two-step approach is followed to obtain the optimal solution. First, a simple method is proposed to find the optimal set of users to receive service for a given fixed target system load, and then the optimal solution is obtained as a global search within a certain range. Furthermore, a stochastic approximation method is presented to effectively identify the required control parameters. The performance evaluation reveals the advantages of our proposed policy over other existing ones and confirms that it achieves very high throughput while maintains fairness among the users, under different channel conditions and requirements.

Reduced-Complexity Iterative Markov Chain MBER Detection for MIMO Systems

A novel Markov chain (MC) representation aided minimum bit error rate (MBER) detection method is proposed for an M-QAM modulated SDM/SDMA uplink system. Compared to the conventional MBER scheme, the proposed MC-MBER scheme is capable of reducing the complexity imposed with the aid of its efficient detection candidate set generation assisted by the Markov chain process. Our performance results demonstrate that the MC-MBER multiuser detection (MUD) is capable of reducing the computational complexity by a factor of eight in comparison to the conventional MBER MUD in a rank-deficient system transmitting four 4-QAM substreams with the aid of two receive antennas, while achieving a BER performance comparable to that of the MBER MUD.

Investigation of capacity and call admission control schemes in TD‐SCDMA uplink systems employing smart antenna techniques

AbstractDue to the TDMA (time division multiple access)/time division duplex (TDD) specialization in the uplink (UL) of code division multiplex access (CDMA) systems, some advanced techniques, such as smart antenna (SA) and multi‐user detection (MUD), are utilized conveniently in time division‐synchronous code division multiplex access (TD‐SCDMA) systems. These advanced techniques have great impacts on the capacity and radio resource management (RRM) schemes. In this paper, the UL capacity and load models specified for TD‐SCDMA systems are proposed, in which the impacts from SA and MUD techniques are considered, and the UL load can be estimated based on the total received power in the base station. According to the proposed theoretical capacity and load evaluation models, the call admission control (CAC) strategies suitable for TD‐SCDMA systems are presented. Since there are two kinds of SA schemes (i.e., tracking beam antenna (TBA) and switched beam antenna (SBA)) utilized in TD‐SCDMA systems, the efficient CAC algorithms suitable for these two SA schemes are designed and evaluated, which are based on principles of the interference increase estimation. All simulation results show that the proposed CAC strategies can work efficiently and improve performances of TD‐SCDMA systems dramatically. Copyright © 2009 John Wiley & Sons, Ltd.

Uplink capacity analysis for multihop TDD-CDMA cellular system

Multihop TDD-CDMA cellular system becomes a promising candidate for fourth generation (4G) cellular systems because it provides wider coverage and higher throughput. In this paper, we analyze the uplink system capacity of a multihop TDD-CDMA cellular system. We first analyze a one-hop TDDCDMA cellular system with different base station's (BS's) direct coverage area. We find that the system capacity can be improved if a BS only covers a center small area of the cell. Besides, the high downlink transmission power of the surrounding BSs can degrade the capacity severely. Thus, we propose a multihop TDD-CDMA cellular system to overcome the high downlink inter-cell interference. In our system, a cell area is divided into two regions: inner-region and outer-region. The mobile users in the inner- and outer- region of the cell communicate with BS and relay station (RS), respectively. Based on this model, the uplink system capacity in both regions with the assumption that the inter-cell and intra-cell interference follow the Gaussian distribution is obtained. A comparison equation to provide a fair comparison with conventional one-hop TDD-CDMA cellular system is presented. From the results, we conclude that multihop TDD-CDMA cellular system can improve capacity significantly.

A Novel Interference Cancellation Approach for Interleaved OFDMA Uplink System

In this paper, the issue of carrier frequency offset (CFO) compensation in interleaved orthogonal frequency division multiple access (OFDMA) uplink system is investigated. To mitigate the effect of multiple access interference (MAI) caused by CFOs of different users, a new parallel interference cancellation (PIC) compensation algorithm is proposed. This scheme uses minimum mean square error (MMSE) criterion to obtain the estimation of interference users, then circular convolutions [6] are employed to restore MAI and compensate CFO. To tackle the complexity problem of circular convolutions, an efficient MAI restoration and cancellation method is developed. Simulations illustrate the good performance and low computational complexity of the proposed algorithm.

Self-Encoded Multiple Access Multiuser Convolutional Codes in Uplink and Downlink Cellular Systems

Self-encoded spread spectrum eliminates the need for traditional pseudo noise (PN) code generators. In a self-encoded multiple access (SEMA) system, the number of users is not limited by the number of available sequences, unlike code division multiple access (CDMA) systems that employ PN codes such as m-, Gold or Kassami sequences. SEMA provides a convenient way of supporting multi-rate, multi-level grades of service in multimedia communications and prioritized heterogeneous networking systems. In this paper, we propose multiuser convolutional channel coding in SEMA that provides fewer cross-correlations among users and thereby reducing multiple access interference (MAI). We analyze SEMA multiuser convolutional coding in additive white Gaussian noise (AWGN) channels as well as fading channels. Our analysis includes downlink synchronous system as well as asynchronous system such as uplink mobile-to-base station communication.

Adaptive space-time reduced-rank estimation based on diversity-combined decimation and interpolation applied to interference suppression in CDMA systems

An adaptive low-complexity space-time reduced-rank processor is proposed for interference suppression in asynchronous DS code division multiple access (CDMA) systems based on a diversity-combined decimation and interpolation method. The novel design approach for the processor employs an iterative procedure to jointly optimise the interpolation, decimation and estimation tasks for reduced-rank parameter estimation. Joint iterative least squares design parameter estimators are described and low-complexity adaptive recursive least squares (RLS) algorithms for the proposed structure are developed. To design the decimation unit, the optimal decimation scheme based on the counting principle is presented and low-complexity decimation structures are proposed. Linear space-time receivers with antenna arrays based on the proposed reduced-rank processor are then presented and investigated to mitigate multi-access interference and intersymbol interference in an asynchronous DS-CDMA system uplink scenario. An analysis of the convergence properties of the proposed space-time processor is carried out and analytical expressions are derived to predict the mean squared error performance of the proposed processor with RLS algorithms. Simulations show that the proposed processor outperforms the best known reduced-rank schemes at substantially lower complexity.

Cooperative diversity technique for MIMO-OFDM uplink in wireless interactive broadcasting

OFDMA is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. Recently, to enhance the performance of OFDMA systems, MIMO techniques have been investigated, and the OFDMA systems have been extended to MIMO architectures. Although transmit diversity is clearly advantageous on OFDMA uplink system in interactive broadcasting, it may not be possible that practical OFDMA system of wireless interactive broadcasting have more than two antennas due to size, cost, or hardware limitations. Cooperative diversity techniques are proposed to overcome the limitations. Cooperative networks aim to utilize the available terminals as a collection of distributed antennas for the transmission of signals from one end of the network to the other. Therefore, this paper focuses on design and performance evaluation of MIMO-OFDMA uplink system with cooperative diversity for wireless interactive broadcasting. For high-rate and reliable transmission, efficient transmission codes are proposed for two transmission cases (one-sided transmission case and two-sided transmission case). And, we present our investigations of the effects of line-of-sight. In the wireless communication environment, line-of-sight is very important to improve the system performance. Under the condition of utilizing the line-of-sight of cooperative partner, the performance of a terminal without line-of-sight can be improved. Taking into account the effects of utilizing line-of sight of cooperative partner, the BER performance is investigated for a MIMO-OFDMA uplink scenario.

Practical Considerations on Channel Estimation for Up-Link MC-CDMA Systems

Channel parameters, which are usually estimated at the receiver, are required in signal detection in multi-carrier (MC) code division multiplex access (CDMA). The accuracy of channel estimation directly affects the performance of the overall system. In this paper, we present some practical techniques to improve channel estimation in up-link MC-CDMA systems. For most of multipath channels, there are only a few significant taps in time domain that determine the frequency response. Based on this characteristic, we detect and keep these significant taps and discard the trivial ones, which are usually very noisy or contain only noise components. After channel parameters are estimated at the pilot blocks, channels at the data blocks can be obtained by interpolation. To improve the performance of simple linear interpolation, we apply optimal interpolation, which takes channel estimation error and channel correlation into consideration. We then investigate the impact of channel estimation error on minimum mean-square-error (MMSE) successive interference cancellation (SIC) detector and find that more accurate channel is required to further improve the performance of the MMSE SIC detector. Therefore, we develop soft-decision-directed (SDD) channel estimation, which exploits the information at the data blocks to improve channel estimation.