Showing papers on "Telecommunications link published in 2008"

Performance Analysis of Slotted Carrier Sense IEEE 802.15.4 Medium Access Layer

Abstract: Advances in low-power and low-cost sensor networks have led to solutions mature enough for use in a broad range of applications varying from health monitoring to building surveillance. The development of those applications has been stimulated by the finalization of the IEEE 802.15.4 standard, which defines the medium access control (MAC) and physical layer for sensor networks. One of the MAC schemes proposed is slotted carrier sense multiple access with collision avoidance (CSMA/CA), and this paper analyzes whether this scheme meets the design constraints of those low-power and low-cost sensor networks. The paper provides a detailed analytical evaluation of its performance in a star topology network, for uplink and acknowledged uplink traffic. Both saturated and unsaturated periodic traffic scenarios are considered. The form of the analysis is similar to that of Bianchi for IEEE 802.11 DCF only in the use of a per user Markov model to capture the state of each user at each moment in time. The key assumptions to enable this important simplification and the coupling of the per user Markov models are however different, as a result of the very different designs of the 802.15.4 and 802.11 carrier sensing mechanisms. The performance predicted by the analytical model is very close to that obtained by simulation. Throughput and energy consumption analysis is then performed by using the model for a range of scenarios. Some design guidelines are derived to set the 802.15.4 parameters as function of the network requirements.

Method of performing random access procedure in wireless communication system

Abstract: A method includes transmitting a random access preamble, receiving a random access response as a response of the random access preamble, wherein the random access response comprises an uplink resource assignment and a request for transmission of a Channel Quality Indicator (CQI), and transmitting the CQI in the uplink resource assignment.

Single Carrier FDMA: A New Air Interface for Long Term Evolution

Abstract: Single Carrier Frequency Division Multiple Access (SC-FDMA) is a novel method of radio transmission under consideration for deployment in future cellular systems; specifically, in 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) systems. SC-FDMA has drawn great attention from the communications industry as an attractive alternative to Orthogonal Frequency Division Multiple Access (OFDMA). Introduction to Single Carrier FDMA places SC-FDMA in the wider context of wireless communications, providing the reader with an in-depth tutorial on SC-FDMA technology. The book introduces the reader to this new multiple access technique that utilizes single carrier modulation along with orthogonal frequency multiplexing and frequency domain equalization, plus its applications in communications settings. It considers the similarities with and differences from orthogonal frequency division modulation, multiplexing, and multiple access used extensively in cellular, broadcasting, and digital subscriber loop applications. Particular reference is made to the peak power characteristics of an SC-FDMA signal as an added advantage over OFDMA. Provides an extensive overview of the principles of SC-FDMA and its relation to other transmission techniques. Explains how the details of a specific implementation influence the tradeoffs among various figures of merit. Describes in detail the configuration of the SC-FDMA uplink transmission scheme published by 3GPP. Features link level simulation of an uplink SC-FDMA system using MATLAB. This is an essential text for industry engineers who are researching and developing 3GPP LTE systems. It is suitable for engineers designing wireless network equipment, handsets, data cards, modules, chipsets, and test equipment as well as those involved in designing LTE infrastructure. It would also be of interest to academics, graduate students, and industry researchers involved in advanced wireless communications, as well as business analysts who follow the cellular market.

Limited feedback-based block diagonalization for the MIMO broadcast channel

Abstract: Block diagonalization is a linear preceding technique for the multiple antenna broadcast (downlink) channel that involves transmission of multiple data streams to each receiver such that no multi-user interference is experienced at any of the receivers. This low-complexity scheme operates only a few dB away from capacity but requires very accurate channel knowledge at the transmitter. We consider a limited feedback system where each receiver knows its channel perfectly, but the transmitter is only provided with a finite number of channel feedback bits from each receiver. Using a random quantization argument, we quantify the throughput loss due to imperfect channel knowledge as a function of the feedback level. The quality of channel knowledge must improve proportional to the SNR in order to prevent interference-limitations, and we show that scaling the number of feedback bits linearly with the system SNR is sufficient to maintain a bounded rate loss. Finally, we compare our quantization strategy to an analog feedback scheme and show the superiority of quantized feedback.

Base station device and communication control method

Abstract: A base station device which performs communication with a mobile station by using a downlink shared channel includes: reception means for receiving transmission acknowledgement information for a downlink shared channel from the mobile station by a control channel which is frequency-multiplexed with an uplink shared channel; measurement means for measuring radio quality of the control channel; and judgment means for judging the reliability of the downlink radio quality information according to the radio quality. Moreover, a communication control method in the base station device which performs communication with the mobile station by using the downlink shared channel includes: a step of receiving transmission acknowledgement information for the downlink shared channel from the mobile station by the control channel which is frequency-multiplexed with the uplink shared channel; a step of measuring radio quality of the control channel; and a step of judging whether the mobile station has transmitted the downlink radio quality information according to the radio quality.

Multicell downlink capacity with coordinated processing

Abstract: We study the potential benefits of base-station (BS) cooperation for downlink transmission in multicell networks. Based on a modified Wyner-type model with users clustered at the cell-edges, we analyze the dirty-paper-coding (DPC) precoder and several linear precoding schemes, including cophasing, zero-forcing (ZF), and MMSE precoders. For the nonfading scenario with random phases, we obtain analytical performance expressions for each scheme. In particular, we characterize the high signal-to-noise ratio (SNR) performance gap between the DPC and ZF precoders in large networks, which indicates a singularity problem in certain network settings. Moreover, we demonstrate that the MMSE precoder does not completely resolve the singularity problem. However, by incorporating path gain fading, we numerically show that the singularity problem can be eased by linear precoding techniques aided with multiuser selection. By extending our network model to include cell-interior users, we determine the capacity regions of the two classes of users for various cooperative strategies. In addition to an outer bound and a baseline scheme, we also consider several locally cooperative transmission approaches. The resulting capacity regions show the tradeoff between the performance improvement and the requirement for BS cooperation, signal processing complexity, and channel state information at the transmitter (CSIT).

Performance of Uplink Fractional Power Control in UTRAN LTE

Abstract: UTRAN long term evolution is currently being standardized in 3GPP with the aim of more than twice the capacity over high-speed packet access. The chosen multiple access for uplink is single carrier FDMA, which avoids the intra-cell interference typical of CDMA systems, but it is still sensitive to inter-cell interference. As a result, the role of the power control becomes decisive to provide the required SINR, while controlling at the same time the interference caused to neighboring cells. This is the target of the fractional power control (FPC) algorithm lately approved in 3GPP. This paper evaluates in detail the impact of a FPC scheme on the SINR and interference distributions in order to provide a sub-optimal configuration tuned for both interference- and noise-limited scenarios.

Wireless communication system for monitoring physical downlink control channel

Abstract: A method of monitoring a physical downlink control channel (PDCCH) in a wireless communication system is provided. A user equipment monitors a set of PDCCH candidates for a search space in a subframe. The search space includes a common search space monitored by all user equipments in a cell and a UE-specific search space monitored by at least one UE in the cell.

Energy Efficient Design in Wireless OFDMA

Abstract: Energy-efficient transmission is an important aspect of wireless system design due to limited battery power in mobile devices. We consider uplink energy-efficient transmission in OFDMA systems since mobile stations are battery powered. We account for both circuit and transmit power when designing energy-efficient communication mechanisms and emphasize energy efficiency over peak rates or throughput. Both link adaptation and resource allocation schemes are developed to optimize the overall bits transmitted per Joule of energy, which allows for maximum energy savings in a network. Our simulation results show that the proposed schemes significantly improve energy efficiency.

In-Band Interference Effects on UTRA LTE Uplink Resource Block Allocation

Abstract: In this paper we investigate the impact of in-band interference on the uplink multiple access of UMTS Terrestrial Radio Access, long term evolution (UTRA LTE). In- band and out-of-band interference arise as a result of transmitter imperfections. Out-of- band, or adjacent channel, interference can be controlled by a spectral shaping filter. However, the shaping filter cannot control the leakage within the system bandwidth, and the resulting leakage can interfere with other users' transmission in the uplink multiple access. The situation is emphasized when the resource block allocation size is small, and when the interfering signal is received at higher power spectral density (PSD). The effect of frequency offset and different PSD level from the UE interferers to a victim UE is studied. The impact on different UE resource block size allocation is also investigated. The results are obtained from an LTE uplink link level simulation. It is shown that a significant SNR degradation is observed at the victim UE with smaller resource block size, when evaluated in a realistic channel. Considering that the frequency offset can be maintained below 200 Hz and PSD level offset below 10 dB, the SNR loss at 10% BLER target is less than 1 dB.

Dual-polarized wireless communications: from propagation models to system performance evaluation

Abstract: In this paper, we address the potential benefits of dual-polarized arrays in multi-antenna wireless systems. After an extensive literature overview of experimental data, we present a new and simple analytical framework to model dual-polarized Rayleigh and Ricean fading channels for arbitrary array sizes. The model relies on a limited number of physical parameters, such as the channel spatial correlations, the channel co-polar and the cross-polar ratios and the antenna cross-polar discrimination. Then, we investigate the multiplexing advantage of dual-polarized transmissions through the evaluation of the ergodic mutual information, for both TITO and MIMO systems. Finally, the performance of two space-time coding schemes (Alamouti OSTBC and uncoded spatial multiplexing) is evaluated via a detailed analysis of the pairwise error probability.

Distributed Downlink Beamforming With Cooperative Base Stations

Abstract: In this paper, we consider multicell processing on the downlink of a cellular network to accomplish ldquomacrodiversityrdquo transmit beamforming. The particular downlink beamformer structure we consider allows a recasting of the downlink beamforming problem as a virtual linear mean square error (LMMSE) estimation problem. We exploit the structure of the channel and develop distributed beamforming algorithms using local message passing between neighboring base stations. For 1-D networks, we use the Kalman smoothing framework to obtain a forward-backward beamforming algorithm. We also propose a limited extent version of this algorithm that shows that the delay need not grow with the size of the network in practice. For 2-D cellular networks, we remodel the network as a factor graph and present a distributed beamforming algorithm based on the sum-product algorithm. Despite the presence of loops in the factor graph, the algorithm produces optimal results if convergence occurs.

Satellite communications systems engineering : atmospheric effects, satellite link design, and system performance

Abstract: List of Acronyms. Preface. Chapter 1 Introduction to Satellite Communications. 1.1 Early History of Satellite Communications. 1.2 Some Basic Communications Satellite System Definitions. 1.3 Regulatory Process for Satellite Communications. 1.4 Overview of Book Structure and Topics. References. Chapter 2 Satellite Orbits. 2.1 Kepler's Laws. 2.2 Orbital Parameters. 2.3 Orbits in Common Use. 2.4 Geometry of GSO Links. References. Problems. Chapter 3 Satellite Subsystems. 3.1 Satellite Bus. 3.2 Satellite Payload. References. Chapter 4 The RF Link. 4.1 Transmission Fundamentals. 4.2 System Noise. 4.3 Link Performance Parameters. References. Problems. Chapter 5 Link System Performance. 5.1 Link Considerations. 5.2 Uplink. 5.3 Downlink. 5.4 Percent of Time Performance Specifications. References. Problems. Chapter 6 Transmission Impairments. 6.1 Radiowave Frequency and Space Communications. 6.2 Radiowave Propagation Mechanisms. 6.3 Propagation Below About 3 GHz. 6.4 Propagation Above About 3 GHz. 6.5 Radio Noise. References. Problems. Chapter 7 Propagation Effects Modeling and Prediction. 7.1 Atmospheric Gases. 7.2 Clouds and Fog. 7.3 Rain Attenuation. 7.4 Depolarization. 7.5 Tropospheric Scintillation. References. Problems. Chapter 8 Rain Fade Mitigation. 8.1 Power Restoral Techniques. 8.2 Signal Modification Restoral Techniques. 8.3 Summary. References. Problems. Chapter 9 The Composite Link. 9.1 Frequency Translation (FT) Satellite. 9.2 On-Board Processing (OBP) Satellite. 9.3 Comparison of FT and OBP Performance. 9.4 Intermodulation Noise. 9.5 Link Design Summary. References. Problems. Chapter 10 Satellite Multiple Access. 10.1 Frequency Division Multiple Access. 10.2 Time Division Multiple Access. 10.3 Code Division Multiple Access. References. Problems. Chapter 11 The Mobile Satellite Channel. 11.1 Mobile Channel Propagation. 11.2 Narrowband Channel. 11.3 Wideband Channel. 11.4 Multi-Satellite Mobile Links. References. Appendix A Satellite Signal Processing Elements. A.1 Analog Systems. A.2 Digital Baseband Formatting. A.3 Digital Source Combining. A.4 Digital Carrier Modulation. A.5 Summary. References. Appendix B Error Functions and Bit Error Rate. B.1 Error Functions. B.2 Approximation for BER. Index.

Capacity Bounds for Two-Way Relay Channels

Abstract: We provide achievable rate regions for two-way relay channels (TRC). At first, for a binary TRC, we show that the subspace-sharing of linear codes can achieve the capacity region. And, for a Gaussian TRC, we propose the subset-sharing of lattice codes. In some cases, the proposed lattice coding scheme can achieve within 1/2-bit the capacity and is asymptotically optimal at high signal-to-noise ratio (SNR) regimes.

Sparsed U-TDOA wireless location networks

Abstract: In an overlay, U-TDOA-based, Wireless Location System, LMUs typically co-located with BTSs, are used to collect radio signaling both in the forward and reverse channels. Techniques are used to compensate for sparse LMU deployments where sections of the U-TDOA service area are uplink demodulation or downlink beacon discovery limited.

Rate Optimization for Multiuser MIMO Systems With Linear Processing

Abstract: This paper focuses on linear transceiver design for rate optimization in multiuser Gaussian multple-input multiple-output (MIMO) systems. We focus on two design criteria: 1) maximizing the weighted sum rate subject to a total power constraint; 2) maximizing the minimum user rate subject to a total power constraint. For these problems, new power allocation strategies are derived, which can be formulated as geometric programs (GPs) involving mean square errors (MSEs). Based on these solutions, we propose iterative algorithms, where each iteration contains the optimization of the uplink power, uplink receive filters, and downlink receive filters. Monotonic convergence of the algorithms is proved. Simulations show that the algorithms outperform existing linear schemes. Additionally, we extend the results to other variations of the problems, such as, the problem of sum-rate constrained or user-rate constrained power minimization, and the problem of sum-rate maximization subject to user-rate constraints and a total power constraint.

Channel-aware scheduling algorithms for SC-FDMA in LTE uplink

Abstract: Single-carrier frequency division multiple access (SC-FDMA) has been selected as the uplink access scheme in the UTRA Long Term Evolution (LTE) due to its low peak-to-average power ratio properties compared to orthogonal frequency division multiple access. Nevertheless, in order to achieve such a benefit, it requires a localized allocation of the resource blocks, which naturally imposes a severe constraint on the scheduler design. In this paper, three new channel-aware scheduling algorithms for SC-FDMA are proposed and evaluated in both local and wide area scenarios. Whereas the first maximum expansion (FME) and the recursive maximum expansion (RME) are relative simple solutions to the above-mentioned problem, the minimum area-difference to the envelope (MADE) is a more computational expensive approach, which, on the other hand, performs closer to the optimal combinatorial solution. Simulation results show that adopting a proportional fair metric all the proposed algorithms quickly reach a high level of data-rate fairness. At the same time, they definitely outperform the round-robin scheduling in terms of cell spectral efficiency with gains up to 68.8% in wide area environments.

Adaptive Soft Frequency Reuse for Inter-Cell Interference Coordination in SC-FDMA Based 3GPP LTE Uplinks

Abstract: This paper proposes a decentralized adaptive soft frequency reuse scheme for the uplink of 4G long-term evolution (LTE) systems. While universal frequency reuse (UFR) is being targeted for next generation multi-cellular wireless networks, ongoing efforts supporting the LTE standard have proved that actual implementations of UFR in LTE lead to unacceptable interference levels experienced by user equipments near the cell edge area in a multi-cellular configuration. The herein proposed adaptive soft frequency reuse scheme is a step forward towards effective inter-cell interference coordination (ICIC) in next-generation wireless networks. Our solution to the uplink ICIC problem stands out for its two essential features that consist of physical resource block (PRB) reuse avoidance/minimization and cell-edge bandwidth breathing which can be implemented at the cost of a negligible information exchange over the X2 interface (backbone). The PRB reuse avoidance feature significantly decreases inter-cell interference levels while improving the achievable average throughput per user, especially for those identified as cell-edge ones. The cell-edge bandwidth breathing strategy allows to track and adapt to semi-static changes in traffic loading and user distributions within each cell which drastically reduces the blocking probability of incoming calls under cell-edge bandwidth constrained traffic.

Antenna combining for the MIMO downlink channel

Abstract: A multiple antenna downlink channel where limited channel feedback is available to the transmitter is considered. In a vector downlink channel (single antenna at each receiver), the transmit antenna array can be used to transmit separate data streams to multiple receivers only if the transmitter has very accurate channel knowledge, i.e., if there is high-rate channel feedback from each receiver. In this work it is shown that channel feedback requirements can be significantly reduced if each receiver has a small number of antennas and appropriately combines its antenna outputs. A combining method that minimizes channel quantization error at each receiver, and thereby minimizes multi-user interference, is proposed and analyzed. This technique is shown to outperform traditional techniques such as maximum-ratio combining because minimization of interference power is more critical than maximization of signal power in the multiple antenna downlink. Analysis is provided to quantify the feedback savings, and the technique is seen to work well with user selection and is also robust to receiver estimation error.

Power-Efficient Impedance-Modulation Wireless Data Links for Biomedical Implants

Abstract: We analyze the performance of wireless data telemetry links for implanted biomedical systems. An experimental realization of a bidirectional half-duplex link that uses near-field inductive coupling between the implanted system and an external transceiver is described. Our system minimizes power consumption in the implanted system by using impedance modulation to transmit high-bandwidth information in the uplink direction, i.e., from the implanted to the external system. We measured a data rate of 2.8 Mbps at a bit error rate (BER) of <10-6 (we could not measure error rates below 10-6 ) and a data rate of 4.0 Mbps at a BER of 10-3. Experimental results also demonstrate data transfer rates up to 300 kbps in the opposite, i.e., downlink direction. We also perform a theoretical analysis of the bit error rate performance. An important effect regarding the asymmetry of rising and falling edges that is inherent to impedance modulation is predicted by theory and confirmed by experiment. The link dissipates 2.5 mW in the external system and only 100 muW in the implanted system, making it among the most power-efficient inductive data links reported. Our link is compatible with FCC regulations on radiated emissions.

OFDMA vs. SC-FDMA: performance comparison in local area imt-a scenarios

Abstract: The system requirements for IMT-A are currently being specified by the ITU. Target peak data rates of 1 Gb/s in local areas and 100 Mb/s in wide areas are expected to be provided by means of advanced MIMO antenna configurations and very high spectrum allocations (on the order of 100 MHz). For the downlink, OFDMA is unanimously considered the most appropriate technique for achieving high spectral efficiency. For the uplink, the LTE of the 3GPP, for example, employs SCFDMA due to its low PAPR properties compared to OFDMA. For future IMT-A systems, the decision on the most appropriate uplink access scheme is still an open issue, as many benefits can be obtained by exploiting the flexible frequency granularity of OFDMA. In this article we discuss the suitability of using OFDMA or SC-FDMA in the uplink for local area high-data-rate scenarios by considering as target performance metrics the PAPR and multiuser diversity gain. Also, new bandwidth configurations have been proposed to cope with the 100 MHz spectrum allocation. In particular, the PAPR analysis shows that a localized (not distributed) allocation of the resource blocks (RBs) in the frequency domain shall be employed for SC-FDMA in order to keep its advantages over OFDMA in terms of PAPR reduction. Furthermore, from the multiuser diversity gain evaluation emerges the fact that the impact of different RB sizes and bandwidth configurations is low, given the propagation characteristics of the assumed local area environment. For full bandwidth usage, OFDMA only outperforms SC-FDMA when the number of frequency multiplexed users is low. As the spectrum load decreases, instead, OFDMA outperforms SC-FDMA also for a high number of frequency multiplexed users, due to its more flexible resource allocation. In this contex different channel-aware scheduling algorithms have been proposed due to the resource allocation differences between the two blocks chemes.

Robust Transceiver Optimization in Downlink Multiuser MIMO Systems with Channel Uncertainty

Abstract: The problem of transceiver optimization in multiuser multiple-input multiple-output downlink wireless systems is considered. The base station is assumed to possess only estimated, erroneous values of channel coefficients. The exact channels lie in uncertainty regions, specified by the Frobenius norms of the error matrices. An iterative optimization of the transmit and receive filters is performed with the goal of minimizing the total transmit power, while satisfying the users' mean square error (MSE) targets for all channels from the uncertainty regions. Each iteration consists of two steps that can be equivalently rewritten as semidefinite programs with efficient numerical solutions. It is shown that the whole algorithm converges. The proposed framework can be applied for solving robust counterparts of several related MSE-optimization problems. The modifications of the proposed algorithms for accommodating box-like disturbances are analyzed, as well.

On transceiver design and channel quantization for downlink multiuser MIMO systems with limited feedback

Abstract: We consider a MIMO broadcast channel where both the transmitter and receivers are equipped with multiple antennas. Channel state information at the transmitter (CSIT) is obtained through limited (i.e., finite-bandwidth) feedback from the receivers that index a set of precoding vectors contained in a predefined codebook. We propose a novel transceiver architecture based on zero-forcing beamforming and linear receiver combining. The receiver combining and quantization for CSIT feedback are jointly designed in order to maximize the expected SINR for each user. We provide an analytic characterization of the achievable throughput in the case of many users and show how additional receive antennas or higher multiuser diversity can reduce the required feedback rate to achieve a target throughput.We also propose a design methodology for generating codebooks tailored for arbitrary spatial correlation statistics. The resulting codebooks have a tree structure that can be utilized in time-correlated MIMO channels to significantly reduce feedback overhead. Simulation results show the effectiveness of the overall transceiver design strategy and codebook design methodology compared to prior techniques in a variety of correlation environments.

Low complexity subcarrier and power allocation for utility maximization in uplink OFDMA systems

Abstract: We consider the joint subcarrier and power allocation problem with the objective of maximizing the total utility of users in the uplink of an OFDMA system. Our formulation includes the problems of sum rate maximization, proportional fairness and max-min fairness as special cases. Unlike some previous algorithms, which are iterative and time consuming, our proposed one is non-iterative and with time complexity of only O(KN log2 N), where K and N are the number of users and subcarriers respectively. We prove that it provides a solution that is Pareto optimal within a large neighborhood of itself. Besides, we derive an efficiently computable upper bound of the optimal solution. Simulation results show that our algorithm is nearly optimal.

On the Design of Linear Transceivers for Multiuser Systems with Channel Uncertainty

Abstract: We consider the design of linear transceivers for multiuser communication systems in the presence of uncertain channel state information (CSI), with an emphasis on downlink systems with a single antenna at each receiver. For systems with uplink-downlink reciprocity, we consider a stochastic model for the channel uncertainty, and we propose an efficient algorithm for the joint design of the linear preceding matrix at the base station and the equalizing gains at the receivers so as to minimize the average mean-square-error (MSE) over the channel uncertainty. The design is based on a generalization, derived herein, of the MSE duality between the broadcast and multiple access channels (MAC) to scenarios with uncertain CSI, and on a convex formulation for the design of robust transceivers for the dual MAC. For systems in which quantized channel feedback is employed, we consider a deterministically-bounded model for the channel uncertainty, and we study the design of robust downlink transceivers that minimize the worst- case MSE over all admissible channels. While we show that the design problem is NP-hard, we also propose an iterative local optimization algorithm that is based on efficiently-solvable convex conic formulations. Our framework is quite flexible, and can incorporate different bounded uncertainty models as well as a variety of power constraints. In particular, we study a "system-wide" uncertainty model, and although the resulting design problem is still NP hard, it does result in a significantly simpler iterative local design algorithm than the "per-user" uncertainty model. Our approaches to the minimax design for the downlink can be extended to the uplink, and we provide explicit formulations for the resulting uplink designs. Simulation results indicate that the proposed approaches to robust linear transceiver design can significantly reduce the sensitivity of the downlink to uncertain CSI, and can provide improved performance over that of existing robust designs.

Resource allocation for multicast services in multicarrier wireless communications

Abstract: We consider a multicast resource allocation problem for the downlink in OFDM-based wireless cellular network systems. In a conventional multicast system, to accommodate users with bad channel conditions, the transmission is based on the worst case user. We show that such a multicast system saturates the capacity when the number of users increases in fading environments. We exploit the multicarrier nature of OFDM and advances in coding techniques such as MDC (multiple description coding), in which arbitrary combinations of layers can be decoded at the receiver. Different MDC layers are carried over different subcarriers and users with good channels receive data from more subcarriers than users with poor channel conditions. We present an optimal subcarrier/bit allocation method requiring full search of possible candidates. To reduce the complexity, we propose a two-step suboptimum algorithm by separating subcarrier allocation and bit loading. Numerical results show that the proposed heuristics significantly outperform the conventional multicast transmission scheme. The difference between optimum and heuristic solutions is less than 5%.

Multiple antenna MMSE based downlink precoding with quantized feedback or channel mismatch

Abstract: In this paper, we consider the downlink of a multiuser wireless communication system with multiple antennas at the base station and users each with a single receive antenna. It is known that when channel state information (CSI) is available at the transmitter a large performance gain can be achieved. In a system employing time-division duplexing (TDD), CSI can be obtained at the base station if there is reciprocity between the forward and reverse channels. CSI can also be conveyed from the users to the base station via a limited-rate feedback channel in a frequency-division duplexing (FDD) system. In any case, channel estimation errors are inevitable due to the presence of background noise in the estimated signal and due to the finite number of feedback bits used in a limited-rate feedback system model. In this paper, we first consider the general case when partial CSI is available at the transmitter. We derive an MMSE based precoding technique that considers channel estimation errors as an integral part of the system design. Using rate-distortion theory and the generalized Lloyd vector quantization algorithm, we then specialize our results for the more practical limited-rate feedback system model. Compared to previously proposed precoding techniques such as channel inversion and regularized channel inversion, it is shown that the proposed precoding technique significantly improves the average bit error rate (BER) in the system. Furthermore, the performance of the proposed technique is investigated in the high signal-tonoise ratio (SNR) regime. It is shown that the proposed technique suffers from a ceiling effect that asymptotically limits the system performance.

Method of transmitting scheduling information in tdd system

Abstract: A method of transmitting scheduling information in time-division-duplex (TDD) system is provided. The method comprises configuring a radio frame, the radio frame comprising at least one downlink subframe and at least one uplink subframe, wherein a downlink subframe is reserved for downlink transmission and an uplink subframe is reserved for uplink transmission, and transmitting scheduling information on a downlink control channel in a downlink subframe, the scheduling information comprising an uplink indicator and uplink resource assignment, the uplink indicator indicating which at least one uplink subframe the uplink resource assignment is valid for. Data can be efficiently transmitted by using an uplink indicator which indicates a specific location of a subframe.

Method of transmitting sounding reference signal in wireless communication system

Abstract: A method of transmitting a sounding reference signal includes generating a physical uplink control channel (PUCCH) carrying uplink control information on a subframe, the subframe comprising a plurality of SC-FDMA (single carrier-frequency division multiple access) symbols, wherein the uplink control information is punctured on one SC-FDMA symbol in the subframe, and transmitting simultaneously the uplink control information on the PUCCH and a sounding reference signal on the punctured SC-FDMA symbol. The uplink control information and the sounding reference signal can be simultaneously transmitted without affecting a single carrier characteristic.

Method and apparatus for handling random access procedure in a wireless communications system

Abstract: A method for handling a random access procedure in a user equipment, called UE hereinafter, of a wireless communications system, the UE assigned a dedicated preamble for the random access procedure, the method includes deciding whether to reuse the dedicated preamble when the UE doesn't receive a downlink message successfully or the random access procedure is failed (302).