Pre-equalization in combination with transmit diversity for OFDMA code-division multiplexing systems in fading channels

Massive MIMO has been proposed as a means of achieving huge spectral efficiency in cellular networks. However, its implementation is usually based on linear precoding schemes using Channel State Information (CSI). For this reason, the use of the Time Division Duplex (TDD) mode is typically assumed since it removes the dependence on the number of antennas at the transmitter to acquire CSI. Unfortunately, most of the cellular network operate in the Frequency Division Duplex (FDD) mode in which the acquisition of CSI depends proportionally to the number of transmit antennas. For this reason, the implementation of Massive MIMO is handicapped to few transmit antennas for the current cellular networks. In this paper we study the use of reconfigurable antennas that can switch among a set of preset modes, i.e., radiation patterns, through Blind Interference Alignment schemes. We show that it is a solution to exploit a very large number of antennas at the transmit side, overcoming the bottleneck that today constitutes the acquisition of CSI in Massive MIMO systems operating in FDD mode.

The acquisition problem in the wideband-CDMA (WCDMA) system proposed by the 3rd Generation Partnership Project (3GPP) to the ITU consists of determining both the timing and the identity of the particular scrambling code being received. A fast search mechanism to tackle this problem, based on cyclically permutable (CP) codes is proposed by Sriram and Hosur (see IEEE Inter. Conf. on Communications (ICC), June 1999) for the frequency-division-duplex (FDD) component of WCDMA. The CP codes for the FDD mode of WCDMA are not optimum for the time-division duplex (TDD) mode and have to be redesigned. In this paper we design optimum CP codes for the TDD mode. The new CP codes can also identify both frame timing and long code group information using only one synchronization slot. Thus, they can be used for GSM to WCDMA TDD mode handover. The new CP codes have been incorporated into the 3GPP WCDMA TDD standard.

In the project FRAMES (Future Radio Wideband Multiple Access Systems) of the European research program ACTS (Advanced Communications Technologies and Services) an air interface proposal for UMTS (Universal Mobile Telecommunications System) has been developed. In January 1998 ETSI (European Telecommunications Standards Institute) reached a consensus on a UMTS air interface standard termed UTRA (UMTS Terrestrial Radio Access), which is mainly based on the proposal developed within FRAMES. UTRA consists of an FDD (frequency division duplex) mode applying the multiple access scheme W-CDMA (wideband CDMA) and a TDD (time division duplex) mode applying the multiple access scheme TD-CDMA (time division CDMA). For the different services to be supported a set of channel coders has been proposed which have to be further evaluated. In consideration of this need, in this paper a low-complexity rate-compatible punctured turbo code (RCPTC) for high-quality circuit switched data services in the TDD-mode is investigated depending on its main parameters. The criterion in these investigations is the bit error ratio (BER). The performance of the considered turbo code (TC) is analyzed and compared to a convolutional code (CC) of equal complexity.

In this paper, we study pre-equalization for sparse index multiple access (SIMA), which was proposed to enable one-shot identification and detection in random access, over frequency-selective fading channels in time division duplexing (TDD) mode. Each device is to decide a transmit vector to adapt to its channel for pre-equalization based on the channel reciprocity. Due to pre-equalization, the receiver does not need to estimate the channel state information (CSI) of active devices. We further modify pre-equalization with multiple precoding matrices for a better performance with a lower transmission power. For low signaling overhead in random access, we also consider one-shot identification and detection to perform active device identification and their signal detection simultaneously with a low-complexity algorithm by exploiting the sparsity of device activity as well as unique identification sequences.

In this paper, we establish a general framework on the reduced dimensional channel state information (CSI) estimation and pre-beamformer design for frequency-selective massive multiple-input multiple-output (MIMO) systems employing single- carrier (SC) modulation in time division duplex (TDD) mode by exploiting the joint angle-delay domain channel sparsity in millimeter (mm) wave frequencies. First, based on a generic subspace projection taking the joint angle-delay power profile and user-grouping into account, the reduced-rank minimum mean square error (RR-MMSE) instantaneous CSI estimator is derived for spatially correlated wideband MIMO channels. Second, the statistical pre-beamformer design is considered for frequency-selective SC massive MIMO channels. We examine the dimension reduction problem and subspace (beamspace) construction on which the RR-MMSE estimation can be realized as accurately as possible. It is observed that the proposed techniques show remarkable robustness to the pilot interference (or contamination) with a significant reduction in pilot overhead.

In the 3/sup rd/ Generation Partnership Project (3GPP) time division CDMA (TD-CDMA) has been selected as the air interface for the TDD (time division duplexing) bands of 3/sup rd/ generation (3G) mobile radio systems. In this paper a novel multiuser transmission scheme for the TD-CDMA downlink is presented which in contrast to state of the art TD-CDMA transceivers utilizes the reciprocity of the mobile radio channels of the uplink and the downlink. It is shown that according to this scheme no channel estimation is necessary at the MSs and the data detection effort is dramatically reduced. The theoretical analysis of the paper is supported by computer simulation results which illustrate the superiority of the novel scheme in terms of the BER (bit error rate) performance and the system spectrum efficiency in the downlink of TD-CDMA.

In this paper, a comparison study of various UMTS data transmission techniques is performed. An analysis of the system in the presence of geographically non-uniform multi-service traffic profiles is done for three transmission techniques: Frequency Division Duplex (FDD), Time Division Duplex (TDD), and High Speed Downlink Packet Access (HSDPA). Several combinations were selected: FDD, TDD, FDD combined with HSDPA, FDD combined with TDD, and FDD combined with TDD and HSDPA. The TDD mode is the technique with better results, but the introduction of HSDPA is clearly beneficial for data services, as expected. The distribution of the services by the three techniques is the best scenario, namely for some traffic profiles. For example, for a threshold of 128 kbps (for HSDPA services) and 50 % of the power reserved to HSDPA, it is possible to get 1.4 GB of carried traffic.

The advanced knowledge of forward link channel information enables the optimal transmit waveform to be designed for a multiple input multiple output (MIMO) antenna system to exploit the channel's characteristics fully. The reciprocal property of the radio channel makes time division duplex (TDD) an attractive air interface design since the forward link channel information can be derived from the reverse link signal without any explicit feedback. Among the many practical factors that may invalidate the channel reciprocity in TDD mode, this paper focuses on those that are associated with the mismatches among the radio frequency (RF) circuits in the many transmit/receive chains. With a simple model, the sources for the RF mismatches are identified and their calibration requirements are established using simulation.

The 3GPP proposal for 3rd generation mobile phone systems comprises not only frequency division duplex (FDD) but also time division duplex (TDD) as duplex method for the JD-CDMA system. The TDD mode is an addition to the FDD mode to facilitate flexible, asymmetric data transfer of up- and downlink. Special structured pilot sequences are included into the data bursts and can be exploited to perform an efficient joint channel estimation of all active users as basis for multiuser detection. In this paper linear receiver concepts for the TDD mode are examined in terms of performance with focus on the influence of real, non-ideal channel estimation. RAKE receiver, zero forcing and minimum mean-square-error multiuser detectors are introduced and investigated by means of simulation for different environments using appropriate ITU channel models. Furthermore, an improved channel estimation procedure is presented, yielding significant performance enhancements for the multiuser detection algorithms.

To support a high density of mobile devices and its connections by means of rapid increase of mobile devices as well as small cells, various studies related to random access such as the random access load, better uplink resource utilization and interference among random access attempts have been developed for enhanced long-term evolution (LTE) system and 5G communication technology. In this paper, we discuss about the technical issues of random access resource configuration in time division duplex (TDD) mode and propose an improved method of it to make better uplink resource utilization on frequency domains and to minimize interference among random access attempts for physical random access channel (PRACH) resource allocation point of view, but it can also easily extend the LTE system. The proposed method is compared with LTE TDD PRACH configuration method through theoretical analysis. It shows that the proposed method provides better resource utilization for PRACH and physical uplink shared channel resources (PUSCH), allows flexible resource utilization in uplink scheduling, and is intuitive and effective in cell deployment with random access channel optimization in terms of random access resource configuration.

Multi-user multiple-input multiple-output (MU-MIMO) system operating in frequency division duplex (FDD) mode has an advantage over that operating in time division duplex (TDD) mode in achieving a lower return time latency as both the uplink and downlink data streaming can occur simultaneously. However, unlike TDD, the reciprocity of the uplink and downlink channels does not hold and hence the system typically needs to rely on channel feedback which reduces the uplink channel capacity. Reducing the channel feedback while maintaining the channel estimation accuracy is a critical factor for a successful implementation of MU-MIMO FDD. In addition, when combined with orthogonal frequency division multiple access (OFDMA), channel feedback needs to occur independently for different sub-channels assigned to different user terminals. This paper proposes the use of discrete cosine transformation (DCT) based channel compression for the purpose of MU-MIMO-OFDMA channel feedback and shows its advantage over conventional discrete Fourier transform (DFT) based method. Compared to ideal channel feedback without channel estimation or compression errors, the proposed method achieves within 1 dB loss in uncoded bit error performance for a MU-MIMO-OFDMA system using 32 antennas at the central access point supporting 16 user terminals each with a single antenna.

Orthogonal frequency division multiplexing (OFDM) is sensitive to carrier frequency offset and channel estimation error, which destroy the subcarrier orthogonality and give rise to intercarrier interference (ICI). This paper addresses the performance degradation due to imperfect parameters in an orthogonal frequency and code division multiplexing (OFDM-CDM) system with multiple transmit antennas. For the performance evaluation, the average bit error rate (BER) impairment due to imperfect channel information and frequency offset is investigated taking into account the effect of time-varying channels. Derived results show that a space-time block coded (STBC) OFDM-CDM system experiences a severe BER degradation regardless of the number of users.

We develop an analytical method to evaluate the symbol error rate (SER) of Tomlinson-Harashima precoding (THP) for a multiple transmit antenna system operating in a time division duplex (TDD) mode. By making small error assumption the noise due to the channel estimation error for a given channel is investigated thoroughly, which enables one to calculate the SER with channel estimation error.

We design non-codebook-based Multiple-Input Multiple-Output (MIMO) precoding schemes using multiple cell-specific reference signals patterns for the time division duplex (TDD) mode of LTE, where channel reciprocity can be exploited. Previously proposed non-codebook-based precoding schemes typically use UE specific reference signals for demodulation. Cell specific reference signals are however always allocated for the transmission of common control signalling, mobility measurements and downlink channel quality measurements. In order to save the resources occupied by UE specific reference signals, and to simplify UE implementation, a precoder-estimation scheme and a codebook-assisted scheme are designed. The codebook-assisted scheme has the novelty of using a codebook in equalization. Link throughput simulations indicate that the codebook-assisted scheme is preferable compared to the precoder-estimation scheme and the scheme using UE specific reference signals in scenarios with moderate UE velocity.

In this paper, we first analyze the differences of underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) systems and conventional terrestrial OFDM system, and give a simple introduction of the backgrounds. By considering the real UWA channel environments, the measured channel data is used to generate the UWA channel model and calculate the relative parameters for underwater OFDM systems. Practical least square (LS) based channel estimation with linear interpolation are adopted to obtain the channel state information (CSI) at receiver side. As multi-input multi-output (MIMO) processing techniques, Alamouti code is implemented and evaluated to perform for space time block coding (STBC) and space frequency block coding (SFBC) for UWA OFDM systems with the MIMO configuration of , at the same time, maximum ratio combining (MRC) is performed for the purpose of comparison. The simulation results show that, with perfect channel estimation, SFBC failed to work duo to the serious frequency selectivity of UWA channel environments. When the practical channel estimation is applied, in the case of STBC, the proposed 4-column pilot pattern gives better performance about 7dB than SISO system.