Time-division duplex CDMA communications

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 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.

In the universal mobile telephony system (UMTS) the frequency division duplex (FDD) and time division duplex (TDD) modes have adjacent carriers at 1920 MHz. This creates adjacent channel interference (ACI) between the two different air interfaces. Since different duplexing modes are used, the implications for each system are different, with respect to capacity and coverage: these implications are investigated in the paper. The separation distance of the TDD and FDD base station and the load in each system are varied and a symmetrical speech service in both systems is considered, with non-ideal power control assumed. It is found that for an FDD cell radius of 1000 m, a TDD cell radius of 50 m and 10% maximal tolerable outage, the effects of ACI on capacity can be compensated by dynamically increasing the required power at each BS, without affecting the coverage.

The potential of combining channel pre-equalization with transmit diversity for orthogonal frequency-division multiple-access code-division multiplexing (OFDMA-CDM) systems is investigated. The pre-equalization exploits the channel knowledge which is presumed to be available at the transmitter (Tx). The transmission signal is pre-distorted according to the channel state information (CSI) which can be made available, for example, by using time division duplex (TDD) mode. In TDD mode and under the assumption that CSI changes slowly in time, CSI estimated from a received slot can be used for pre-equalization of the next transmission slot. Concerning transmit diversity, we consider space-frequency block coding (SFBC) and transmit selection diversity (TSD). It turns out that TSD significantly outperforms SFBC. Moreover, systems with pre-equalization and TSD exhibit lower complexity than systems with pre-equalization and SFBC, since in the former case CSI is needed only at Tx, while in the latter case CSI is required at both Tx and receiver (Rx). In addition, it is shown that TSD is the optimal transmit diversity technique in the sense of bit error rate (BER) minimization in the case when CSI is available at Tx.

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.

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.

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.

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 IEEE 802.16 standard defines two main duplexing modes: Time Division Duplexing (TDD) and Frequency Division Duplexing (FDD). The FDD can be divided further into Full-FDD (F-FDD) and Half-FDD (H-FDD). F-FDD requires full duplexing support from subscriber stations and works with two frequency bands. TDD operates a single frequency band, but it does not need full-duplexing support. H-FDD is a combination of these: it works with two frequency bands and does not require full-duplexing support. The cost of this is a more complicated scheduling and added overhead. Still, H-FDD might be the only possible duplexing mode in some occasions. In this paper, these duplexing modes are compared and the H-FDD specific features, such as group balancing, are presented. The simulation results show that H-FDD does not quite match the performance of F-FDD or TDD. In the downlink direction, F-FDD and TDD have similar performance but in the uplink direction F-FDD can benefit from subchannelization gain with fewer bursts per frame.

Spectrum and infrastructure sharing among multiple mobile network operators have recently drawn significant attention from the industry and the academia sectors. Recent advances in the area of Software Defined Networking (SDN) launch the design of innovative SDN-based solutions for resource sharing in multi-tier networks of macrocell and small-sized base stations, a.k.a. HetNet. In this paper, we present a novel SDN-based framework that enables efficient and elastic spectrum utilization among multiple operators in 3GPP LTE-A HetNet scenario. Assuming a multi-operator environment of Frequency Division Duplex (FDD) macrocells complemented by multi-tenant Time Division Duplex (TDD) pico cells, we present a SDN-based architecture that allows efficient resource sharing among the TDD and FDD systems in a dynamic way. A TDD frame re-configuration mechanism is also employed, to optimize the ratio of uplink and downlink slots in the TDD frame of picocells. System-level simulation results demonstrate that the combination of these two functional enhancements in the LTE-A HetNet, significantly reduces the application layer delay for both the FDD macrocell and TDD picocell systems, leading to highly efficient and dynamic spectrum sharing among multiple network operators.

It has been demonstrated that code division multiple access (CDMA) provides great flexibility by enabling efficient multiuser access in a cellular environment. In addition, time division duplex (TDD) as compared to frequency division duplex (FDD) represents an appropriate method to cater for the asymmetric use of a duplex channel. However, the TDD technique is subject to additional interference mechanisms compared to an FDD system, in particular if neighboring cells require different rates of asymmetry. If TDD is combined with an interference limited multiple access technique such as CDMA, the additional interference mechanism represents an important issue. This issue poses the question of whether a CDMA/TDD air-interface can be used in a cellular environment. The problems are eased if a hybrid time division multiple access (TDMA)/CDMA interface (TD-CDMA) is used. The reason for this is that the TDMA component adds another degree of freedom which can be utilized to avoid interference. This, however, requires special channel assignment techniques. A notable example of a system which uses a TD-CDMA/TDD interface is the Universal Mobile Telecommunications System (UMTS). This paper presents a novel centralized dynamic channel assignment (DCA) algorithm for a TD-CDMA/TDD air-interface. The DCA algorithm exploits a new technique which is termed "TS-opposing technique." The key result is that the new DCA algorithm enables neighboring cells to adopt different rates of asymmetry without a significant capacity loss.

Major goal of ultra reliable and low latency communication (URLLC) is to reduce the latency down to a millisecond (ms) level while ensuring reliability of the transmission. Since the current uplink transmission scheme requires a complicated handshaking procedure to initiate the transmission, to meet this stringent latency requirement is a challenge in wireless system design. In particular, in the time division duplexing (TDD) systems, supporting the URLLC is difficult since the mobile device has to wait until the transmit direction is switched to the uplink. In this paper, we propose a new approach to support a low latency access in TDD systems, called channel aware sparse transmission (CAST). Key idea of the proposed scheme is to encode a grant signal in a form of sparse vector. This together with the fact that the sensing mechanism preserves the energy of the sparse vector allows us to use the compressed sensing (CS) technique in CAST decoding. From the performance analysis and numerical evaluations, we demonstrate that the proposed CAST scheme achieves a significant reduction in access latency over the 4G LTE-TDD and 5G NR-TDD systems.

The continuous growth in mobile data traffic and limited license wireless spectrum have led to dramatically increase the demand of the radio spectrum. It is widespread the concern about the coexistence of long term evolution (LTE) and Wi-Fi in the unlicensed band. There are several techniques have been proposed to enable the coexistence of LTE and Wi-Fi in the unlicensed band, but these works are targeted on the impact of the LTE to the Wi-Fi network performance. An experiment is carried out in this work to evaluate the impact of Wi-Fi signal on the video streaming in the LTE network. The experimental test comprised of the national instrument (NI) universal software radio peripheral (USRP) 2953R that is controlled by the LabVIEW Communication LTE application framework. Extensiveexperiments are carried out under two scenarios, i.e. (1) Coexistence of LTE and Wi-Fi signal, (2) LTE signal only. Performance evaluations are carried out with different Modulation and coding schemes (MCS) values and different mode of operations, i.e. frequency division duplex (FDD) and time division duplex (TDD) mode. The results illustrated that the interference from Wi-Fi signal caused the performance degradation of the LTE network in throughput and the power received by user equipment (UE).

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.

Unmanned aerial vehicles (UAVs) are promising and powerful aerial platforms that can execute a variety of complex tasks. However, the increasing complexity of tasks and number of UAV nodes pose significant challenges for UAV sensing networks, such as limiting the spectral resources and increasing device complexity. A potential solution is to implement full-duplex (FD) technology in UAV sensor network transceivers. Although appropriate self-interference (SI) cancellation techniques have been employed in the digital domain, the amplitude of the signal of interest (SoI) is relatively small and can be obscured by SI, especially over longer distances. Moreover, the introduction of phase offsets when filtering measurement signals can lead to signal distortion, resulting in estimation errors in the measurement results. To address these issues, this paper presents a joint transmit (TX) and receive (RX) beamforming algorithm based on the penalty dual decomposition (PDD) algorithm, which considers the constraints of transmission power, reception power, and residual SI power. The simulation analyses demonstrate that with a limited number of antennas, the proposed joint TX-RX beamforming algorithm can effectively suppress SI by up to 140 dB, yielding high-precision measurements in UAV sensor networks without compromising the accuracy of the control signals. Compared with that of the traditional frequency-division duplex (FDD) mode, the measurement accuracy is not decreased; compared with those of the time-division duplex (TDD) mode, the distance and speed measurement accuracies of the UAVs are increased by 10 m and 1.5 m/s, respectively, in the FD mode because there is no interruption of the tracking loop and no continuous retracking in the FD mode.