Orthogonal Pilot Sequences Research Articles

Adaptive pilot decontamination in multi-cell massive MIMO networks

This paper considers the contaminated channel estimation in multicell very large multiuser multiple-input multiple-output networks. To provide desirable spectral efficiency of these systems, we propose two adaptive algorithms which their implementations do not require orthogonal pilot sequences. The first algorithm (i.e., normalized least mean square) has low complexity and also gives an acceptable spectral efficiency in channels with low attenuation. The second algorithm (i.e., recursive least square) is more robust against pilot contamination and provides the desirable spectral efficiency in very large number of antennas at the base stations. In simulations, the performance of the proposed algorithms is evaluated and also compared with the performance of the conventional minimum mean square error estimation.

Group-Blind Detection for Uplink of Massive MIMO Systems

When paired with traditional channel estimation and detection, massive MIMO is severely affected by pilot contamination. While sticking to the traditional structure of the training phase where orthogonal pilot sequences are reused in different cells, we propose a group-blind detector that takes into account the presence of pilot contamination. Our detector uses the excess antennas to partially remove interference during the data transmission phase. We derive asymptotic expressions for the SINR gain and the achievable rate in the massive regime, i.e., when the number of antennas tends to infinity while keeping the number of users per cell fixed. Implementing the group-blind detector requires an estimate of the aggregate out-of-cell channel covariance. We propose a simple scheme, referred to as method of silences , to obtain such estimate. Numerical results confirm our analysis in practical scenarios, and show cases where the method of silences achieves a large fraction of the promised SINR gain over conventional detectors.

Analysis and Design of Secure Massive MIMO Systems in the Presence of Hardware Impairments

To keep the hardware costs of future communications systems manageable, the use of low-cost hardware components is desirable. This is particularly true for the emerging massive multiple-input multiple-output (MIMO) systems which equip base stations (BSs) with a large number of antenna elements. However, low-cost transceiver designs will further accentuate the hardware impairments, which are present in any practical communication system. In this paper, we investigate the impact of hardware impairments on the secrecy performance of downlink massive MIMO systems in the presence of a passive multiple-antenna eavesdropper. Thereby, for the BS and the legitimate users, the joint effects of multiplicative phase noise, additive distortion noise, and amplified receiver noise are taken into account, whereas the eavesdropper is assumed to employ ideal hardware. We derive a lower bound for the ergodic secrecy rate of a given user when matched filter data precoding and artificial noise (AN) transmission are employed at the BS. Based on the derived analytical expression, we investigate the impact of the various system parameters on the secrecy rate and optimize both the pilot sets used for uplink training and the AN precoding. Our analytical and simulation results reveal that: 1) the additive distortion noise at the BS may be beneficial for the secrecy performance, especially if the power assigned for AN emission is not sufficient; 2) all other hardware impairments have a negative impact on the secrecy performance; 3) despite their susceptibility to pilot interference in the presence of phase noise, so-called spatially orthogonal pilot sequences are preferable unless the phase noise is very strong; and 4) the proposed generalized null-space AN precoding method can efficiently mitigate the negative effects of phase noise.

Random Access Protocols for Massive MIMO

5G wireless networks are expected to support new services with stringent requirements on data rates, latency and reliability. One novel feature is the ability to serve a dense crowd of devices, calling for radically new ways of accessing the network. This is the case in machine-type communications, but also in urban environments and hotspots. In those use cases, the high number of devices and the relatively short channel coherence interval do not allow per-device allocation of orthogonal pilot sequences. This article addresses the need for random access by the devices to pilot sequences used for channel estimation, and shows that Massive MIMO is a main enabler to achieve fast access with high data rates, and delay-tolerant access with different data rate levels. Three pilot access protocols along with data transmission protocols are described, fulfilling different requirements of 5G services.

Adaptive Pilot Clustering in Heterogeneous Massive MIMO Networks

We consider the uplink of a cellular massive MIMO network. Acquiring channel state information at the base stations (BSs) requires uplink pilot signaling. Since the number of orthogonal pilot sequences is limited by the channel coherence, pilot reuse across cells is necessary to achieve high spectral efficiency. However, finding efficient pilot reuse patterns is non-trivial especially in practical asymmetric BS deployments. We approach this problem using coalitional game theory. Each BS has a few unique pilots and can form coalitions with other BSs to gain access to more pilots. The BSs in a coalition thus benefit from serving more users in their cells, at the expense of higher pilot contamination and interference. Given that a cell's average spectral efficiency depends on the overall pilot reuse pattern, the suitable coalitional game model is in partition form. We develop a low-complexity distributed coalition formation based on individual stability. By incorporating a base station intercommunication budget constraint, we are able to control the overhead in message exchange between the base stations and ensure the algorithm's convergence to a solution of the game called individually stable coalition structure. Simulation results reveal fast algorithmic convergence and substantial performance gains over the baseline schemes with no pilot reuse, full pilot reuse, or random pilot reuse pattern.

Performance of OPS-SAP technique for PAPR reduction in IEEE 802.11p scenarios

Vehicular Ad Hoc Networks (VANETs) are wireless networks that emerged thanks to the rapid evolution of wireless technologies and the automotive industry. The IEEE 802.11p standard is part of a group of standards related to all layers of protocols for Wireless Access in Vehicular Environment (WAVE) communications, which defines Medium Access Control (MAC) and Physical (PHY) levels. The PHY layer of IEEE 802.11p is essentially based on Orthogonal Frequency Division Multiplexing (OFDM) due to its advantages. However, OFDM signal suffers from high Peak-to-Average Power Ratio (PAPR) at the transmitter side, which causes a significant power efficiency penalty. An efficient peak power reduction technique is Simple Amplitude Predistortion aided by Orthogonal Pilot Sequences (OPS-SAP), which consists in moving certain outer constellation points of the frequency-domain OFDM symbol. In this paper, we propose the application of this OPS-SAP scheme in the IEEE 802.11p scenario, and, moreover, its evaluation under a complete PHY layer.

Communication Using a Large-Scale Array of Ubiquitous Antennas: A Geometry Approach

The recent trends of densification and centralized signal processing in radio access networks suggest that future networks may comprise ubiquitous antennas coordinated to form a network-wide gigantic array, referred to as the ubiquitous array (UA). In this paper, the UA communication techniques are designed and analyzed based on a geometric model. Specifically, the UA is modeled as a continuous circular/spherical array enclosing target users and free-space propagation is assumed. First, consider the estimation of multiuser UA channels induced by user locations. Given single pilot symbols, a novel channel estimation scheme is proposed that decomposes training signals into Fourier/Laplace series and thereby translates multiuser channel estimation into peak detection of a derive function of location. The process is shown to suppress noise. Moreover, it is proved that estimation error due to interference diminishes with the increasing minimum user-separation distance following the power law, where the exponent is 1/3 and 1 for the circular and spherical UA, respectively. If orthogonal pilot sequences are used, channel estimation is found to be perfect. Next, consider channel-conjugate data transmission that maximizes received signal power. The power of interference between two users is shown to decay with the increasing user-separation distance sub-linearly and super-linearly for the circular and spherical UA, respectively. Furthermore, a novel multiuser precoding design is proposed by exciting different phase modes of the UA and controlling the mode weight factors to null interference. The number of available degrees of freedom for interference nulling using the UA is proved to be proportional to the minimum user-separation distance.

Downlink Hybrid Information and Energy Transfer With Massive MIMO

We consider a downlink massive MIMO system, where the base station simultaneously sends information and energy to information users and energy users, respectively. The aim is to maximize the minimum harvested energy among the energy users while meeting the rate requirements of information users. With perfect channel state information (CSI), the problem is solved by obtaining the asymptotically optimal power allocation of information users and the combination coefficients of the energy precoder. For the CSI estimation in time-division duplex systems, orthogonal pilot sequences are employed by information users during the uplink, and one common pilot sequence is shared by all energy users. It is shown that the energy-harvesting performance of such a shared pilot scheme is always better than that of the orthogonal pilot scheme. Further, exploiting the intercell interference in multicell systems, a joint precoder is proposed for cooperative energy transfer, for which both the centralized and distributed implementations are given. Results indicate that the cooperative energy transfer always outperforms the noncooperative scheme with either perfect or estimated CSI.

Energy-efficient beamforming for two-tier massive MIMO downlink

Heterogeneous networks (HetNets) consisting of macro cells with very large antenna arrays and a secondary tier of small cells with a few antennas each can well tackle the contradiction of large coverage of the network and high data rate at the hot spots. However, it is not permissible to assign orthogonal pilot sequences for all the supported users due to the large number. Hence, we propose a pilot reduction scheme based on the heterogeneous system configurations and the unique topology of this HetNet. The reusing of pilot sequences causes the presence of the contaminated channel state information (CSI) and results in receivers' Quality of Service (QoS) outage. With the contaminated CSI, we provide an energy-efficient beamforming based on minimizing the total power consumption while keeping the QoS constraints satisfied and restricting the QoS outage probability below a given specification. By applying the approach of Bernstein approximation and semi-definite relaxation, we transform the original intractable chance constrained program to a convex problem conservatively. Numerical results show that the average power consumption of the proposed beamforming for our pilot reduction scheme is close to that of the perfect CSI case. Since our scheme will greatly compress the length of pilot sequence especially for those highly densified network with large number of small cells, it will be crucially helpful to put such two-tier massive multiple-input and multiple-output (MIMO) systems into practice.

Energy efficient peak power reduction in OFDM with amplitude predistortion aided by orthogonal pilots

The high Peak-to-Average Power Ratio (PAPR) is a main drawback of Orthogonal Frequency Division Multiplexing (OFDM) systems. We propose a twostep technique to reduce the PAPR consisting of a metricbased constellation extension method, such as Simple Amplitude Predistortion (SAP) algorithm, aided by Orthogonal Pilot Sequences (OPS) in a previous step, where we also provide a low-complex implementation of OPS scheme. We show that our proposal, named OP-SAP, outperforms previous approaches in terms of PAPR reduction, due to joining the benefits of Orthogonal Pilots with SAP algorithm. Moreover, it is energy efficient within two aspects: transmitted energy and implementation energy. OP-SAP saves up to 57% of transmitted energy per predistorted symbol compared to SAP. Regarding implementation energy, PAPR reduction techniques introduce some additional computational complexity, which requires extra cycles in the processor that demand energy consumption. We present an exhaustive analysis on computational power cost that shows the low power consumption of OP-SAP compared to other methods as SeLected Mapping (SLM), what yields a remarkable energy saving in its practical implementation1.

Estimation of residual carrier and sampling frequency offsets in OFDM-SDMA uplink transmissions

This paper investigates the estimation of multiple residual carrier frequency offsets (RCFOs) and sampling frequency offsets (SFOs) in the uplink of a multiuser OFDM network with space division multiple access (SDMA). The proposed solutions are based on the presence of some dedicated pilot tones within the signal spectrum, which are traditionally employed in OFDM systems to track channel variations and residual frequency errors. The main obstacle is the large number of parameters involved in the estimation process, which makes the uplink synchronization a rather challenging task. A practical solution to this problem relies on the separation of the received signals before the estimation procedure is started. This way, the RCFOs and SFOs of different users are estimated independently with affordable complexity. We propose two alternative approaches for users' separation. The former is based on the use of orthogonal pilot sequences, while the latter exploits the distinct spatial signatures of the uplink signals. Computer simulations and theoretical analysis are employed to assess the performance of the proposed schemes and to make comparison with existing alternatives.

Analysis of Pilot-Assisted Channel Estimators for OFDM Systems With Transmit Diversity

In this paper, we analyze the performance of pilot-assisted least square (LS) and minimum mean squared error (MMSE) channel estimators for orthogonal frequency division multiplexing (OFDM) systems with transmit antenna diversity. We first provide a design of orthogonal pilot sequences to simplify the estimators. We then analyze the mean squared error (MSE) performance, and study the leakage effect. When a channel tap is not sample-spaced, our analysis shows that the power of the channel tap will leak to not only other taps of the same antenna, but also taps belonging to other antennas. The leakage across antennas is mainly determined by the phase separation between pilot sequences, which is further related to the ratio between the number of pilots and number of antennas. We demonstrate that the MSE performance can be improved if more pilots are used, or fewer channels are estimated simultaneously.

Whitening-rotation-based semi-blind MIMO channel estimation

This paper proposes a whitening-rotation (WR)-based algorithm for semi-blind estimation of a complex flat-fading multi-input multi-output (MIMO) channel matrix H. The proposed algorithm is based on decomposition of H as the matrix product H=WQ/sup H/, where W is a whitening matrix and Q is unitary rotation matrix. The whitening matrix W can be estimated blind using only received data while Q is estimated exclusively from pilot symbols. Employing the results for the complex-constrained Cramer-Rao Bound (CC-CRB), it is shown that the lower bound on the mean-square error (MSE) in the estimate of H is directly proportional to its number of unconstrained parameters. Utilizing the bounds, the semi-blind scheme is shown to be very efficient when the number of receive antennas is greater than or equal to the number of transmit antennas. Closed-form expressions for the CRB of the semi-blind technique are presented. Algorithms for channel estimation based on the decomposition are also developed and analyzed. In particular, the properties of the constrained maximum-likelihood (ML) estimator of Q for an orthogonal pilot sequence is examined, and the constrained estimator for a general pilot sequence is derived. In addition, a Gaussian likelihood function is considered for the joint optimization of W and Q, and its performance is studied. Simulation results are presented to support the algorithms and analysis, and they demonstrate improved performance compared to exclusively training-based estimation.

Peak power reduction for OFDM systems with orthogonal pilot sequences

In this paper, a novel peak-to-average power reduction approach for orthogonal frequency division multiplexing (OFDM) has been addressed. Two-dimensional pilot-symbol assisted modulation (2D-PSAM) is employed in coherent OFDM for channel estimation, and it is based on inserting known symbols spread throughout the 2D time-frequency grid. These pilot symbols are employed to simultaneously perform distortionless peak power reduction with a suboptimum technique named orthogonal pilot sequences (OPS), which reduces additional system complexity and side information compared to optimum pilot values. This proposal attains a further step over other previous works, since this set of sequences allows blind detection at the receiver without prior knowledge of any side information.

Per-survivor processing-based decoding for space-time trellis code

The problem of adaptive decoding of space-time trellis codes on time-variant wireless channels is considered. We show that per-survivor processing (PSP) can be adopted to obtain approximated adaptive maximum-likelihood sequence detection (MLSD) of space-time trellis codes when there is no periodically inserted orthogonal pilot sequence. Then we propose a self-tuning least mean square (LMS)-based PSP decoder and a second-order LMS-based one. The former has the advantage that there are only fading rate-independent parameters to be predetermined, while the latter can offer fairly good performance on moderately fast time-varying channels.

Second-Order LMS Based Per-Survivor Processing Decoder for Space-Time Trellis Code

An adaptive space-time trellis code (STTC) decoder with the second-order LMS (SOLMS) based channel tracking is studied in this paper. We first show that per-survivor processing (PSP) can be adopted as the approximation of the adaptive maximum likelihood sequence detection (MLSD) of STTC when there are no periodically inserted orthogonal pilot sequences (PIOPS). With this presupposition, we propose a new decoder for STTC, called SOLMS based PSP decoder, in which we use min-max optimum SOLMS algorithm for channel tracking when some ``boundary'' statistical knowledge of the fading channel is available. Intensive computer simulations have been done to compare the performance of the proposed decoder with other known decoding methods as applied to long burst transmissions.

Optimal design and placement of pilot symbols for channel estimation

The problem of designing and placing pilot symbols for the estimation of frequency-selective random channels is considered. The channel is assumed to be a block-fading model with finite impulse response (FIR). For both single-input single-output (SISO) and multiple-input multiple-output (MIMO) channels, under the assumption of independent and identical distributed channel taps, the Cramer-Rao bound (CRB) on the mean square error (MSE) of semi-blind channel estimators is derived and minimized with respect to pilot symbols and their placement. It is shown that the optimal strategy is to place pilot symbols satisfying certain orthogonality condition in such a way that data and pilot symbols with higher power are in the middle of the packet. The results also indicate that the optimal pilot placements are independent of channel probability distribution. For constant modulus symbols, we show that the quasi-periodic placement and its generalization in the multiuser case turn out to be optimal. We further consider estimating channels with correlated taps and show that the previous placement strategy is also optimal among orthogonal pilot sequences.

A space-time coding modem for high-data-rate wireless communications

This paper presents the theory and practice of a new advanced modem technology suitable for high-data-rate wireless communications and presents its performance over a frequency-flat Rayleigh fading channel. The new technology is based on space-time coded modulation (STCM) with multiple transmit and/or multiple receive antennas and orthogonal pilot sequence insertion (O-PSI). In this approach, data is encoded by a space-time (ST) channel encoder and the output of the encoder is split into N streams to be simultaneously transmitted using N transmit antennas. The transmitter inserts periodic orthogonal pilot sequences in each of the simultaneously transmitted bursts. The receiver uses those pilot sequences to estimate the fading channel. When combined with an appropriately designed interpolation filter, accurate channel state information (CSI) can be estimated for the decoding process. Simulation results of the proposed modem, as applied to the IS-136 cellular standard, are presented. We present the frame error rate (FER) performance results as a function of the signal-to-noise ratio (SNR) and the maximum Doppler frequency, in the presence of timing and frequency offset errors. Simulation results show that for a 10% FER, a 32-state eight-phase-shift keyed (8-PSK) ST code with two transmit and two receive antennas can support data rates up to 55.8 kb/s on a 30-kHz channel, at an SNR of 11.7 dB and a maximum Doppler frequency of 180 Hz. Simulation results for other codes and other channel conditions are also provided. We also compare the performance of the proposed STCM scheme with delay diversity schemes and conclude that STCM can provide significant SNR improvement over simple delay diversity.