Minimum Mean Square Error Turbo Equalization Research Articles

Experimental Evaluation of JANUS Fast Modes in Very High Acoustic Frequency Bands

This article presents extensive field tests of a fast-mode operation of the JANUS acoustic communication standard where the signals occupy high-frequency bands of 38 kHz spanning from 96 to 134 kHz instead of the nominal frequency band of 4.1 kHz spanning from 9.44 to 13.6 kHz specified in the standard. The fixed 32-chip preamble and the 144-chip baseline JANUS packet utilize the frequency-hopped binary frequency shift keying with 13 frequency pairs as defined in the standard while the cargo packets use the single-carrier <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary phase shift keying modulation with a center frequency of 115 kHz and a symbol rate of 23 ksps or up to 34.5 kb/s information data rate with high-order phase shift keying (PSK) and rate-1/2 forward error correction codes. The original JANUS receiver algorithm is modified to improve the frame/symbol synchronization for the fast mode and increase the decoding success rate of the baseline JANUS packet in difficult multipath channels. More than 10 experiments were conducted using a field-programmable-gate-array-based hardware platform consisting of a single transmit projector and a single receive hydrophone. The experiment results show that the JANUS fast mode worked well with both the original JANUS receiver algorithm and the modified receiver algorithm, yielding zero bit error in most of the baseline JANUS packets. The modified receiver algorithm is able to reduce 30% of error packets to zero error in the experiment experiencing difficult multipath channels where the original JANUS receiver algorithm suffers from large bit error rates (BER). Meanwhile, the cargo packets utilize the linear minimum mean-square error turbo equalizer and achieve a BER around 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> .

Filtered Multitone Modulation Underwater Acoustic Communications Using Low-Complexity Channel-Estimation-Based MMSE Turbo Equalization

Filtered multitone (FMT) modulation divides the communication band into several subbands to shorten the span of symbols affected by multipath in underwater acoustic (UWA) communications. However, there is still intersymbol interference (ISI) in each subband of FMT modulation degrading communication performance. Therefore, ISI suppression techniques must be applied to FMT modulation UWA communications. The suppression performance of traditional adaptive equalization methods often exploited in FMT modulation UWA communications is limited when the effect of ISI spans tens of symbols or large constellation sizes are used. Turbo equalization consisting of adaptive equalization and channel decoding can improve equalization performance through information exchanging and iterative processes. To overcome the shortcoming of traditional minimum mean square error (MMSE) equalization and effectively suppress the ISI with relatively low computation complexity, an FMT modulation UWA communication using low-complexity channel-estimation-based (CE-based) MMSE turbo equalization is proposed in this paper. In the proposed method, turbo equalization is first exploited to suppress the ISI in FMT modulation UWA communications, and the equalizer coefficients of turbo equalization are adjusted using the low-complexity CE-based MMSE algorithm. The proposed method is analyzed in theory and verified by simulation analysis and real data collected in the experiment carried out in a pool with multipath propagation. The results demonstrate that the proposed method can achieve better communication performance with a higher bit rate than the FMT modulation UWA communication using traditional MMSE adaptive equalization.

MMSE Turbo Equalization and Detection for Multicarrier Faster-Than-Nyquist Signaling

In this paper, two minimum mean-squared error (MMSE) turbo equalization and detection schemes for spectral efficient multicarrier faster-than-Nyquist (MFTN) signaling system are presented. First, the two-dimensional (2-D) soft input and soft output (SISO) MMSE equalization intended for magnetic recording channels, i.e., 2-D intersymbol interference (ISI), is extended to the detection of MFTN signals. Although the 2-D MMSE turbo equalization exhibits an attractive bit-error-rate (BER) performance under certain time-frequency spacing packing between adjacent symbols/subcarriers, the BER performance will be degraded when introducing serious intercarrier interference. Second, a 1-D SISO MMSE equalization combined with soft successive interference cancellation (SIC) is further proposed for the detection of MFTN signals, and it shows that the MMSE equalization combined with SIC could asymptotically approach the maximum a posterior equalization combined with SIC with negligible BER performance loss. Our computational complexity analysis and numerous simulation results demonstrate that the proposed MMSE turbo equalization schemes may be more preferred choice in the practical detection of MFTN signals.

Low Complexity Turbo Receiver for LTE Uplink with Transmitter IQ Imbalance

In this paper, we investigate two improved turbo receivers for the Long Term Evolution (LTE) uplink in the presence of transmitter (Tx) in-phase and quadrature-phase imbalance (IQI) with parameters known at eNodeB. For multiuser multiple-input multiple-output (MU-MIMO) single-carrier frequency division multiple access (SC-FDMA) systems, we derive a optimal joint linear minimum mean square error (MMSE) turbo multiuser detector (MUD) based on the mirror symmetry clusters. For the single use SC-FDMA system with Tx IQI, we derive an optimal widely linear MMSE (WLMMSE) turbo equalizer. Both receivers are implemented in the discrete frequency domain and only slightly increase the computational complexity compared to the conventional turbo receivers. Monte Carlo simulations show that the proposed receivers significantly outperform the conventional turbo receivers. The simulation results are then confirmed by the extrinsic information transfer (EXIT) chart analysis.

Adaptive Linear Turbo Equalization Over Doubly Selective Channels

Over the last decade, tremendous gains, leading to near-capacity achieving performance, have been shown for a variety of communication systems through the application of the turbo principle, i.e., the exchange of extrinsic information between constituent algorithms for tasks such as channel decoding, equalization, and multiple-input–multiple-output (MIMO) detection. In this paper, we study the practical application of such an iterative detection and decoding (IDD) framework to underwater acoustic communications. We explore complexity and performance tradeoffs of a variety of turbo equalization (TEQ)-based receiver architectures. First, we elaborate on two popular but suboptimal turbo equalization techniques: a channel-estimate-based minimum mean-square error TEQ (CE-based MMSE-TEQ) and a direct-adaptive TEQ (DA-TEQ). We study the behavior of both TEQ approaches in the presence of channel estimation errors and adaptive filter adjustment errors. We confirm that after a sufficient number of iterations, the performance gap between these two TEQ algorithms becomes small. Next, we demonstrate that an underwater receiver architecture built upon the least mean squares (LMS) DA-TEQ technique can leverage and dramatically improve the performance of the conventional implementation based on the decision-feedback equalizer at a feasible complexity. To maintain performance gains over time-varying channels, the slow convergence speed of the LMS algorithm has been improved via two methods: 1) repeating the weight update for the same set of data with decreasing step size and 2) reducing the dimensionality of the equalizer by capturing sparse channel structure. This receiver architecture was used to process collected data from the SPACE 08 experiment (Martha's Vineyard, MA). Receiver performance for different modulation orders, channel codes, and hydrophone configurations is examined at a variety of distance, up to 1 km from the transmitters. Experimental results show great promise for this approach, as data rates in excess of 15 kb/s could readily be achieved without error.

Improved turbo equalization based on soft ISI cancellation

Some turbo equalizers with perfect performance, such as the maximum a posteriori probability (MAP) equalizer and the minimum mean squared error (MMSE) turbo equalizer, are difficult to apply to certain wireless communication systems with large delay spread, due to their prohibitive complexity. By contrast, turbo equalization based on soft intersymbol interference (ISI) cancellation is much simpler; however, its performance is very poor. To avoid the shortcomings of the above equalizers, a novel equalization scheme is presented in this paper. The proposed method lies on an extension of soft ISI cancellation paradigm in the sense that the matched filter output is used to estimate the transmitter symbol. While the original method employs single output in the estimation, the number of outputs in the proposed method can vary depending on the delay spread of the channel. The extrinsic information transfer (EXIT) chart analyses and the simulation results show that the performance of the presented equalizer is much better than that of the original turbo equalization based on soft ISI cancellation, and it is only slightly more complex. The new turbo equalization scheme can perform as well as MMSE turbo equalization, but is simpler.

Nonlinear frequency domain MMSE turbo equalization using probabilistic data association

This letter proposes a new frequency domain turbo equalization algorithm based on nonlinear minimum-mean- squared-error (MMSE) estimation. The conceptual basis of the proposed equalizer is the probabilistic data association (PDA) filtering. It is shown that by performing internal equalizer iterations, of which structure is resulted from the algorithm derivation according to the PDA concept, the convergence properties, analytical and verified by simulations, can be significantly improved over conventional MMSE turbo equalization techniques. Results of the simulations conducted to demonstrate the superiority of the proposed algorithm and to verify the accuracy of the analysis are presented in this letter.

Nonlinear frequency domain MMSE turbo equalization using probabilistic data association

This letter proposes a new frequency domain turbo equalization algorithm based on nonlinear minimum-mean- squared-error (MMSE) estimation. The conceptual basis of the proposed equalizer is the probabilistic data association (PDA) filtering. It is shown that by performing internal equalizer iterations, of which structure is resulted from the algorithm derivation according to the PDA concept, the convergence properties, analytical and verified by simulations, can be significantly improved over conventional MMSE turbo equalization techniques. Results of the simulations conducted to demonstrate the superiority of the proposed algorithm and to verify the accuracy of the analysis are presented in this letter.

Low-complexity MMSE turbo equalization: a possible solution for EDGE

This paper deals with a low complexity receiver scheme where equalization and channel decoding are jointly optimized in an iterative process. We derive the theoretical transfer function of the infinite length linear minimum mean square error (MMSE) equalizer with a priori information. A practical implementation is exposed which employs the fast Fourier transform (FFT) to compute the equalizer coefficients, resulting in a low-complexity receiver structure. The performance of the proposed scheme is investigated for the enhanced general packet radio service (EGPRS) radio link. Simulation results show that significant power gains may be achieved with only a few (3-4) iterations. These results demonstrate that MMSE turbo equalization is an attractive candidate for single-carrier broadband wireless transmissions in long delay-spread environments.