Bandwidth expansion

Abstract: For the purpose of various synchronization tasks (including carrier phase, time, frequency, and frame synchronization), one may add a known pilot sequence, typically a pseudo-noise sequence, to the unknown data sequence. This approach is known as a spread-spectrum pilot technique or as a superimposed pilot sequence technique. In this paper, we apply the superimposed pilot sequence technique for the purpose of channel estimation (CE). We propose and verify a truly coherent receiver based on the Viterbi algorithm, which is optimal in the sense of per-survivor processing. We also suggest a generic low-cost receiver structure based on reduced-state sequence estimation. Among the distinct advantages compared to conventional pilot-symbol-assisted CE are (i) a lack of bandwidth expansion and (ii) a significantly improved performance in fast fading environments. The proposed Viterbi receiver may also be used as an alternative receiver for pilot-symbol-assisted CE.

Abstract: An ultra-wide bandwidth time-hopping spread-spectrum code division multiple-access system employing a binary PPM signaling has been introduced by Scholtz (1993), and its performance was obtained based on a Gaussian distribution assumption for the multiple-access interference. In this paper, we begin first by proposing to use a practical low-rate error correcting code in the system without any further required bandwidth expansion. We then present a more precise performance analysis of the system for both coded and uncoded schemes. Our analysis shows that the Gaussian assumption is not accurate for predicting bit error rates at high data transmission rates for the uncoded scheme. Furthermore, it indicates that the proposed coded scheme outperforms the uncoded scheme significantly, or more importantly, at a given bit error rate, the coding scheme increases the number of users by a factor which is logarithmic in the number of pulses used in time-hopping spread-spectrum systems.

Abstract: We propose a deep neural network (DNN) approach to speech bandwidth expansion (BWE) by estimating the spectral mapping function from narrowband (4 kHz in bandwidth) to wideband (8 kHz in bandwidth). Log-spectrum power is used as the input and output features to perform the required nonlinear transformation, and DNNs are trained to realize this high-dimensional mapping function. When evaluating the proposed approach on a large-scale 10-hour test set, we found that the DNN-expanded speech signals give excellent objective quality measures in terms of segmental signal-to-noise ratio and log-spectral distortion when compared with conventional BWE based on Gaussian mixture models (GMMs). Subjective listening tests also give a 69% preference score for DNN-expanded speech over 31% for GMM when the phase information is assumed known. For tests in real operation when the phase information is imaged from the given narrowband signal the preference comparison goes up to 84% versus 16%. A correct phase recovery can further increase the BWE performance for the proposed DNN method.

Abstract: Trellis coded modulation with two or multidimensional signal constellations, together with coherent maximum-likelihood detection, is considered an attractive solution for communications over the additive white Gaussian noise (AWGN) channel. In this paper a new noncoherent communication system is introduced called noncoherent coded modulation (NCM) as an alternative to coherent coded modulation. NCM achieves almost the same power efficiency, without bandwidth expansion or an extensive increase in complexity. As a noncoherent system, the method does not need carrier phase estimation. Nonetheless, differential encoding is not required. High performance noncoherent detection is achieved by using multiple symbol observations. Unlike previous approaches, a sliding window for the observations is used, with each observation covering several branches of the trellis, such that the observations are time-overlapped. We define a new type of noncoherent maximum-likelihood sequence estimator (MLSE), and analyze its performance over the AWGN channel by numerical calculation of the union bound. We perform a computerized search and present new codes for noncoherent detection with their performance. The new codes cover many useful rates and complexities and achieve higher performance than existing codes for noncoherent detection. The method can also be used for multiple symbol demodulation of MDPSK with better results than existing methods.