Digital block-based predistortion (PD) is an efficient scheme to linearize power amplifiers (PA). The performance of this method highly depends on the estimation accuracy of the amplifier characteristics. In this paper, the PA characteristics are approximated by piecewise continuous polynomials in the form of splines. The approximation is based on a block of observed data at the input and output of the PA. This approach leads to a linear parametric approximation problem with moderate implementation complexity, and it also gives the best possible fit to a given set of observations. With cubic splines it is possible to get a fairly accurate estimate even with a short block of data, and hence a fast convergence of the PD algorithm. The performance of the method has been evaluated for an example selected from an application in mobile satellite communications with burst transmission. We use a model of a 2 Watts class A PA that is based on measurements, and the calculations are performed for 16-QAM modulation. The mean square approximation error, the power spectrum and the error vector magnitude (EVM) of the received signal have been analyzed by simulations. For 6 dB back-off, the adjacent channel power ratio (ACPR) level is reduced by approximately 15 dB relative to a transmitter without predistortion, after an adaptation based on only 25 transmitted symbols.

This paper presents a method for calculating the reduction in crosstalk power sum obtained by canceling the M strongest interferers in a multi-pair cable. This approach uses Monte Carlo simulations, and it is based on an improved crosstalk model where all individual pair combinations in the cable are modeled separately. All parameters of the model are based on measurements, and an extensive set of crosstalk measurements is necessary in order to describe all relevant pair combinations. The results for far end crosstalk apply to both single carrier and multi- carrier systems. However, the results for near end crosstalk are valid only for multicarrier systems using per sub-channel cancellation. The method has been applied to three cable types of different sizes, which are all based on 10-pair binder groups. The results predict somewhat smaller crosstalk reductions than previously reported for cables containing up to ten pairs. However, it is shown that crosstalk cancellation in larger cables is far more challenging. Our results indicate that it may be difficult to obtain crosstalk reductions of more than 5 - 10 dB for cables of size 100 pairs or more for a realistic number of cancellers (M les 20).

New blind carrier frequency offset (CFO) estimation methods based on the correlation function of the subchannel signals are presented for OFDM/OQAM systems. The proposed estimators are robust to multipath effects due to the narrowband property of the subchannels in OFDM systems. The performance of the estimators is evaluated by asymptotic analysis and simulation results. Our results show that methods based on estimation in subchannels have better performance than method based on the signal before demodulation.

The high sensitivity to carrier frequency offset of OFDM systems compared to single carrier systems is well known. In this paper we study how this can be remedied for OFDM/OQAM systems by appropriate pulse shaping. Building on earlier methods for minimizing out-of-band energy, a procedure for pulse shape design is derived. The new pulses are optimal with respect to interference in OFDM/OQAM systems. It is also demonstrated that, in this respect, OFDM/OQAM outperforms OFDM/QAM systems using rectangular pulses.

This work presents a new and more accurate crosstalk model that is used for calculating the probability distributions of crosstalk power sum for M randomly selected pairs in an N-pair twisted pair cable. The calculation is based on averaged measurements of crosstalk in each individual pair combination. Probability distributions are calculated both for near end crosstalk (NEXT) and far end crosstalk (FEXT) in one specific type of commonly used cable that contains one 10-pair binder group. The results for the 10-pair cable show the same type of variation for the 99% point of crosstalk power sum as in the standard model used for DSL systems, which is based on 50-pair binder groups. However, the parameters are different for the two cases. For the 10-pair cable, there is a smaller variation in crosstalk as a function of the active number of pairs. The results for FEXT in a 10-pair group are in close correspondence with the standard model, while the level of NEXT is 1 2.5 dB weaker in the 10-pair group than in the standard model.

This paper presents pulses of finite duration for use in orthogonal frequency division multiplexing (OFDM) using offset QAM modulation. In such a system the pulses will overlap in both frequency and time without intersymbol or interchannel interference. The pulses are generated by an optimization procedure which minimizes the out-of-band energy under the constraints of zero intersymbol and interchannel interference. Low out-of-band energy is important to achieve high bandwidth efficiency, especially when the number of channels is small. The optimization procedure leads to an integral equation which is solved by expanding the pulse in truncated prolate spheroidal wave functions. Examples of optimized pulses and their spectrum are presented for lengths of 2 and 4 symbol intervals.

An approach to reduced-complexity detection of partial response continuous phase modulation (CPM) on a linear multipath channel is presented. The method, referred to as decision feedback sequence estimation (DFSE), is based on a conventional Viterbi algorithm (VA) using a reduced-state trellis combined with decision feedback (DF). By varying the number of states in the VA, the receiver structure can be changed gradually from a DF receiver to the optimal maximum-likelihood sequence estimator (MLSE). In this way different tradeoffs between performance and complexity can be obtained. Results on the receiver performance, based on minimum distance calculations and bit error rate simulations, are given for Gaussian minimum-shift keying modulation on typical mobile radio channels. It is shown that for channels with a long memory, a significant complexity reduction can be achieved at the cost of a moderate degradation in performance.>

It is shown how to improve the tracking properties of least squares (LS) estimation for a time-variant channel estimation (system identification) problem by introducing a parametric time-variant model instead of the conventional model, which is a constant impulse response in each iteration. A recursive LS algorithm is developed for the case that the model varies linearly with time. Simulation results are given for examples where the unknown channel has linear, sinusoidal, and stochastic variations with time. A significant improvement of the tracking properties compared to LMS and conventional LS algorithms is demonstrated for cases with high signal to noise ratio and smooth variations of the unknown channel.>

A new type of digital echo canceler for two-wire digital transmission is presented. The new principle involves very simple signal processing and is thus an interesting alternative for digital transmission on subscriber lines. The principle is compared with other echo cancellation techniques, and it is shown how choice of line code, equalization, and carrier recovery are affected by the new echo canceler. A theoretical analysis of the principle is given, taking into account finite accuracy, jitter, noise, and correlated data streams. The echo canceler can be used for line attenuation up to 40 dB. At 80 kbits/s this corresponds to at least 7 km 0.6 mm cable and is sufficient to cover more than 99 percent of the existing Norwegian subscriber lines.

A comparison between different line codes suitable for transmission of 140 Mbit/s digital signals over 1.2/4.4 mm coaxial cable has been made using the computer optimization program, "OPUT." One alternative, the two level partial response class 1 (PR1), is chosen and a laboratory system has been built and tested. A block diagram of the system followed by a description is given. Performance of the system is presented and commented on. It is concluded that a PR1 system can be implemented with reasonable noise margin with a repeater spacing of 2.15 km for a 1.2/4.4 mm coaxial cable.