The present invention relates to the field of transmitting and receiving digital signals over a radio link. In particular, it relates to a method and circuit for properly controlling the peak power of a filtered signal in a single carrier data transmission system.
In order to make efficient use of the microwave radio spectrum, state-of-art digital radio systems employ highly bandwidth-efficient modulation techniques. Today, several commercial systems employ a 128 QAM signal constellation and some 512 QAM prototypes are currently being field-tested.
Moreover, adjacent channel interference specifications require the use of tightly band limited signaling formats: to comply with these requirements, pulse shapes with very small roll-off factor are usually used. Unfortunately, these pulse shapes result in a substantial increase of the PAR (peak to average power ratio).
As the number of the points of the signal constellation grows, the system becomes more and more sensitive to all types of linear and nonlinear signal distortion. A particularly critical issue in bandwidth-efficient QAM systems is the non-linearity of the high power amplifier (HPA) used at the transmitter. A non-linear HPA results in self interference and, due to the growth of the transmitted spectrum out of the band allocated to the transmitted signal, in adjacent channel interference.
In principle, the effect of HPA non-linearity could be reduced by backing-off the output signal level from the amplifier saturation point. Unfortunately, this reduces the transmitted signal power and, consequently, the radio link flat-fade margin. Moreover, the HPA back-off has to be increased when pulse shapes with small roll-off factor are used.
To increase the fade margin, the HPA must be driven as close to its saturation point as possible. Thus, amplifier efficiency can be increased by using a signal with a lower ratio of peak-to-average power (and, of course, with the same bandwidth efficiency). At the present the problem of reducing the peak-to-average power ratio is faded in one of the following four ways.
The first way consists in using “shaping” constellation techniques in order to reduce the ratio between the peak power and the average power of the unfiltered signal. Such techniques are disclosed, for instance, in G. David Formey, “Trellis Shaping”, IEEE Trans. Inform. Theory, vol. 38, no. 2, pages 281-300, March 1992.
Such a solution provides, under certain circumstances, unsatisfactory performances. In particular it is not able to remarkably reduce the peak power of the filtered signal and thus does not increase the efficiency of HPA's.
The second possibility consists in providing analog signal predistortion. In other words, a non-linear circuit having a characteristic inverse to the one of the HPA is inserted in the path of the analog signal.
Such a second arrangement is not able to realize exactly the inverse characteristic of the HPA. Moreover, this response may vary with temperature and aging.
The third arrangement consists in performing channel equalization and non-linear cancellation of Inter Symbol Interference (ISI), namely the receiving equalizer tries to cancel the interference connected with non-linearity.
The main disadvantage of this third solution is that it does not operate out of band emission. Moreover, with small values of the roll-off factor, it would require very complex equalizers.
Finally, the last known solution to the above problem is providing codes to control the peak power of the filtered signal at the amplifier input. This solution is disclosed in A. Spalvieri, A. Sandri, D. Mapelli, “Codes for the peak power constrained channel”, GLOBECOM 1995, IEEE, Vol. 1, pages 639-643. The basic idea under such a solution is avoiding transmission of sequences having an associated high peak power of the filtered signal, and replacing them by more suitable ones namely by sequences with a lower power of the filtered signal. The possibility of carrying out this replacement is given by the increasing of the dimension of the alphabet of the transmitted points. Of course, in reception, the unwanted sequences, suppressed in transmission, are reconstructed in their original form.
Also this last solution has disadvantages, as it requires very complex codes in order to achieve significant gains.