BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which
FIG. 1 is a block diagram of a frequency domain enhanced base band filtering according to a first embodiment of the present invention;
FIG. 2 shows a portion of FIG. 1;
FIG. 3 is a block diagram of a frequency domain enhanced base band filtering in systems with time domain base band signal generation according to a second embodiment of the present invention;
FIG. 4 is a graph showing an amplitude versus frequency response of a 65 tap FIR low-pass filter;
FIG. 5 is a graph showing an amplitude versus frequency response of a 22 tap FIR low-pass filter;
FIG. 6 is graph showing the impulse response of the 65 tap FIR low-pass filter;
FIG. 7 is graph showing the impulse response of the 22 tap FIR low-pass filter;
FIG. 8 is a graph showing an OFDM base band spectrum of 600 sub-carriers with 15 kHz sub-carrier offset at Nyquist frequency using reference filtering with the 65 tap FIR low-pass filter and an enhanced filtering with the 22 tap FIR low-pass filter; and
FIG. 9 shows an enlarged extract portion of FIG. 8.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a simplified block diagram of a frequency domain enhanced base band filtering according to an embodiment of the present invention. Starting from the base band signal generation and ending at a radio frequency transmitter. After a modulation, the base band signal is provided in frequency domain. In this embodiment, the way of obtaining a simpler filtering, either in digital or analog domain, is to allow significantly greater amplitude variation in the pass band of the filter while maintaining a large enough attenuation in the stop band. In order to maintain the original frequency response of the base band, the frequency domain base band signal after modulation is multiplied by an inverse transfer function of the simplified filtering in frequency domain. In FIG. 1, the inverse transfer function is shown as a coefficient vector F−1ANA(f) (analog domain) and F−1DIG(f) (digital domain). In the following, the filter used for the simplified filtering will be referred to as “enhanced” or “simplified” filter.
As further shown in FIG. 1, the signal which consists of the result of the multiplication is converted from frequency domain to time domain by an inverse fast fourier transform process. This process is carried out in a circuit element IFFT of FIG. 1. The output of the IFFT is a time domain signal. The process carried out in the embodiment shown in FIG. 1 and described so far has been done in the digital domain.
According to the embodiment shown in FIG. 1, a cyclic prefix is added to the output of the IFFT in the time domain. The addition of a cyclic prefix in this part of the base band processing does not have an impact on the transmitter. Namely, the addition of a cyclic prefix to a or each symbol solves both inter-symbol-interferences and inter-channel-interferences. Assuming that the channel impulse response has a known length L, the prefix consists simply of copying the last L-1 values from each symbol and appending them in the same order to the front of the symbol. By doing so, the convolution of the impulse response with the signal at the end of a symbol does not affect any of the actual data at the beginning of the next symbol. The above described enhanced filtering can be implemented in transmitters which use a cyclic prefix, wherein, after a symbol in the transmitted signal is converted from frequency to time domain, a part of the signal from the end is copied to the beginning of the symbol.
The embodiment of FIG. 1 further comprises a digital low pass filter LPF having a digital filter transfer function FDIG(t) in time domain wherein the time domain output signal of IFFT is input. The output of the low pass filter LPF is converted from digital to analog domain by a digital-to-analog converter DAC and thereafter input into an analog low pass filter LPF having an analog filter transfer function FANA(t). The output of the analog low pass filter LPF is finally transmitted by the radio frequency transmitter designated in FIG. 1 as RF transmitter.
In addition to simplifying a filter that includes a single filter element, it is also possible to simplify a filter that consists of several filter elements and multiply the frequency domain signal with a coefficient vector where the inverse transfer function of all filter elements are combined. A digital base band filter that is enhanced can also operate at an oversampled frequency.
Let Y be a vector with n complex numbers representing a base band signal sample (symbol) in frequency domain. Respectively, A is the coefficient vector to compensate the filter transfer function, with n real or complex numbers. Real numbers can be used if only the amplitude response of the filter is compensated; complex numbers can be used if both the amplitude and phase response need to be compensated. X is the frequency domain compensated base band signal which is generated by multiplication of A and Y, as shown as follows:
x
2
=a
1
·y
1
x
2
=a
2
·y
2
X=Ā· Y
or
x
n−1
=a
n−1
·y
n−1
x
n
=a
n
·y
n
The coefficients xn, an, and yn are at the same base band frequency; i.e. an represents the filter inverse transfer function at the same frequency as xn; and y represents the base band signal. This is shown in FIG. 2.
A represents the required samples from the inverse of the filter frequency domain transfer function. In case of multiple filters, A is the product of the transfer functions of those filters.
The multiplication can be done at any point before the signal is in time domain. It may be best to have the multiplication as close to the signal conversion from frequency to time domain if the representation of X requires more bits than Y.
The implementation requires that the base band signal is represented in frequency domain before the filtering. In systems where the base band signal is generated in time domain, it is possible to make a time-to-frequency domain conversion such as fast fourier transform process carried out by a circuit element FFT, and then complete the compensation in the same way, as described above, and to convert the signal back to time domain as shown in FIG. 3.
The operation of the base band filtering in accordance with embodiments of the invention is demonstrated by two simulations of an OFDM base band signal generation with traditional base band filtering and with the enhanced base band filtering, with reference to FIGS. 4 to 9. Both cases are using QPSK modulated OFDM with 600 sub-carriers with 15 kHz spacing i.e. the 10 MHz channel bandwidth case of EUTRA downlink transmission. The base band clocking frequency is 15.36 MHz. The reference case uses a 65 tap Nyquist rate FIR (finite impulse response) filter with 0.1 dB pass band variation and 38 dB stop band attenuation. The enhanced case uses a 22 tap Nyquist rate FIR filter with 10 dB pass band variation and 38 dB stop band attenuation. The amplitude response of these filters is shown in FIG. 4 and FIG. 5. The enhanced filter has not been optimized for this case but was chosen as an example of what could be easily possible to implement.
The impulse responses of the reference and enhanced filter are shown in FIG. 6 and FIG. 7. The impulse response of the filter defines how the filtered signal energy spreads in time. The signal spreading is highly unwanted for example in OFDM since it increases the inter-symbol interference and therefore limits the maximum achievable data rates. The impulse response of the enhanced filter is only about one third of the reference case.
FIG. 8 shows the spectra of the two base band signal cases overlapped in the same figure. The reference signal has a worse performance on the transition region from pass band to stop band. As can be seen, there is no significant difference in the pass band and stop band attenuation so that the performance of the filters is equally good at those areas. The noise in the stop band is larger for the reference case, but the minimum attenuation in the stop band is very similar.
FIG. 9 shows a close-up of FIG. 8. In OFDM there are discontinuities of the signal in time domain between the modulated symbols which causes signal spreading which can be regarded as noise or interference occurring both on the transmit channel and the adjacent channel. Since the enhanced filter affects this interference already starting from the pass band, the performance is better in the transition band. As an example, the better filtering in the transition band can be used to increase the number of subcarriers in OFDM.
The better base band performance achieved by enhanced filtering can be used to improve the spectrum efficiency, i.e. the data transfer speed in communication systems. It is also possible to trade off the good base band performance to ease the implementation of other transmitter parts such as the power amplifier and the clipping algorithms.
Other base band signal processing, e.g. signal clipping and pre-distortion, should take into account that the base band signal has been altered between the multiplication and the last enhanced filter if it modifies the signal at this part of the base band chain.
Embodiments of the invention may require an increased number of bits to represent the frequency domain signal after the multiplication with the coefficient vector. The accuracy of the base band signal in time domain depends on the number of used bits after the multiplication.
Finally, it should be noted that the above preferred descriptions are of preferred examples for implementing embodiments of the present invention, but the scope of the present invention should not necessarily be limited by this description. It will be apparent to those skilled in the art that a person understanding this invention may conceive of changes or other embodiments or variations, which utilize the principles of this invention without departing from the broader spirit and scope of the invention as set forth in the appended claims. All are considered within the sphere, spirit, and scope of the invention.
Patent Application
20070258526
