Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-05-26
2003-10-21
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S296000
Reexamination Certificate
active
06636555
ABSTRACT:
FIELD OF INVENTION
The present invention relates to limiting the amplitude of a transmission signal, e.g., a telecommunication signal to be transmitted via a radio station.
BACKGROUND OF THE INVENTION
In telecommunications systems, usually a large number of communication channels is transmitted together via the same transmission medium, e.g., a radio frequency band. Various access schemes for placing communication channels on the transmission medium are known. A well-known scheme is CDMA (Code Division Multiple Access) where a number of different communication channels is transmitted simultaneously in a radio frequency band in such a way that they overlap in both the time domain and the frequency domain.
In order to distinguish each communication channel signal from the other communication channel signals, each communication channel signal is encoded with one or more unique spreading codes, as is well-known in the art. By modulating each of the communication channel signals with a spreading code, the sampling rate (i.e., the “chip rate”) may be substantially increased in accordance with a spreading factor. For example, each communication channel signal is modulated in accordance with a digital modulation scheme, e.g., a quadrature amplitude modulation (QAM) or a phase shift keying (PSK) technique. Consequently, an in-phase and quadrature component signal is produced for each communication channel signal. QAM and PSK are well known in the art. The in-phase and quadrature component signals associated with each of the communication channels are then encoded using a unique spreading code sequence. The resulting in-phase and quadrature component signal pairs are sampled (i.e., at the chip rate) and individually weighted. The in-phase and quadrature component signals are eventually combined to form a composite in-phase signal and a composite quadrature signal. The composite in-phase signal and the composite quadrature signal are then separately filtered by a low-pass, pulse shaping filter. Subsequent to filtering, the composite in-phase signal and the composite quadrature signal are modulated by a cosine-carrier and a sine-carrier respectively and combined into a single, multicode transmission signal, e.g., a CDMA signal. The single, multicode transmission signal is then upconverted by a carrier frequency and the signal power associated with the transmission signal is boosted by a high power amplifier prior to transmission. At the receiving unit, the baseband signal associated with each of the communication channel signals is extracted from the transmission signal by demodulating and decoding the transmission signal using the carrier frequency and the various spreading codes. Furthermore, it will be understood that in a typical cellular telecommunications system, the transmission source may, for example, be a high power base station, and the receiving entity may, for example, be a mobile station (i.e., a mobile telephone).
When there is an especially large number of communication channel signals, it is sometimes preferable to generate two or more transmission or carrier signals, wherein each of the two or more carrier signals is modulated with its own unique carrier frequency. The two or more modulated carrier signals are then independently amplified by a corresponding high power amplifier prior to transmission, or alternatively, the two or more modulated carrier signals are combined into a single, complex transmission signal, which is then amplified by a single, high power amplifier prior to transmission.
As one skilled in the art will readily appreciate, CDMA substantially increases system bandwidth, which in turn, increases the network's traffic handling capacity a whole. In addition, combining independent carrier signals into a single complex transmission signal, as described above, is advantageous in that a single high power amplifier is required rather than a separate high power amplifier for each independent carrier signal. This is advantageous because high power amplifiers are expensive, and employing one high power amplifier in place of many will result in a substantial cost savings.
Despite the advantages associated with CDMA, combining multiple communication channel signals and/or independent carrier signals, in general, significantly increases the peak-to-average power ratio associated with the resulting transmission signal. More specifically, the peak-to-average power ratio for a transmission signal can be determined in accordance with the following relationship:
PR
PTA
=PR
F
+10*log (
N
)
wherein PR
PTA
represents the peak-to-average power ratio of the corresponding composite signal, PR
F
represents the power ratio of the low pass, pulse shaping filter and N represents the number of communication channels which make up the carrier (CDMA) signal.
The problem associated with large peak-to-average power ratio is that it diminishes the efficiency of the high power amplifier in the transmitter. Efficiency as one skilled in the art will readily understand, is measured in terms of the amount of output power (i.e., Pmean) divided by the amount of input power (i.e., Pdc+Ppeak). As Ppeak (i.e., peak power) increases relative to Pmean, the efficiency of the high power amplifier decreases.
One possible solution is to simply limit or clip the amplitude (i.e, Ppeak) of the carrier signal. Unfortunately, this is likely to result in the generation of intermodulation products and/or spectral distortions. Intermodulation products and/or spectral distortions are, in turn, likely to cause interference between the various communication channel signals. Accordingly, this is not a preferred solution.
Another possible solution is to design a more complex high power amplifier, one that can tolerate and more efficiently amplify (CDMA) carrier signals that exhibit large peak-to-average ratios. However, this too is not a preferred solution as the cost of high power amplifiers are generally proportional to complexity. Accordingly, this solution would result in driving up the cost of the telecommunications device that houses the high power amplifier.
U.S Pat. No. 5,621,762 (“Miller et al.”) offers yet another possible solution for the peak-to-average power ratio problem, that is to limit the peak-to-average power ratio before the soon-to-be transmitted telecommunications signal is filtered and subsequently amplified. More specifically, Miller describes a peak power suppression device for reducing the peak-to-average power ratio of a single code sequence at the input of the high power amplifier. The peak power suppression device employs a digital signal processor (DSP) which receives the single code sequence, maps the code sequence onto a symbol constellation diagram, predicts an expected response from the pulse shaping filter and limits the amplitudes appearing on the symbol constellation diagram in accordance with the expected response of the pulse shaping filter.
The primary problem with the solution offered in Miller is that the peak power suppression device is incapable of coping with the high data bit rates encountered in telecommunications systems such as CDMA. Further, the device is incapable of coping with multiple carrier channel signals and/or multi-code sequences. For example, the peak power suppression device described in Miller is inherently slow, as evidenced by the fact that it employs a DSP (Digital Signal Processor), and by the fact that the DSP has the time necessary to execute a pulse shaping filter prediction algorithm. Therefore, a need exists for a telecommunications signal amplitude limitation device that is capable of limiting the peak-to-average power ratio of a telecommunications signal before it is filtered and subsequently amplified, and additionally, is capable of handling significantly higher bit rates, multiple code sequences, and multiple CDMA carrier signals.
SUMMARY OF THE INVENTION
It is therefore object of the invention to provide a method and apparatus for limiting the amplitude of a complex transmission signal comprising a plurality of
Frank Georg
Niegel Michael
Rödel Udo
Jenkens & Gilchrist P.C.
Pham Chi
Telefonaktiebolaget LM Ericsson(publ)
Tran Khanh Cong
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