Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1998-10-16
2001-12-11
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S298000
Reexamination Certificate
active
06330289
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to base station multi-channel power amplifier (also called a multi-channel linear power amplifier or MCLPA) performance.
BACKGROUND TO THE INVENTION
We will discuss the invention with reference to an example cellular system for which the invention is beneficial, namely IS-54-B Cellular System Dual-Mode Mobile Station—Base Station Compatibility Standard (800 MHz AMPS/TDMA cellular systems). Existing Multi-Channel Power Amplifiers (MCLPAs) used in IS-54-B cellular systems must function with both analog and digital modulation schemes. MCLPAs must provide a highly linear response over a large dynamic range. This wide dynamic range is required since the characteristics of the signals amplified by the MCLPA include a very large peak-to-average signal power.
Individual signals being fed into the amplifier have an inherently high peak-to-average signal power. The largest component of the peak signal power occurs whenever a modulation symbol goes through transition. Multiple input signals increase the total peak-to-average signal power whenever modulation transitions on multiple signals occur at the same time effectively increasing the amplitude of the peak transient.
AMPS analog transmission is based upon one traffic channel per Radio Frequency (RF) carrier. In TDMA mode, three traffic channels are imposed on one RF carrier using digital modulation techniques. The ensuing linearity requirements in TDMA mode call for a highly linear power amplifier to maintain modulation mask integrity.
A problem with TDMA modulation when applied to an MCLPA environment is periodic correlation of symbol transitions. In a typical system, a base station is connected to the rest of the wireless network by one or more T
1
connections. Multiple signals are carried by each T
1
link, and there is a correlation between the multiple signal streams in the form of synchronized data bit transitions. For example, bits arriving on a T
1
span will have an inherent correlation to the T
1
span clock when they arrive at the base station span adapter. Modulation symbols represent one or more data bits (in the case of IS-54-B one modulation symbol represents two data bits). Timing correlations between multiple data bit streams at the span adapter will translate into timing correlations of the subsequent modulation symbols.
This correlation sometimes increases the peak input power far above the average power, and drives the MCLPA into a less linear state, generating Intermodulation Distortion (IMD) above the levels specified in IS-54-B.
MCLPAs are typically advertised with a continuous maximum rated power using a Continuous Wave (CW) carrier, such as Maximum 50 Watts CW. In order to avoid this problem in the past, many approaches have been attempted to ensure that adequate linearity is maintained. For example, cellular operators running these MCLPAs in TDMA mode have had to reduce the input drive (effectively the overall output power) to ensure that adequate linearity is maintained at the peak signal power levels. However, reducing total output power negatively affects cost, performance, and network capacity.
The prior art has seen manufacturers of cellular base station amplifier systems attempt many solutions to this problem. These prior art solutions have focused on design concepts to increase amplifier linearity. Error amplification, feedback, pre-distortion and feed-forward control techniques are commonly employed to increase the range of linearity of the amplifiers in order to avoid these problems. All of these techniques to enhance linearity of the MCLPA are accompanied by increased cost. While necessary for TDMA systems, this increased cost offers little benefit during AMPS operation, as the required degree of linearity for analog transmission is much less than digital requirements needed to support TDMA signals.
Therefore, there exists a need for an improved system which will overcome these problems at a reduced cost.
SUMMARY OF THE INVENTION
An object of the present invention is to provide improved base station multi-channel power amplifier (MCLPA) performance. However, unlike prior art solutions which have focused on enhancing the linearity of the MCLPA, the inventors herein have designed a system which decreases the linearity requirements for the MCLPA while operating in digital mode. This is achieved by reducing the peak-to-average signal power of the combined signals before amplification at the MCLPA. By reducing the peak signal power, this invention can advantageously reduce the cost per channel per Watt compared to conventional systems.
According to one aspect of the invention, there is provided a system which produces a number of bit stream signals to be modulated into said number of symbol stream signals, said system including means for introducing an offset/time delay to said symbol stream signals to avoid correlation of the symbol transitions in the MCLPA. By doing this, the system avoids multiple symbols going into transition at the same time. By preventing the signal peaks from occurring at the same time and reinforcing to form a larger peak signal, the system reduces the maximum peak signal power, without adversely affecting the average signal power. This reduces the peak power requirements placed on the MCLPA. Preferably, the sum total of all offsets (i.e., the maximum delay) is less than one symbol period. Note that the terms offset and time delay are used synonymously in this specification.
According to one embodiment these time delays can be introduced into the bit stream signals prior to modulation. In such a system the sum total of all offsets (i.e., the maximum delay) is preferably less than one bit period unless the system accounts for symbol boundaries in order to ensure no overlap of symbol transitions between the individual symbol streams.
Alternatively, said unique offsets/delays can be introduced to the symbol streams after symbol formation. By offsetting the symbol streams directly, the maximum delay introduced is extended to one symbol period from one bit period. The offset can be introduced by hardware or software at any point in the system after span adaption and prior to signal combining.
In accordance with a broad aspect of the present invention there is provided apparatus for reducing intersymbol correlation comprising: means for receiving a plurality of bit stream signals; means for modulating said plurality of bit stream signals to produce a plurality of symbol stream signals for amplification by a multi-channel power amplifier (MCLPA); and means for introducing a time offset in said symbol stream signals prior to amplification.
In accordance with another aspect of the invention, there is provided apparatus for reducing symbol correlation comprising: a span adapter whose output comprises a plurality of bit stream signals; a plurality of channel elements, wherein each channel element receives one of said plurality of bit stream signals and produces a corresponding symbol stream signal; and delay means for introducing a time offset in each of said symbol stream signals prior to amplification.
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Fayeski Eric Stephen
Keashly Dusty
Al-Beshrawi Tony
Chin Stephen
Measures Jeff
Nortel Networks Limited
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