GB2380079A

GB2380079A - A mobile phone base-station multicarrier amplifier with feedforward linearisation

Info

Publication number: GB2380079A
Application number: GB0218400A
Authority: GB
Inventors: John Domokos
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2001-09-20
Filing date: 2002-08-06
Publication date: 2003-03-26
Anticipated expiration: 2022-08-06
Also published as: GB0218400D0; GB2380079B; GB0122661D0

Abstract

A number of digital signals 2-4 in a multi-channel base-station transmitter are combined 14, converted 16 to an analogue signal and amplified by power amplifier 18. The output of the power amplifier 18 is compared 22 with a reference signal to generate a correction signal for adding 21 to the output of the amplifier 18. The reference signal is generated by providing a separate DAC 8-10 in each channel and summing the DAC outputs. The equaliser 15 matches the delay of the main path and the reference path and responds adaptively to the correction signal from element 15.

Description

Radio Communications Apparatus This invention relates to radio communications apparatus for transmitting multiple carriers particularly, but not exclusively, in a mobile telecommunications system.

Mobile telecommunications systems such as GSM and EDGE include a number of base stations at separate locations to provide coverage to mobile units over a large geographic area. The base stations serve areas referred to as cells. Each base-station is usually configured to transmit on a number of different carrier frequencies to enable a large number of connections to be supported. The different carriers are combined in the basestation at a component called a combiner and fed to the same antenna.

A problem occurs at the combiner where not all the input power is efficiently output. A lot of power is absorbed in the combiner and lost in the form of heat and this inefficiency rises with the number of carriers carried such that the power output is 1/N where N is the number of separate carriers. In an effort to reduce this power loss the step of combination has been carried out before the output power amplifier. In this way the combined signal is applied to the power amplifier. A problem with this architecture is that the power amplifier has to cater for a broader frequency range than if it was configured to cater for amplifying a single carrier. Practical power amplifiers have performance characteristics which depart from the ideal linear characteristic that means that the gain of the power amplifier varies with desired output away from a straight-line. The performance will also vary across the frequency range. In order to alleviate this problem one attempt at a

solution has been to use a signal cancellation loop and to feed forward the error signal from the input of the power amplifier to its output. A problem with this arrangement is that the single digital to analogue converter used to convert the combined carrier signals has a performance that varies both by amplitude and frequency.

According to the invention there is provided a radio transmitter section comprising carrier signal inputs to which in use at least two carrier signals are applied, a first signal path comprising a first combiner for combining the carrier signals to produce a combined signal output, a digital to analogue converter for converting the combined signal output to analogue form, a power amplifier for amplifying the analogue form of the combined signal to form a power amplified signal, and a second signal path comprising at least two digital to analogue converters operably coupled to the carrier signal inputs to produce analogue carrier signals, a second combiner operably coupled to the digital to analogue converters to combine the analogue carrier signals to produce a reference signal, a comparitor for comparing the reference signal of the second signal path with the output of the power amplifier of the first signal path to produce a comparison signal, an error amplifier responsive to the comparison signal to produce a correction signal to the output of the power amplifier to correct the power amplified signal.

A specific embodiment of the invention will now be described, by way of example only, with reference to the drawing in which Figure 1 shows in block diagram form a transmitter section of a radio communications base-station in accordance with the invention.

As is shown in figure 1, a transmitter section lincludes a number of carrier inputs 1 to n.

In this case n is three but in other embodiments may be a different number. The carrier inputs are labelled 2 to 4. Each carrier input is connected to a carrier generator not shown that encodes the message to be transmitted onto a carrier wave in a way that will be familiar to a person skilled in the art. The encoded signal is coupled from the carrier input 2-4 via a delay equaliser 5, 6 and 7 to a respective digital to analogue converter 8, 9 and 10. The delay equalisers 5, 6 and 7 have introduce a delay to match that introduced by an equaliser 15 in the second signal path. The resulting analogue signals are then summed together in a combiner 11 and the combined output passed to a block up-converter 12.

The block up-converter 12 produces an ideal reference signal 26 at radio frequencies to be used by a component in another branch of the transmitter section.

The other branch of the transmitter section comprises connections to each of the carrier inputs to a second combiner 14 the connections being made before the delay equalisers 5 to 7 in the first branch. The combined carrier signal is passed to an equaliser 15. The equaliser 15 works on the amplitude and the phase of the signal to make the linear response of the two signal paths the same. The equalised signal is coupled to a digital to analogue converter 16 and the resulting analogue signal passed to a block up-converter 17.

The output of the block up-converter is passed tot he power amplifier 18. The output of the power amplifier is coupled to an antenna 19 via a block T. Block T is a delay line.

Between the power amplifier 18 and the antenna 19 are two coupling lines 20 and 21. The first of these, coupler 20, couples a portion of the output signal to a comparitor 22. The comparitor 22 compares the coupled signal to that of the"ideal"signal provided by block up-converter 12 to provide an error signal to an error amplifier 23. The error amplifier 23 amplifies the signal and inputs it to the coupler 21 where it adds or subtracts to the output signal thus affecting the signal output radiated by the antenna 19. The delay line T replicates the inherent propagation delay of the error amplifier so that when the error correction signal is added back via the coupler 21 it is synchronised to the main signal.

The result of the comparison is also fed to a squarer 24 which squares the error and passes the squared error to a minimiser 25. The minimiser 25 seeks a set of coefficients to apply to the equaliser 15 to control the variation of the magnitude and phase of the signal. These are then applied by the equaliser 15.

Claims

Claims Claim 1. A radio transmitter section comprising: carrier signal inputs to which in use at least two carrier signals are applied; a first signal path comprising: a first combiner for combining the carrier signals to produce a combined signal output; a digital to analogue converter for converting the combined signal output to analogue form; and

a power amplifier for amplifying the analogue form of the ZD combined signal to form a power amplified signal; and a second signal path comprising: at least two digital to analogue converters operably coupled to the carrier signal inputs to produce analogue carrier signals; a second combiner operably coupled to the digital to analogue converters to combine the analogue carrier signals to produce a reference signal; a comparitor for comparing the reference signal of the second signal path with the output of the power amplifier of the first signal path to

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produce a comparison signal, an error amplifier responsive to the Z-1 comparison signal to produce a correction signal to correct the power amplified signal.
Claim 2. A transmitter section as claimed in claim 1 wherein the correction signal is applied to the output signal of the power amplifier.
Claim 3. A transmitter section as claimed in claim 1 or claim 2 wherein at least one equaliser is provided to match the transfer function of the first and the second signal paths.
Claim 4. A transmitter section as claimed in claim 1, 2 or 3 wherein the at least one equaliser is provided in the first signal path.
Claim 5. A transmitter section as claimed in claim 4 wherein the at least one equaliser is provided in the first path operably coupled at its input to a carrier signal input to provide an equalised output to the analogue to digital converter.
Claim 6. A transmitter section as claimed in claim 5 wherein a delay is provided for each carrier input.

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Claim 7. A transmitter section as claimed in any preceding claim wherein a z : l minimiser search engine is provided operably coupled to the equaliser in the second signal path to control the equaliser.
Claim 8. A transmitter section substantially as hereinbefore described with reference to an as illustrated by the drawing.
Claim 9. A radio communications transmitter including a transmitter section as claimed in any preceding claim.