U.S. patent application number 13/558837 was filed with the patent office on 2014-01-30 for multi-band observation receiver.
This patent application is currently assigned to TELEFONAKTIEBOLAGET L M ERICSSON (PUBL). The applicant listed for this patent is Bradley John Morris, Somsack Sychaleun. Invention is credited to Bradley John Morris, Somsack Sychaleun.
Application Number | 20140029683 13/558837 |
Document ID | / |
Family ID | 49626989 |
Filed Date | 2014-01-30 |
United States Patent
Application |
20140029683 |
Kind Code |
A1 |
Morris; Bradley John ; et
al. |
January 30, 2014 |
Multi-Band Observation Receiver
Abstract
Transmitter observation receivers and methods are described that
can predistortion-compensate transmitters capable of operating in
multiple communication bands and frequency ranges. Such observation
receivers and method involve generating at least one compensation
signal such that a signal to be transmitted that is within a
bandwidth that simultaneously encompasses multiple frequency ranges
is compensated.
Inventors: |
Morris; Bradley John;
(Ottawa, CA) ; Sychaleun; Somsack; (Ottawa,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Morris; Bradley John
Sychaleun; Somsack |
Ottawa
Ottawa |
|
CA
CA |
|
|
Assignee: |
TELEFONAKTIEBOLAGET L M ERICSSON
(PUBL)
Stockholm
SE
|
Family ID: |
49626989 |
Appl. No.: |
13/558837 |
Filed: |
July 26, 2012 |
Current U.S.
Class: |
375/267 ;
375/296 |
Current CPC
Class: |
H04B 2001/0425 20130101;
H03F 3/24 20130101; H03F 1/3241 20130101; H03F 1/3247 20130101;
H04B 1/62 20130101; H04L 27/367 20130101; H04L 27/2626
20130101 |
Class at
Publication: |
375/267 ;
375/296 |
International
Class: |
H04B 1/62 20060101
H04B001/62; H04L 27/26 20060101 H04L027/26 |
Claims
1. An arrangement for a pre-distortion-compensated transmitter for
a communication system, comprising: an electronic processor circuit
configured for converting a base-band signal to be transmitted to a
spectrally shifted, pre-distorted signal to be transmitted based on
at least one compensation signal; a power amplifier configured for
generating an amplified version of the spectrally shifted,
pre-distorted signal to be transmitted, wherein the amplified
version is in one frequency range of a plurality of frequency
ranges used in the communication system; a coupler configured for
generating a sample signal from the amplified version; and a
transmitter observation receiver (TOR) configured for receiving the
sample signal and generating at least one compensation signal based
on the sample signal; wherein the at least one compensation signal
is generated such that a signal to be transmitted that is within a
bandwidth that simultaneously encompasses multiple frequency ranges
is compensated, and the electronic processor circuit converts the
base-band signal to be transmitted such that a relationship between
the base-band signal to be transmitted and the sample signal is
substantially linear with constant phase.
2. The arrangement of claim 1, wherein the TOR includes a wideband
analog-to-digital converter (ADC) configured for converting the
sample signal into a digital sample signal, and at least one
digital down-converter configured for generating the at least one
compensation signal; and the electronic processor circuit converts
the base-band signal to be transmitted such that a relationship
between the base-band signal to be transmitted and the sample
signal is substantially linear with constant phase.
3. The arrangement of claim 2, wherein the TOR includes a plurality
of digital down-converters, each of which is optimized for a
respective frequency range in the plurality of frequency
ranges.
4. The arrangement of claim 3, wherein each frequency range
correspond to a respective one of a plurality of communication
bands.
5. The arrangement of claim 1, wherein the TOR is configured for
tuning to different frequency ranges in the bandwidth by
selectively adjusting at least one tuning component of the TOR.
6. A method of pre-distortion-compensating a signal to be
transmitted for a communication system, comprising: converting a
base-band signal to be transmitted to a spectrally shifted,
pre-distorted signal to be transmitted based on at least one
compensation signal; generating an amplified version of the
spectrally shifted, pre-distorted signal to be transmitted, wherein
the amplified version is in one communication band of a plurality
of communication bands used in the communication system; generating
a sample signal from the amplified version; and generating the at
least one compensation signal based on the sample signal such that
a signal to be transmitted that is within a bandwidth that
simultaneously encompasses multiple frequency ranges is
compensated; wherein the base-band signal is converted such that a
relationship between the base-band signal to be transmitted and the
sample signal is substantially linear with constant phase.
7. The method claim 6, wherein generating the at least one
compensation signal includes converting the sample signal into a
digital sample signal, and generating the at least one compensation
signal based on the digital sample signal; and the base-band signal
to be transmitted is converted such that a relationship between the
base-band signal to be transmitted and the sample signal is
substantially linear with constant phase.
8. The method of claim 6, wherein generating the at least one
compensation signal includes generating a plurality of compensation
signals, each of which is optimized for a respective frequency
range in the plurality of frequency ranges.
9. The method of claim 8, wherein each frequency range corresponds
to a respective one of a plurality of communication bands.
10. The method of claim 6, wherein generating the at least one
compensation signal includes tuning to different frequency ranges
in the bandwidth by selecting at least one tuning component.
Description
TECHNICAL FIELD
[0001] This invention relates to electronic communication systems
and more particularly to pre-distortion-compensated transmitters in
such systems and even more particularly to observation receivers
and techniques in such transmitters.
BACKGROUND
[0002] Many current electronic communication systems use quadrature
modulation schemes, which involve in-phase (I) and quadrature (Q)
signal components and do not have constant envelopes. Examples of
such communication systems are cellular radio telephone systems
that use wideband code division multiple access (WCDMA), orthogonal
frequency division multiple access (OFDMA), and their variants.
Thus, part of the communicated information is encoded in the
amplitude (envelope) of the transmitted signal and part is encoded
in the phase of the transmitted signal.
[0003] To avoid distorting communicated information, the power
amplifier (PA) and various other components of a radio transmitter
have to be linear, which is to say for example that the functional
relationship between the output power of the PA and the input power
of the PA is a straight line for all possible power levels. In
addition, the phase shift of the input signal through the PA has to
be constant for all possible power levels.
[0004] Departures from amplitude linearity and phase constancy
introduce distortion into the PA's output signal, such as spectral
broadening that can disturb nearby communication channels.
Amplitude/phase distortion (vector distortion) in the transmitter
can also increase the bit error rate (BER) of the communication
system, e.g., degrading the audio quality of a voice call or
reducing the speed of an internet connection.
[0005] In general, the likelihood of proper transmitter performance
can be increased by including in the transmitter a transmitter
observation receiver (TOR) that samples the output signal of the PA
and generates a compensation signal that is fed back to the
modulator, PA, and/or other transmitter components to correct the
PA's output signal. In effect, the compensation signal pre-distorts
the transmitter input signal such that the PA's output signal is
apparently undistorted. Since transmitter distortion typically
arises mainly in the PA, a signal acquired after the PA is fed back
and compared with the transmitter input signal as part of the
pre-distortion process.
[0006] FIG. 1 is a block diagram of an arrangement 100 that is an
example of a pre-distortion-compensated transmitter having an
antenna 102, a coupler 104, a power amplifier 106, a modulator 108,
and a TOR 110. The PA 106 and modulator 108 can be considered the
"transmit path" of the arrangement 100. It will be understood that
the modulator 108 typically includes oscillators and other
components not shown and that the modulator 108 generally
represents the base-band processing and up-conversion processing
applied to the input signal. As seen in FIG. 1, the TOR 110 samples
the transmitted signal generated by the transmit path through the
operation of the coupler 104 and provides a compensation signal to
the modulator 108.
[0007] Currently available pre-distortion-compensated transmitters
are generally designed to operate over a small range of transmitted
frequencies, such as a communication band of a communication
system. For example, the Long Term Evolution (LTE) communication
system currently being standardized by the Third Generation
Partnership Project (3GPP) has a communication Band 1 that covers
2110-2170 megahertz (MHz). Both the forward transmit path and the
feedback compensation path in the transmitter are effectively tuned
to the same range of frequencies, and cannot be deployed to support
other frequency ranges, e.g., other communication bands. The
typical transmitter operation is constrained to a single (narrow)
frequency range of interest as a result of spectral linearity
limitations of its various tuned circuits (e.g., narrow-band
filters) and tunable circuits (e.g., voltage-controlled local
oscillators). For example, amplitude and phase variation over
frequency makes linearization (pre-distortion) difficult over a
broad range of frequencies, and an oscillator may be able to tune
over only a few hundred MHz.
[0008] FIG. 2 is a block diagram that depicts a known way to use a
single TOR in a single-frequency, multi-transmitter arrangement.
The multiple transmitters generate respective signals having the
same carrier frequency, e.g., any carrier in a communication band,
of a communication system. In the arrangement 200 depicted in
[0009] FIG. 2, an antenna 202-1 receives output signals of a Tx 1
PA 206-1 and an antenna 202-2 receives output signals of a Tx 2 PA
206-2 that have a respective Tx 1 modulator 208-1 and a Tx 2
modulator 208-2. Couplers 204-1, 204-2 provide portions of the
output signals of the PAs to a single TOR 210 through operation of
a switch 212. The TOR 210 samples the output signal connected to it
by the switch 212 without needing tuning and provides a
compensation signal to the respective modulator. In this way, the
single TOR 210 is essentially time-shared sequentially between the
PAs 206-1, 206-2, each PA producing a signal in the same frequency
range. It is believed that such an arrangement was available from
Nortel in its CDMA tri-sector radio.
[0010] The frequency limitations of TORs and
pre-distortion-compensated transmitters are becoming more serious
problems as the number and range of available communication bands
around the world increases. Currently available
pre-distortion-compensated transmitters require redesign,
modification and re-banding to operate in new communication bands,
and this increases the cost of designing and supporting
communication systems.
SUMMARY
[0011] Problems and disadvantages of previous transmitters are
overcome by methods and arrangements in accordance with this
invention.
[0012] In accordance with aspects of this invention, there is
provided an arrangement for a pre-distortion-compensated
transmitter for a communication system. The arrangement includes an
electronic processor circuit configured for converting a base-band
signal to be transmitted to a spectrally shifted, pre-distorted
signal to be transmitted based on at least one compensation signal;
a power amplifier configured for generating an amplified version of
the spectrally shifted, pre-distorted signal to be transmitted,
where the amplified version is in one frequency range of a
plurality of frequency ranges used in the communication system; a
coupler configured for generating a sample signal from the
amplified version; and a transmitter observation receiver (TOR)
configured for receiving the sample signal and generating at least
one compensation signal based on the sample signal. The at least
one compensation signal is generated such that a signal to be
transmitted that is within a bandwidth that simultaneously
encompasses multiple frequency ranges is compensated. The
electronic processor circuit converts the base-band signal to be
transmitted such that a relationship between the base-band signal
to be transmitted and the sample signal is substantially linear
with constant phase.
[0013] Also in accordance with aspects of this invention, there is
provided a method of pre-distortion-compensating a signal to be
transmitted for a communication system. The method includes
converting a base-band signal to be transmitted to a spectrally
shifted, pre-distorted signal to be transmitted based on at least
one compensation signal; generating an amplified version of the
spectrally shifted, pre-distorted signal to be transmitted, where
the amplified version is in one frequency range of a plurality of
frequency ranges used in the communication system; generating a
sample signal from the amplified version; and generating at least
one compensation signal based on the sample signal such that a
signal to be transmitted that is within a bandwidth that
simultaneously encompasses multiple frequency ranges is
compensated. The base-band signal to be transmitted is converted
such that a relationship between the base-band signal to be
transmitted and the sample signal is substantially linear with
constant phase.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The several objects, features, and advantages of this
invention will be understood by reading this description in
conjunction with the drawings, in which: FIG. 1 is a block diagram
of a known single-frequency pre-distortion-compensated
transmitter;
[0015] FIG. 2 is a block diagram of a single-frequency,
multi-transmitter arrangement having a shared observation
receiver;
[0016] FIG. 3 is a block diagram of a multi-band
pre-distortion-compensated transmitter having a wideband analog
observation receiver;
[0017] FIG. 4 is a block diagram of a multi-band
pre-distortion-compensated transmitter having multiple observation
receivers;
[0018] FIG. 5A is a block diagram of a pre-distortion-compensated
transmitter having a tunable analog observation receiver;
[0019] FIG. 5B is a block diagram of a tuning block suitable for a
pre-distortion-compensated transmitter having a tunable observation
receiver;
[0020] FIGS. 6A, 6B are block diagrams of multi-band
pre-distortion-compensated transmitters having tunable digital
observation receivers;
[0021] FIG. 7 is a schematic diagram of a programmable digital
down-converter for a pre-distortion-compensated transmitter;
[0022] FIG. 8 is a diagram of an example of an LTE cellular
communication system; and
[0023] FIG. 9 is a flow chart of a method of
pre-distortion-compensating a signal to be transmitted for a
communication system.
DETAILED DESCRIPTION
[0024] This invention can be implemented in many types of
communication system that use pre-distortion compensation of a
signal transmitter. This description of examples of embodiments of
the invention refers to the accompanying drawings, in which the
same or similar reference numbers in different drawings identify
the same or similar components.
[0025] In response to the increasing number and range of available
communication bands around the world, transmitters capable of
operating in multiple communication bands are beginning to be
developed. TORs can improve such multi-band transmitters, and can
be included in multi-band transmitters in a number of ways.
[0026] FIG. 3 is a block diagram of an arrangement 300 that is an
example of a pre-distortion-compensated transmitter having a single
TOR that must have sufficient bandwidth to see all relevant
communication bands and all related distortion if it is to be able
to compensate a signal to be transmitted that is within a bandwidth
that simultaneously encompasses all relevant communication bands.
The transmitter 300 has an antenna 302, a coupler 304, a PA 306,
and a base-band digital processor 308. The PA 306 and processor 308
can be considered the transmit path of the arrangement 300. A TOR
310 depicted in FIG. 3 includes a wideband down-converter 312, such
as a wideband quadrature demodulator, and a wideband
analog-to-digital converter (ADC) 314 that converts the wideband
analog signal produced by the down-converter 312 into a digital
wideband compensation signal provided to the processor 308.
[0027] As described above, a TOR is generally specifically tuned to
operate in one frequency range, such as a part or all of one
communication band, due to very stringent analog performance (gain
and phase) requirements, and this currently makes it difficult to
implement the single TOR 310 for operation over multiple
communication bands. The bandwidth needed by the TOR 310 depends on
the frequency range within which the signal to be transmitted by
the transmitter 300 can be found, which can be a bandwidth that
simultaneously encompasses a plurality of communication bands.
[0028] For example, if the transmitter 300 is configured for
dual-band operation, e.g., to generate a 40-MHz-wide signal in Band
3 and a 40-MHz-wide signal in Band 1, then the TOR 310 must
generate a compensation signal such that those signals to be
transmitted can be compensated, which in this example is a
compensation signal within a bandwidth that simultaneously
encompasses both Band 1 and Band 3. The compensation signal is thus
generated in a bandwidth of at least 1095 MHz (i.e., (2170-1805
MHz).times.3). To have such a wide bandwidth, the TOR 310 requires
significant power, circuit area, and cost, and optimization (for
gain flatness, phase linearity, etc.) of the TOR 310 over such a
wide bandwidth is difficult. The difficulties increase dramatically
as the bandwidth within which the compensation signal must be
generated increases, e.g., in a dual-band transmitter that is
expected to operate in any two communication bands over a wide
frequency range, such as 1805-2170 MHz, or Bands 3, 9, 35, 39, 33,
37, 2, 36, 34, 4, and 1 in an LTE communication system. It will be
appreciated that other frequency ranges and communication bands can
be used as examples.
[0029] One way to overcome the difficulties of a single, wideband
TOR 310 is to use multiple TORs, each optimized for a respective
communication band or portion of the total transmitter bandwidth.
Such an arrangement is depicted in FIG. 4, which is a block diagram
of an arrangement 400 that is an example of a
pre-distortion-compensated transmitter having two TORs 410-1,
410-2. The transmitter 400 has an antenna 402, couplers 404-1,
404-2, a PA 406, and a base-band digital processor 408. Each of the
TORs 410-1, 410-2 includes a respective down-converter 412-1, 412-2
and a respective ADC 414-1, 414-2 that need to operate over only
respective frequency ranges Range 1, Range 2, each of which is
typically much less than the transmitter's total bandwidth.
Accordingly, the down-converters 412 and ADCs 414 can be
implemented more easily than the wideband down-converter 312 and
wideband ADC 314.
[0030] Compared with the transmitter 300, the transmitter 400
eliminates the requirement for a TOR 310 that generates a
compensation signal suitable for a signal to be transmitted that is
within a very wide bandwidth, e.g., within a bandwidth that
simultaneously encompasses plural communication bands. For example,
if the transmitter 400 is configured to generate a 40-MHz-wide
signal in Band 3 and a 40-MHz-wide signal in Band 1, then the
bandwidth of each of the TORs 410-1, 410-2 needs to be only at
least 120 MHz (40 MHz.times.3). Although it is easier to optimize
the TORs 410 relative to the TOR 310, the transmitter 400 must have
two TORs, and in general as many TORs as signals in the
transmitter's multi-band signal to be transmitted, which imposes
their associated significant power, area, and cost requirements on
the transmitter 400. In addition, the multiple TORs in the
transmitter 400 still must be optimized for specific frequencies or
frequency ranges.
[0031] The arrangements depicted in FIGS. 3 and 4 can be further
improved to enable compensating a signal to be transmitted that is
within a wide simultaneous bandwidth and achieve efficient
multi-band transmitter operation as described below.
[0032] FIG. 5A is a block diagram of an arrangement 500 that is an
example of a pre-distortion-compensated transmitter having a single
TOR that is sequentially time-shared among multiple frequency
ranges, resulting in generation of a compensation signal suitable
for compensating a transmitted signal that is within a bandwidth
that can simultaneously encompass a plurality of communication
bands of the communication system. The transmitter 500 has an
antenna 502, a coupler 504, a PA 506, and a base-band digital
processor 508. A TOR 510 depicted in FIG. 5A includes a
down-converter 512, an ADC 514, and a local oscillator (LO) 516 or
other device that selects the operating frequency range of the
down-converter 512. It will be understood that the LO 516 can be
considered to part of the down-converter 512.
[0033] The base-band processor 508 is typically configured to
receive a complex-valued input signal and the fed-back compensation
signal, and to output a pre-distorted, up-converted signal.
Although FIG. 5A depicts an up-converted, pre-distorted signal
provided directly to the PA 506, it will be understood that the
pre-distorted signal can be a base-band or intermediate-frequency
(IF) signal that is spectrally shifted by suitable components (not
shown) as appropriate. Thus, it will further be understood that the
processor 508 includes one or more suitable ADCs and
digital-to-analog converters (DACs) for converting signals from
analog form to digital form and vice versa, as needed.
[0034] It will also be understood that the pre-distorted signal
generated by the processor 508 is obtained by applying a suitable
pre-distortion function to the input signal, advantageously in the
digital domain. The pre-distortion function is such that the
relationship between the input signal and samples of the PA output
signal is substantially linear with constant phase. The
pre-distortion function initially can be a predetermined function
(e.g., based on a model of the PA) that can then be adapted based
on the comparison of the complex input signal with the fed-back
sample of the output signal. In this way, compensation signals are
generated in the digital domain, even compensation signals that do
not strictly comply with the Nyquist criterion and even
compensation signals that may linearize transmitted signals in
multiple bands based on the transmitted signal in one of those
bands.
[0035] The power amplifier generates an amplified version of the
spectrally shifted, pre-distorted signal to be transmitted in one
communication band of a plurality of communication bands used in
the communication system. As depicted in FIG. 5A, the TOR 510
advantageously can be optimized for a bandwidth or frequency range
that is sufficient to cover the widest communication band of
interest to the transmitter 500, and by suitably tuning the LO 516,
that bandwidth can be time-shared among all communication bands
covered by the transmitter 500. It will be appreciated that the
success of the arrangement depicted in FIG. 5A depends on the
optimization of the TOR 510 for all communication bands covered by
the transmitter 500. The selection of which band to observe is
nominally achieved by simply tuning the LO 516, but other tuning
components 518 may need to be added or removed from the TOR 510 to
compensate for different circuit operation/performance in the
different bands and obtain desired performance at the different
frequencies.
[0036] FIG. 5B depicts an example of an optional tuning block 518
that is suitable for a pre-distortion-compensated transmitter
having a tunable observation receiver, such as the arrangement
depicted in FIG. 5A. In general, tuning components can include
capacitors and/or inductors that are selectively included/excluded
from the TOR 510 by one or more suitable switches or multiplexers,
although as depicted in FIG. 5B, the tuning components 518 can be
organized into an amplitude equalizer, comprising a
resistor-inductor-capacitor (RLC) network having values appropriate
for the particular frequency range, and a group-delay equalizer,
comprising an LC network with suitable values, that need not be
switched in and out of the TOR 510. Even a well-designed wideband
analog TOR will show a reduction in gain as the input frequency
increases (e.g., as the TOR changes from observing frequencies
around Band 3 to frequencies around Band 1). Accordingly, the
tuning block 518 is one form of an analog frequency equalizer that
can compensate for gain and phase variations with frequency of the
TOR. It will be understood that many electrically equivalent
arrangements can be used. An important advantage of a transmitter
such as that depicted in FIG. 5A is its reduction of the number of
observation receivers to one receiver that is configured for
receiving the samples of the PA output signal and generating at
least one compensation signal based on the samples such that a
signal to be transmitted that can be anywhere within a bandwidth
that simultaneously encompasses multiple communication bands is
compensated. This saves power/area/cost in exchange for requiring
the receiver 510 to be tuned to cover the desired frequency range
of the transmitted signal, e.g., multiple communication bands or
even multiple portions of a single band.
[0037] The arrangement in FIG. 5A can thus be seen as time-sharing
a single TOR 510 across more than one communication band or
portions of a frequency range by changing the LO frequency and
possibly adjusting tuning components. As described above, tuning an
analog TOR typically requires different matching components for
different frequency ranges of operation, but as illustrated by FIG.
5A, an analog TOR can be used for multiple frequency ranges by
setting the LO to new frequencies and selecting and adjusting
suitable tuning components.
[0038] The arrangement depicted in FIG. 5A can be further improved
as described below in connection with FIGS. 6A, 6B. FIG. 6A is a
block diagram of an arrangement 600 that is an example of a
pre-distortion-compensated transmitter having a TOR 610 that
includes a digital down-converter 612, such as a suitably
programmed digital processor circuit, and a wideband ADC 614. The
TOR 610 is configured for receiving the samples of the PA output
signal and generating at least one compensation signal based on the
samples. The at least one compensation signal is generated such
that a signal to be transmitted that is within a bandwidth that
simultaneously encompasses multiple communication bands or
frequency ranges is compensated. It will be noted in comparing the
TORs 510, 610 that the sampling rate of the ADC 614 is generally
greater than the sampling rate of the ADC 514. Wideband ADCs 614
suitable for use in current cellular radio communication systems
are commercially available, for example from National Semiconductor
Corp., which is now part of Texas Instruments Inc., Dallas, Tex.,
U.S.A. It will also be noted that FIG. 6A does not explicitly show
a LO as FIG. 5A does because the TOR 610 preferably can be
implemented with a fixed-frequency LO, with tuning of the TOR 610
implemented digitally rather than by tuning the LO. Nevertheless,
the artisan will understand that other arrangements are
possible.
[0039] In the transmitter 600, filtering and tuning of the sampled
transmitted signal preferably is moved to the digital domain. In
this way, the repeatability and configurability of digital-domain
processing enables easily changing which frequency range, e.g.,
which communication band, is observed by the TOR 610. By using a
digital down-converter 612, errors that would be caused by analog
components (e.g., due to time, voltage, and/or temperature
variations) are not promulgated back through signals on the
transmit path. Moreover, the response of the transmitter can be of
the same quality across a wide frequency range, such as a plurality
of communication bands. As noted above, TORs that employ analog
components generally must be carefully optimized even for a single
communication band, and behave differently (and introduce errors)
when used at other frequencies. The higher quality of the
compensation signal enables the base-band digital processor 608 to
achieve a higher quality relationship between the transmitter's
input signal and sampled output signal.
[0040] The wideband ADC 614 and digital down-converter 612 enable
the arrangement 600 to operate in multiple communication bands in a
time-shared way as the filter/tuner stage 612 selectively observes
one communication band at a time. Thus, the arrangement 600 has
power and space advantages over a single-band
pre-distortion-compensated transmitter, such as that described in
U.S. patent application No. 13/128,466 filed on Sep. 21, 2011, by
Bradley John Morris et al. for "Method and Frequency Agile
Pre-Distorted Transmitter Using Programmable Digital Up and Down
Conversion", which is a national phase of International Application
PCT/IB2010/002941 filed on Nov. 18, 2010. U.S. patent application
No. 13/128,466 is incorporated in this application by
reference.
[0041] Moreover, the arrangement 600 also has advantages over the
transmitter 500 described above in that difficulties arising from
re-tuning a TOR for different communication bands can be
substantially eliminated by the digital down-converter 612, whose
tuning parameters, filter response, etc. can easily be configured
as necessary for each band. A suitable digital down-converter 612
is described in U.S. patent application No. 13/130,211 filed on
Sep. 9, 2011, by Bradley John Morris et al. for "Methods and
Systems for Programmable Digital Down-Conversion", which is a
national phase of International Application PCT/IB2010/002927 filed
on Nov. 18, 2010. U.S. patent application No. 13/130,211 is
incorporated in this application by reference.
[0042] FIG. 6B is a block diagram of an arrangement 600' that is an
example of a pre-distortion-compensated transmitter having a TOR
610' that includes plural digital down-converters 612-1, 612-2 and
a wideband ADC 614', such as the circuits described above in
connection with FIG. 6A. The TOR 610' is configured for receiving
the sample signal and generating at least one compensation signal
based on samples of the PA output signal. The at least one
compensation signal is generated such that a signal to be
transmitted that is within a bandwidth that simultaneously
encompasses multiple communication bands or frequency ranges is
compensated. As in FIG. 6A, it will be noted that FIG. 6B does not
explicitly show a LO as FIG. 5A does because the TOR 610'
preferably can be implemented with a fixed-frequency LO, with
tuning of the TOR 610' implemented digitally rather than by tuning
the LO.
[0043] The plural down-converters 612-1, 612-2 can be configured in
several ways for continuous observation of a given frequency range,
such as a communication band or plural communication bands. It will
be understood that FIG. 6B depicts two down-converters 612 but more
can be included in the arrangement 600'. The arrangement 600' is
relatively more efficient than other possible arrangements in that
it shares its analog components and wideband ADC among replicated
digital functionality. In addition, the transmitter 600' can be
advantageous with respect to the transmitter 600 in that the TOR
610' can continuously observe the transmitted signal by using as
many digital down-converters 612 as needed.
[0044] FIG. 7 is a schematic diagram of a programmable digital
down-converter 612 that is suitable for a
pre-distortion-compensated transmitter, such as the arrangements
600, 600'. The converter 612 includes a complex frequency range or
band selection filter 702, a digital down-sampler 704, and a
complex base-band tuner 706. The complex baseband tuner 706 can
alternatively be included in the base-band digital processor 608,
608'. The functionality of the filter 702 and down-sampler 704 can
be implemented with a polyphase filter.
[0045] The down-sampler 704 is configured to generate a
down-sampled signal that includes one sample for each N samples in
a digital signal input to the down-sampler, where N is an integer
that is greater than or equal to two. Of course, it is preferable
for N to be an integer power of two, but a rate-change filter can
be included in the down-converter 612 to handle conversion of the
sampling rate of the input signal provided to the down-converter
divided by N to a desired sampling rate of the output signal
generated by the down-converter.
[0046] It will be appreciated that a filter is not required before
the ADC 614 when there is minimal interference (e.g., something
other than the transmitted signal) from the antenna 602 coupled
into the feedback path. All required filtering can then be achieved
digitally during down-conversion, for example, by judicious
selection of polyphase filter coefficients in the programmable
digital down-converter 612.
[0047] FIG. 8 is a diagram of an example of an LTE cellular
communication system 800 that includes user equipments (UEs) 810,
820, a radio access network (RAN) that includes a plurality of
evolved Node B (eNodeBs0, or base stations, 130 1, 130 2, . . . ,
130 N, and a core network (CN) that includes a serving gateway
(SGW) node 140 and a packet data network 150. Other nodes can also
be provided in the system 800.
[0048] Each eNodeB 130 1, 130 2, . . . , 130 N serves a respective
geographical area that is divided into one or more cells. An eNodeB
can use one or more of the pre-distortion-compensated transmitters
described above and antennas at one or more sites to transmit
information into its cell(s), and different antennas can transmit
respective, different pilot and other signals. Neighboring eNodeBs
are coupled to each other by an X2-protocol interface that supports
active-mode mobility of the UEs. An eNodeB controls various radio
network functions, including for example single-cell radio resource
management (RRM), such as radio access bearer setup, handover, UE
uplink/downlink scheduling, etc. Each eNodeB also carries out the
Layer-1 functions of coding, decoding, modulating, demodulating,
interleaving, de-interleaving, etc.; and the Layer-2 retransmission
mechanisms, such as hybrid automatic repeat request (HARQ), and
functions of radio link control (RLC) and RRC. The eNodeBs 130 1,
130 2, . . . , 130 N are coupled to one or more SGWs 140 (only one
of which is shown in FIG. 8).
[0049] The network 800 can exchange information with one or more
other networks of any type, including a local area network (LAN); a
wide area network (WAN); a metropolitan area network; a telephone
network, such as a public switched terminal network or a public
land mobile network; a satellite network; an intranet; the
Internet; or a combination of networks. It will be appreciated that
the number of nodes illustrated in FIG. 8 is simply an example.
Other configurations with more, fewer, or a different arrangement
of nodes can be implemented. Moreover, one or more nodes in FIG. 8
can perform one or more of the tasks described as being performed
by one or more other nodes in FIG. 8. For example, parts of the
functionality of the eNodeBs can be divided among one or more base
stations and one or more radio network controllers, and other
functionalities can be moved to other nodes in the network.
[0050] FIG. 9 is a flow chart of an example of a method of
pre-distortion-compensating a signal to be transmitted for a
communication system. The method includes converting (step 902) a
base-band signal to be transmitted to a spectrally shifted,
pre-distorted signal to be transmitted based on at least one
compensation signal, for example by a base-band digital processor
608, 608'. The method also includes generating (step 904) an
amplified version of the spectrally shifted, pre-distorted signal
to be transmitted, for example by a PA 606, 606', where the
amplified version is in one communication band of a plurality of
communication bands used in the communication system. The method
also includes generating (step 906) a sample signal from the
amplified version, for example by a coupler 604, 604'. The method
also includes generating (step 608) at least one compensation
signal based on the sample signal such that a signal to be
transmitted that is within a bandwidth that simultaneously
encompasses multiple communication bands or frequency ranges is
compensated, for example by a TOR 610, 610'. Generating the at
least one compensation signal includes converting the sample signal
into a digital sample signal, and generating the at least one
compensation signal based on the digital sample signal. As
described above, the base-band signal to be transmitted is
converted such that a relationship between the base-band signal to
be transmitted and the sample signal is substantially linear with
constant phase.
[0051] Also as described above, generating the at least one
compensation signal can include generating a plurality of
compensation signals, each of which is optimized for a respective
frequency range in a plurality of communication bands, and each
frequency range can correspond to a respective one of the plural
communication bands in a communication system, such as that
depicted in FIG. 8. The sample signal can be converted into the
digital sample signal by a wideband ADC 614, 614' or equivalent
device. Generating the at least one compensation signal can also
include tuning to different frequency ranges in the bandwidth by
selecting at least one tuning parameter and filter response or
analog compensation component.
[0052] It is expected that this invention can be implemented in a
wide variety of environments, including for example mobile
communication devices. It will be appreciated that procedures
described above are carried out repetitively as necessary. To
facilitate understanding, many aspects of the invention are
described in terms of sequences of actions that can be performed
by, for example, elements of a programmable computer system. It
will be recognized that various actions could be performed by
specialized circuits (e.g., discrete logic gates interconnected to
perform a specialized function or application-specific integrated
circuits), by program instructions executed by one or more
processors, or by a combination of both. Many communication devices
can easily carry out the computations and determinations described
here with their programmable processors and application-specific
integrated circuits.
[0053] Moreover, the invention described here can additionally be
considered to be embodied entirely within any form of
computer-readable storage medium having stored therein an
appropriate set of instructions for use by or in connection with an
instruction-execution system, apparatus, or device, such as a
computer-based system, processor-containing system, or other system
that can fetch instructions from a medium and execute the
instructions. As used here, a "computer-readable medium" can be any
means that can contain, store, or transport the program for use by
or in connection with the instruction-execution system, apparatus,
or device. The computer-readable medium can be, for example but not
limited to, an electronic, magnetic, optical, electromagnetic,
infrared, or semiconductor system, apparatus, or device. More
specific examples (a non-exhaustive list) of the computer-readable
medium include an electrical connection having one or more wires, a
portable computer diskette, a RAM, a ROM, an erasable programmable
read-only memory (EPROM or Flash memory), and an optical fiber.
[0054] Thus, the invention may be embodied in many different forms,
not all of which are described above, and all such forms are
contemplated to be within the scope of the invention. For each of
the various aspects of the invention, any such form may be referred
to as "logic configured to" perform a described action, or
alternatively as "logic that" performs a described action.
[0055] It is emphasized that the terms "comprises" and
"comprising", when used in this application, specify the presence
of stated features, integers, steps, or components and do not
preclude the presence or addition of one or more other features,
integers, steps, components, or groups thereof.
[0056] The particular embodiments described above are merely
illustrative and should not be considered restrictive in any way.
The scope of the invention is determined by the following claims,
and all variations and equivalents that fall within the range of
the claims are intended to be embraced therein.
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