U.S. patent application number 10/250473 was filed with the patent office on 2004-07-15 for automatic gain control.
Invention is credited to Grasbeck, Ari, Kondradsson, Per, Nystrom, Christian.
Application Number | 20040137868 10/250473 |
Document ID | / |
Family ID | 20283019 |
Filed Date | 2004-07-15 |
United States Patent
Application |
20040137868 |
Kind Code |
A1 |
Nystrom, Christian ; et
al. |
July 15, 2004 |
Automatic gain control
Abstract
The invention relates to gain control for an amplifier for
received radio signals by means of an automatic gain control signal
(c). A down converter (103) produces signal components (I; Q) that
are amplified in a respective amplifier (104, 105). Then, the
resulting signal components (I.sub.P-LP; Q.sub.P-LP) are digitised
in A/D-converters (110; 111) for further processing in a digital
signal processor (112) and are also fed to a respective signal
level detector (113; 114) whose output signals ([I]; [Q]) are
combined (115) into a combined signal (V.sub..SIGMA.). This signal
(V.sub..SIGMA.) is forwarded to a gain control unit (116), which
produces the automatic gain control signal (c). The automatic gain
control signal (c) controls the gain of the amplifiers (104; 105)
such that the resulting signal components (I.sub.P-LP; Q.sub.P-LP)
are maintained at signal level ([I]; [Q]) less than a predetermined
limit level being adapted to the A/D-converters (110; 111).
Inventors: |
Nystrom, Christian;
(Sollentuna, SE) ; Kondradsson, Per; (Jarfalla,
SE) ; Grasbeck, Ari; (Solna, SE) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Family ID: |
20283019 |
Appl. No.: |
10/250473 |
Filed: |
December 19, 2003 |
PCT Filed: |
February 13, 2002 |
PCT NO: |
PCT/SE02/00237 |
Current U.S.
Class: |
455/232.1 ;
455/127.2 |
Current CPC
Class: |
H03G 3/3052 20130101;
H03G 3/001 20130101 |
Class at
Publication: |
455/232.1 ;
455/127.2 |
International
Class: |
H04B 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 16, 2001 |
SE |
0100528-9 |
Claims
1. A method for controlling gain of an amplifier for received radio
signals in a radio receiver, the radio receiver having a
controllable gain responsive to an automatic gain control signal
(c), the method comprising: amplifying received radio frequency
signals (RF), level detecting at least one down converted signal
(I.sub.P-LP; Q.sub.P-LP) resulting from amplified radio frequency
signals (RF.sub.P), and generating the automatic gain control
signal (c) in response to at least one down converted signal level
([I]; [Q]) such that the controllable gain maintains the at least
one down converted signal level ([I]; [Q]) at less than a
predetermined limit level, characterised by varying a time constant
(.tau.) of the automatic gain control signal (c) in response to a
time derivative parameter (V.sub..SIGMA.) of the at least one down
converted signal (I.sub.P-LP; Q.sub.P-LP) such that the automatic
gain control signal (c) is adapted to a power variation rate of the
received radio frequency signals (RF).
2. A method according to claim 1, characterised by allocating a
relatively small value to the time constant (.tau.) if the time
derivative parameter (V.sub..SIGMA.) has a comparatively high
value, and allocating a relatively large value to the time constant
(.tau.) if the time derivative parameter (V.sub..SIGMA.) has a
comparatively low value.
3. A method according to claim 2, characterised by generating a
pulse signal (R) if the time derivative parameter (V.sub..SIGMA.)
exceeds a threshold value (V.sub.T), the pulse signal (R) at least
being indicative of a message start in the received radio frequency
signals (RF).
4. A method according to claim 3, characterised by during reception
of a message the pulse signal (R) being indicative of an erroneous
setting of the controllable gain.
5. A method according to any one of the claims 3 or 4,
characterised by setting the threshold value (V.sub.T) on basis of
a power level (P.sub.RF) of the received radio frequency signals
(RF).
6. A method according to any one of the claims 3-5, characterised
by digitally processing the pulse signal (R) to produce a time
constant value (.tau..sub.set-D), the time constant value
(.tau..sub.set-D) forming a basis for the time constant (.tau.) of
the automatic gain control signal (c).
7. A method according to claim 6, characterised by updating the
time constant value (.tau..sub.set-D) after a specific time
period.
8. A method according any one of the claims 1-7, characterised by
combining two signal components of the at least one down converted
signal (I.sub.P-LP; Q.sub.P-LP) to form a combined signal
(V.sub..SIGMA.), and producing the time derivative parameter
(V.sub..SIGMA.) by estimating a time derivative of the combined
signal (V.sub..SIGMA.).
9. A method according to claim 8, characterised by estimating time
derivative of the combined signal (V.sub..SIGMA.) by high pass
filtering the combined signal (V.sub..SIGMA.).
10. A method according to any one of the claims 8 or 9,
characterised by deriving the two signal components (I.sub.P-LP;
Q.sub.P-LP) by means of quadrature demodulation of the received
radio frequency signals (RF).
11. A method according to any one of the claims 6-10, characterised
by controlling the generation of the automatic gain control signal
(c) on basis of the time constant (.tau.), a desired value
(V.sub.des) and an average value of the combined signal
(V.sub..SIGMA.).
12. A computer program directly loadable into the internal memory
of a digital computer, comprising software for performing the steps
of any of the claims 1-11 when said program is run on a
computer.
13. A computer readable medium, having a program recorded thereon,
where the program is to make a computer perform the steps of any of
the claims 1-11.
14. An arrangement for controlling gain of an amplifier for
received radio signals in a radio receiver, comprising at least one
amplifier (102) for amplifying received radio frequency signals
(RF), a gain control loop (112-120) for controlling gain of at
least one amplifier (104; 105) in response to an automatic gain
control signal (c), the gain control loop (112-120) including at
least one signal level detector (113, 114) for receiving at least
one down converted signal (I.sub.P-LP; Q.sub.P-LP) resulting from
amplified radio frequency signals (RF.sub.P) and producing in
response thereto at least one down converted signal level ([I];
[Q]), and a gain control signal generator (116) for receiving the
at least one down converted signal level ([I], [Q]; V.sub..SIGMA.)
and producing in response thereto the automatic gain control signal
(c) such that the gain control loop (112-120) maintains the at
least one down converted signal level ([I]; [Q]) at less than a
predetermined limit level, characterised in that the gain control
signal generator (116) is adapted for varying a time constant
(.tau.) of the automatic gain control signal (c) in response to a
time derivative parameter (V.sub..SIGMA.) of the at least one down
converted signal (I.sub.P-LP; Q.sub.P-LP) to compensate for a power
variation rate of the received radio frequency signals (RF).
15. An arrangement according to claim 14, characterised in that the
gain control signal generator (116) is adapted for allocating a
relatively small value to the time constant (.tau.) if the time
derivative parameter (V.sub..SIGMA.) has a comparatively high
value, and allocating a relatively large value to the time constant
(.tau.) if the time derivative parameter (V.sub..SIGMA.) has a
comparatively low value.
16 An arrangement according to claim 15, characterised in that the
gain control loop (112-120) includes a ramp detector (118) for
generating a pulse signal (R) if the time derivative parameter
(V.sub..SIGMA.) exceeds a threshold value (V.sub.T).
17. An arrangement according to claim 16, characterised in that the
gain control loop (112-120) includes a digital signal processor
(112) for receiving the pulse signal (R) and producing in response
thereto a time constant value (.tau..sub.set-D), the time constant
value (.tau..sub.set-D) forming a basis for the time constant
(.tau.) of the automatic gain control signal (c).
18. An arrangement according to any one of the claims 16 or 17,
characterised in that the digital signal processor (112) is adapted
to set the threshold value (V.sub.T) on basis of a power level
(P.sub.RF) of the received radio frequency signals (RF).
19. An arrangement according to any one of the claims 14-18,
characterised in that the gain control loop (112-120) includes at
least two signal level detectors (113, 114), which each produces a
respective down converted signal level ([I]; [Q]), a combiner for
receiving the down converted signal levels ([I]; [Q]) and producing
in response thereto a combined signal (V.sub..SIGMA.), and a
derivation unit (117) for receiving the combined signal
(V.sub..SIGMA.) and producing in response thereto a time derivative
parameter (V.sub..SIGMA.).
20. An arrangement according to claim 19, characterised in that the
derivation unit (117) includes a high pass filter.
21. An arrangement according to any one of the claims 19 or 20,
characterised in that the radio receiver comprises a quadrature
demodulator (103) and a frequency oscillator (121) for deriving two
signal components (I.sub.P-LP; Q.sub.P-LP) by means of quadrature
demodulation of the amplified received radio frequency signals
(RF.sub.P).
22. An arrangement according to any one of the claims 17-21,
characterised in that the gain control loop (112-120) includes
means (119; 120) for receiving the time constant value
(.tau..sub.set-D) and producing the time constant (.tau.) in
response thereto, the digital signal processor (112) is adapted for
producing a desired value (V.sub.des) indicative of an appropriate
combined signal level, and the gain control signal generator (116)
is adapted for receiving the time constant (.tau.), the desired
value (V.sub.des) and a time limited average value of the combined
signal (V.sub..SIGMA.) and producing in response thereto the
automatic gain control signal (c).
Description
THE BACKGROUND OF THE INVENTION AND PRIOR ART
[0001] The present invention relates generally to automatic gain
control in a radio frequency receiver. More particularly the
invention relates to a method of controlling gain of an amplifier
for received radio signals in a radio receiver, according to the
preamble of claim 1. The invention also relates to a computer
program, for instance in the form of a digital processor algorithm,
according to claim 12, a computer readable medium according to
claim 13 and an arrangement according to the preamble of claim
14.
[0002] A problem that arises in most radio communications receivers
concerns the wide variation in power level of the radio signals
received at the antenna. This variation is due to a variety of
causes. For example, the distance between the transmitter and the
receiver can vary considerably. Different transmitters may also
utilise different power levels. Since, disregarding the influence
of any screening objects, the received signal power decreases as
the square of the distance to the transmitter, wide variations in
received power level are likely to arise in many situations.
Furthermore, these variations may occur very rapidly due to changes
in the radio conditions. Movements of the receiver station and/or
transmitter station or repositioning of objects between the
stations are typical situations in which the conditions for the
radio channel can change dramatically.
[0003] In radio design it is therefore common practice to include
an automatic gain control (AGC) circuit in the receiver. The AGC
circuit utilises feedback to maintain a fixed (or at least as
stable as possible) signal power level within the receiver even
though the signal level at the antenna varies widely. The AGC is
achieved by using an amplifier whose gain can be controlled by an
external signal, e.g. a voltage or a current.
[0004] In analogue receivers it is known to incorporate AGC
circuits that operate on a down converted intermediate frequency
(IF) signal, i.e. a signal component, which has been frequency
transformed down from a received radio frequency (RF) signal and
which is to be further frequency transformed down in a following
frequency down conversion step.
[0005] Receivers in which the radio signal is digitally processed,
in most cases after frequency down conversion, usually perform the
AGC operation by digitally assisted processing. Thus, the AGC loop
implies both analogue to digital conversion and digital to analogue
conversion. For many of today's applications this gives a
satisfying compensation for the power level variations in the
received radio signals.
[0006] However, besides capable A/D- and D/A-converters, digital
AGC also requires an amount of processing power, which in turn is
correlated with power consumption and costs. For large signal
bandwidths this effect becomes especially pronounced.
[0007] Moreover, a large bandwidth places relatively demanding
requirements on the A/D- and D/A-converters, particularly if a high
digital resolution is necessary.
[0008] In a radio communication system for so-called bursty
communication, for instance in the form of data packets, short
pieces of information are passed between transmitter stations and
receiver stations at irregular and generally unpredictable time
instances. A particular station can, in most such systems, act both
as a transmitter station and as a receiver station. With some
exceptions, this means that every station in the system is a
potential receiver of a radio message at any time. The station must
therefore be capable of tuning its AGC circuit to received radio
signals very rapidly.
[0009] The IEEE: 802.11a, 802.11b and ETSI: Hiperlan/2 constitute
specific examples of wireless LAN protocols where extremely quick
and accurate AGC-tuning is demanded. (IEEE=The Institute of
Electrical and Electronics Engineers, ETSI=The European
Telecommunications Standards Institute, LAN=Local Area Network,
Hiperlan=High performance radio local area network). 802.11b
specifies packet data exchange at speeds up to 11 Mbps/channel
(under direct sequence modulation at 2.4 GHz) and Hiperlan/2 makes
possible wireless access to the Internet and real time video
services at speeds of 54 Mbps (at 5 GHz). In order to meet the
hardest requirements of these standards a radio receiver must be
capable of calibrating its receiver circuitry to the power level of
received radio signals within 10 .mu.s from start of transmission.
This means that the AGC function must control the receiver
amplifier gain to a suitable level within 10 .mu.s or less.
[0010] However, it is both technically complicated and expensive to
accomplish an AGC function involving digital signal level detection
with sufficient accuracy and within such short time limits by
utilising today's A/D converters, D/A converters and digital signal
processors.
SUMMARY OF THE INVENTION
[0011] It is therefore an object of the present invention to solve
the problems above and thus provide an improved solution for
controlling the gain of an analogue variable gain amplifier in a
radio receiver, which is quick and accurate enough to meet the
requirements of the modern packet data protocols.
[0012] According to one aspect of the invention the object is
achieved by a method for controlling gain of an amplifier for
received radio signals in a radio receiver as initially described,
which is characterised by varying a time constant of an automatic
gain control signal in response to a time derivative parameter of
the at least one down converted signal. The time constant is at
least varied such that the automatic gain control signal is adapted
to a power variation rate of the received radio frequency signals.
The time constant may, of course, also be varied on basis of
arbitrary additional parameters depending on, for instance, the
protocol and frame structure according to which the particular
receiver operates.
[0013] According to another aspect of the invention these objects
are achieved by a computer program directly loadable into the
internal memory of a digital computer, comprising software for
controlling the method described in the above paragraph when said
program is run on a computer.
[0014] According to yet another aspect of the invention these
objects are achieved by a computer readable medium, having a
program recorded thereon, where the program is to make a computer
perform the method described in the penultimate paragraph
above.
[0015] According to an additional aspect of the invention the
object is achieved by an arrangement for controlling gain of an
amplifier for received radio signals in a radio receiver as
initially described, which is characterised in that a gain control
signal generator is adapted for varying a time constant of the
automatic gain control signal in response to a time derivative
parameter of at least one down converted signal. The variation of
the time constant is thus adjusted to a power variation rate of the
received radio frequency signals.
[0016] The invention thereby provides a very fast, flexible and
reliable AGC, which places moderate demands both on the analogue
and the digital components. As a consequence, the solution may be
realised at a comparatively low cost. The power consumption also
becomes reasonable.
[0017] Moreover, the solution is scalable with respect to frequency
range and power level, such that it may be applied to many
technologies in addition to the packet data protocols mentioned.
Hence, the invention is also applicable to known TDMA- and
CDMA-standards for public mobile data communication, such as GSM,
EDGE, GPRS, IS-95B, IS-136, cdma2000, W-CDMA and IMT-2000.
(TDMA=Time Division Multiple Access, CDMA=Code Division Multiple
Access, GSM=Global System for Mobile Communication, EDGE=enhanced
data rate for GSM Evolution, GPRS=General Packet Radio Service in
GSM, IS-95B: a packet mode version of the direct sequence
CDMA-standard IS-95 used in North America, IS-136: a TDMA cellular
system standard predominantly used in North America, cdma2000: a
proposed standard in the USA for the next generation of public
mobile communication, WCDMA=Wideband CDMA, IMT-2000: harmonisation
initiative in International Mobile Telecommunications in 2000
between USA, Europe and Asia under the Radiocommunication
Standardization Sector of the International Telecommunication Union
(ITU-R)).
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The present invention is now to be explained more closely by
means of preferred embodiments, which are disclosed as examples,
and with reference to the attached drawings.
[0019] FIG. 1 shows a block diagram over an arrangement according
to an embodiment of the invention,
[0020] FIG. 2 illustrates a first aspect of the automatic gain
control algorithm according to the invention by means of a power
diagram,
[0021] FIG. 3 shows a diagram over a combined signal on which an
automatic gain control is based according to an embodiment of the
invention,
[0022] FIG. 4 shows a diagram over a time derivative parameter on
which the automatic gain control may also be based according to an
embodiment of the invention, and
[0023] FIG. 5 illustrates, by means of a flow diagram, an
embodiment of the method according to the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0024] FIG. 1 shows a block diagram over a receiver arrangement,
which comprises a gain control loop according to an embodiment of
the invention. The arrangement includes at least one radio
frequency amplifier 102 for receiving radio frequency signals RF
and producing corresponding amplified signals RF.sub.P. A down
converter 103 generates down converted signals I and Q from the
amplified radio frequency signals RF.sub.P. According to a
preferred embodiment of the invention, the down converter 103
produces two quadrature signal components I and Q respectively by
multiplying the amplified signal RF.sub.P with a reference
frequency LO from a local oscillator 121 respective with a phase
shifted version of the reference frequency LO.
[0025] The quadrature signal components I and Q are passed to a set
of amplifiers 104; 105, whose gain is controllable in response to
an automatic gain control signal c and which generate corresponding
amplified signal components I.sub.P and Q.sub.P. According to a
preferred embodiment of the invention the amplified signal
components I.sub.P and Q.sub.P are also filtered and further
amplified in subsequent low pass filters 106; 107 and amplifiers
108; 109 respectively. As a result, filtered and amplified signals
I.sub.P-LP; Q.sub.P-LP are created, which represent information
contained in the received radio frequency signals RF. The signals
I.sub.P-LP; Q.sub.P-LP are received by respective A/D-converters
110; 111 where they are converted into a digital format I.sub.D;
Q.sub.D for further processing in a digital signal processor
112.
[0026] Nevertheless, the filtered and amplified signals I.sub.P-LP;
Q.sub.P-LP are also included in the gain control loop and thus fed
back to indirectly control the gain of the amplifiers 104; 105. A
first signal level detector 113 receives a first signal component
Q.sub.P-LP and produces in response thereto a first signal level
[Q] representing the level or envelop of the first signal component
Q.sub.P-LP, i.e. a first order approximation. Naturally, any higher
order of approximation of the signal [Q] may likewise be made, such
as a second order power estimation. Correspondingly, a second
signal level detector 114 receives a second signal component
I.sub.P-LP and produces a second signal level [I] representing the
level or envelop of the second signal component I.sub.P-LP.
[0027] A combiner 115 receives both the first and the second signal
level [Q]; [I] and generates a combined signal V.sub..SIGMA., which
represents the sum of the signal levels [Q] and [I]. The combined
signal V.sub..SIGMA. is passed on to a gain control signal
generator 116 that produces an automatic gain control signal c on
basis of i.a. the combined signal V.sub..SIGMA..
[0028] The gain control signal generator 116 may include an
integrator, which influences a time constant .tau. of the automatic
gain control signal c in response to time properties of the
combined signal V.sub..SIGMA., such that a quickly varying combined
signal V.sub..SIGMA. results in a comparatively short time constant
.tau., and vice versa, a slowly varying combined signal
V.sub..SIGMA. results in a comparatively long time constant .tau..
During search for a radio frequency signal RF to receive or at
other instances when a message is expected, the gain control signal
generator 116 allocates a comparatively low value to the time
constant .tau. (i.e. a short time constant .tau.). Otherwise, the
gain control signal generator 116 sets a time constant .tau. value
that is adapted to an actual power variation rate of the received
radio frequency signals RF. More generally, the gain control signal
generator 116 is adapted for varying the time constant .tau. in
response to a time derivative parameter of the at least one down
converted signal, i.e. I.sub.P-LP or Q.sub.P-LP, which describes a
power variation rate of the received radio frequency signals RF.
Naturally, this does not preclude that the time constant may be
varied on basis of arbitrary additional parameters, which for
instance, depend on the protocol and frame structure according to
which the particular receiver operates.
[0029] On basis of a suitable input level of the first and second
signal components I.sub.P-LP and Q.sub.P-LP to the dynamic range of
the A/D-converters 110 and 111, the digital signal processor 112
produces a desired value V.sub.des of the combined signal
V.sub..SIGMA.. This value V.sub.des is also fed to the gain control
signal generator 116 to influence the automatic gain control signal
c.
[0030] The automatic gain control signal c is fed in parallel to
both the amplifiers 104 and 105 where it controls the gain, i.e.
the amplification factor. The automatic gain control signal c is
repeatedly updated to a value that is expected to make the
amplifiers 104 and 105 respective 108 and 109 deliver the down
converted signals at levels [I]; [Q] sufficiently close to a
desired signal level. In practice, this also means that the
automatic gain control signal c influences the amplifiers 104,
105,108 and 109 to deliver the down converted signals at levels
[I]; [Q] less than a predetermined limit level. Preferably, the
predetermined limit level is also optimised with respect to the
dynamic range of the A/D-converters 110 and 111.
[0031] A derivation unit 117 also receives the combined signal
V.sub..SIGMA.. This unit produces a time derivative parameter
V.sub..SIGMA. by estimating a time derivative of the combined
signal V.sub..SIGMA., for instance by high pass filtering the
combined signal V.sub..SIGMA.. The time derivative parameter
V.sub..SIGMA.is received in a ramp detector 118, for instance a
comparator, together with a threshold value V.sub.T from the
digital signal processor 112. If the time derivative parameter
V.sub..SIGMA. exceeds the threshold value V.sub.T, the comparator
118 generates a pulse signal R. The digital signal processor 112
receives the pulse signal R and interprets the presence of the
pulse signal R as an indication of a message start in the received
radio frequency signals RF. The pulse signal R is activated i.a.
when the RF power level rises sufficiently quickly from a
relatively low, or even zero/noise floor level, to a relatively
high level. By default, the time constant .tau. here has a
comparatively low value and receiver is therefore well prepared for
receiving the incoming message.
[0032] In an alternative embodiment of the invention, one or both
of the derivation unit 117 and the ramp detector 118 are realised
by functions within the digital signal processor 112.
[0033] In either case, after receiving the pulse signal R, the
digital signal processor 112 computes a relevant starting value for
the time constant .tau. (i.e. a short value) and delivers a
corresponding digital value .tau..sub.set-D on an output. This
value .tau..sub.set-D is forwarded to a conversion unit 119 being
adapted to the digital signal format used. The conversion unit 119
creates a corresponding analogue parameter .tau..sub.set. Hence,
the unit 119 may be a conventional D/A-converter, a low pass filter
or a pulse generator depending on the resolution requirements and
the format of the digital signal .tau..sub.set-D. If necessary, a
following time constant generator 120 produces an actual time
constant .tau. on basis of the analogue parameter .tau..sub.set,
for instance, by means of a table look-up in a bank of stored time
constant values .tau..
[0034] During reception of a message (e.g. a data packet) the
signal level of the received radio frequency signals RF should only
vary to small extent. The automatic gain control signal c should
namely have calibrated the gain of the amplifiers 104 and 105 to
this signal level, such that the time derivative parameter
V.sub..SIGMA. is approximately zero, or at least below the
threshold value V.sub.T. Consequently, the pulse signal R should
now be inactive. If, nevertheless, the digital signal processor 112
registers a pulse signal R during reception of a message, this is
interpreted as an indication of an erroneous setting of the
controllable gain in the amplifiers 104 and 105. Of course, this
can in turn be a consequence of an external fact, such as a radical
change in the radio environment. In any case, the affected message
will normally be damaged or lost and will thus probably have to be
re-transmitted.
[0035] This could imply that the receiver enters a search mode, in
which a relevant portion of the radio frequency spectrum is scanned
for possible radio frequency signals RF to receive. Alternatively,
the receiver resets one or more of its parameters and attempts to
re-establish the message reception. In any case, the digital signal
processor 112 allocates a comparatively short time constant
.tau..sub.set-D.
[0036] The pulse signal R in turn constitutes a basis for the time
constant value .tau..sub.set-D. The pulse signal R history, and
indirectly the threshold value V.sub.T history, namely indicate
that a particular stage in a typical message reception cycle has
been reached. A short time constant value .tau..sub.set-D should
always be applied in the beginning of the reception of a message. A
large rise in the pulse signal R accompanied by a rise in the power
level of the received radio frequency signals RF constitute
indications of a message start and thus a short time constant value
.tau..sub.set-D is likely to be successful.
[0037] A relatively long time constant value .tau..sub.set-D should
be applied whenever the radio frequency signals RF are received at
a relatively high and constant power level and if simultaneously
the absence of a pulse signal R indicates that a message is
currently being received. A long time constant value
.tau..sub.set-D here means that the time constant value
.tau..sub.set-D may also be infinitely long. The received radio
frequency power level can be determined via the digital signals
I.sub.D; Q.sub.D and the value of the automatic gain control signal
c. The value of the automatic gain control signal c can in turn be
detected by the digital signal processor 112 via an A/D-converter
(not shown).
[0038] A relatively short time constant value .tau..sub.set-D
should be applied whenever no radio frequency signals RF are being
received or in other situations when a new message is expected, for
instance, in case of a message being lost or damaged. Consequently,
based on variations in the power level of the received radio
frequency signals RF and the pulse signal R, the digital signal
processor 112 can allocate a adequate time constant value
.tau..sub.set-D for the automatic gain control signal c. After
having received a complete message, the digital signal processor
112 typically allocates a short time constant value .tau..sub.set-D
and thus prepares the receiver for reception of a new message.
[0039] FIG. 2 shows a power diagram that illustrates a first aspect
of the automatic gain control algorithm according to the invention.
The horizontal axis represents time t and the vertical axis shows a
transmitted/received signal power level in decibel. Naturally, the
power values differ substantially depending on whether they reflect
transmitted or received signal power. However, the
interrelationship between the relevant levels and thresholds are
substantially the same in both cases. A first power level PSS
denotes the average signal level during transmission/reception of
the payload information in the message. The transmitter should
reach the level P.sub.SS at latest at a rise time t.sub.r after
initiating the transmission and the receiver should reach its level
P.sub.SS at a somewhat later time t.sub.t after that the
transmission has been initiated. However, it is preferable if also
the receiver has reached its level P.sub.SS already at the end of
the rise time t.sub.r. The period between the rise time t.sub.r and
the time t.sub.t is namely utilised for calibrating certain units
in the receiver, such as an equaliser. Thus, a so-called training
sequence is typically sent between t.sub.r and t.sub.t. The
802.11a-protocol specifies a relatively short rise time t.sub.r of
2 .mu.s from initiating of the transmission. Since the receiver
station cannot know when a transmission is in fact started, it uses
a ramp detector, which here is illustrated by the threshold value
V.sub.T for the time derivative V.sub..SIGMA. of the combined
signal V.sub..SIGMA.. Hence, the receiver station deduces that a
new message is to be received whenever the time derivative
V.sub..SIGMA. rises above the threshold value V.sub.T. The receiver
station construes that it has detected a so-called signal power
ramp, which starts at point in time t.sub.R, when the threshold
value V.sub.T is exceeded. Provided that the rise time t.sub.r is a
system parameter known by the receiver station (e.g. t.sub.r=2
.mu.s), the receiver station can expect the signal power ramp to
end within a time period equal to the rise time t.sub.r after
t.sub.R.
[0040] When the rise time t.sub.r has come to an end, the power P
of the radio frequency signal preferably levels out at the first
power level P.sub.SS. During the period from t.sub.r to t.sub.t,
when for instance training sequence is sent, the signal power level
P typically shows a variation of .+-..DELTA.P.sub.t=3 dB around the
first power level P.sub.SS (i.e. peak-to-average (PAP)=3 dB).
According to 802.11a t.sub.t is 8 .mu.s and 802.11b t.sub.t
stipulates a t.sub.t of 20-48 .mu.s.
[0041] As of the time t.sub.t and onwards the radio frequency
signals should have an average power P level close to the first
power level P.sub.SS until the transmission is ended. The variation
from this level is normally .+-..DELTA.P.sub.D=10 dB (i.e. PAP=10
dB).
[0042] Even though the expected variation in power level P is lower
between t.sub.r and t.sub.t than after t.sub.t, it is generally
preferable if the time constant .tau. of the automatic gain control
signal c is shorter between t.sub.r and t.sub.t than after t.sub.t.
It is namely not desirable if the receiver "compensates" for the
variations in the signal power after t.sub.t, since such power
variations may represent payload information in the message.
Regardless of the time constant, the automatic gain control signal
c should always have a value such that a down converted signal
level is maintained below a limit level. This level is illustrated
by means of a corresponding power value P.sub.lim in the
diagram.
[0043] FIG. 3 shows a diagram over an exemplary combined signal
V.sub..SIGMA. on which an automatic gain control c may be based
according to an embodiment of the invention. The horizontal axis
represents time t and the vertical axis shows the combined signal
level V.sub..SIGMA., for instance in volts. The FIG. 1 shows an
embodiment of the invention where the combined signal V.sub..SIGMA.
contains a sum of the signal levels [Q]; [I] of a first down
converted signal component Q and a second down converted signal
component I. If the down converted signal is a sine wave with a
constant amplitude .vertline.s.vertline., the combined signal level
V.sub..SIGMA. will then vary between {square root}{square root over
(2)}.vertline.s.vertline. and .vertline.s.vertline. also when the
received signal level is constant. Consequently, a certain degree
of variation in the combined signal level V.sub..SIGMA. must be
tolerated without activation of the ramp detector 118. Of course,
an alternative level detection and/or signal combination may be
performed, which results in a smaller "undesired" variation in the
combined signal level V.sub..SIGMA.. It is, nevertheless, still
preferable that a moderate variation (i.e. non-zero time
derivative) in the combined signal level V.sub..SIGMA. be
accepted.
[0044] FIG. 4 shows a diagram over an exemplary time derivative
parameter V.sub..SIGMA. according to an embodiment of the
invention. Provided that the combined signal V.sub..SIGMA. is
produced according to the design in FIG. 1 and that the down
converted signal is a sine wave with a constant amplitude, the time
derivative parameter V.sub..SIGMA. will also vary to some extent,
for instance between a highest value .DELTA. and a lowest value
-.DELTA.. In order not to generate a pulse signal R (or alter the
time constant .tau.) in response to these variations, the threshold
value V.sub.T must lie sufficiently above .DELTA. to also exclude
variations due to the down converted signal being a non-sine wave
signal or a signal composed of more than one sine wave. A pulse
signal R should only be generated when the RF power level rises
relatively quickly. However, as soon as the derivative parameter
V.sub..SIGMA. exceeds the threshold value V.sub.T, here at
t=t.sub.R, the ramp detector 118 produces a pulse signal R.
[0045] In order to sum up, a method for controlling gain of an
amplifier for received radio signals in a radio receiver according
to an embodiment of the invention will now be described with
reference to a flow diagram in the FIG. 5. It should be borne in
mind that even though the steps in the flow diagram are executed
sequentially, this is merely true with respect to an infinitesimal
fraction of a received signal. While, for instance a certain
segment of the signal is being down converted, a somewhat later
segment of the same signal is being received, and so on.
[0046] Radio frequency signals are received and amplified in a
first step 501. A following step 502 down converts the signals into
quadrature components, which are then amplified. Subsequently, a
step 503 computes a time derivative parameter on basis of the
quadrature components. A following step 504 determines an automatic
gain control signal having a time constant value, which is based on
the time derivative parameter computed in the step 503. Finally,
the amplification of the quadrature components is controlled in a
step 506, on basis of the automatic gain control signal, such that
down converted signal components to be digitised are maintained at
signal level less than a predetermined limit level.
[0047] All of the process steps, as well as any sub-sequence of
steps, described with reference to the FIG. 5 above may be
controlled by means of a computer program, such as a digital signal
processor algorithm, being directly loadable into the internal
memory of a general computer, a digital signal processor, a
baseband processor or an ASIC (Application Specific Integrated
Circuit), which includes appropriate software for controlling the
necessary steps when the program is run on a computer/digital
signal processor. The computer program can likewise be recorded
onto arbitrary kind of computer readable medium.
[0048] The term "comprises/comprising" when used in this
specification is taken to specify the presence of stated features,
integers, steps or components. However, the term does not preclude
the presence or addition of one or more additional features,
integers, steps or components or groups thereof.
[0049] The invention is not restricted to the described embodiments
in the figures, but may be varied freely within the scope of the
claims.
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