Signal offset elimination

Details Signal offset elimination Signal offset elimination

1997-12-19

2001-07-10

Pham, Chi (Department: 2631)

Pulse or digital communications

Receivers

Angle modulation

C375S332000, C329S304000

Reexamination Certificate

active

06259748

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method for estimating the DC offset and the phase offset for a radio receiver operable in a digital passband transmission system.
DESCRIPTION OF THE PRIOR ART
FIG. 1
shows a general arrangement of a known digital passband transmission system in which a data source
10
emits a data symbol m
n
periodically every T seconds. The rate at which symbols are generated, transmitted and received in the system remains constant and is equal to 1/T, the symbol rate. An encoder
11
operates on the output (m
0
, m
1
, m
2
, . . . ) of the data source
10
by encoding each data symbol m
n
into a constellation point s
n
, which may be represented geometrically in signal space. The number of distinct possible constellation points is finite, and the set of constellation points which correspond to the set of possible data symbols is known as the signal constellation. For each generated constellation point s
n
, a modulator
12
constructs a distinct modulated signal s
n
(t) of duration T seconds as the representation of the data symbol m
n
. An antenna
13
of the transmitter then transmits, in series, each modulated signal s
n
(t) on a communications channel of the transmission system. A receiver operating in the digital passband transmission system receives via an antenna
14
each of the transmitted signals s
n
(t) as a received signal r
n
(t) for a duration of T seconds. In other words, the receiver samples the received radio signal at the symbol rate of the transmission system to produce a series of data samples or received signals r
n
(t). The task of the receiver is to reproduce the series of emitted data symbols m
n
by processing the series of received signals r
n
(t). This is accomplished in two stages. The first stage is a detector
15
that operates on the received signal r
n
(t) for a duration of T seconds to produce an observation or received signal point r
n
which, like the constellation point, may be represented geometrically in signal space. A decoder
16
constitutes the second stage of the receiver and performs a decision making process on the received signal point r
n
to produce an estimate m′
n
of the original message signal m
n
. The encoding and modulating process which occurs in the transmitter of
FIG. 1
typically involves switching (keying), for example, the phase of a carrier wave
17
in some fashion in accordance with the incoming data samples m
n
.
A further discussion of digital passband transmission systems may be found in chapter 8 of “Communication Systems”, Wiley, 3rd Edition, by Simon Haykin.
A popular type of digital passband data transmission used in cellular telephone systems is M-ary phase shift keying (MPSK) or more specifically quadrature phase shift keying (QPSK). A known QPSK transmitter is shown in FIG.
2
. Each message m
n
emitted from the data source
10
is converted into a constellation point s
n
defined in the QPSK system by the two quadrature signals s
n
[I] and s
n
[Q]. The two quadrature signals modulate two orthonormal carrier waves which have the same frequency &ohgr;
c
but differ in phase by 90°. These two modulated signals s
n
(t)[I] and s
n
(t)[Q] are then summed to produce the transmission signal s
n
(t).
FIG. 3
shows a known QPSK receiver which may receive each transmitted signal s
n
(t) as a received signal r
n
(t). The received signal r
n
(t) is then fed into an I correlator
31
which retrieves the I component r
n
[I] of the received signal, and a Q correlator
32
which retrieves the Q component r
n
[Q] of the received signal. The signals r
n
[I] and r
n
[Q] define the received signal point r
n
.
The constellation points s
n
and the received signal point r
n
of the QPSK system may be plotted in a signal space diagram with the x axis representing the in-phase I component and the y axis representing the &pgr;/2 out-of-phase quadrature Q component.
FIG. 4
is an example of a signal space diagram showing a set of four received signal point r
1
, r
2
, r
3
, and r
4
and their associated constellation points s
1
, s
2
, s
3
, and s
4
.
In digital passband transmission systems employing MPSK and QPSK modulation schemes, it is known for a received signal point r
n
to deviate from its associated constellation point s
n
. This may be the result of random errors which typically occur due to noise components being added to the transmission signal s
n
(t) in the communications channel. Deviation of the received signal points may also be the result of systematic errors present in the received signal points r
n
. These systematic errors may be the result of a phase offset in the transmission system which rotates each of the received signal points in signal space. The systematic errors may also be the result of a DC offset in the transmission system which translates each of the received signal points in signal space.
The total DC offset present in the received signal points r
n
may be the result of a combination of DC offset sources, such as:
a) The base band (data source and encoder) of the transmitter producing a DC offset.
b) Intermediate frequency or carrier residue at the transmitter.
c) Intermediate frequency or carrier residue at the receiver.
d) The base band of the receiver producing a DC offset.
e) Interference from a system clock appearing as a DC offset superimposed on a particular channel.
Deviation of a received signal point r
n
from its associated constellation point s
n
can be detrimental to the performance of the receiver. Optimisation of the receiver helps to reduce the deviation so that the decoder
16
can perform the decision making process more accurately. Optimisation may involve improving the signal-to-noise ratio so that the random error component in the deviation resulting from noise is reduced. Optimisation may also involve reducing the systematic errors in the deviation by estimating the systematic errors in the system and then eliminating the estimated systematic error from the received signal.
A known method for estimating the DC offset in a receiver is to average the received signal points r
n
in the base band of the receiver to obtain a value for the complex DC offset. For example, the I components r
n
[I] of the received signal points may be added together and the sum divided by the number of samples to yield an estimation for the I component of the DC offset, and the Q components r
n
[Q] of the received signal points may be added together and the sum divided by the number of samples to yield an estimation for the Q component of the DC offset. In order for the method to yield an accurate estimation for the DC offset, the number of received signal points N must be large enough to significantly reduce the random errors in the received signal points. Also, to avoid any biasing errors, the method depends on the set of original constellation points associated with the set of received signal points averaging substantially to zero.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a method of estimating the DC offset and the phase offset for a radio receiver operable in a digital passband transmission system, the method comprising; sampling a received radio signal at the symbol rate of the transmission system to produce a set of data samples, processing each data sample in the set in order to determine a received signal point in signal space for each data sample, determining an associated constellation point in signal space for each received signal point, calculating the DC offset and the phase offset which minimises, for the set of data samples, the sum of the square of the errors between each received signal point and its associated constellation point.
According to a second aspect of the present invention there is provided a method of estimating the DC offset for a radio receiver operable in a digital passband transmission system, the method comprising; sampling a received radio signal at t

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