METHOD AND APPARATUS FOR DETERMINING SIGNAL-TO-NOISE
RATIO
BACKGROUND OF THE INVENTION
AWideband Code Division Multiple Access (WCDMA) system provides
that a user equipment sends multiple control channels and data
channels. These channels are made orthogonal to each other through
the use of channelization codes. Therefore, these channels may be
transmitted simultaneously, and a receiver can separate them using
the different channelization codes.
However, in a multipath scenario, any two channels from different
fingers (multipaths) are no longer orthogonal because their arrival
timing at the receiver is mismatched due to the latency between the
two fingers. Therefore, while a receiver separates a current channel
from other channels in the same finger using the associated
channelization code, the channels from other fingers may leak into the
current channel. This leakage is called cross-finger interference. The
energy of the cross-finger interference is the original energy in the
other finger reduced by a despreading factor determined by the
symbol size of the channelization code in the current finger of the
receiver.
Fig. 1 illustrates a channel model for a Wideband Code Division
Multiple Access (WCDMA) system. Here, a user equipment (UE) has
multiple concurrent channels, such as Sc(t) for a control channel
signal (e.g., a Dedicated Physical Control Channel or DPCCH in
WCDMA) and Sd(t) for a data channel signal (e.g., E-DCH Dedicated
Physical Data Control Channel or E-DPDCH in WCDMA). While the
UE may send many concurrent channels, only two channels are
shown for the sake of brevity and simplicity of disclosure. In
particular, the UE applies respective orthogonal codes at spreaders
10, 12 for each signal, and a transmitter 14 transmits the multiple
signals over a medium such as an air interface. Noise n(t) is an
additive channel noise added by transmission over the medium. A
base station or NodeB therefore receives the following waveform at its
receiver 20:
x(t)=Sc(t)+Sd(t)+n(t)
In this example, if the control signal and the data signal come from
different fingers (or multi-paths), then power of one signal may leak
into the power of another signal causing cross-finger interference. This
interference significantly impacts receiver performance, particularly
when determining a signal-to-noise ratio (SIR) and the operations
such as uplink power control which rely on the SIR determination.
To better understand this impact, uplink power control will be briefly
discussed. As is well-known, during uplink power control according to
WCDMA, a NodeB and UE establish a power control loop only based
on the DPCCH. The NodeB measures the SIR (DPCCH Sc(t) power to
n(t) noise power ratio) and compares the measured SIR to a target SIR.
The NodeB instructs the UE to adjust the uplink DPCCH power level
to meet the desired SIR range and a desired bit error rate (BER)
performance received, for example, from a radio network controller
(RNC). After this power control closed loop is established, the UE will
generate a DPCCH uplink transmission gain factor for the DPCCH
channel. The UE uses the gain factor as a base to calculate the
transmission power for the other uplink channels. For example, for
the E-DPDCH channel, a desired power ratio is defined by TPR= (EDPDCH
power)/(DPCCH power). The E-DPDCH transmission power
gain factor ed is then determined according to the following
expression:
ed 2 = TPR * 2
In this approach, the closed loop power control should not include EDPDCH
signal power. Any interference leaked from the E-DPDCH into
the DPCCH will cause the DPCCH SIR measurement to have an offset
and result in misleading the power control loop. If the cross-finger
interference is too strong, the problem is that the power control loop
diverges and can not set a correct power gain factor for each channel.
The cross-finger interference could be very strong and significantly
impact system performance. For example, in WCDMA system, EDPDCH
power may be 100s of times stronger than the DPCCH. For
the DPCCH, the cross-finger interference from the E-DPDCH could be
even stronger than DPCCH itself. For additive channel noise power
estimation, the cross-finger interference could cause a huge offset for
the noise power estimation. This cross-finger interference may cause a
NodeB to miss-measure the SIR and lead to power diverge in uplink
power control loop.
Furthermore, existing techniques for estimating SIR rely on a
correlation function as shown by the expressions below. First, assume
the output symbol at the receiver for the control signal Sc is:
y(t)=Sc(t)+n(t), where t is a symbol index.
At the receiver, the control signal power E[Sc2] is determined based on
correlation function as follows:
E[Sc2] ~= y (t )*y(t- l )
= [Sc(t)+n(t)]*[Sc(t- l)+n(t- l)]
= Sc(t)Sc(t- l ) + {Sc(t)n(t- l)+Sc(t- l)n(t)+n(t)n(t- l ) }
= Sc(t)Sc(t- l ) + Oc(Sc, n )
-> Sc2
where oc(Sc, n ) is called a correlation remainder.
The receiver determines the noise power E[n2] according to the
following expression:
E[n2] ~= y(t) 2 - y(t)y(t- l )
= [ Sc(t)+n(t) - [Sc2 + Oc(Sc, n)]
= [Sc(t) 2 + n(t) 2 + 2Sc(t)n(t)] - [Sc2 + oc(Sc, n)]
= n(t) 2 + on(Sc, n )
-> n2
where on(Sc, n ) is also called correlation remainder.
These measurements include both signal power and cross-finger
interference power, which may be represented as:
Ek[Sc(t) 2] = EkSc + Ecross2 _k, where k represents a finger index
Ek[n(t) 2] = Ekn + Ecrossl _ k
where Ek[Sc(t) 2] is the received control signal power, EkSc represents
the portion of the received control signal power due to the sent control
signal, Ecross2 _ k represents the portion of the control signal power
due to the cross-finger interference, Ek[n(t) 2] is the received noise
power, Ekn represents the portion of the received noise power due to
the additive channel noise, and Ecrossl_k represents the portion of
the received noise power due to the cross-finger interference. In view
of the above, the SIR may be expressed as:
fi E,_Sc +E _
Both the signal power and noise power estimation output contain
cross-finger interference since the cross-finger interference exists in
the input of the correlation function, or output of DPCCH despreader.
In a High Speed Uplink Packet Access (HSUPA) environment, the
cross-finger interference could be much stronger than noise power:
Ecmss _k » E [n(t)2]
This causes SIR measurements to have a huge random offset, and
significantly downgrades throughput.
SUMMARY OF THE INVENTION
At least some example embodiments relate to a method of determining
a signal-to-noise ratio in a wireless network.
In one embodiment, the method includes despreading the received
signals by applying an unused channelization code, determining noise
power based on output of the despreading, and determining a signalto-
noise ratio, SIR, based on the noise power and at least one of the
received signals.
In one embodiment, the method further includes canceling the noise
power from power of the at least one of the received signals to produce
a modified received signal power. Here, the SIR is determined based
on the noise power and the modified received signal power.
In one embodiment, the despreading, the determining noise power and
the canceling are performed for a plurality of fingers of a receiver.
In one embodiment, the determining a SIR includes determining an
estimated noise power for each of the plurality of fingers based on the
noise power determined for each of the plurality of fingers and a total
antenna power for each of the plurality of fingers, and determining a
SIR based on the estimated noise powers for each of the fingers and
the modified received signal power for each of the plurality of fingers.
In one embodiment, the noise power includes power from noise and
cross-finger interference, and the estimated noise power includes
power from the noise but not from the cross-finger interference.
In one embodiment, the determining an estimated noise power
determines the estimated noise power, for a particular finger from the
plurality of fingers, by subtracting a first sum from a second sum. The
first sum is a sum of the total antenna power for each of the plurality
of fingers except the particular finger. The second sum is a sum of the
noise power determined for each of the plurality of fingers.
In one embodiment, the determining an estimated noise power
determines the estimated noise power, for a particular finger from the
plurality of fingers, by subtracting a first sum from the total antenna
power for the particular finger. The first sum is a sum of signal powers
for signals received by the particular finger.
In one embodiment, determining a SIR includes determining, for each
of the plurality of fingers, a finger SIR based on the modified received
signal power for the finger and the estimated noise power for the
finger; and adding the finger SIRs for the plurality of fingers.
In one embodiment, the method further includes performing uplink
power control based on the determined SIR.
Another embodiment of the method includes determining a noise
power for each of a plurality of fingers, determining at least one signal
power associated with each of the plurality of fingers, and estimating a
noise power at each of the plurality of fingers based on the noise
power determined for each of the plurality of fingers and a total
antenna power for each of the plurality of fingers. A signal-to-noise
ratio, SIR, is determined based on the estimated noise powers for the
plurality of fingers and the determined signal powers associated with
the plurality of fingers.
In one embodiment, the noise power includes power from noise and
cross-finger interference, and the estimated noise power includes
power from the noise but not from the cross-finger interference.
In one embodiment, the determining an estimated noise power
determines the estimated noise power, for a particular finger from the
plurality of fingers, by subtracting a first sum from a second sum. The
first sum is a sum of the total antenna power for each of the plurality
of fingers except the particular finger, and the second sum is a sum of
the noise power determined for each of the plurality of fingers.
In one embodiment, the method further includes canceling, for each
finger, the noise power from the determined signal power to produce a
modified signal power. Here, the finger SIR is determined based on the
modified signal power for the finger and the estimated noise power for
the finger.
In one embodiment, the determining an estimated noise power
determines the estimated noise power, for a particular finger from the
plurality of fingers, by subtracting a first sum from the total antenna
power for the particular finger. The first sum is a sum of signal powers
for signals received by the particular finger.
In one embodiment, the determining a SIR includes determining, for
each of the plurality of fingers, a finger SIR based on the modified
received signal power for the finger and the estimated noise power for
the finger; and adding the finger SIRs for the plurality of fingers.
In one embodiment, the method further includes performing uplink
power control based on the determined SIR.
At least some embodiment relate to a receiver.
In one embodiment, the receiver includes a plurality of fingers and an
SIR generator. Each finger is configured to despread received signals
by applying an unused channelization code, and is configured to
determine a noise power based on output of the despreading. The SIR
generator is configured to generate a SIR based on the noise powers
and at least one of the received signals.
In another embodiment, the receiver includes a plurality of fingers and
an SIR generator. Each finger is configured to determine a noise power
and an associated signal power. The SIR generator is configured to
estimate a noise power at each of the plurality of fingers based on the
noise power determined for each of the plurality of fingers and a total
antenna power received by each of the plurality of fingers, and is
configured to determine a SIR based on the estimated noise powers for
the plurality of fingers and the signal powers associated with the
plurality of fingers.
BRIEF DESCRIPTION OF THE DRAWINGS
The example embodiments will become more fully understood from
the detailed description given herein below and the accompanying
drawings, wherein like elements are represented by like reference
numerals, which are given by way of illustration only and thus are not
limiting of the example embodiments and wherein:
Fig. 1 illustrates a channel model for a WCDMA system.
Fig. 2 illustrates a portion of an uplink receiver for determining signalto-
noise ratio (SIR) in a WCDMA system according to an embodiment.
Fig. 3 illustrates a portion of an uplink receiver for determining signalto-
noise ratio (SIR) in a WCDMA system according to another
embodiment.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Detailed illustrative embodiments are disclosed herein. However,
specific structural and functional details disclosed herein are merely
representative for purposes of describing example embodiments. An
embodiment may, however, be embodied in many alternate forms and
should not be construed as limited to only the embodiments set forth
herein.
It will be understood that, although the terms first, second, etc. may
be used herein to describe various elements, these elements should
not be limited by these terms. These terms are only used to
distinguish one element from another. For example, a first element
could be termed a second element, and, similarly, a second element
could be termed a first element, without departing from the scope of
example embodiments. As used herein, the term "and/or" includes
any and all combinations of one or more of the associated listed items.
It will be understood that when an element is referred to as being
"connected" or "coupled" to another element, it can be directly
connected or coupled to the other element or intervening elements
may be present. In contrast, when an element is referred to as being
"directly connected" or "directly coupled" to another element, there are
no intervening elements present. Other words used to describe the
relationship between elements should be interpreted in a like fashion
(e.g., "between" versus "directly between", "adjacent" versus "directly
adjacent", etc.).
The terminology used herein is for the purpose of describing particular
embodiments only and is not intended to be limiting of example
embodiments. As used herein, the singular forms "a", "an" and "the"
are intended to include the plural forms as well, unless the context
clearly indicates otherwise. It will be further understood that the
terms "comprises", "comprising,", "includes" and/or "including", when
used herein, specify the presence of stated features, integers, steps,
operations, elements, and/ or components, but do not preclude the
presence or addition of one or more other features, integers, steps,
operations, elements, components, and/ or groups thereof.
Exemplary embodiments are discussed herein as being implemented
in a suitable computing environment. Although not required,
exemplary embodiments will be described in the general context of
computer-executable instructions, such as sections, program modules
or functional processes, being executed by one or more computer
processors or CPUs. Generally, sections, program modules or
functional processes include routines, programs, objects, components,
data structures, etc. that performs particular tasks or implement
particular abstract data types. The sections, program modules and
functional processes discussed herein may be implemented using
existing hardware in existing communication networks. For example,
sections, program modules and functional processes discussed herein
may be implemented using existing hardware at existing network
elements, servers or control nodes. Such existing hardware may
include one or more digital signal processors (DSPs), applicationspecific-
integrated-circuits, field programmable gate arrays (FPGAs)
computers or the like.
In the following description, illustrative embodiments will be described
with reference to acts and symbolic representations of operations (e.g.,
in the form of flowcharts) that are performed by one or more
processors, unless indicated otherwise. As such, it will be understood
that such acts and operations, which are at times referred to as being
computer-executed, include the manipulation by the processor of
electrical signals representing data in a structured form. This
manipulation transforms the data or maintains it at locations in the
memory system of the computer, which reconfigures or otherwise
alters the operation of the computer in a manner well understood by
those skilled in the art.
It should also be noted that in some alternative implementations, the
functions/ acts noted may occur out of the order noted in the figures.
For example, two figures shown in succession may in fact be executed
substantially concurrently or may sometimes be executed in the
reverse order, depending upon the functionality/ acts involved.
As used herein, the term "user equipment (UE)" may be considered
synonymous to, and may hereafter be occasionally referred to, as a
mobile, mobile unit, mobile station, mobile user, access terminal (AT),
subscriber, user, remote station, receiver, etc., and may describe a
remote user of wireless resources in a wireless communication
network. The term "base station (BS)" may be considered
synonymous to and/or referred to as a base transceiver station (BTS),
NodeB, access node (AN), eNodeB, etc. and may describe equipment
that provides data and/ or voice connectivity between a network and
one or more users.
As is well-known in the art, each of the user equipment and the base
station may have transmission and reception capabilities.
Transmission from the base station to the UE is referred to as
downlink or forward link communication. Transmission from the UE
to the base station is referred to as uplink or reverse link
communication.
According to at least some example embodiments, instead of using a
correlation function, the receiver creates a new virtual noise channel
by despreading received signals using an unused channelization code.
This noise channelization code has not been used in transmission by
the user equipment (UE) transmitter and may be thought of, therefore,
as a virtual noise channel. For example, the channelization code may
be from the orthogonal channel code set defined by the standard.
Therefore, this noise channelization code is orthogonal to all user
control and data channels. Since the UE transmitter never physically
sends out a signal with the noise channelization code, the receiver
noise channel output will not include any user channel signals, and
therefore, represents the channel noise. However, this channel noise
may include cross-finger interference. According to at least one
embodiment, a noise power estimation having the cross-finger
interference cancelled there from is determined in real time from the
channel noise. Based on this noise power estimation, the SIR may also
be determined in real time.
Fig. 2 illustrates a portion of an uplink receiver for determining signalto-
noise ratio (SIR) in a WCDMA system according to an embodiment.
For example, the uplink receiver may form part of a NodeB or base
station. Fig. 2 illustrates a plurality of fingers 50- 1 to 50-N that each
receive the signals received by an antenna 52. While each finger 50
may have different timings associated with different multipaths, the
structure of each finger 50 may be the same. Accordingly, only the
structure of finger 50- 1 is illustrated and will be described for the
sake of brevity.
As shown, a noise despreader 60 receives signals received by the
antenna 52. The noise despreader 60 applies an unused orthogonal
channel code to the received signals. The orthogonal code may be any
well-known orthogonal code such as a Walsh code, etc. As mentioned
above, various standards may provide a set of orthogonal codes. The
noise despreader 60 selects a code from this set that is not used in
transmitting the signals received at the antenna 52. Accordingly,
output from the first despreader will be just the noise. There is no
correlation remainder on(Sc, n). A noise power determining unit 62
determines the power of the noise signal En Ch on a symbol-by-symbol
basis. For example, the noise power determining unit 62 may square
the noise signal output from the noise despreader 60.
The finger 50 also includes first - Mth signal despreaders 70- 1 to 70-
M using respective orthogonal codes to each despread a respective
signal such as a control signal or a data signal, respectively. A noise
cancellation unit 80 receives the noise power and the signals output
by the signal despreaders 70. The noise cancellation unit 80 includes
power determining units 82- 1 to 82-M, each determining the power of
a signal output by a respective signal despreader 70 on a symbol-bysymbol
basis. For example, in one embodiment, the power
determining units 82 may square the respective signal to obtain the
signal power. The noise cancellation unit 80 also includes combiners
84- 1 to 84-M. Each combiner subtracts the noise power output by the
noise power determining unit 62 from the signal power output by an
associated power determining unit 82 to cancel the noise power from
the signal power and output a modified signal power Esig from which
noise has been cancelled. In Fig. 2, Esig represents the signal power
for the jth signal in the ith finger. It should be noted that this noise
power determination and cancellation works for any signal symbol
without a need to know the signal bits. However, if the signal is
known, like a pilot signal, the pilot symbols may be adjusted to line up
on the same bits and accumulated. This is called coherent
accumulation. The coherent accumulation may be squared to obtain a
coherent power. Accordingly, the power determined units 62 may
perform coherent or non-coherent power estimation based on whether
the received signal is known.
As shown in Fig. 2, the uplink receiver further includes a SIR
generator 100 that generates a SIR based on the signals produced by
the fingers 50. In particular, the SIR generator 100 includes a total
power detector 110, a noise power estimator 120, and a SIR resolver
130. The total power detector 110 determines the total antenna power
for each finger. Namely, each finger is associated with a different
arrival time (e.g., multipath). The total power detector 110 determines
the total power received at the antenna 52 at the arrival time
associated with a finger as the total antenna power of the finger. The
total antenna power is determined on a symbol basis for each finger.
Namely, the power over the total number of chips forming a symbol is
determined as the total antenna power.
The noise power estimator 120 estimates a clean noise power
associated with each finger based on the noise power for each finger
and the total antenna power for each finger. As will be recalled, the
noise power includes noise plus cross-finger interference. By contrast
the estimated noise power provides an estimate of the noise power
without the cross-finger interference. In particular, the noise power
estimator 120 determines the estimated noise power according to the
following expression:
fingers
E n oise_k = Eant_k Eant i - Ench
i=0
where E n 0ise_k is the estimated noise power for the kth finger, Eant_k is
the total antenna power for the kth finger, and E n Ch k is the noise
power from the power determining unit 62 for the kth finger. As this
expression indicates, the estimated noise power for the kth finger is
the (1) the sum of the total antenna power for all the fingers except the
kth finger subtracted from (2) the sum of the noise power E n Ch
determined for all the fingers.
The SIR resolver 130 receives the estimated noise power for each
finger and the modified signal power for a signal of interest from each
finger. For example, in WCDMA, the signal of interest may by the
DPCCH. The SIR resolver 130 determines the SIR of each finger. For
example, using the DPCCH a s an example, the finger SIR for the kth
finger is determined according to the following expression:
SIRk = EDPCCH_k/ Enoise_k.
where EDPCCH k is the signal power of the DPCCH for the kth finger.
The SIR resolver 130 sums the finger SIRs to determine a total SIRtot
a s the output SIR.
Fig. 2 further shows that the receiver of the NodeB includes a power
controller 150. The power controller 150 performs power control in
any well-known manner except that the measured or detected SIR
used in the power control operation is the SIRtot output from the SIR
generator 100.
Fig. 3 illustrates a portion of an uplink receiver for determining signalto-
noise ratio (SIR) in a WCDMA system according to another
embodiment. For example, the uplink receiver may form part of a
NodeB or base station. The embodiment of Fig. 3 is the same a s the
embodiment of Fig. 2 except that the SIR generator 100 of Fig. 2 has
been replaced with a SIR generator 170 in Fig. 3 . Accordingly, for the
sake of brevity, only these differences will be described.
The SIR generator 170 in Fig. 3 is the same a s the SIR generator 100
in Fig. 2 except that the noise power estimator 120 of Fig. 2 has been
replaced with a noise power estimator 180 in Fig. 3 . Accordingly, for
the sake of brevity, only these differences will be described.
The noise power estimator 180 receives the total antenna powers from
the total power detector 110. The noise power estimator 180 also
receives all of the uplink signal powers Esig after noise cancellation.
For each finger k , the noise power estimator 180 determines the
estimated noise power Enoise kaccording to the expression below:
alluplinkchannels
Enoise_k = Eant_k - (EsigkJ)
i=0
Accordingly, the sum of the signal powers for a finger are subtracted
from the total antenna power for the finger to determine the estimated
noise power for the finger.
The SIR resolver 130 receives the estimated noise powers for the
fingers and determines the finger SIR and total SIRtot a s described
above with respect to Fig. 2 . Furthermore, the power controller 150
may perform power control based on this total SIRto t as described
above with respect to Fig. 2.
In wireless communication systems such as a WCDMA system, there
are many signal processing operations in addition to power control
that rely on accurate noise power, signal power, and/or SIR
estimation. Accordingly, the noise power, signal power and/or SIR
generated according to the above embodiments may be used in any of
these well-known operations. For example, DTX detection may be
performed in the same manner as described in US Patent No.
7,782,820, the entire contents of which are hereby incorporated by
reference, except that the noise power, signal power and/or the
determined SIR may be replaced with those determined according to
any of the above embodiments.
The invention being thus described, it will be obvious that the same
may be varied in many ways. For example, while the embodiments
were described as applied to a WCDMA system, the invention is not
limited to WCDMA. Also, while described a applied to the uplink, the
embodiments may also be applied to the downlink. Such variations
are not to be regarded as a departure from the invention, and all such
modifications are intended to be included within the scope of the
invention.
WE CLAIM:
1. A method of determining a signal-to-noise ratio in a wireless
network, comprising:
determining (62) a noise power for each of a plurality of fingers
(50);
determining (82) at least one signal power associated with each
of the plurality of fingers;
estimating (120) a noise power at each of the plurality of fingers
based on the noise power determined for each of the plurality of
fingers and a total antenna power for each of the plurality of fingers;
and
determining (130) a signal-to-noise ratio, SIR, based on the
estimated noise powers for the plurality of fingers and the determined
signal powers associated with the plurality of fingers.
2. The method of claim 1, wherein the noise power includes power
from noise and cross-finger interference, and the estimated noise
power includes power from the noise but not from the cross-finger
interference.
3. The method of claim 1, wherein the determining an estimated noise
power determines the estimated noise power, for a particular finger
from the plurality of fingers, by subtracting a first sum from a second
sum, the first sum is a sum of the total antenna power for each of the
plurality of fingers except the particular finger, and the second sum is
a sum of the noise power determined for each of the plurality of
fingers.
4. The method of claim 3, wherein the determining a SIR comprises:
determining, for each of the plurality of fingers, a finger SIR
based on the determined signal power for the finger and the estimated
noise power for the finger; and
adding the finger SIRs for the plurality of fingers.
5. The method of claim 4, further comprising:
canceling, for each finger, the noise power from the determined
signal power to produce a modified signal power; and wherein
the determining, for each of the plurality of fingers, a finger SIR
determines the finger SIR based on the modified signal power for the
finger and the estimated noise power for the finger.
6. The method of claim 1, wherein the determining an estimated noise
power determines the estimated noise power, for a particular finger
from the plurality of fingers, by subtracting a first sum from the total
antenna power for the particular finger, and the first sum is a sum of
signal powers for signals received by the particular finger.
7. The method of claim 6, wherein the determining a SIR comprises:
determining, for each of the plurality of fingers, a finger SIR
based on the modified received signal power for the finger and the
estimated noise power for the finger; and
adding the finger SIRs for the plurality of fingers.
8. The method of claim 7, further comprising:
canceling, for each finger, the noise power from the determined
signal power to produce a modified signal power; and wherein
the determining, for each of the plurality of fingers, a finger SIR
determines the finger SIR based on the modified signal power for the
finger and the estimated noise power for the finger.
9. The method of claim 1, further comprising:
performing uplink power control based on the determined SIR.
10. A receiver, comprising:
a plurality of fingers (50), each finger configured to determine a
noise power and an associated signal power;
a signal-to-noise ratio, SIR, generator (100) configured to
estimate a noise power at each of the plurality of fingers based on the
noise power determined for each of the plurality of fingers and a total
antenna power received by each of the plurality of fingers, and
configured to determine a SIR based on the estimated noise powers for
the plurality of fingers and the signal powers associated with the
plurality of fingers.