System and method for demodulating multiple QAM signals

A system and method demodulate N QAM signals (N being a positive integer equal to or greater than 1) substantially simultaneously using, for example, one or two oscillators, regardless of how many QAM signals need to be demodulated.

BACKGROUND

1. Field of the Invention

The present invention is related to a system and method for demodulating multiple Quadrature Amplitude Modulation (QAM) signals.

2. Background Art

In one example, the need to demodulate several QAM (Quadrature Amplitude Modulation) carriers or signals simultaneously arises when channel bonding is introduced in the future specifications of Docsis 3.0 (Data-Over-Cable Service Interface Specification 3.0). Channel bonding is a technology that combines two or more physical channels into a single virtual channel, effectively doubling or greater the data transfer speeds. When a receiver needs to demodulate several carrier signals (e.g., QAM signals), it needs to generate a local oscillator (sine and cosine) for each of the QAM signals. Conventionally, implementation of this requirement resulted in using an equal number of NCOs (Numerically Controlled Oscillators), or other forms of implementation for generating the local carriers, as there were QAM signals. An NCO can been seen as referring to a block or device that generates a local carrier consisting of cosine and sine. For example, typically N numbers of NCOs were required for N number of QAM signals to be demodulated. If N becomes too large the implementation will become very costly.

Therefore, what is needed is a system and method that reduces a number of oscillators needed to demodulate multiple carrier signals.

SUMMARY

An embodiment of the present invention provides a system that demodulates carrier signals comprising at least a first oscillator, and in one example a second oscillator, and at least first through third demodulators.

In the example using two oscillators, the first oscillator produces first and second signals, the first signal having a frequency that is phase separated 90 degrees from a frequency of the second signal. The second oscillator produces third and fourth signals, the third signal having a frequency that is phase separated 90 degrees from a frequency of the fourth signal. The first demodulator receives the first and second signals and first and second carrier signals, the first demodulator generating a first pair of demodulated signals therefrom. The second demodulator receives the third and fourth signals and the first pair of demodulated signals, the second demodulator generating a second pair of demodulated signals therefrom. The third demodulator receives the fourth signal and the first and second pair of demodulated signals, the third demodulator generating a third pair of demodulated signals therefrom. The first, second and third pair of demodulated signals are transmitted along first, second, and third channels, respectively.

In another example, when the frequency of the first carrier signal is equal to the distance between the first and second carrier signals, the second oscillator is eliminated from the system. When the second oscillator is eliminated from the system the second demodulator receives the first and second signals and the first pair of demodulated signals and the third demodulator receives the first and second signals and the first and second pair of demodulated signals.

In another embodiment, a method comprises: (a) determining a spacing between signals and a frequency of the signals in a received block of signals, (b) initiating one or both oscillation devices in a pair of oscillation devices based on the spacing and the frequency of the signals, (b) determining what demodulator in a set of demodulators begins demodulation of the signals based on which signal in the block of signals is first received by the set of demodulation devices, and (c) demodulating the block of signals.

In yet another embodiment of the present invention, there is provided a method of demodulating a plurality of input channels comprising: generating a first oscillating signal to demodulate a first channel of the plurality of input channels, generating a second oscillating signal to demodulate a second channel of the plurality of input channels, and demodulating a third channel of the plurality of input channels based on the first and second oscillating signals.

The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers may indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number may identify the drawing in which the reference number first appears.

DETAILED DESCRIPTION

Overview

While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the pertinent art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the present invention. It will be apparent to a person skilled in the pertinent art that this invention can also be employed in a variety of other applications.

Embodiments of the present invention provide a system and method for demodulating N QAM signals (N being a positive integer equal to or greater than 1) substantially simultaneously using, for example, one or two oscillators, regardless of how many QAM signals need to be demodulated.

This approach will be more cost efficient and save some silicon area, and hence power consumption, especially if N is large.

In one example, the system and method are not limited to digital implementation, and in another example the system and method can be applied to analog demodulation.

The digital example is based on the assumption that an RF signal of the multiple QAM carriers that are to be demodulated are already block converted and digitized through an analog to digital converter (A/D). It is also assumed that the frequency spacing between adjacent carriers for all carriers is the same.

Overall System

FIG. 1shows a system100, according to one embodiment of the present invention. System100includes a carrier signal source102coupled to a converter/demodulator104(e.g., a set top box, a modem, or the like). N signals106are transmitted between carrier signal source102and converter/demodulater104, where N is an integer greater than or equal to 1. Converter/demodulator104then spreads the N signals onto N channels108. For example, in a cable TV environment, N digital or analog cable carrier signals106(e.g., QAM carriers) are received at a converter/demodulator104(e.g., modem) and demodulated into N signals along channels108to be used by a display device (not shown). In one example, the N signals106are sampled at a first frequency ωA (e.g., a frequency demodulation will start at) and spaced a distance ωB apart. For example in the US ωB is 6 MHz and in Europe ωB is 8 MHz, while in other countries other spacings can be used.

FIG. 2shows a system200, according to one embodiment of the present invention. In system200, similar to system100, N signals106are received, but in this embodiment signals106are analog signals that are converted to digital signals210through use of a converter213, for example an analog-to-digital converter. In one example, digital signals210are referred to as RF or radio frequency signals. Digital signals210are received by a demodulator204, which also receives first and second oscillator signals212and214, respectively, from first and second oscillators216and218. For example, these signals can be considered first and second clock signals that are 90 degrees apart. Demodulator204demodulates digital signals210using first and second oscillator signals212and214, as is described in more detail below, to produce demodulated signals along channels220.

In one example, oscillators216and/or218(and the other oscillators described below) are programmable oscillators, such as numerically controlled oscillators that are programmed by receiving a certain value corresponding to a desired frequency. In other examples, oscillators216and/or218can be dedicated to a single frequency.

In this embodiment, only two oscillators216and218are required for demodulator204to demodulate N number of digital signals210, as is described in more detail below with respect toFIGS. 3 and 4. This is in contrast to conventional systems, as described above, in which N oscillators would have been required for N signals.

In one example, where ωA and ωB are the same, only one oscillator signal from one oscillator, for example oscillator216, would be required to have demodulator204demodulate N number of digital signals210, as is described in more detail below with respect toFIGS. 5 and 6.

It is to be appreciated the systems and methods described herein can be used to simultaneously demodulate a block of signals with constant spacing between the signals in any environment.

Exemplary Operation

FIGS. 3 and 4show a demodulator304, according to one embodiment of the present invention. Demodulator304is one embodiment of demodulator204. In this embodiment, a first oscillator316produces a first oscillator signal or first signal cos(ωA) and a second oscillator signal or second signal sin(ωA). A second oscillator318produces a first oscillator signal or first signal cos(ωB) and second oscillator signal or second signal sin(ωB). Both oscillators' signals are received by demodulator304that comprises N demodulators304-0to304-N. In one example, an optional demodulator304-(−N) is also within demodulator304. Demodulator304also receives signal106or210as Io and Qo.

In one example, S(n) is defined as the digitized multiple QAM carrier RF signal (e.g., either106or210) that is demodulated and we will define ωAas the channel carrier frequency and use it as a starting point for demodulation. We will also define ωBas the channel spacing frequency.

In this embodiment, for channel320-0when carrier frequency ωAis used, the demodulation process in demodulator304-0produces:
S*cos(ωA), for inphase.  (1)
S*sin(ωA), for quadrature.  (2)

Where signals cos(ωA) and sin(ωA) are the Inphase and Quadrature local oscillators, respectively. Note that for simplicity the time index (n) notation has been dropped.

If we proceed to the next higher frequency channel320-1, then for carrier frequency ωA+ωB, the required Inphase and Quadrature local oscillators output from demodulator304-1will be
cos(ωA+ωB), for inphase.  (3)
sin(ωA+ωB), for quadrature.  (4)

Equation (3) and (4), respectively, can be expanded as
cos(ωA)*cos(ωB)−sin(ωA)*sin(ωB)=2*cos(ωA)*cos(ωB)−cos(ωA−ωB)  (5)
and
sin(ωA)*cos(ωB)+cos(ωA)*sin(ωB)=2*sin(ωA)*cos(ωB)−sin(ωA−ωB)  (6)

If we proceed further to the next higher frequency channel320-2, then for carrier frequency channel of ωA+ωB+ωB, the required local oscillators produced from demodulator304-2will be

Then moving on to the next channel320-3channel frequency of ωA+3ωB, the required local oscillators produced by demodulator304-3will be
cos(ωA+3ωB)=2*cos(ωA+2ωB)*cos(ωB)−cos(ωA+ωB)  (9)
sin(ωA+3ωB)=2*sin(ωA+2ωB)*cos(ωB)−sin(ωA+ωB)  (10)

From equations (7) to (10), we can generalize them as the following expressions for further demodulators304-N after304-3
cos(ωA+NωB)=2*cos(ωA+(N−1)ωB)*cos(ωB)−cos(ωA+(N−2)ωB)  (11)
sin(ωA+Nωb)=2*sin(ωA+(N−1)ωB)*cos(ωB)−sin(ωA+(N−2)ωB)  (12)

In one example, demodulation may not start at a beginning of a block of carrier signals, and can start anywhere within a block of carrier signals. Thus, the system described above and below is very flexible as to what signals can be demodulated. In order to make this work a −N demodulator (e.g., an N less than 0 demodulator) can be used. It is to be appreciated the −N demodulator is not required if demodulation starts at the beginning of the block, thus it can be seen as an optional demodulator.

For the case of N<0, e.g., for a demodulator304-(−N) the expressions will be
cos(ωA+NωB)=2*cos(ωA+(N+1)ωB)*cos(ωB)−cos(ωA+(N+2)ωB)  (13)
sin(ωA+NωB)=2*sin(ωA+(N+1)ωB)*cos(ωB)−sin(ωA+(N+2)ωB)  (14)

In this embodiment, based on equations (11) to (14), carrier demodulation can be done by using the computed values of the 2 adjacent channels. Thus, in this embodiment, only two oscillators316and318are used, one for generating cos(ωA) and sin(ωA), and the other for generating cos(ωB) and sin(ωB).

FIG. 4shows an implementation of the demodulators304-(−N) to304-N, according to one embodiment of the present invention. In this example, the above operational parameters are met.

Demodulator304-(−N) includes first and second mixers422and423, first and second shifters424and425, and first and second adders426and427.

For example, in this digital example the shifters424and425, as well as other shifters discussed below, left shift the signal leaving the mixers422and423by 1 bit. This is to produce the 2*cos and 2*sin variables in equations (13) and (14), and similar variables in the other equations. In an analog implementation, the shifter would be replaced with a gain device where the signal would be multiplied by 2.

In this example, through receipt, mixing, shifting, and adding of I(−N+1), I(−N+2), Q(−N+1)and Q(−N+2), and cos(ωB), respectively as shown, desired output signals as shown in equations (13) and (14) are produced, as discussed above, as signals I−Nand Q−Nalong channel320-(−N).

Demodulator304-0includes first and second mixers428and429that mix RFin signals (e.g., signal106or210) with first and second signals cos(ωA) and sin (ωA), respectively, to produce the output signals shown in equations (1) and (2), discussed above, as signals Io and Qo along channel320-0and as inputs into subsequent demodulators304. This demodulator is optional, as discussed above,

Demodulator304-1includes first, second, third and fourth mixers430,431,432, and433, respectively, and first and second adders434and435that receive signals from demodulator304-0and from second oscillator318, respectively as shown, to produce signals shown in equations (3) and (4), discussed above, as signals I1and Q1along channel320-1and as inputs to subsequent demodulators304. This demodulator uses two additional mixers431and433as compared to other demodulators in this embodiment because, as seen in the related equations, the equations rely on N and N−1 previous demodulator outputs, which do not exist at this point, so to begin the process the two extra demodulators are used. However, from this point on, only one pair of mixers is required since there will be N and N−1 demodulator output signals.

Demodulators304-2to304-N include first and second mixers436-N and437-N, first and second shifters438-N and439-N, and first and second adders440-N and441-N, which respectively receive signal cos(ωB) from second oscillator318and signals IN'1, IN−2, QN−1, and QN−2from previous demodulators304-(N−1) and304-(N−2) to produce signals shown in equations (11) and (12), as discussed above, as signals INand QNalong channel320-N as, if appropriate, as inputs to subsequent demodulators304.

It is to be appreciated this is merely one implementation of demodulator304that can be used to produce the desired signals.

FIGS. 5 and 6show details of a demodulator504, according to one embodiment of the present invention. In this embodiment, ωA and ωB are the same, so they are shown as ω, thus only one oscillator, for example oscillator516, is required to have demodulator504demodulate N number of digital signals210. Oscillator516produces a first oscillator signal or first signal cos(ω) and second oscillator signal or second signal sin(ω), which is received by demodulator504that comprises N demodulators504-1to504-N.

Thus, in this example when ωA=ωB, there is a special case in which equations (11) and (12) will become
cos(NωA)=2*cos((N−1)ωA)*cos(ωA)−cos((N−2)ωA)  (15)
sin(NωA)=2*cos((N−1)ωA)*sin(ωA)+sin((N−2)ωA)  (16)

In this case, only one oscillator516is used to generate cos(ω) and sin(ω) in order to demodulate other N QAM carrier frequencies.

FIG. 6shows an implementation of the demodulators504-1to504-N, according to one embodiment of the present invention. In this example, the above operational parameters are met.

Demodulator504-1includes first and second mixers642and643that mix RFin signals (e.g., signal106or210) with first and second signals cos(ω) and sin (ω), respectively, to produce the output signals shown in equations (15) and (16), discussed above, as signals I1and Q1, along channel520-1and as inputs into subsequent demodulators504.

Demodulator504-2includes first and second mixers644and645, first and second shifters646and647, and first adder648that receives RFin signals from demodulator504-1and from oscillator518, respectively as shown, to produce signals shown in equations (15) and (16), discussed above, as signals I2and Q2along channel520-2and as inputs to subsequent demodulators504.

Demodulator504-3to504-N include first and second mixers649-N and650-N, first and second shifters651-N and652-N, and first and second adders653-N and654-N, which respectively receive signal cos(ω) and sin(ω) from oscillator518and signals IN−1, IN−2, and QN−2from previous demodulators504-(N−1) and504-(N−2) to produce signals shown in equations (15) and (16) discussed above, as signals INand QNalong channel520-N, if appropriate, as inputs to subsequent demodulators504.

It is to be appreciated this is merely one implementation of demodulator504that can be used to produce the desired signals.

Exemplary Methods

FIG. 7shows a flowchart depicting a method700, according to one embodiment of the present invention. In one example, method700is carried out using one or more of the systems described above. In step702, a spacing between signals and a frequency of signals in a received block of signals is determined. In step704, one or both oscillation devices in a pair of oscillation devices is initiated based on the spacing and the frequency of the signals. In step706, what demodulator in a set of demodulators begins demodulation of the signals is determined based on which signal in the block of signals is first received by the set of demodulation devices. In step708, the block of signals is demodulated.

FIG. 8shows a flowchart depicting a method800, according to one embodiment of the present invention. In one example, method800is carried out using one or more of the systems described above. Method800can be a method of demodulating a plurality of input channels. In step802, a first oscillating signal is generated to demodulate a first channel of the plurality of input channels. In step804, a second oscillating signal is generated to demodulate a second channel of the plurality of input channels. In step806, a third channel, and in some examples subsequent channels, of the plurality of input channels is demodulated using the first and second oscillating signals. This allows for a reduction in the required amount of oscillating signals in order to demodulate any number of channels.

In one example, generating of the first oscillating signal is performed by mixing an input signal with a first signal, which can include sine and cosine components.

In one example, generating of the second oscillating signal is performed by mixing first and second portions of the first oscillating signal with a second signal, which can include sine and cosine components, to produce first through fourth mixed signals. Then, the first and third mixed signals are added and the second and fourth mixed signals are added to produce the second oscillating signal.

In one example, generating of the third oscillating signal is performed by mixing first and second portions of the second oscillating signal with a portion of the second signal, for example a cosine portion of the second signal, to produce first and second mixed signals. The first and second mixed signals are delayed, and then added to the first and second portions of the first oscillating signal to produce the third oscillating signal.

CONCLUSION