WO2004034666A1

WO2004034666A1 - Multi carrier qam modulator with direct up-conversion

Info

Publication number: WO2004034666A1
Application number: PCT/IB2003/004066
Authority: WO
Inventors: Hendricus T. G. M. Penning De Vries
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-10-11
Filing date: 2003-09-12
Publication date: 2004-04-22
Also published as: AU2003260864A1

Abstract

The invention relates to a method for creating complex intermediate frequency (IF) output signals within a quadrature amplitude modulator (QAM), with the steps of creating an in-phase (I) signal and a quadrature (Q) phase signal of input signals, creating a pair of IF signals of said I-signal, and creating a pair of IF signals of said Q-signal. To enable easy integration and simple combination of a plurality of QAM signals onto one single broadcast carrier, it is proposed that first IF signals of said I- and said Q-signals are combined within a first adder and put out as a first combined IF signal, second IF signals of said I- and said Q-signals are combined within a second adder and put out as a second combined IF signal.

Description

Multi carrier QAM modulator with direct up-conversion

The invention relates to a method for creating intermediate frequency (IF) output signals within a quadrature amplitude modulator (QAM), with the steps of, creating an in-phase (I) signal and a quadrature (Q) phase signal of input signals, creating a pair of IF signals of said I-signal, and creating a pair of IF signals of said Q-signal. The invention further relates to an integrated circuit comprising mapping means for mapping an input signal onto an in-phase path and a quadrature phase path, said in-phase path comprising upconversion means for converting in-phase path signals into a pair of IF signals, and said quadrature path comprising upconversion means for converting quadrature phase path signals into a pair of IF signals. The invention also relates to the use of such a method and such an integrated circuit.

In future network architectures, it is likely that a plurality of quadrature amplitude modulated (QAM) carriers are used.

WO 01/03440 Al discloses a method that allows the combination of intermediate frequency (IF) QAM signals for two or more different channels in a cable television system so that the resulting composite signal can be upconverted to a radio frequency by a single upconversion circuit for broadcast. According to this document, at least two QAM modulators are provided, each receiving, respectively, a video signal for a separate channel, and each outputting, respectively, a QAM signal at a different IF carrier frequency. The QAM signals comprise signal components referred to as the in-phase (I) and quadrature (Q) phase signals. The I- and Q-phase signals are modulated onto a single carrier. The I-phase signal is multiplied by a cosine wave at the carrier frequency, and the Q-phase signal by a sine wave at carrier frequency. The I- and Q-phase signals are combined to form a composite I/Q modulated signal.

The composite I/Q modulated signals are then converted from the digital to the analog domain. Further a combiner is provided, for combining the D/A converted QAM signals output by the at least two QAM modulators. For upconverting the composite IF carrier signal output by the combiner, a single upconverter is required.

Upconversion from IF to RF frequency may be implemented in the analog domain. The commonly used superheterodyne upconvertor uses two mixers and several image filters. This upconvertor architecture is not suited for integration. An alternative class of upconvertor architectures uses Hilbert transforms and or complex filtering techniques.

For upconverting in the analog domain, the combined signal is send to a single sideband (SSB) modulator in the analog domain. A SSB modulator may be implemented by Hilbert transformer, multipliers and an adder. The Hilbert transformer implements a 90 degree phase shift to the input signal. This 90 degree phase shift makes it possible to reject mirror frequencies in the output signal. The multipliers shift the intermediate frequency carrier to a radio frequency at center frequency.

These architectures are suited for integration. However, implementation and integration of Hilbert transformers and/or complex filters is complicated.

A wideband SSB modulator, which is able to upconvert signals comprising a plurality of IF carrier signals is difficult to implement. The Hilbert transformer has to work properly over a wide frequency band. As requirements on the Hilbert transformer are serious in case of a composite signal comprising multiple QAM IF signals, implementation becomes more difficult in the analog domain.

It is thus an object of the invention to allow easy conversion of complex QAM IF signals to a desired radio frequency. It is a further object to allow upconversion without the use of a Hilbert transformer in the analog domain. It is yet a further object of the invention to enable easy integration without the use of complex filters.

These and other objects of the invention are solved by a method characterized in that first IF signals of said I- and said Q-signals are combined within a first adder and put out as a first combined IF signal, second IF signals of said I- and said Q-signals are combined within a second adder and put out as a second combined IF signal.

By using this method, a complex IF output signal is generated, which can be easily upconverted into a desired radio frequency, even when multiple QAM signals have to be upconverted into a single carrier. An input signal is first split into an in-phase signal and an quadrature phase signal. The in-phase signal is then split and upconverted into a pair of LF signals. This may be carried out by multipliers, multiplying the in-phase signal with +/-cos(ω₁t) for generation of a first IF signal and +/-sin(ωιt) for generation of a second IF signal, with an the local oscillator frequency.

The same applies for the quadrature phase signal. This signal is also split and upconverted into a pair of IF signals by multiplying +/-cos(ωιt) +/-sin(ω₁t) to the quadrature phase signal, respectively.

The first signals generated from the in-phase signal and the quadrature phase signal are fed to a first adder, which combines these signals. Also the second intermediate frequency signals generated from the in-phase signal and the quadrature phase signal are fed to a second adder, which combines these signals.

The whole splitting, upconversion to intermediate frequency and adding may be carried out in the digital domain, which makes integration easier. The output from the adders may then be shifted to radio frequency in the analog domain without the use of a Hilbert transformer. The upconverted analog signals may then be easily added by a third adder in the analog domain. Thus implementation becomes less complicated.

A method according to claim 2 is preferred. The output of the two adders represents one complex IF signal in the digital domain. The adders may add intermediate frequency signals from a plurality of input signals. All these intermediate frequency signals should have different intermediate frequencies provided by their local oscillators. The output of multiple QAM modulators may be combined into a single complex IF signal, which may be upconverted after D/A conversion onto a radio frequency without the need of a Hilbert transformer. This is a reason for easy implementation.

Methods according to claims 3 and 4 are also preferred. The integration and implementation is easy in the digital domain. The complex intermediate frequency signals can be generated as accurate as desired.

A method according to claim 5 enables interfacing between the digital QAM modulator and an analog upconvertor.

With a method according to claim 6, the signals may be upconverted to the desired RF output frequency to be transmitted.

The combination of the radio frequency signals is best carried out with a method according to claim 7. To avoid interference between intermediate frequency signals from different input signals, a method according to claim 8 is proposed.

An optimum transmit filter within the in-phase and the quadrature phase path coming from the input signal is provided by a method according to claim 9. To map bits from the input data stream into QAM symbols for splitting the input signals onto an in-phase signal and a quadrature phase signal, a method according to claim 10 is advantageous.

To allow high quality combination of intermediate frequency signals, a method according to claim 11 is proposed. During DA and upconversion, distortion might be introduced to the signal. In case the signal distortion, such as phase and/or amplitude error, can be measured, it would also be possible to design a correction algorithm that pre-distorts the input signals. Such a predistortion would operate either on the complex intermediate frequency signals or on the I/Q baseband signals.

A further aspect of the invention is an integrated circuit. The objects of the invention are solved when a first adder is provided for combining first IF signals from said pairs of IF signals and a second adder is provided for combining second IF signals from said pairs of IF signals. The QAM modulator may be implemented with two integrated circuits. A first integrated circuit generates a complex, digital IF signal. The second integrated circuit implements digital/analog conversion and upconversion. Yet a further aspect of the invention is the use of a pre-described method or a pre-described integrated circuit to generate a plurality of QAM signals as is required in headends for CATN, Voice over IP, Video over IP, Video on Demand, DVB, DOCISIS related applications.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment described hereinafter. The only figure shows a combined QAM modulator according to the invention.

The QAM modulator according to the invention generates a complex intermediate frequency output in such a way that this signal can be easily upconverted to the desired radio frequency. Multiple QAM signals can be added and send to the single upconverter.

Figure 1 shows a QAM modulator capable of combining QAM signals of a plurality of input signals. The QAM converter comprises a symbol mapper 2, half raised cosine filters 8, a first adder 14, a second adder 16, digital analog converters 22, a third adder 24, and multipliers 9. The QAM converter works as follows:

An input signal x (t) may comprise a stream of bits representing a video signal or a voice signal or any other data in the digital domain. The signal ^t) is fed to symbol mapper 2. Symbol mapper 2 maps the bits from the input signal x^t) into QAM symbols. These QAM symbols are provided as in-phase signals on in-phase path 4 and quadrature phase signals on quadrature phase path 6. In-phase signals and quadrature phase signals are filtered by half raised cosine filters 8, allowing optimal transmission. The filtered in-phase and quadrature phase signals are fed to multipliers 9 a-d.

Within in-phase path 4, said in-phase signal is multiplied with cos(ω₁t) at multiplier 9a and with -sin(ωit) (inverted sin(ω_xt)) at multiplier 9b. The output of multiplier 9a is a first intermediate frequency signal 10a. The output of multiplier 9b is a second intermediate frequency signal 12a.

Within quadrature phase path 6, said quadrature phase signal is multiplied with sin(ω_ϊt) at multiplier 9c and with cos^t) at multiplier 9d. The output of multiplier 9c is a first intermediate frequency signal 10b. The output of multiplier 9d is a second intermediate frequency signal 12b.

The first intermediate frequency signals 10 are fed to first adder 14. The second intermediate frequency signals 12 are fed to second adder 16.

Adder 14 allows to add more intermediate frequency in-phase signals 10c, lOd, .... The adder 14 combines all intermediate frequency signals 10 onto one IF signal 18. It should be noted, that the IF signals 10c, lOd from different input signals should have different intermediate frequencies.

Adder 16 allows to add more intermediate frequency signals 12c, 12d, .... The adder 16 combines all intermediate frequency signals 12 onto one IF signal 20. It should be noted, that the IF signals 12c, 12d from different input signals should have different intermediate frequencies.

The IF signals 18 and 20 represent one complex IF signal in the digital domain. This signal is digital/analog converted within D/A converters 22, respectively. The D/A converted signals are upconverted onto the radio frequency ω_rby the multipliers 9e and 9f, respectively. This is done by multiplying the D/A converted signal 18 by cos(ω_xt) and D/A converted signal 20 by sin(ω_xt).

The upconverted radio frequency signals 18 and 20 are then fed to adder 24 which combines the signals and puts out a combined QAM radio frequency signal in the analog domain.

The desired radio frequency QAM signal can be written as

QAM(t)=x(t) * cos(ω_rt) + y(t) * sin(ω_rt) With x(t) the signal on in-phase path 4 and y(t) the signal on quadrature phase path 6, and with the trigonometric identities : cos(a+b) = cos(a)cos(b) - sin(a)sin(b) sin(a+b) = sin(a)cos(b) + cos(a)sin(b) And substitution of ω_r= ωι + ω_x, QAM(t) can be re- written as:

QAM(t)= x(t) * [cos(α>ι t) cos(ω_x t) - sin(ω_! t) sin(ω_x t)] +y(t) * [sin(ω_! t) cos(ω_x t) + cos(ω₁1) sin(ω_x t)], which is implemented by an integrated circuit according to figure 1. This implementation may be within two separate integrated circuits. The first integrated circuit generates a complex, digital IF signal and the second integrated circuit implements D/A and upconversion. The invention can be applied to generate a plurality of QAM signals as is required in headends for CATV, Voice over IP, Video over IP, Video on Demand, DVB, DOCISIS related applications.

Claims

CLAIMS:

1. Method for creating complex intermediate frequency (IF) output signals within a quadrature amplitude modulator (QAM), with the steps of: a) creating an in-phase (I) signal and a quadrature (Q) phase signal of input signals, b) creating a pair of IF signals of said I-signal, and c) creating a pair of IF signals of said Q-signal, characterized in that d) first LF signals of said I- and said Q-signals are combined within a first adder and put out as a first combined IF signal, e) second IF signals of said I- and said Q-signals are combined within a second adder and put out as a second combined IF signal.

2. Method according to claim 1, characterized in that said first combined signal put out by said first adder and said second combined signal put out by said second adder represent one complex IF signal.

3. Method according to claim 1, characterized in that said first IF signals are in- phase signals and that said second IF signals are quadrature phase signals.

4. Method according to claim 1 , characterized in that said steps a)-e) are carried out in the digital domain.

5. Method according to claim 1, characterized in that said combined IF signals put out by said first and second adder are digital analog (D/A) converted.

6. Method according to claim 1, characterized in that said combined IF signal put out by said first and second adder are upconverted to a radio frequency, respectively.

7. Method according to claim 1, characterized in that said upconverted IF signals are combined within a third adder, and that said third adder puts out a radio frequency signal.

8. Method according to claim 1, characterized in that said intermediate frequencies for creating said pairs of IF signals differ for each input signal.

9. Method according to claim 1, characterized in that a half raised cosine filter filters said I-signals and said Q-signals, respectively.

10. Method according to claim 1, characterized in that said I-signals and said Q- signals from an input signal are created by symbol mapping an input signal.

11. Method according to claim 1, characterized in that said LF signals are predistorted in the digital domain to compensate for distortions and imperfections during further processing, respectively.

12. Integrated circuit comprising:

- mapping means for mapping an input signal onto an in-phase path and a quadrature phase path, - said in-phase path comprising upconversion means for converting in-phase path signals into a pair of IF signals, and

- said quadrature path comprising upconversion means for converting quadrature phase path signals into a pair of IF signals, characterized in that - a first adder is provided for combining first signals from said IF signals, and

- a second adder is provided for combining second signals from said IF signals.

13. Use of a method according to claim 1 or an integrated circuit according to claim 12 to generate a plurality of QAM signals as is required in headends for CATV, Voice over IP, Video over LP, Video on Demand, DVB, DOCISIS related applications.