2N-QAM macrocell

Details 2N-QAM macrocell 2N-QAM macrocell

1998-10-20

2002-08-13

Pham, Chi (Department: 2631)

Pulse or digital communications

Systems using alternating or pulsating current

Plural channels for transmission of a single pulse train

C375S259000, C332S103000, C332S104000

Reexamination Certificate

active

06434199

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a quadrature amplitude modulation (QAM) circuit and more particularly to a QAM circuit which utilizes current signal processing which makes it amenable to being formed as a microwave monolithic integrated circuit (MMIC) or integrated circuit (IC) thus reducing the part count and complexity of the circuit while providing a relatively higher symbol rate than known QAM circuits.
2. Description of the Prior Art
Various modulation techniques are known for modulating a carrier signal with various types of information. Due to limited bandwidth allocations in some applications, modulation techniques have been developed to increase the amount of information that can be transmitted per frequency. One such technique is known as quadrature phase shift keying (QPSK). Such QPSK modulation techniques are known in the art and described in U.S. Pat. Nos.: 5,440,259; 5,615,230; 5,440,268; 5,550,868; 5,598,441; 5,500,876 and 5,485,489; hereby incorporated by reference. In general, in such a modulation technique, the phase of both the real and quadrature components of a carrier are modulated in order to enable two bits, each having two states, to be transmitted over a single frequency. As such, at each frequency, the carrier can be modulated into one of four different phase states, known as symbols, which form a constellation as generally shown in FIG.
1
A. The QPSK modulation technique is thus able to provide twice the information per frequency relative to conventional amplitude and frequency modulation techniques such as BPSK, making it suitable for applications where bandwidth allocations are relatively limited as in, for example, satellite communications systems.
In order to further increase the amount of information transmitted per frequency, other modulation techniques have been developed, such as quadrature amplitude modulation (QAM). Such QAM modulation techniques are relatively well known in the art. Examples of such QAM modulation circuits are disclosed in U.S. Pat. Nos.: 5,612,651; 5,343,499; 5,363,408; and 5,307,377; hereby incorporated by reference. Such QAM modulation techniques essentially involve amplitude modulation of a QPSK signal to provide constellations of symbols of 8, 16, 32 and 64 and more per frequency, for example, as illustrated in FIG.
1
B.
A known QAM modulation circuit is shown in FIG.
2
and generally identified by the reference numeral 20 and disclosed in, S. Nam, A. E. Ashtiani, I. D. Robertson, S. Lucyszyn, Micorwaves and RF, April 1998, Vol. 37, No. 4, p. 113, hereby incorporated by reference. The QAM circuit
20
includes a digital mapping chip
22
, a phase shifter
24
, and a variable attenuator
26
. The QAM circuit
20
is adapted to generate a constellation of 2
N
symbols, where N is the number of bits in the data word applied to the digital mapping chip
22
. As shown, in
FIG. 2
, N=6, therefore the QAM circuit
20
is adapted to provide a constellation of 64 symbols. The N bit wide data word is applied to the digital mapping chip, typically an application specific integrated circuit (ASIC), which, in turn, provides signals to the phase shifter
24
and variable attenuator
26
to provide the desired constellation. The output of the QAM circuit
20
is an RF signal at the local oscillator (LO) frequency that during a symbol period, 1/R, where R is the data symbol rate, has an amplitude and phase that corresponds to a specific transmitted word. Demodulators on the receiving end operate by measuring the RF carrier amplitude and phase during a symbol period and performing a reverse mapping operation.
The QAM circuit architecture illustrated in
FIG. 2
is relatively simple in nature since it requires very few devices to implement and is known to be low in power consumption. Unfortunately, the symbol transfer rate is known to be limited by the speed of the phase shifter
24
and the variable attenuator
26
, currently in the 100 ksps range (100,000 symbols per second), making the system unsuitable for high data rate applications in the millimeter wave bands including the recently allocated 38 GHz band. Another disadvantage of the system illustrated in
FIG. 2
, is the requirement of a separate digital mapping chip.
FIG. 3
illustrates a QAM circuit which utilizes a commercially available I-Q modulator, along with a digital mapping circuit and a pair of digital to analog converters (DAC)
34
and
36
. As discussed in detail by W. H. Pratt in RF Design, 1994, the I-Q modulator
30
provides relatively improved performance and combines the variable attenuator and phase shifter used in the architecture illustrated in
FIG. 2
on a single monolithic device. More particularly, the I-Q modulator
30
may utilize Gilbert transconductance cells formed from GaAs or InP heterojunction bipolar transistor (HBT) technology, thus providing relatively higher symbol rates, for example, up to about 100 Msps. Unfortunately, the QAM circuit
28
requires a digital mapping chip
32
and pair of DACs
34
and
36
; both of which consume a relatively large amount of power for high symbol rate applications(i.e. 1-3 watts each).
In order to provide an increased symbol rate for use in the higher frequency bands and eliminate the need for the mapping circuit and DACs, a wideband QAM circuit
40
as illustrated in
FIG. 4
has been developed. The QAM circuit
40
provides a relatively higher symbol rate, in excess of one giga symbol per second (1 Gsps), utilizing wide bandwidth double-balanced diode mixer in the multiplier sections. As can be seen from
FIG. 4
, the QAM circuit
40
enables the digital data word (D
0
, D
1
, D
3
, D
4
and D
5
) to be fed directly into the multipliers
42
-
47
, thus eliminating a digital mapping chip. The QAM circuit
40
also eliminates the need for DACs, thus reducing the power consumption of the circuit. Unfortunately, the QAM circuit
40
requires a relatively larger number of discrete components to implement. For example, the 64 QAM modulator illustrated in
FIG. 4
requires 10-20 discrete devices, making the modulator
40
relatively complex and expensive to implement. Another disadvantage of the QAM modulator
40
is that it requires extensive tuning to correct for errors produced in the manufacturing processes of the discrete devices as well as circuit interactions.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve various problems in the prior art.
It is yet another object of the present invention to provide a quadrature amplitude modulation (QAM) circuit which provides increased symbol rates.
It is yet a further object of the present invention to provide a QAM circuit that is amenable to be formed on a single microwave monolithic integrated circuit (MMIC) or integrated circuit (IC).
It is yet a further object of the present invention to provide a QAM circuit with reduced power consumption.
Briefly, the present invention relates to a 2
N
-QAM integrated macrocell for generating constellations with 2
N
symbols, where N is the number of bits in the digital data word [D
0
:D
N−1
] applied to the integrated macrocell. Since the digital data word is applied directly to the integrated macrocell, the need for a digital mapping chip is eliminated thus reducing the complexity and power consumption of the circuit. An important aspect of the invention is that the 2
N
-QAM integrated macrocell is amenable to being fabricated as a single microwave monolithic integrated circuit (MMIC) or integrated circuit (IC). As such, a QAM circuit can be formed from one or two MMICs; the 2
N
-QAM macrocell and a quadrature phase shifting device formed on the same MMIC or a separate MMIC to form a complete 2
N
-QAM circuit. For lower frequency applications (<1 GHz) the QAM circuit can also be formed on a single IC, using standard silicon BJT processes. The phase shifting device is used to provide the in-phase and quadrature phase components of the local oscillator signal and may be reali

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