Patent Description:
In order for radio signals to be transmitted over distances it is necessary for them to be amplified before transmission. Power amplifiers are typically used for this purpose. It can be desirable for the power amplifiers to amplify the signal linearly so that non-linear distortions are not created in the amplified signal.

However, linear amplifiers are inefficient and can be expensive.

<CIT> discloses a radio frequency power amplifier characterized by an efficiency and operative to produce an output signal at a desired output power, including an outphasing system with shunt reactance and a controller.

<CIT> discloses a method of generating quantized signals comprising generating a quantized phase domain relating to quantized phases of an input signal.

<CIT> discloses a signal component separator; at least one digital phase modulator; a non-isolated power combiner; and a mismatch compensator.

<CIT> discloses a multi-antenna beam-forming system for transmitting constant envelope signals decomposed from a variable envelope signal.

<CIT> discloses (abstract): "High efficiency transmitters for electronically scanned arrays (also known as electronically steered antennas or ESAs). The transmitters. utilize outphasing power amplification. In one embodiment, the outphasing transmitter includes a phase shifting device. The phase shifting device is configured for phase shifting an input signal and providing a pair of phase shifted signals for each emitting element of the EAS. The outphasing transmitter also includes an outphasing power amplifier associated with each emitting element. Each outphasing power amplifier is configured for separately amplifying the pair of shifted signals for each emitting element and combining the separately amplified signals for each emitting element for transmission.

The invention is defined by the appended independent claims <NUM> and <NUM>.

In some but not necessarily all examples, the means for discrete phase control in each outphasing path is configured to define, for the outphasing path, a constellation point in a constant envelope constellation diagram.

In some but not necessarily all examples, the system comprises means for providing discrete phase control in each outphasing path to define, for each outphasing path, a constellation point in the constant envelope constellation diagram and thereby define a target constellation point in the non-constant-envelope constellation diagram for the output signal.

In some but not necessarily all examples, each outphasing path comprises a phase controller that enables discrete phase control in the outphasing path, wherein the phase controller is configured to phase modulate a carrier wave.

In some but not necessarily all examples, the amplifier in each outphasing path is a non-linear amplifier configured for amplification of the substantially constant-envelope component received by the outphasing path.

In some but not necessarily all examples, the system comprises means for controlling, for each antenna, discrete phase control in each outphasing path to define, for each outphasing path, a constellation point in a constant envelope constellation diagram and thereby define a target constellation point in a non-constant-envelope constellation diagram for each of the combined output signals provided to each of the antennas.

In some but not necessarily all examples, the means for decomposing an input signal to multiple substantially constant-envelope components is configured to decompose the input signal to multiple substantially constant-envelope components for all antennas.

In some but not necessarily all examples, the cost function is dependent upon total transmission power of the plurality of antennas and a measure of a difference between information intended to be received and information estimated to be received, as a consequence of providing the respective combined output signals of the outphasing paths to a respective antenna for transmission.

In some but not necessarily all examples, the cost function is dependent upon efficiency, for example, power efficiency of the transmitter such as power added efficiency of the amplifier used.

In some but not necessarily all examples, the cost function is additionally dependent upon interference, for example out of band transmission.

In some but not necessarily all examples, a base station comprises the system.

According to various, but not necessarily all, embodiments there is provided a method comprising:.

According to various, but not necessarily all, embodiments there is provided an apparatus comprising:.

In some but not necessarily all examples, the decomposing means for decomposing is configured to provide for joint spatial precoding and outphasing decomposition.

In some but not necessarily all examples, each antenna input signal for a respective antenna defines a constellation point.

In some but not necessarily all examples, the means for providing discrete phase control in each outphasing path is configured to define, for each outphasing path associated with an antenna, a constellation point in a constant envelope constellation diagram and thereby define a target constellation point in a non-constant-envelope constellation diagram for the input signal for the antenna.

In some but not necessarily all examples, the transmitted signals from each antenna are combined over the air to generate a symbol of the input signal at a receiver.

The following Figures disclose various examples of an apparatus <NUM>, system <NUM> or base station <NUM> comprising:.

The decomposing described is used for outphasing and can therefore be described as outphasing decomposition. In some examples, described later, non-linear precoding provides for joint (spatial) precoding and outphasing decomposition. The joint (spatial) precoding allows use of discrete phase control (instead of continuous phase control) to control each transmit antenna to transmit a constellation point (rather than a symbol). The signals from each antenna are then combined over the air to generate the symbol at the receiver. Thus in at least some examples, decomposition is achieved by joint (spatial) precoding and outphasing-decomposition.

<FIG> illustrates an example of an apparatus <NUM> comprising:.

In the example illustrated the apparatus <NUM> comprises:.

In this example, the decomposing means <NUM> decomposes an input signal <NUM> to two substantially constant-envelope components S1, S2 per transmit antenna. There are two outphasing paths <NUM><NUM>, <NUM><NUM> per transmit antenna, the outputs of which are combined by the combining means <NUM>. However, in other examples, the decomposing means <NUM> decomposes an input signal <NUM> to M (M≥<NUM>) substantially constant-envelope components S1, S2. SM per transmit antenna and there are M outphasing paths <NUM><NUM>, <NUM><NUM>. <NUM>m per transmit antenna the outputs of which are combined by the combining means <NUM> for transmission by a transmit antenna.

Circuitry <NUM> is outphasing transmit chain circuitry. In this example the outphasing transmit chain circuitry <NUM> comprises one outphasing branch per transmit antenna. In this example, the outphasing branch comprises multiple (two in this example) outphasing paths <NUM>n and combining means <NUM>. Each outphasing path <NUM>n provides for discrete phase control (discrete phase control means <NUM>n) and for magnitude control (amplifier <NUM>n). In the example illustrated the discrete phase control means <NUM>n and amplifier <NUM>n are illustrated as series connected in a particular order. This order can, however, be reversed.

Each outphasing path <NUM>n provides linear amplification, even when the amplifiers <NUM>n in the outphasing paths <NUM>n are non-linear. The apparatus <NUM> can therefore achieve linear amplification using cheaper and more energy efficient non-linear amplifiers rather than using linear amplifiers.

The amplifier <NUM>n in each outphasing path <NUM>n is configured for linear amplification of the substantially constant-envelope component Sn received by the outphasing path <NUM>n. In some examples, the amplifiers <NUM>n are non-linear power amplifiers. In some examples, the same non-linear amplifier <NUM>n is used in each outphasing path <NUM>n. In some examples, the non-linear amplifiers <NUM>n are class E or class F.

The apparatus can be any suitable transmitter or transceiver. It can, for example, be a network node such as a base station, for example a cellular base station e.g. node-B in 3GPP, or can be a terminal, for example a cellular terminal e.g. mobile equipment (ME) or user equipment (UE) in 3GPP.

As illustrated in <FIG>, the signal S, to be amplified, is decomposed into multiple substantially constant envelope signals S1, S2 such that their vector sum would substantially reproduce the original signal S (S=S1+S2). As the decomposed signals S1, S2 are substantially constant envelope they can be amplified at the substantially constant envelope value in a linear fashion by the amplifier <NUM>n even if the amplifier <NUM>n has non-linear characteristics over a larger range of magnitudes. Therefore, each amplifier <NUM>n can operate in an efficient, non-linear mode. The outputs of the multiple amplifiers <NUM>n are then summed, for example with a power combiner <NUM>n or an isolating combiner <NUM>n to produce an amplified version of the input signal as an output signal (e.g. G. S2, where G is amplifier gain).

The sum of the multiple substantially constant-envelope components S1, S2 when transmitted enables substantial reproduction of the input signal <NUM> at a receiver.

The multiple amplifiers <NUM>n are constant-envelope and therefore consume a fixed amount of power.

Magnitude variation in the signal S over time can be accommodated by varying, over time, the phase (angle) between S and S1 and between S and S2 without needing to vary the magnitude of S1 or the magnitude of S2. Thus S1 and S2 are constant envelope.

A substantially constant-envelope component Sn can fluctuate only within a narrow efficiency range of operation of the amplifier <NUM>n of the outphasing path <NUM>n that receives the constant-envelope component Sn.

The term "substantially" is used to indicate that some small variation in magnitude can, in some examples, be accommodated. The term "exactly" is used to indicate that there is no variation in magnitude. The envelopes can be substantially constant or exactly constant. Where the word substantially is not explicitly used, for example some Patent Offices do not allow the term in the claims, it should be inferred.

Thus, the signal S(t) can be decomposed from a sequence of inputs {S(tn)} to a sequence of multiple components, each sequence of components {S1(tn)}, {S2(tn)} has a constant magnitude, over time, but has a phase that can be varied discretely over the sequence.

The substantially constant-envelope components S1, S2 can be provided as substantially constant-envelope phasor components (digital domain) or substantially constant-envelope analogue signal components (analogue domain). Each outphasing path <NUM>n comprises means for:.

The discrete phase control means <NUM>n for discrete phase control in each outphasing path <NUM>n is configured to define, for the outphasing path <NUM>n, a constellation point <NUM> in a constant-envelope constellation diagram <NUM>. An example of a suitable constant-envelope constellation diagram <NUM> is illustrated in <FIG>.

Each outphasing path <NUM>n has an associated (constant magnitude) constellation diagram <NUM>. In some but not necessarily all examples, each outphasing path <NUM>n has the same associated (constant magnitude) constellation diagram <NUM>.

The combining means <NUM>, for combining output signals from the outphasing paths <NUM>n, is configured to provide, as output <NUM>, a signal associated with a constellation point <NUM> in a non-constant-envelope constellation diagram <NUM>. The combining means <NUM> can, for example, add the output signals from the outphasing paths <NUM>n,. An example of a non-constant-envelope constellation diagram <NUM> is illustrated in <FIG>. The non-constant-envelope constellation diagram <NUM> is the constellation diagram formed by combining two signals that have the same associated constant magnitude constellation diagram <NUM> illustrated in <FIG>.

The constant-envelope constellation diagram <NUM> is characterized by quantized phases and a fixed magnitude. The constellation points lie on a circle, centered at the origin. In this example the constellation points are evenly distributed on the circumference of that circle.

The discrete phase control means <NUM>n is configured to add a quantum of phase 2π. m/N where <NUM>≤m<N and N is the number of constant magnitude constellation points <NUM>, for example, N can in some example equal <NUM>" where n is a natural number. In some examples, n is greater than or equal to <NUM>. Adjacent constellation points <NUM> in the constant-envelope constellation diagram <NUM> are separated by a low resolution or quantized phase of 2π/N.

The non-constant-envelope constellation diagram <NUM> is characterized by quantized phases and quantized magnitudes. The constellation points lie on the origin or circles of different magnitude radius, centered at the origin. In this example the constellation points <NUM> that are distributed on the circumferences of the circles are evenly distributed on the circumferences of the circles.

If the N constellation points <NUM> for S1 are defined by exp(2π. m1/N) and the N constellation points <NUM> for S2 are defined by exp(2π. m2/N), then the constellation points <NUM> for the combined signal are exp(2π. m1/N) + exp(2π.

The non-constant-envelope constellation diagram <NUM> has more constellation points <NUM> and a denser distribution of constellation points <NUM> than the constant-envelope constellation diagram <NUM>.

The discrete phase control means <NUM>n defines, for its outphasing path <NUM>n, a constellation point <NUM> in the constant-envelope constellation diagram <NUM>. In combination, they define a target constellation point <NUM> in the non-constant-envelope constellation diagram <NUM> for the output signal <NUM>.

In <FIG>, each outphasing path <NUM>n comprises a phase controller <NUM>n, controlled by the decomposed signals S1, S2, that enables discrete phase control in the outphasing path <NUM>n. The decomposition means <NUM> is configured to control the discrete phase control in each phase controller <NUM>n.

In these examples, the phase controller <NUM>n is configured to phase modulate a carrier wave.

The carrier wave is provided by a local oscillator (LO) <NUM> to each of the multiple phase controllers <NUM>n. The phase of the carrier wave is controlled by the phase controller <NUM>n , via the decomposed signal Sn, to have a constellation point <NUM> in a constant envelope constellation diagram. Each phase controller <NUM>n therefore provides discrete (e.g. quantized) phase control for its outphasing path <NUM>n. The signal, after discrete phase control is amplified by the respective amplifier <NUM>n of the outphasing path <NUM>n. The combining means <NUM> combines the amplified output signals from the outphasing paths <NUM>n to produce, as output, an amplified version of the carrier wave that has a target constellation point <NUM> in a non-constant envelope constellation diagram <NUM>. The combined output signal <NUM> of the outphasing paths <NUM>n is provided to an antenna <NUM> for transmission.

Circuitry <NUM> is outphasing transmit chain circuitry. In this example the outphasing transmit chain circuitry <NUM> comprises one outphasing branch per transmit antenna. In this example, each outphasing branch comprises multiple (two in this example) outphasing paths <NUM>n and comprises combining means <NUM>.

In these examples, the signal <NUM> is provided via an Evolved Common Public Radio Interface (eCPRI). This is an interface specification between Radio Equipment Control (REC) and Radio Equipment (RE) of radio base stations used for cellular wireless networks. The interface can transport baseband I/Q signals to the radio equipment. Other interfaces can be used.

In this example, the combining means <NUM> is an isolating combiner. Other combiners can be used.

In this example, the decomposition means <NUM> comprises circuitry for precoding. The circuitry decomposes the input signal <NUM> to multiple substantially constant-envelope components Sn for multiple respective outphasing paths <NUM>n.

The circuitry for precoding can provide additional phase control. For example, controlling the signals provided to the outphasing paths <NUM>n so that when the signals from the outphasing paths <NUM>n are combined by combining means <NUM>, for all outphasing paths of all antennas, a target symbol, for example an OFDM symbol, is obtained. An OFDM symbol cannot be generated at each antenna <NUM> when only low-resolution phase shifters <NUM>n are used. Instead, the signals from each antenna <NUM> are combined 'over the air' to generate the OFDM symbol at the UE.

The (over-the-air) sum of all outphasing branches (not only the ones that are combined in a single Tx path) re-creates the wanted signal, input signal <NUM>, in a pre-determined spatial direction (i.e., beam). This multi-antenna/over-the-air combining enables the use of efficient/quantized outphasing paths and branches.

Outphasing transmit chain circuitry <NUM> provides, per transmit antenna <NUM>, one outphasing branch which comprises multiple (two in this example) outphasing paths <NUM>n and combining means <NUM>.

In some examples, there are many outphasing branches, each consisting of a plurality of outphasing paths <NUM>n, a nonlinear power amplifier per outphasing path, and a single combiner. The output of each outphasing branch is a constellation point <NUM> from the constellation <NUM>. The input signal <NUM> is decomposed such that the sum of all outphasing paths (from all outphasing branches) reconstructs the wanted signal (in a specific beam direction). In this example there are at least two outphasing branches, and at least two outphasing paths per outphasing branch, and at least four constant-envelope signals S1, S2, S3 and S4, whose superposition (over the air) resembles the input signal <NUM>.

The decomposition means <NUM> serves via multiple outphasing branches respective multiple antennas <NUM>.

In this example, the decomposition means <NUM> is circuitry for non-linear precoding and provides additional phase control. For example, controlling the signals provided to the outphasing paths <NUM>n to control beam-forming by the multiple antennas <NUM> as described above. For example, controlling the signals provided to the outphasing paths <NUM>n so that when the signals from the outphasing paths <NUM>n are combined in <NUM>, and the resulting signals from all outphasing branches are combined over the air when transmitted by the antennas <NUM>, a target symbol is obtained.

The discrete phase control means <NUM>n, for an outphasing path <NUM>n can for example be provided by a radio frequency phase shifter, a phase-modulated phase locked loop (PLL), a base band phase shifter or a Gaussian Minimum Shift Key (GMSK) chain.

<FIG> illustrates an example in which the discrete phase control means <NUM>n, for an outphasing path <NUM>n is provided by a Gaussian Minimum Shift Key (GMSK) chain comprising a digital to analogue converter <NUM>n, a Gaussian filter <NUM>n and a quadrature modulator <NUM>n. The quadrature modulator comprises the local oscillator and the phase controller. A single local oscillator can be shared by multiple outphasing paths. The quadrature modulator provides the discrete phase control means for discrete phase control in each outphasing path <NUM>n.

<FIG> illustrates a system <NUM> comprising: decomposing means <NUM> for decomposing an input signal <NUM> to multiple substantially constant-envelope components S1, S2 that is shared by multiple antennas <NUM>.

The S1, S2 signals are generated for each antenna. So, they differ for the individual antennas.

The system <NUM> comprises, for each one of a plurality of antennas, an outphasing branch of outphasing transmit chain circuitry <NUM> comprising (as previously described) an outphasing path <NUM>n for each substantially constant-envelope component Sn. Each outphasing path <NUM>n has discrete phase control means <NUM>n for discrete phase control in each outphasing path <NUM>n and an amplifier <NUM>n in each outphasing path <NUM>n and combining means <NUM> for combining output signals from the outphasing paths <NUM>n. The combined output signals of the outphasing paths <NUM>n are provided to a respective antenna <NUM> for transmission. Each outphasing path <NUM>n in the system <NUM> amplifies a constant-envelope component. The magnitude of the constant-envelope component, in each outphasing path <NUM>n can, in some examples be the same such that all outphasing paths <NUM>n amplify components that have the same magnitude.

The decomposing means <NUM> controls, for each antenna <NUM>, discrete phase control in each outphasing path <NUM>n to define, for each outphasing path <NUM>n, a constellation point <NUM> in a constant-envelope constellation diagram <NUM> and thereby define a target constellation point in a non-constant-envelope constellation diagram <NUM> for each of the combined output signals provided to each of the antennas <NUM>.

The decomposing means <NUM> is configured to decompose the input signal <NUM> to multiple substantially constant-envelope components Sn jointly for all antennas <NUM>.

<FIG> illustrates an example of the system <NUM> used for beam-forming. It can, for example, be configured for massive multiple input multiple output (mMIMO).

For example, each antenna <NUM> is arranged in a regular array. The signal for each antenna is provided by a respective outphasing branch of outphasing transmit chain circuitry <NUM> as illustrated in <FIG>.

The array can for example comprise <NUM>*X antennas where X=<NUM>^n and n=<NUM>, <NUM>, <NUM>, <NUM>. e.g. there can, in some examples, be <NUM>, <NUM>, <NUM>, <NUM>. antennas <NUM> in the array.

<FIG> illustrates an example of a base station <NUM> comprising the system <NUM> as previously described. The base station can be cheaper to build and cheaper to operate because less expensive and/or more power efficient amplifiers <NUM> can be used.

The decomposing means <NUM> is configured to optimize a cost function, across the plurality of antennas <NUM>, to determine the discrete phase control used for each outphasing path <NUM>n.

An example of a cost function is: <MAT> subject to <MAT> <MAT>.

Where fn is the nth column of the DFT matrix, NxN DFT matrix has element (m,n) equal to <MAT>.

I(eff(A)≥η) is the indicator function defined, for example, as <MAT>.

The constraint <MAT> constrains the efficiency of the outphasing architecture, so that an efficiency of at least η is achieved.

In this example the cost function is dependent upon total transmission power of the plurality of antennas. In the example cost function above, the dependency upon total transmission power is provided by: <MAT>.

In this example the cost function is dependent upon a measure of a difference between information intended to be received and information estimated to be received, as a consequence of providing the respective combined output signals of the outphasing paths 120n to a respective antenna for transmission.

The measure of difference can, for example, be signal noise and distortion ratio (SNDR) or error vector magnitude (EVM) that quantify a precision of the generated signal. SNDR is a ratio of the total signal power level (Signal + Noise + Distortion) to unwanted signal power (Noise + Distortion).

In the example cost function above, a measure of difference is provided by a squared difference between the information intended to be received u[n] and the information estimated to be received αH[n]x[n], summed over all antennas: <MAT>.

In this example cost function, there is a constellation constraint in that the phase control can be quantized in a predetermined manner. In the example cost function above, optimization of the cost function is subject to the constraint <MAT>.

The cost function can, in some examples, be dependent upon efficiency. For example, there may be a requirement for efficiency to be above a threshold value.

In the example cost function above, the dependence upon efficiency is provided by: <MAT>.

The cost function can, in some examples, be additionally dependent upon interference, for example out of band transmission. For example, there may be a requirement for out of band transmission to be below a threshold value. This can be particularly useful when orthogonal frequency division multiplexing (OFDM) is in use.

In the example cost function above, the dependence upon out of band transmission is provided by: <MAT>.

The optimization of the cost function can be solved using a greedy optimization algorithm. A greedy optimization algorithm treats each component of the cost function as independent and iteratively finds the optimized value for each component of the cost function.

The optimization finds an optimal A, where each element of A belongs to one of the constellation points <NUM> in the set χ For example, A = S1 + S2, where S1 and S2 are constellation points <NUM> in <FIG> and A is a constellation point <NUM> in <FIG>.

The consequences of using a properly specified cost function that balances decreasing transmission power while maintaining efficiency, can be seen in <FIG> is a histogram of the signal constellation points <NUM> at any antenna <NUM>. The outer, greater magnitude, constellation points <NUM> from outphasing are selected more often when compared with the inner constellation points. The outer constellation points have higher output power, and therefore higher efficiency than the inner constellation points.

The multiple amplifiers <NUM>n are constant-envelope and therefore consume a fixed amount of power. Decreasing output power, while keeping the power consumed fixed reduces efficiency.

<FIG> illustrates an example of a method <NUM> as previously described. The method comprises:.

The separate discrete phase control 404A and amplification 404B of each of the constant-envelope components {Sn} can occur contemporaneously in parallel.

The blocks illustrated in the <FIG> may represent steps in a method and/or sections of code in a computer program. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some blocks to be omitted.

The precedingly described apparatus <NUM>, system <NUM> and base station <NUM> comprise: decomposing means <NUM> for decomposing an input signal <NUM> to multiple substantially constant-envelope components S1, S2;.

Although embodiments have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the claims.

Claim 1:
A system (<NUM>) comprising:
means for decomposing an input signal (<NUM>) to multiple substantially constant-envelope components (S1, S2);
for each one of a plurality of antennas (<NUM>), an apparatus (<NUM>) comprising:
an outphasing path (120n) for each substantially constant-envelope component;
means for discrete phase control (112n) in each outphasing path;
an amplifier (114n) in each outphasing path;
means for combining output signals (<NUM>) from the outphasing paths;
means (<NUM>) for providing the combined output signal of the outphasing paths of the apparatus to a respective one of the plurality of antennas for transmission; and
means for optimizing a cost function, across the plurality of antennas, to determine the discrete phase control used for each outphasing path, wherein the optimization finds an optimal A, where A is the time-domain signal across all antennas, wherein the specified cost function is arranged to balance decreasing transmission power while maintaining efficiency.