Patent Application
20130016634
The present application relates generally to frequency agile duplexers used in radio systems and, more specifically, to frequency agile electronic duplexers which make use of feedforward cancellation techniques.
The design of wireless base station front ends offers unique challenges. For example, a number of limitations and practical challenges need to be overcome in the areas of high-power filtering, frequency agility, linearity and low insertion loss.
Certain techniques have been devised to attempt to reject the high power transmit signal reflection from the antenna port. A classic arrangement is to establish a Feed Forward Cancellation Loop path between the transmit port and the receive port of the antenna coupling network. One of the only practical method to match such a delay is to use a spool of coax cable in the feedforward path of the FFCL to match the round-trip delay of the transmit signal antenna reflection in the antenna feeder cable. However, given the broad and unpredictable range of feeder cable lengths for each base station deployment, it would be impractical to attempt to control the delay mismatch variation of a feedforward cancellation arrangement with a feedforward path between the transmit and receive ports of the antenna coupling network. Furthermore, even if the feedforward path coax delay line was implemented with smaller gauge cable, the volume occupied by the delay line could easily exceed that of a typical duplexer for large towers (long feeder lengths) and occupy a significant portion of the base station footprint. Additional factors that limit the performance of feedforward cancellation circuits over wide frequency bands is the delay mismatch between the main path and the cancellation path and the inherent frequency dependence of circuit components in terms of amplitude and phase ripple over a given frequency range.
Conventional filter duplexers can be used to isolate the transmit and receive circuitry but unusually strong, close-in interferers may be very difficult to deal with. Additionally, conventional filters are not easily adaptable to new operating frequencies. Existing adaptive/agile/electronic duplexer designs only address one of the noise or emissions problems. Usually this is the broadband transmit noise emissions in the receive path, or even more specifically, just the transmit noise emissions in the receive band of the receive path. Existing feedforward linearization deals specifically with high level distortion resulting from nonlinearity of the power transistors in a power amplifier, but does not deal with broadband noise emissions introduced by the power transistors.
For these reasons, traditional feedforward cancellation arrangements are not sufficient to implement a frequency agile duplexer architecture, especially in a radio platform which can be reconfigured to operate at high power levels in multiple modes and in multiple frequency bands.
The present invention is directed to alleviating the problems of the prior art.
The present invention overcomes the problems of the prior art by providing an electronic duplexer which is able to correct for broadband emission noise introduced by power amplifier, reduce interference caused by the transmit signal and observed in the receive path and identify and correct interference signals other than those created by the transmit signal. In particular, the invention provides an electronic duplexer for sharing at least one antenna between at least one transmitter in a transmit path and at least one receiver in a receive path. The electronic duplexer comprises an electronic duplexer input for receiving at least one input transmit signal from the transmit path and an electronic duplexer output for providing at least one desired output signal to the receive path. An antenna interface has a transmit portion for transmitting an at least one desired transmit signal over the at least one antenna and a receive portion for receiving an at least one receive signal over the at least one antenna. A transmit antenna emissions correction circuit has an input coupled to the antenna interface. The transmit antenna emissions correction circuit correcting broadband noise emissions from the transmit path in the at least one input transmit signal thereby providing an at least one corrected transmit signal. A transmit interference correction circuit has an input coupled to the transmit portion of the antenna interface and an output coupled to the receive portion of the antenna interface. The transmit interference correction circuit correcting interference of the at least one transmit signal in the receive path thereby providing a first at least one corrected receive signal. An arbitrary interferer correction circuit has an input coupled to the receive portion of the antenna interface and an output coupled to the electronic duplexer output. The arbitrary interferer correction circuit correcting interference of signals other than the broadband noise emissions from the transmit path and the interference of the at least one transmit signal in the receive path thereby providing the at least one output receive signal.
Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
In order to lighten the following description, the following acronyms will be used:
As indicated above, the present invention addresses the issues brought out by the aforementioned prior art.
A preferred embodiments presented is shown in
The electronic duplexer 110 is comprised of a first FFCL 120 disposed at the output of the PA 111. The FFCL 120 is used to correct broadband noise emissions, that is, those that do not include linearity close-in emissions, from the PA 111. A second FFCL 121 is used at the antenna coupler 122 to reduce the interference of the transmit signal in the receive path 114 of the antenna 118. A third FFCL 123 is used at the input of the LNA 116 to identify interference signals other than those identified at the transmit end 112 and to correct those additional interfering signals.
A first filter circuit 124 is placed between the first FFCL 120 and the second FFCL 121. A second filter circuit 125 is placed between the second FFCL 121 and the third FFCL 123. The second FFCL 121 includes transmit interference correction block 132 which operates as a filter to remove signal interference or unwanted noise. Such a filter is described in U.S. Pat. No. 7,702,295. The third FFCL 123 includes an arbitrary interferer correction filter circuit 133. Such a filter is described in detail in published international patent application WO 2010/063097.
It will be understood by those knowledgeable in the art that the position of the main path filters 124 and 125 may be chosen advantageously within the transmit and receive paths around the correction combining points depending on the most suitable choices for noise budget, power, gain and linearity of signal processing components.
In a reduced order system, the main path filters are designed for a conventional passband (typically covering one operating band or sub-band). The lack of rejection from the main path filters resulting from the reduced order is recovered through the correction from the electronic correction circuits. In a frequency agile system, the main path filters are designed to whatever order is required for a passband that covers all of the necessary operating frequencies. Where the passband filters cover multiple operating bands, then the FFCL provide the signal attenuation required to meet operational requirements.
With reference to the first FFCL 120, the output of PA 111 is coupled into a transmit antenna emission correction block 130. The emissions correction block 130 manipulates the coupled signal to eliminate the modulated transmit signal so as to capture substantially all of the broadband noise emissions of the PA 111. In particular, the broadband noise emissions are then phase shifted 134, amplitude scaled 135, and a buffer 136 such that when added back 131 into the main path, the broadband noise emissions are substantially eliminated from the PA output signal.
Referring now to
Referring now to
Those skilled in the art will understand that the location at which the output of second stage correction block 304 is added into the receive path may change depending on the noise, gain, power, linearity and interactions with other correction loops.
With reference to
Each TX branch has its own TAEC and each RX branch has it's own AIC. In the most general case of N×M MIMO (N TX, M RX) then a TIC is needed for each TX to every RX.
