Currently, with rapid development of global mobile communications, telecommunication operators are continuously interested in reducing total cost of ownership (TCO) and looking for cost-effective methods for expanding mobile networks. This is necessary for coping with the rapidly growing demands of users. In order to reduce the TCO, a base station can adopt a multi-carrier technique, e.g., a base station can use a number of carriers, or carrier frequencies, within its transceiver. Such base station can be referred to as a multi-carrier base station. A multi-carrier base station can have multiple carriers in one or more transmission frequency bands for one or more radio access technologies (RATs). A multi-carrier base station needs to support higher output power and broader bandwidth than legacy base stations to provide improved coverage and capacity.
Typically, a transceiver includes non-linear components, which means that the output of each of these components is a non-linear function of its input. Given a multi-carrier signal to be transmitted (hereinafter referred to as “transmission signal”), a non-linear component will produce, at its output, additional spectral components at frequencies different from the carrier frequencies of the transmission signal. These spectral components are known as inter-modulation (IM) products and have a specific relation in frequency with a transmission signal to be transmitted by the transceiver. The IM products include active IM (AIM) products and passive IM (PIM) products.
In order to cancel the IM product caused by the transmission signal from the signal actually transmitted by a transceiver, an analog or digital pre-distortion technique has been proposed. FIG. 1 shows a structure of a transceiver 100 provided with a digital pre-distortion (DPD) circuit 110. The DPD circuit 110 has a coupler 111 for obtaining a copy of a radio frequency (RF) signal output from a power amplifier (PA) 121 and feed it back to a demodulation and down-conversion circuit 112. The copy of the RF signal contains the transmission signal and an IM product which is caused by the transmission signal and produced in a transmission signal path 120 (including e.g., a Digital-to-Analog Converter (DAC) 123, a modulation and up-conversion circuit 122 and the PA 121). The demodulation and down-conversion circuit 112 down-converts the copy of the RF signal into an intermediate frequency (IF) signal. The DPD circuit 110 also includes an Analog-to-Digital Converter (ADC) 113 for converting the IF signal into a digital signal which is then processed by a Digital Signal Processor (DSP) 114. The transmission signal is delayed at a delay circuit 115 and the delayed signal is input to the DSP 114 which then estimates the IM product. Finally, the transmission signal is gain adjusted at a gain adjustment circuit 116 according to the estimated IM product from the DSP 114. In this way, the transmission signal is pre-distorted to cancel the IM product. The transceiver 100 also has an antenna front end 130 including a transmission filter 131, a reception filter 132 and an antenna 133. Here, the band pass transmission filter 131 is provided for filtering the IM product before the transmission signal is fed to the antenna 133. The respective functions of the PA 121, the modulation and up-conversion circuit 122, the DAC 123, the transmission filter 131, the reception filter 132 and the antenna 133 are well known and thus the details thereof will be omitted here. FIG. 2 is a schematic diagram of signal spectra with and without DPD. It can be seen from FIG. 2 that, with DPD, the IM product is significantly reduced. After being filtered by the transmission filter, the IM product is much lower than the transmission signal (Carrier 1 (C1) and Carrier 2 (C2) as shown in FIG. 2).
Since the IM products are extended over the frequency spectrum out of the transmission frequency bands, there is a risk that some of the IM products will end up in the reception frequency band of the transceiver, thereby causing distortion in a signal received at the transceiver. Since a multi-carrier base station transceiver supports a high transmit power and has a number of carriers arranged over a broad bandwidth, the IM products may cause a considerable distortion in a received signal. For example, it can also be seen from FIG. 2 that a residual IM product falls into the reception frequency band of the transceiver.
Furthermore, there are further IM products that cannot be canceled by the DPD or filtered by the transmission filter. For example, these IM products are produced at an antenna, a connector connected to the antenna, a connector connected to the transmission filter, a feeder cable connecting the connectors, and the transmission filter itself, as shown in FIG. 3. Although these IM products are much lower than the IM products caused by e.g. the power amplifier, they may cause distortion to a received signal when the IM products fall into the reception frequency band of the transceiver, especially if the received signal is weak. FIG. 4 shows an example of signal spectra in which an IM product falling into the reception frequency band is comparable to the received signal.
Patent document U.S. Pat. No. 8,055,235B1 describes a method for interference cancellation in which a cancellation signal and an interference signal is summed in analog domain. However, this method is only applicable to cancel large interference signals, but is not applicable to cancel IM products at very low level, especially PIM products produced at the passive components shown in FIG. 3. Patent documents US2011/0149714 and US20090068974 describe similar interference cancellation techniques that cancel interferences in digital domain. But again, they are not applicable to cancel IM products at very low level, especially PIM products. Patent document CA2658070 describes a method for blindly detecting PIM products. However, it fails to provide any solution for canceling them. Patent document US2011/0075754A1 describes a method for removing PIM products generated in a wave guide and an antenna that fall into a reception frequency band. However, it cannot detect or cancel IM products produced at the components shown in FIG. 3 and cannot detect or cancel IM products at very low level, such as −100 dBm.
There is thus a need for an improved solution for canceling IM products caused by a transmission signal from a received signal.