Method and apparatus for detection of intermodulation products

1. A method of testing for generation of passive intermodulation (PIM) in a wireless network, the method comprising:

connecting a PIM test apparatus to a radio head of a base station of the wireless network using a baseband data link;

generating a first test signal and a second test signal at the PIM test apparatus;

sending the first test signal and the second test signal via the baseband data link to the radio head for transmission by the radio head, the first test signal being for transmission at a first radio frequency and the second test signal being for transmission at a second radio frequency;

receiving at baseband at the PIM test apparatus, via the baseband data link, a waveform received at the radio head at a third radio frequency;

generating, at the PIM test apparatus at baseband, on the basis of at least the first signal and the second signal, at least one interference signal comprising a simulated intermodulation product;

correlating, at the PIM test apparatus said at least one interference signal with the waveform received at the radio head to produce data representing a correlation; and

detecting the generation of PIM in the wireless network in dependence on the data representing the correlation.

2. The method of claim 1 , wherein the baseband data link is a Common Public Radio Interface (CPRI) link.

3. The method of claim 1 , further comprising providing an indication, on the basis of the data representing the correlation, of an amplitude of the PIM generated by at least the first test signal and the second test signal.

4. The method of claim 1 , further comprising:

generating, on the basis of at least the first signal and the second signal at baseband, a plurality of delayed interference signals comprising simulated intermodulation products generated using a plurality of delay values, wherein said at least one interference signal is one of the plurality of delayed interference signals;

correlating each of the delayed interference signals with the received waveform to produce data representing a correlation for each delayed interference signal; and

detecting the generation of PIM in the wireless network in dependence on the data representing the correlations.

5. The method of claim 4 , further comprising providing an indication, on the basis of the data representing the correlations, of a delay of the generated PIM in the wireless network.

6. The method of claim 1 , further comprising frequency shifting baseband intermodulation products of the first and second test signals to produce the simulated intermodulation product, whereby to align the simulated intermodulation product in frequency with an intermodulation product in the received waveform.

7. The method of claim 6 , wherein said frequency shifting comprises shifting by a frequency determined by:

adjusting the frequency shift to a plurality of frequency shift values; and

selecting a frequency for said frequency shifting on the basis of data representing a correlation of an interference signal comprising a simulated intermodulation product with the received waveform for each of the plurality of frequency shift values.

8. The method of claim 1 , further comprising modulating the first and second test signals with a code.

9. A test apparatus for testing for generation of passive intermodulation (PIM) in a wireless network, the apparatus comprising:

an interface configured to connect to a baseband data link, the baseband data link being configured to connect the test apparatus to a radio head of a base station of the wireless network; and

one or more processors configured to cause the test apparatus to:

generate a first test signal and a second test signal;

send the first test signal and the second test signal via the baseband data link to the radio head for transmission by the radio head, the first test signal being configured for transmission at a first radio frequency and the second test signal being configured for transmission at a second radio frequency;

receive at baseband, via the baseband data link, a waveform received at the radio head at a third radio frequency;

generate, on the basis of at least the first signal and the second signal at baseband, at least one interference signal comprising a simulated intermodulation product;

correlate said at least one interference signal with the waveform received at the radio head to produce data representing a correlation; and

detect the generation of PIM in the wireless network in dependence on the data representing the correlation.

10. The test apparatus of claim 9 , wherein the baseband data link is a Common Public Radio Interface (CPRI) link.

11. The test apparatus of claim 9 , wherein the test apparatus is configured to provide an indication, on the basis of the data representing the correlation, of an amplitude of the PIM generated by at least the first test signal and the second test signal.

12. The test apparatus of claim 9 , wherein the test apparatus is configured to generate, on the basis of at least the first signal and the second signal at baseband, a plurality of delayed interference signals comprising simulated intermodulation products generated using a plurality of delay values, wherein said at least one interference signal is one of the plurality of delayed interference signals;

correlate each of the delayed interference signals with the received waveform to produce data representing a correlation for each delayed interference signal; and

detect the generation of PIM in the wireless network in dependence on the data representing the correlations.

13. The test apparatus of claim 12 , wherein the test apparatus is configured to provide an indication, on the basis of the data representing the correlations, of a delay of the generated PIM in the wireless network.

14. The test apparatus of claim 9 , wherein the test apparatus is configured to frequency shift baseband intermodulation products of the first and second test signals to produce the simulated intermodulation product, whereby to align the simulated intermodulation product in frequency with an intermodulation product in the received waveform.

15. The test apparatus of claim 14 , wherein the test apparatus is configured to frequency shift the baseband intermodulation products of the first and second test signals by a frequency determined by:

adjusting the frequency shift to a plurality of frequency shift values; and

selecting a frequency for said frequency shifting on the basis of data representing a correlation of an interference signal comprising a simulated intermodulation product with the received waveform for each of the plurality of frequency shift values.

16. The test apparatus of claim 9 , wherein the test apparatus is configured to modulate the first and second test signals with a code.