Techniques for correcting for phase and amplitude offsets in a MIMO radio device
Techniques to correct for phase and amplitude mismatches in a radio device in order to maintain channel symmetry when communicating with another device using MIMO radio communication techniques. Correction for the amplitude and phase mismatches among the plurality of transmitters and plurality of receivers of a device may be made at baseband using digital logic (such as in the modem) in the receiver path, the transmitter path or both paths of that device. In a device, amplitude and phase offsets are determined among the plurality of radio transmitter and radio receiver paths by measuring phase and amplitude responses when supplying a signal to a transmitter in a first antenna path of the device and coupling the radio signal from a first antenna to a second antenna path of that device where the signal is downconverted by a receiver associated with the second antenna path, and similarly coupling a signal from the second antenna path to the first antenna path. Measurements are obtained between the first antenna path and each of the other antenna paths when coupling a signal in both directions between them. Phase and amplitude offset correction values are computed from the phase and amplitude measurements during a self-calibration operation or mode of the device, and are used during a run-time operation or mode when processing the baseband transmit and/or receive signals to compensate for the phase and amplitude offsets among the plurality of transceiver paths of a device. Amplitude offset correction may not be necessary (or optional) for certain radio implementations or MIMO radio algorithms. The device may execute the self-calibration mode on device power-up, and then periodically thereafter. Self-calibration may also be performed at the factory on a device.
1. A method for calibrating a radio device that comprises a plurality of antennas, a corresponding plurality of receivers and a corresponding plurality of transmitters, the method comprising steps of:
a. supplying a reference signal to an input of a receiver circuit associated with a first antenna path to enable the receive circuit to process the reference signal and produce a first baseband signal that is converted to a first digital signal by an analog-to-digital converter associated with the first antenna path;
b. coupling the first digital signal to a digital-to-analog converter associated with a transmitter circuit of a second antenna path, wherein the transmitter circuit associated with the second antenna path outputs an analog signal;
c. measuring the phase difference between the reference signal supplied to the input of the receiver circuit associated with the first antenna path and the analog signal output by the transmitter circuit associated with the second antenna path to produce a first phase difference;
d. supplying a reference signal to an input of a receiver circuit associated with the second antenna path to enable the receiver circuit to process the reference signal and produce a second baseband signal that is converted to a second digital signal by an analog-to-digital converter associated with the second antenna path;
e. coupling the second digital signal to a digital-to-analog converter associated with a transmitter circuit of the first antenna path, wherein the transmitter circuit associated with the first antenna path outputs an analog signal;
f. measuring the phase difference between the reference signal supplied to the input of the receiver circuit associated with the second antenna path and the analog signal output by the transmitter circuit associated with the first antenna path to produce a second phase difference; and
g. computing a phase mismatch value from the first phase difference and the second phase difference, whereby the phase mismatch value represents the phase mismatch between the first antenna path and the second antenna path and is used when processing baseband signals that are transmitted and/or received via the plurality of antenna paths.
2. The method of claim 1 , and further comprising repeating steps (a) through (g) to compute a phase mismatch value between the first antenna path and antenna path i, for i=3 to N, where N is the total number of antenna paths.
3. The method of claim 2 , wherein the steps (a)–(h) are repeated when a reference signal is supplied at each of a plurality of frequencies within a bandwidth of a baseband signal.
4. The method of claim 3 , wherein the steps (c) and (f) of measuring comprise measuring the phase difference at each of a plurality of sub-carriers of the multi-carrier reference signal.
5. The method of claim 1 , wherein the steps (a) and (d) of supplying a reference signal comprise supplying a continuous wave reference signal.
6. The method of claim 1 , wherein the steps (a) and (d) of supplying a reference signal comprise supplying a multi-carrier reference signal.
7. The method of claim 1 , and further comprising the step of storing in a memory information to allow for adjustment of phase mismatch values according to gain settings of the plurality of transmitters and/or gain settings of the plurality of receivers.