Implementations of a multiple-antenna beamformer may require the use of multiple radio circuits that feed these antennas. Some implementations are based on an array of discrete mixers controlled by a common LO (Local Oscillator) where the number of radio circuits is unlimited, but cost, power consumption, real estate and reliability are not optimal. Some other implementations offer an integrated RFIC (Radio-Frequency-Integrated-Circuit) controlled by a common integrated LO, yet, the number of radios is typically to few limited combinations e.g. 2 or 4 or 8 integrated radio circuits in a commercially available RFICs. Hence when a larger number of radio circuits is required, staking an array of RFICs together may be a preferable approach, provided the LOs in each of the RFICs can be locked to each other via external circuitry and procedures.
Such an array of RFICs may maintain frequency lock when all LOs are using a common crystal reference, yet in some cases phase lock mechanism must be implemented each time the radios are retuned, as well on a periodic basis that will counter the effect of phase drift.
FIG. 1 depicts a beamforming TD-LTE base station according to the prior art which includes multiple radio circuits 120-1, 120-2, up to 120-M, that feed or provide input to an antenna array of M elements 110-1, 110-2, up to 110-M, where the multiple radio circuits and antennas are used for beamforming FIG. 1 further illustrates an example where the aforementioned beamforming capability may be used for the implementation of Multi-User MIMO, where up to N data streams are simultaneously served (e.g. in data communication with) by N baseband entities 140-1, 140-2, up to 140-N, where N<M. FIG. 1 further depicts an example implementation using a common local oscillator 150 configured to feed or provide input to the multiple radio circuits, so that digital beamformer 130 weights applied to baseband signals, are routed through the multiple radio frequency (RF) chains with a wells controlled resultant phases.
FIG. 2 depicts another possible block diagram similar to the one in FIG. 1, also in accordance with the prior art, where the radio circuits used are integrated Radio-Frequency-Integrated-Circuits (RFICs) 220-1, 220-2, up to 220-M, and where the LO are implemented inside and fed by a common reference clock 250; however, such a solution guarantees frequency lock but does not provide phase lock across the array, which makes it unfit for beamforming. The problem stems from the fact that while LOs are frequency locked, their relative phases may drift. Therefore, for beam forming purposes, the architecture shown in FIGS. 1 and 2 will not work properly.