Cancelling Noise in Multi-Band Digital-Intensive Transmitters


Digital-intensive RF transmitters using delta-sigma modulation have a compact physical footprint, enabling a low-cost mobile MIMO transmitter for 5G systems, but suffer from a high noise floor due to quantization errors. In order to eliminate the need of high-Q multi-band RF filters for attenuating such high noise floor, this paper for the first time presents a multi-band noise cancellation technique for digital-intensive transmitters based on concurrent multi-band delta-sigma modulation. Using an asymmetric RF power combiner, an experimental implementation realizes the bandpass noise cancellation for non-contiguous interband carrier aggregation of 835 MHz and 1450 MHz bands. For 40-MHz aggregated bandwidth, the proposed noise cancellation technique achieves ACPR better than -47 dB and -42 dB for the low-band and the high-band, respectively. With the dual-band prototype with a 6-GHz sampling rate, the proposed technique needs three times less bandwidth for noise cancellation signal compared to conventional feed-forward techniques, and thus the sampling rate requirement on noise cancellation path can be significantly relaxed.