Concurrent Dual-Band DPD

Unlike independent single-band DPD, this architecture extracts a single set of 2D-DPD coefficients by solving a joint optimization problem. The predistorter models the nonlinear interaction between the two baseband signals, including cross-modulation distortion products. The extraction algorithm processes both feedback signals simultaneously to minimize the error vector magnitude in both bands, accounting for the fact that the power amplifier's nonlinear response is a function of the composite dual-band envelope.

A core capability is the suppression of cross-band intermodulation distortion (IMD). When two carriers at frequencies f1 and f2 are amplified by a shared nonlinear device, they generate intermodulation products at frequencies like 2f1-f2 and 2f2-f1. These products can fall directly into the receive bands of the transceiver. The 2D-DPD model explicitly includes cross-term basis functions that predict and cancel these intermodulation products, a feat impossible with separate single-band predistorters.

The behavioral model extends the standard memory polynomial into a 2D memory polynomial (2D-MP). The predistorted signal for band 1 is a function of both the band 1 and band 2 input envelopes, including their lagged terms:

• In-band terms: |x1(n-m)|^k * x1(n-m)
• Cross-band terms: |x2(n-m)|^j * |x1(n-m)|^k * x1(n-m) This structure captures the envelope memory effect caused by the shared bias network and thermal dynamics affecting both carriers simultaneously.

The architecture is inherently agnostic to the frequency separation between the two carriers. Whether the bands are adjacent in a carrier aggregation scenario or separated by hundreds of megahertz in a multi-standard radio, the 2D-DPD model operates on the complex baseband representations of each signal independently. The predistortion is applied at baseband for each band, and the nonlinear interaction is modeled mathematically without requiring the predistorter to operate at a sampling rate proportional to the total span, avoiding the prohibitive aliasing distortion and ADC sampling rate requirements of a single wideband DPD approach.

A critical implementation advantage is the dramatic reduction in analog-to-digital converter (ADC) sampling rate requirements. A single wideband DPD would need to digitize the entire bandwidth spanning both carriers plus the distortion products. Concurrent dual-band DPD employs two separate observation receivers, each tuned to a single band. Each ADC only needs to capture the bandwidth of its respective carrier, significantly lowering power consumption and hardware cost while avoiding ADC clipping from the high peak-to-average power ratio of the composite signal.

The coefficient adaptation typically employs an indirect learning architecture (ILA) adapted for dual-band operation. A postdistorter is first identified on the power amplifier output, and its coefficients are then copied to the predistorter. The dual-band ILA minimizes the error between the postdistorter input and the attenuated PA output for both bands simultaneously. This closed-loop structure allows the system to track changes in the power amplifier's nonlinear characteristics due to thermal memory effects, aging, or antenna load mismatch without requiring a full model re-extraction.

The primary impairment addressed by concurrent dual-band DPD. When two widely separated carrier frequencies are amplified simultaneously, the PA generates third-order intermodulation products that fall both in-band and out-of-band. Critically, cross-modulation creates distortion products where the envelope of one band modulates the carrier of the other, a phenomenon absent in single-band operation. Volterra series models capture these interactions through cross-kernel terms that couple the two frequency bands.

A behavioral model that extends the standard memory polynomial to handle dual-band excitation. The 2D-MP model expresses the output at each band as a function of both its own input envelope and the envelope of the other band:

• Self-kernels: Capture intra-band nonlinearity and memory
• Cross-kernels: Model inter-band modulation effects
• Complexity scaling: The number of coefficients grows quadratically with nonlinearity order, requiring careful pruning for real-time implementation

While related, carrier aggregation and concurrent dual-band operation are distinct. Carrier aggregation combines multiple component carriers that may be contiguous or non-contiguous within a single band, often sharing a common local oscillator. Concurrent dual-band DPD addresses the harder problem of widely separated bands—potentially across low-band and mid-band frequencies—where harmonic and intermodulation products fall far from the carriers and cannot be filtered by conventional duplexers.

Concurrent dual-band DPD inherently requires multi-rate processing. The predistorter must operate at a sampling rate sufficient to capture distortion products spanning both bands and their intermodulation bandwidth. Key architectural considerations:

• Baseband rate: Processes each band's I/Q data at its native bandwidth
• Composite rate: The combined predistorted signal may require 3-5x the maximum carrier frequency separation
• Interpolation/decimation chains: Critical for aligning sample rates between the predistorter and the dual-band feedback path

The observation receiver in a dual-band DPD system must faithfully capture the PA output across both frequency bands simultaneously. This requires a wideband feedback architecture that is itself linear. Any nonlinearity in the ADC, mixer, or amplifier in the feedback chain corrupts the training data and limits linearization performance. Post-distortion calibration of the observation path is often necessary, using a known training signal to characterize and compensate for receiver impairments before PA model extraction.

Doherty power amplifiers are the dominant architecture for base station transmitters due to their efficiency at backed-off power levels. However, Doherty PAs exhibit strong nonlinear memory effects and complex phase distortions that are exacerbated in dual-band operation. The load modulation between the carrier and peaking amplifiers creates band-dependent AM/AM and AM/PM characteristics. Concurrent dual-band DPD must compensate for these effects across both the carrier and peaking amplifier operating regions simultaneously.