Multi-Band Digital Predistortion (MB-DPD)

The core challenge of MB-DPD is mitigating cross-band distortion, which includes intermodulation products (IMD) and cross-modulation. Unlike single-band DPD, MB-DPD must synthesize correction signals that pre-compensate for distortion products falling both in-band and in the frequency gaps between carriers. This requires models that capture the interaction between the instantaneous envelopes of all concurrent signals.

Two primary architectural topologies exist for MB-DPD:

• Joint DPD Architecture: A single, unified predistorter block processes the composite multi-band signal before upconversion. This is computationally intensive but handles all interactions natively.
• Frequency-Selective DPD: Independent predistorter blocks are applied to each carrier, often combined with dedicated cross-band predistorters that generate cancellation signals specifically for inter-band IMD products. This allows for lower sample rates per branch.

For hardware-efficient implementation on FPGAs, the 2D Look-Up Table (2D-LUT) is a critical technique. Complex gain correction values are indexed by a two-dimensional address derived from the instantaneous magnitudes of two concurrent input signals. This avoids the high computational cost of real-time polynomial evaluation while accurately capturing the envelope-dependent nonlinear behavior across both bands.

Coefficient adaptation in MB-DPD commonly uses the Multi-Band Indirect Learning Architecture (MB-ILA). In this closed-loop system, a post-distorter model is identified from the attenuated PA output and then copied to the predistorter in the forward path. Joint coefficient estimation simultaneously solves for all model parameters, including cross-band terms, in a single optimization step to minimize the error vector magnitude across all bands.

MB-DPD is often co-designed with efficiency enhancement techniques to manage the high peak-to-average power ratio (PAPR) of composite multi-band signals:

• Multi-Band Crest Factor Reduction (MB-CFR): Jointly reduces the PAPR of the composite signal to prevent amplifier saturation.
• Multi-Band Envelope Tracking (MB-ET): Dynamically modulates the PA supply voltage based on the instantaneous composite envelope.
• Dual-Band Doherty DPD: Specialized linearization accounting for the unique load modulation behavior of dual-band Doherty power amplifiers.

The foundational architecture for linearizing a single power amplifier that is simultaneously transmitting two or more widely spaced carrier signals. Unlike traditional single-band DPD, this approach must account for nonlinear interactions between the bands.

• Synthesizes a composite correction signal
• Addresses both in-band and cross-band distortion
• Essential for carrier aggregation in 4G/5G base stations

Nonlinear interference products generated by the interaction of multiple carrier signals within a power amplifier. These unwanted spectral components fall on top of or near the desired transmit bands and cannot be filtered without also removing the desired signal.

• Includes intermodulation distortion (IMD) products
• Requires dedicated cross-band predistorter blocks
• Primary limiter of multi-band transmitter performance

A behavioral model that extends the standard memory polynomial to two dimensions by incorporating cross-terms dependent on the envelope magnitudes of both concurrent bands. This captures cross-band memory effects where the nonlinear behavior in one band is influenced by the past envelope history of a signal in a different band.

• Balances accuracy with computational complexity
• Indexed by |x₁(n)| and |x₂(n)|
• Foundation for many multi-band DPD implementations

A hardware-efficient predistorter implementation where complex gain correction values are stored in a table indexed by a two-dimensional address derived from the instantaneous magnitudes of two concurrent input signals.

• Eliminates real-time polynomial computation
• Requires interpolation for smooth correction
• Ideal for FPGA and ASIC implementations with strict latency budgets

A closed-loop adaptation method where a post-distorter model is identified from the attenuated PA output and then copied to the predistorter in the forward path. This avoids the need for a direct inverse model of the power amplifier.

• Post-distorter trained on PA output
• Coefficients copied to forward predistorter
• Enables online adaptive training during operation

A signal conditioning technique that jointly reduces the peak-to-average power ratio of a composite multi-band signal before amplification. By preventing amplifier saturation, MB-CFR reduces the severity of nonlinear distortion that DPD must subsequently correct.

• Operates on the composite waveform
• Complements DPD for optimal linearity
• Critical for Doherty amplifier efficiency