A Novel Single Feedback Architecture With Time-Interleaved Sampling for Multi-Band DPD

A Novel Single Feedback Architecture with Time-Interleaved Sampling for Multi-Band DPD

Abstract: With the increasing demand for multi-band signal transmission, PA linearization has applied a variety of multiband pre distortion methods. However, as the number of carrier increase, the feedback path will be complex and high sampling rate ADCs will significantly increase system cost. To simplify the system structure and reduce the sampling rate for ADCs, in this letter, a novel DPD method base on a single feedback architecture with time-interleaved sampling method is proposed.

The time interleaved sampling signals based on the low sampling rate can be obtained by two ADCs, whose under-sampling factors are co prime. Then the multi-band signals can be reconstructed due to its band sparsity to calculate the coefficients of the pre distortion. If the frequency differences of the bands are bigger than the BW of the ADCs, the multi-band signals can be still reconstructed.

Compared with the previous approaches regarding the multiband DPD, the proposed method can obtain good linearization performance, and reduce the feedback sampling rate. Experiment results with the multi-band signals are presented to show the improvements.

Existing system: In the meantime, the reference proposed a single feedback loop for multi-band DPD with under-sampling technique, which could greatly reduce the sampling rate. However, if the number of bands increases or the frequency difference of the bands are bigger than the BW of the Analog to- Digital converters (ADCs), which results in some inter modulation products and cross-modulation products cannot be ignored, the method in could not linearize PA well. In order to meet the challenge, this letter proposes novel single feedback architecture with time-interleaved sampling for multi-band DPD. Two ADCs will be used in the feedback loop to achieve the acquisition of useful signals from the amplifier output. Subsequently, using the indirect learning structure, the pre distortion coefficients of each band can be obtained. The experimental platform is built to verify this method.

Proposed system: Adaptive digital pre distortion (DPD) is one of the best methods to compensate for nonlinear distortion of power amplifiers (PAs). Traditional DPD are mainly for single-band transmitters. However, multi-band PAs have different forms of distortion including inter modulation and cross-modulation products, which are more severe than the distortion products of Pas operating in single-band transmitters. In recent years, with the increasing demand, the DPD for multi-band has emerged but faces new challenges. That is a complex feedback path and high sampling rate requirements. Different DPD techniques have been proposed in for the linearization of dual and tri-band PAs. These methods as mentioned above require multiple feedback loops for digital pre-distortion.

Advantages: Corresponding single channel sampling rate will be up to 192MSPS. Otherwise, the filter, ADC, mixer, and LO are needed to add in the feedback path to increase the system complexity and cost.

When the time-delay misalignment between the three signals is 5 ns, the proposed DPD method can still obtain better EVM and ACPR performance, as shown in Table II. The proposed method also has the advantage in terms of complexity in

multi-band DPD. The signal recovery and model complexity of the proposed method are compared with the conventional method, as shown in the Table III. Disadvantages: In this letter, a novel multi-band DPD method with a single feedback path is proposed, which established the time interleaved signals with the low sampling rate based on the recovery of frequency domain sparse signals.

Experiments based on 30MHz bandwidth with two 3-MHz OFDM signals and 500MHz bandwidth with three 10-MHz OFDM signals prove that the ACPR of the PA output signals can be reduced to about -45 dBc with the proposed method.

Compared with the conventional multi-band DPD method, this method can obtain good linearization performance with the reduced sampling rate, anti-time-delay misalignment ability and low complexity.

Modules: Digital pre distortion: WITH the increase of communication services, spectrum resources and power resources become scarce. In order to improve the utilization of spectrum resources, high-order modulation methods are widely used. Adaptive digital pre distortion (DPD) is one of the best methods to compensate for nonlinear distortion of power amplifiers (PAs). Traditional DPD are mainly for single-band transmitters.

However, multi-band PAs have different forms of distortion including inter modulation and cross-modulation products, which are more severe than the distortion products of Pas operating in single-band transmitters. In recent years, with the increasing demand, the DPD for multi-band has emerged but faces new challenges. That is a complex feedback path and high sampling rate requirements.

Different DPD techniques have been proposed in for the linearization of dual and tri-band PAs. These methods as mentioned above require multiple feedback loops for digital pre-distortion.

Down- converted carrier co-location technique: In order to optimize the feedback channel structure, the reference adopted a new down-converted carrier co-location technique (DC3), which modulates multi-band in the same subcarrier to reduce the feedback path bandwidth. In the meantime, the

reference proposed a single feedback loop for multi-band DPD with undersampling technique, which could greatly reduce the sampling rate. However, if the number of bands increases or the frequency difference of the bands are bigger than the BW of the Analog to- Digital converters (ADCs), which results in some inter modulation products and cross-modulation products cannot be ignored, the method in could not linearize PA well. In order to meet the challenge, this letter proposes novel single feedback architecture with time-interleaved sampling for multi-band DPD. Two ADCs will be used in the feedback loop to achieve the acquisition of useful signals from the amplifier output. Subsequently, using the indirect learning structure, the pre distortion coefficients of each band can be obtained. The experimental platform is built to verify this method.

Linearization Result of Multi-band DPD: Two 3-MHz OFDM signals with carrier frequencies of 2130-MHz and 2150-MHz are used to verify the effectiveness of the proposed DPD method. In the feedback loop, the local oscillator (LO) frequency of the down-converter is set to 2140MHz When the proposed method is evaluated, the co prime sampling rates only needs to be set at 19.2 MSPS and 12.8 MSPS to build the time-interleaved signals.

If the traditional 2D-DPD is run, the method requires the addition of a filter, ADC, mixer, and LO, which makes the system more complex. Table I shows the proposed DPD results. The ACPR of the PA output signal can be reduced to -45 dBc with the proposed method. When the time-delay misalignment between the two signals is 5 ns, the proposed DPD method can obtain better EVM in overcoming time-delay misalignment than the conventional method.