In 5G NR, 3GPP introduces a new reference signal, named phase tracking reference signal (PT-RS), to deal with oscillator noise. The noise incurred in the oscillators results in phase modulation of the information signal, leading to significant changes in the frequency spectrum and timing properties of the information signal. This phenomenon related to oscillators is called phase noise. Phase noise produced in local oscillators introduces a significant degradation at millimeter wave (mmWave) frequencies, depending on the power spectral density of phase noise. Phase noise leads to CPE and intercarrier interference (ICI). CPE leads to an identical rotation of a received symbol in each subcarrier. ICI leads to a loss of orthogonality between the subcarriers. Due to the distributed structure of PT-RS in frequency domain, the example primarily uses PT-RS to estimate and minimize the effect of CPE on system performance. The example applies phase noise on the waveform consisting of physical downlink shared channel (PDSCH) and shows the change in EVM and BER without and with CPE compensation. This figure shows the processing chain implemented in this example.

The oscillator power spectral density models the phase noise. This example uses a multipole zero model to approximate the power spectral density of the oscillator. Use the PNModel field of the simParameters structure to select the phase noise model: 'A', 'B', or 'C'. Parameter sets 'A' and 'B' are obtained from practical oscillators operating at 30 GHz and 60 GHz, respectively, as described in TDoc R1-163984. The parameter set 'C' is obtained from the practical oscillator operating at 29.55 GHz, as described in TR 38.803 Section 6.1.10.

The example uses a carrier with a subcarrier spacing of 60 kHz for a transmission bandwidth of 50 MHz.

The example configures PDSCH occupying the complete carrier with modulation scheme set to '64QAM' and number of layers set to 1. The example defaults to a single layer and a single codeword of random uncoded bits.

The waveform is generated for 2 frames and the field NumFrames of simParameters structure controls the number of frames of the waveform. The steps involved are:

Generate random codeword with the bit capacity of PDSCH

Get the PDSCH symbols for the random codeword and map them to grid

Generate and map DM-RS symbols to grid

Generate and map PT-RS symbols to grid

Perform OFDM modulation for the complete grid of all frames

Apply phase noise to the transmitted waveform. To clearly observe the impact of phase noise, the example does not apply any thermal noise or channel model in addition to phase noise. The example applies the same phase noise to all the layers.

Before returning the equalized PDSCH symbols and decoded bits, the receiver performs these steps:

Timing synchronization

OFDM demodulation

Channel estimation

Equalization

CPE estimation and correction

PDSCH decoding

For the CPE estimation and correction step, the receiver uses the logical field CompensateCPE of the simParameters structure. Because the example does not use a propagation channel, the steps of timing synchronization, channel estimation, and equalization are not strictly necessary. However, these steps help investigate the phase noise effects if you introduce a channel.

The example shows the equalized constellation symbols, EVM, and bit error rate, with and without CPE compensation.

To disable the CPE compensation, set the field CompensateCPE of simParameters structure to 0.

To enable the CPE compensation, set the field CompensateCPE of simParameters structure to 1. Use PT-RS to estimate the CPE at all the OFDM symbol locations in a slot. Correct the CPE in the OFDM symbol locations within the range of PT-RS OFDM symbols.

To visualize the impact of phase noise, change the carrier frequency, subcarrier spacing, number of resource blocks, modulation scheme, and the number of frames.

To see the effects of phase noise on the constellation, change the phase noise model.

To analyze the effect of CPE compensation with different configurations, change the time and frequency density of PT-RS.

Visualize the impacts of phase noise by including thermal noise and channel models.

This example demonstrates the impact of phase noise and shows how to estimate and correct CPE with PT-RS. The example also shows that CPE compensation reduces the EVM and improves bit error rate. The displayed constellation plot shows huge ICI in mmWave frequencies, indicating that ICI compensation needs to be performed in addition to CPE compensation.