ghts do not match with any particular physical direction anymore, rather they need to be adjusted according to the generic channel response and this form of multi-antenna-based signal combining is referred to as spatial RX processing. The actual weight selection and optimization, in turn, can be implemented by several methods, which differ in complexity and performance. The simplest method is selection combining where only the signal with the highest instantaneous SNR is used for detection. 424 Chapter 4 SO that it has low density in the frequency domain and high density in the time domain, because the phase rotation caused by CPE is identical for all subcarriers within an OFDM symbol; however, it has minimal correlation across OFDM symbols. The PT-RS is UE-specific, confined in a scheduled resource, and can be beamformed. The number of PT-RS ports can be lower than the total number of ports, and orthogonality between PT-RS ports is achieved by means of frequency-division multiplexing. The PT-RS is configurable depending on the quality of the oscillators, carrier frequency, OFDM subcarrier spacing, and modulation and coding schemes used for transmission. The PT-RS introduced in NR is used for time and frequency tracking as well as estimation of delay spread, and Doppler spread at the UE side. They are transmitted in a confined bandwidth for a configurable time duration controlled by RRC parameters. The time-fre- quency structure of PT-RS depends on the waveform. For OFDM, the first reference symbol (prior to applying any orthogonal sequence) in a PDSCH/PUSCH allocation is repeated every LPT-RS E {1, 4} symbol, starting with the first OFDM symbol in the allocation. The repetition counter is reset at each DM-RS position since there is no need for PT-RS inser- tion immediately following a DM-RS occasion. In the frequency domain, PT-RS are trans- mitted in every second or fourth resource block, resulting in a sparse frequency-domain structure. The density in the frequency domain is dependent on the scheduled bandwidt