M.4 Timing of the FFT window

38.176-23GPPIntegrated Access and Backhaul (IAB) conformance testingNRPart 2: radiated conformance testingRelease 17TS

The FFT window length is FFT size samples per OFDM symbol. For TDD, the number of FFTs performed is the number of uplink symbols in the measurement interval.

The position in time for the FFT shall be determined.

In an ideal signal, the FFT may start at any instant within the cyclic prefix without causing an error. The TX filter, however, reduces the window. The EVM requirements shall be met within a window W < CP. There are three different instants for FFT:

– Centre of the reduced window, called ,

– , and

– .

The value of EVM window length W is obtained from the transmission bandwidth and TS 38.176-1 [3] table 6.5.3.5-2 for 15 kHz SCS, table 6.5.3.5-3 for 30 kHz SCS and table 6.5.3.5-4 for 60 kHz SCS for FR1, and table 6.6.3.5.2-2 for 60 kHz SCS, and table 6.6.3.5.2-3 for 120 kHz SCS for FR2.

The IAB-MT shall transmit a signal according to the test models intended for EVM. The demodulation reference signal of the second ideal signal shall be used to find the centre of the FFT window.

The timing of the measured signal is determined in the pre FFT domain as follows, using and :

1. The measured signal is delay spread by the TX filter. Hence the distinct borders between the OFDM symbols and between data and CP are also spread and the timing is not obvious.

2. In the ideal signal , the timing is known.

Correlation between bullet (1) and (2) will result in a correlation peak. The meaning of the correlation peak is approximately the "impulse response" of the TX filter.

3. The meaning of "impulse response" assumes that the autocorrelation of the ideal signal is a Dirac peak and that the correlation between the ideal signal and the data in the measured signal is 0. The correlation peak, (the highest, or in case of more than one highest, the earliest) indicates the timing in the measured signal.

The number of samples, used for FFT is reduced compared to . This subset of samples is called .

From the acquired samples one timing can be derived.

The timing of the centre is determined according to the cyclic prefix length of the OFDM symbols. For normal CP, there are two values for in a 1 ms period:

– = length of cyclic prefix / 2,

– = Longer CP length – length of cyclic prefix / 2,

Where the length of cyclic prefix is obtained from TS 38.176-1 [3] table 6.5.3.5-2 for 15 kHz SCS, table 6.5.3.5-3 for 30 kHz SCS and table 6.5.3.5-4 for 60 kHz SCS for FR1, and table 6.6.3.5.2-2 for 60 kHz SCS, and table 6.6.3.5.2-3 for 120 kHz SCS for FR2, and the longer CP length is obtained from table M.2.4-1.

As per the example values.

– within the CP of length 288 for most OFDM symbols in 1 ms,

– (= 544 – 144) within the CP of length 544 for OFDM symbol 0 of slot 0 and slot 4.