 mapping The first scrambling is initialized based on the cell ID and the system frame number. The second scrambling is initialized based on the cell ID and the SS block time index. The last channel that we will discuss in this section is the PRACH. Random access preamble - the PRACH As in LTE, the PRACH is based on Zadoff-Chu-based sequences. Two different PRACH types are defined in NR, long-sequence (L=839) and short-sequence (L=139). The long-sequence PRACH is only used below 6 GHz and it is composed of long sequences in time, with low subcarrier spacing. In this case, 1.25 kHz subcarrier spacing with a bandwidth of 1.25 MHz and 5 kHz subcarrier spacing with a bandwidth of 5 MHz are used. Physical channels and signals for the initial access procedure Number of Preamble length Format Numerology repetitions CP length (not incl. CP) 1.25 kHz = 100 us 800 us 1.25 kHz = 680 us 1600 us 1.25 kHz = 150 us 3200 us 5 kHz = 100 us 800 us Figure 5.17 - Long-sequence PRACH formats Four different formats are defined for the long-sequence PRACH. As we realized, this format has a small subcarrier spacing and lasts a very long time. The long-sequence PRACH originates from the LTE preambles. A long-sequence PRACH consists of a Cyclic Prefix (CP), followed by one or more sequences, and finishing with a guard period to align to the slot boundaries. Format o 1.25 kHz Format 1 1.25 kHz Format 3 1.25 kHz Figure 5.18 - The long-sequence PRACH structure Since the subcarrier spacing is larger, the slot duration is shorter. This makes a long-sequence PRACH extremely useful in frequency bands above 6 GHz in combination with beam sweeping. The main difference between the short-sequence and long-sequence PRACH types is that the subcarrier spacing is much higher in a short-sequence PRACH. 15 kHz and 30 kHz subcarrier spacings below 6 GHz frequency and 60 kHz and 120 kHz subcarrier spacings above 6 GHz are used. Format Numerology Number CP length Preamble length of repetitions (not incl. CP) 9.4 us 133 us 18.7 us 267 us 28.1 us 400 us 7.0 us