RACH Channel TLAST - 2.0 English

RFSoC DFE PRACH LogiCORE IP Product Guide (PG391)

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2.0 English

PRACH captures are expected to be scheduled to begin and end throughout the radio frame. The end of a scheduled PRACH capture is indicated by assertion of TLAST on the m_axis_dout interface.

When captures are specified in static mode, their length is specified as a duration in slots and number of repeats, which specifies the number of consecutive captures. The TLAST is asserted at the end of each duration and indicates the boundary between each repeated capture.

When in dynamic mode, the length of a capture is specified as a length field in the dynamic control packet (DCP). The TLAST will indicate the last sample in the dynamic capture. There is no option to specify repeated captures in dynamic mode, although additional capture requests can be sent while a capture is running.

In dynamic mode, for PRACH formats which have multiple preambles and a single CP, it is expected that a single Dynamic capture will span all of the preambles and CP. TLAST will not indicate the boundary between preambles. It will indicate the end of the entire capture. The capture will begin on the requested slot boundary and it is the responsibility of the FFT processing block to extract the preambles from the captured data.

TLAST is generated based upon the schedule and DCP, which dictates when captures begin and end. It does not rely on the s_axis_din0_tlast, s_axis_din1_tlast, or s_axis_din2_tlast inputs.

TLAST will be asserted for each antenna in the ending TID. This is illustrated below for the capture on channel 15, consisting of N samples, the last sample for each antenna is marked by TLAST. A capture on channel 3 is interleaved with the channel 15 samples; it continues unchanged while channel 15 terminates.

Figure 1. Capture on Channel 15

For TLAST which occur between repeated captures (number of repeats is not 0), the TVALID will continue. The next TVALID on a given channel being the first sample out of the next repeat. If there are no more repeats, there will be no further TVALID until the capture pattern restarts, as determined by the pattern period.

If a pattern is configured such that the capture duration and repeats have the same total length (in slots), as the pattern period (frames), then the capture pattern will restart immediately. This configuration can be used to give a continual output of samples separated into packets of length equal to the duration.

In dynamic mode, TLAST indicates the end of a given capture. There will be no more samples for that RCID/antenna unless a new DCP is supplied.

If operating in "Always On" mode, TLAST will only occur when a channel is disabled. When a running channel is disabled in the register map, that channel will generate a TLAST following the Frame Init trigger for its band. TLAST is also output, for each running RCID in "Always On" mode, when the core transitions into "Low Power."

In static mode, TLAST marks the slot boundary for the channel. TLAST appears on the last output sample of the final slot in the static capture. The end of the slot is measured relative to the frame boundary, which is set by the Frame Init trigger. TLAST includes adjustment for latency through the core.

In dynamic mode, to allow channels to be reprogrammed, TLAST occurs 1/64th μs before the end of the final slot requested. So, for example, if a capture of length 1 slot is requested from a CC with SCS of 30 kHz, the actual capture will last 63/64*500 μs = 492.1875 μs. The final 1/64th μs of samples from the slot will not be output. For some combinations of PRACH format and startsymbolid, it is possible that part of the preamble will occur after TLAST. In this case, it is necessary to increment the "length" requested in the DCP to ensure that the full preamble is captured.