RACH Channel TLAST - RACH Channel TLAST - 2.0 English - PG391

PRACH captures are scheduled to begin and end throughout the radio frame. The end of a scheduled PRACH capture is indicated by asserting 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.

In dynamic mode, the length of a capture is specified as a length field in the dynamic control packet (DCP). The TLAST indicates 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 spans all of the preambles and CP. TLAST does not indicate the boundary between preambles. It indicates the end of the entire capture. The capture begins on the requested slot boundary. 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 is asserted for each antenna in the ending TID. This is illustrated in the following figure 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.

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

If the capture duration and repeats have the same total length (in slots) as the pattern period (frames), then the capture pattern restarts immediately. You can use this configuration to get a continual output of samples separated into packets of length equal to the duration.

Dynamic Mode

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

"Always On" Mode

In "Always On" mode, TLAST only occurs when a channel is disabled. Enable "Always On" mode by selecting the "Remove Internal Scheduling" option in the GUI at core generation. When a running channel is disabled in the register map, that channel generates 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."

Static Mode

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.

To allow channels to be reprogrammed in dynamic mode, TLAST occurs 1/64th of a subframe 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 lasts 63/64*500 μs = 492.1875 μs. The final 1/64th of a subframe of samples from the slot are not output. For some combinations of PRACH format and startsymbolid, part of the preamble occurs after TLAST. In this case, increment the "length" requested in the DCP to ensure that the full preamble is captured.