I still do not get the need for this. For the radio phase alignment the current tightest system level requirement is 65ns. Assuming that everything gets tighter (as we have seen) say 5x times, we would still be at >10ns. And you would always be doing the alignment on certain frame boundary over a longer period, not per each sample. If we could even reach close 1ns for real I would say most of the radio people would be extremely happy.
If the rate alignment is the concern then again I do not think there is a need to look at individual sample. You would average over some longer period and some frame structure at minimum.
What kind of equipment you are looking at to be able to provide timestamping services that are sub-ns accurate in reality?
Last, the fronthaul latency requirements out there are way more than 1us. So having max 1us window is not enough.
- Jouni
On 19 Apr 2016, at 22:30, Bross, Kevin <kevin.bross@xxxxxxxxx> wrote:
Richard,
I agree, insofar as the LPF is able to maintain the proper bit spacing. However, I don’t think the LPF will be able to tell which bit position some data is supposed to start on…
· Hypothetical example: suppose there’s a period of successive 0x00 bytes that is suppressed from the front haul to handle quiescent periods. At some point after all these zeroes, there will be a non-zero byte.
o When the next set of data is supposed to go out on the radio, you need to know when the first bit for that next byte is supposed to go out on the radio
o If you send the next byte out one bit position early, the radio will interpret the data incorrectly, as bits will effectively be shifted left
o If you send the next byte out one bit position late, the radio will also interpret the data incorrectly, as bits will effectively be shifted right
· I think you need to be able to specify the presentation time to sufficient accuracy that the RoE device can know the theoretically precise time for when that leading bit should go out.
o For each ¼ ns period specified by the current timestamp granularity, there could be ~6 different bit position that would match a given timestamp. How is the RoE node supposed to know which of these 6 different bit positions is the right time to push out the first bit of that new radio data?
o With future 100 Gbps links, there would be ~25 possible bit positions for each timestamp.
o This is why I proposed re-defining the units of the RoE timestamp to be in picoseconds. This still allows specifying a time up to 1 µs in the future (what all seemed to agree was well beyond the expected transit time for any 4G or 5G fronthaul), while still providing enough precision to specify the timestamp for future links up to 1 Tbps.
--kb
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Kevin Bross Modular Systems Architect
2111 NE 25th Ave, M/S: JF3-466 Phone: 503-696-1411
Hillsboro, OR 97124 mailto:kevin.bross@xxxxxxxxx
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From: stds-1904-3-tf@xxxxxxxx [mailto:stds-1904-3-tf@xxxxxxxx] On Behalf Of Richard Tse
Sent: Thursday, April 14, 2016 12:05 AM
To: Maiden, Richard (Altera) <rmaiden@xxxxxxxxxx>; stds-1904-3-tf@xxxxxxxx
Subject: RE: RF Timing
Richard:
The “push out” of the radio data at the presentation time is a timing event that affects the PLL that regenerates the radio’s clock. This PLL will have a LPF to average out the jitter of these “push out” events.
Rich
From: stds-1904-3-tf@xxxxxxxx [mailto:stds-1904-3-tf@xxxxxxxx] On Behalf Of Richard Maiden
Sent: Wednesday, April 13, 2016 3:58 PM
To: stds-1904-3-tf@xxxxxxxx
Subject: RF Timing
Hi all,
Please find a short presentation attached where I try to frame the requirements from an RF perspective. Ultimately I’d argue that the only thing that matters is that the appropriate sample is put on the air at the appropriate time. The block which determines this is the egress buffer (FIFO) in the radio. This FIFO has 2 controls on the egress side, clock and read address. There are so many ways to generate these signals that I think this topic is out of scope for 1904.3. We just need to make sure that we do not do anything to box ourselves in with the packet definitions which preclude any particular mechanism.
Kevin brings up a good point with the accuracy of our timestamp field. The granularity of its accuracy means that if it is regularly used (rather than a one-time start mechanism), it will be tricky to ensure that we don’t introduce jitter every time we receive a timestamp. Effectively we’d have a moving time-quantization error. For LTE 20MHz for example, we have 30.72MSps for each AxC or 32.5520833333’ns per sample interval. Right now our timestamp granularity is 0.25ns. If we perform a one-time timestamp, we’ll have some quantization error on when we actually send but that would not be a problem. However, on the next presentation time timestamp, we will likely have a different quantization error and so we will in effect jump forwards or backwards in time on when we actually transmit that sample – that’s a problem. Adding more granularity (fractional bits) would reduce this error but I’m not sure how far we’d have to go.
Thanks,
Richard
Richard Maiden
ALTERA (an Intel Company)
101 Innovation Drive
San Jose, CA 95134
Tel: +1 (949) 382-5402