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doc: document burst format

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Thomas Kolb 2024-04-27 22:40:44 +02:00
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@ -17,6 +17,7 @@ data, and adds a fixed preamble to aid synchronization to the signal.
Additionally, a mechanism is defined that allows to group packets in a „burst“,
which reduces overhead.
[#phy_packet_format]
=== PHY Packet Format
A packet encoded by the PHY consists of the following blocks:
@ -80,11 +81,24 @@ The first data symbol follows directly after the last symbol of the PHY header.
Burst transmission allows the transmitter to send multiple packets in quick succession.
tbd.
A burst consists of the following elements:
. Ramp-up sequence
. Pre-packet idle sequence (optional)
. One or more packets as defined in <<phy_packet_format>>
. Ramp-down sequence
The ramp-up, ramp-down and idle sequences consist of alternating +1/-1 BPSK symbols.
During ramp-up, their amplitude stem:[A_u(k)] is scaled up across stem:[L_u] symbols, following the curve stem:[A_u(k) = sin(π/2 * k / L_u)]. Ramp-down follows a similar sinusoidal for stem:[L_d] symbols: stem:[A_d(k) = cos(π/2 * k / L_d)]. The purpose of these sequences is to avoid splatter while turning on and off the transmitted signal.
During the pre-packet idle sequence, the alternating BPSK symbols are continued with constant amplitude 1.0. This sequence may help synchronizers acquire the signal better (especially the gain and timing regulation can benefit), but is not needed if they are fast enough. If and how this part is used is still to be determined.
Each packet in a burst includes a preamble. Packets are directly concatenated, i.e. after the last data symbol follows the first preamble symbol.
=== Pulse Forming
Pulse forming is applied to control the bandwidth of the transmitted signal and to reduce inter-symbol interference at the receiver.
Pulse forming is applied to limit the bandwidth of the transmitted signal and to reduce inter-symbol interference at the receiver.
This system uses Root Raised Cosine (RRC) filters with a roll-off factor of stem:[alpha=0.2]. Therefore the bandwidth is 1.2 times the symbol rate.
@ -99,7 +113,7 @@ The filter is applied to the whole symbol sequence of the burst. The same filter
=== BPSK
In binary phase shift keying, a `1` bit is sent as +1.0 and a `0` bit is sent as -1.0.
In binary phase shift keying, a `1` bit is sent as -1.0 and a `0` bit is sent as +1.0.
=== QPSK