Gr4-packet-modem waveform design

This page documents all the aspects of the waveform used by gr4-packet-modem.

Modulation

The modulation is a single-carrier RRC (root-raised-cosine) filtered PAM (pulse amplitude modulation) waveform. The RRC filter has a roll-off of 0.35.

The following constellations are defined. There are provisions in the modem to use higher-order constellations for the payload data.

• BPSK. Formed by the points 1, -1, which encode the bits 0 and 1 respectively. This is used for the synchronization word.

• QPSK. Formed by the points $(1+i)/{\sqrt {2}}$, $(1-i)/{\sqrt {2}}$, $(-1+i)/{\sqrt {2}}$, $(-1-i)/{\sqrt {2}}$, which encode the values 0, 1, 2, and 3 respectively. This is used for the header and the payload data.

The modem transmitter supports two modes of operation:

• Burst mode. Each packet is radiated as a standalone RF burst, without radiation of any RF signal between packets. In this mode there is amplitude shaping: the amplitude at the beginning and end of the packet are tapered off to avoid amplifier clicks caused by sudden amplitude changes. The taper is done over the duration of 4 symbols using a quarter-sine waveform ($\sin t$ for $0\leq t\leq \pi /2$). At the beginning of the packet, the taper is applied to the first symbols of the synchronization word. At the end of the packet, the payload symbols are extended with pseudorandom QPSK symbols to which the taper is applied, in order to avoid applying the taper to payload symbols. The history of the RRC filter starts filled with zeros at the beginning of each packet and the RRC filter is flushed with zeros at the end of each packet. (Design note: With an RRC filter it is not really necessary to include amplitude shaping, since the RRC pulse shape already provides a smooth amplitude change. The amplitude shaping is included mainly to mimic the operation of other modems in which it is necessary, such as modems using a constant-envelope waveform.)

• Stream mode. The transmitter radiates an RF signal continuously. The RF signal is phase continuous and composed of back-to-back packets. If there are no packets available to be transmitted, idle packets are inserted. All the stream of symbols is RRC filtered as a continuous stream, so the symbols at the beginning and end of a packet affect how the neighboring packets are modulated. This mode emulates the way that a continuous carrier system such as DVB-S2 works.

The receiver in gr4-packet-modem treats these two modes in the same way. It detects and estimates the parameters of each packet independently, without assuming that an RF signal is always present. It would be possible to design a different receiver that takes advantage of the characteristics of the stream mode.

Packet structure

The following table describes the structure of a packet. Each of the sections of the packet is explained in more detail below.

	Symbols	Constellation	Notes
Syncword	64	BPSK	CCSDS 64-bit ASM
Header	128	QPSK	32 bit with r=1/4 LDPC and r=1/2 repetition
Payload	Variable	QPSK/other
CRC-32	32 bits	QPSK/other	CRC-32 of payload
Ramp down	9	QPSK	Only burst mode; pseudorandom