Direct Sequence Spread Spectrum Baseband Modem (SSTM)
40
PRELIMINARY V1.8
Figure 13.
Burst Frame Structures
Each burst cycle also includes 2 “Guard” times to allow for both propagation and RF
transceiver switching time. More specifically, G1 is a 28-bit delay between the time the
Master stops transmission and the Slave commences transmission; G2 is a 28-bit delay
between the time the Slave stops transmission and the Master commences transmission.
These guard times allow for a minimum delay of 328
µsec delay (for a master oscillator
frequency of 16.384 MHz). The total burst cycle is 768 bits long, including 12 bits
internal delay (the transmitter turns off 6 bits after the last data bit is latched into the
transmitter, the master and slave therefore contribute a total of 12-bit internal delay).
During TDD operation, the receiver will go through several stages. Initially, when the 4
UWs of the acquisition burst has been received and decoded correctly, the receiver
(either Master or Slave) declares “locked”. After the empty burst has been decoded, the
receiver declares “rlocked” signifying that the remote device has locked. The behavior
of the receiver after establishing the RLOCK condition depends on whether the internal
state machine is turned on or not (determined by the setting of the configuration word,
bit CI_h[4]). When the state machine is turned off, transmission will be turned off
whenever the UW is not detected. The Slave then waits for a new acquisition burst
while the Master will start the acquisition cycle again by transmitting an acquisition
burst. Note that the Master will continue to broadcast acquisition bursts until it has
received a proper acquisition burst from the Slave in response. The Master will always
revert back to the initial acquisition mode (broadcasting acquisition bursts), whenever it
fails to detect the proper UWs from the slave.
Zero Stuffing
Acquisition Burst Frame Structure
Empty Burst Frame Structure
Data Burst Frame Structure
Preamble
Data
One Stuffing
UW
UW
UW
UW
Preamble
Preamble
UW
UW SW
Signaling Word (SW)
22
32
288
8
Preamble
Data
Unique Word (UW)
Field
Bits
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