SIMPLIFIED 2-state Detectors for SOQPSK-TG and SOQPSK-MIL

We study simple trellis-based detectors for SOQPSK that have a minimal level of complexity. In particular, we show that the state complexity can be cut in half relative to previous approaches—from 4 states down to 2—with asymptotically optimum performance. We give two possible means of achieving this: the pulse amplitude modulation (PAM) technique and the pulse truncation (PT) technique; both of these techniques make use of recent advances in SOQPSK technology based on a continuous phase modulation (CPM) interpretation of SOQPSK. The proposed simplifications are significant since trellisbased SOQPSK detectors are 1–2 dB superior to widely-deployed symbol-by-symbol detectors. These performance gains come at the expense of complexity, and the proposed 2-state detectors minimize this expense. Thus, these simple detection schemes are applicable in settings where high-performance and low complexity are needed to meet restrictions on power consumption and cost. Introduction Shaped-offset QPSK is a type of continuous phase modulation (CPM) [1] which is highly bandwidth efficient. While CPMs have a number of advantages, one advantage in particular is responsible for its widespread deployment: it has a constant signal envelope. This makes it compatible with nonlinear power amplifiers, which are highly efficient in converting limited (i.e. battery) power into radiated power. This in turn allows for a smaller physical size and lower cost for the transmitter. To date, SOQPSK has been incorporated into military and aeronautical telemetry standards, although wider use is merited since it is applicable in any setting where bandwidth-efficient constant-envelope modulations are needed. In addition to its name, SOQPSK shares a number of similarities with conventional offset QPSK (OQPSK). These similarities are exploited at the receiver, where OQPSK-type detectors are the most commonly deployed means of detecting SOQPSK. The advantage of the OQPSK interpretation of SOQPSK is its simplicity. Thus, the low-cost advantage of SOQPSK can be shared by the transmitter and the receiver. However, the disadvantage of OQPSK-type detectors is that they are suboptimal by 1–2 dB,