Impact of timing jitter on the performance of carrier amplitude and phase modulation

Carrier amplitude and phase modulation (CAP) is a highly spectral-efficient multilevel modulation scheme renowned for its implementation simplicity as it offers a low complexity approach to increase spectral efficiency and system throughput. These characteristics has seen CAP being implemented as an alternative to the well-known discrete multitone (DMT). However, the sensitivity of CAP to timing jitter is high due to its use of pulse shaping which eliminate the need for carrier modulation and carrier frequency recovery in its tranceiver architecture. In this paper, the impact of timing jitter on the performance of CAP technique is investigated. It is shown that with increasing timing jitter, the bit error rate (BER) performance of CAP-4 degrades and reaches an error floor when the timing jitter (τ) reaches 20% of the symbol duration, T. Although the timing jitter effect is observed to reduce with increasing roll-off factor, α, the resulting performance degradation increases with the data constellation size. With a 0.02T timing jitter, CAP with 64 data constellation points (CAP-64) and 0.15 roll-off factor reaches an error floor at a BER of 2 × 10¹. The results of this paper show that although CAP offer implementation simplicity, timing synchronization is a critical requirement in its receiver design.