2.3. Passband Transmission
Passband line codes have no energy at or near DC. The reason for their use in DSLs is because DSLs normally are transformer coupled (for purposes of isolation) from transceiver equipment, and thus the transformers pass no DC or low frequencies. This section reviews four of the most popular passband transmission line codes.
2.3.1. Quadrature Amplitude Modulation (QAM)
Quadrature amplitude modulation (QAM) is a N = 2-dimensional modulation method. The two basis functions are (for transmission at time 0):
where ϕ(t) is a baseband modulation function like sinc or a square-root raised cosine pulse shape. The multiplication ...