Question

What concept do things like PSK or QAM fall under and what concept do things like CDMA, OFDMA fall under.....is one a modulation format and the other a modulation scheme or modulation technique? how are the overarching concepts of both groups related. I understand them separately but i get them confused, at to how they are related.

Group1(PSK,QAM) and Group2(CDMA, OFDMA)

Answer 1

PSK and QAM are modulation techniques. Modulation is done to map a lower-frequency message signal to a higher frequency carrier signal so that information can be reliably transmitted over a channel for long distances, and if the modulation scheme is known at the receiver, the low-frequency message signal can be recovered from the transmitted modulated signal.

CDMA, OFDMA etc are multiple access schemes. The goal of multiple access scheme is to utilize the available channel bandwidth to the maximum possible limit while maintaining reliability. Multiple access schemes allow multiple signals being transmitted over the same channel simultaneously while not interfering with each other. Each of these transmitted signals are modulated by some scheme like PSK or QAM.

Modulating a carrier signal with message signal creates the signal that is to be transmitted over a channel.

The channel employs a multiple access scheme to maximize utilization of channel bandwidth, by providing different time slots (TDMA), frequency slot (FDMA), coding scheme (CDMA), subsets of subcarriers (OFDMA) etc to each transmitted signal.

The modulation scheme used before transmission of signal and the multiple access scheme used by channel are independent of each other.