2.2. Receiver and Transmitter
I/Q Transmitters
Direct Upconversion
This direct upconversion is simple, but the modulators (mixers) must operate at excessive high frequency (fRF). At such high frequency, the design of a pair of matched modulators is challenging.
Dual-Upconversion
In this architecture, a signal sIF of intermediate frequency (fIF) is obtained first, then further converted to sRF with a single mixer that takes sIF and local wave of fRF+fIF. The second conversion creates a signal of ωRF and a image of ωIM = ωRF+2ωIF. A bandpass filter can filter out the image.
I/Q Receivers
Dual-Conversion (Superheterodyne) Rx
This receiver architecture stands as the most common architecture in wireless technology. Similar to dual-upconversion transmitter, a intermediate frequency is selected for the ease of designing a matched pair of I/Q demodulators. Here, ωLO1 = ωRF – ωIF. The BW of the two LPFs must be low enough to block 2ωIF, but sufficient to pass the I/Q signals at Rs. Note that typically Rs < 2fIF.
Direct-Conversion (zero-IF) Rx
Similar to direct upconversion architecture for Tx, this architecture is simple, but it requires designing a matched pair of I/Q mixers at a high fRF.
Carrier Recovery
In all coherent (synchronous) demodulation schemes, a replica of the transmitted carrier is required to make possible of the detection of the transmitted data stream at Rx. This carrier replica can be recovered from the received signal itself either by the use of a PLL, or by a simpler method as shown below:
