Wireless Transceiver Architecture
Wireless Transceiver Architecture
The Digital Communications Point of View
When detailing how to dimension a transceiver, it can seem natural to first clarify what is expected from such a system. This means understanding both the minimum set of functions that need to be implemented in a transceiver line-up as well as the minimum performance expected from them. In practice, these requirements come from different topics which can be sorted into three groups. We can indeed refer to the signal processing associated with the modulations encountered in digital communications, to the physics of the medium used for the propagation of the information, and to the organization of wireless networks when considering a transceiver that belongs to such system, or alternatively its coexistence with such systems.
The last two topics are discussed in Chapter 2 and Chapter 3 respectively, while this chapter focuses on the consequences for transceiver architectures of the signal processing associated with the digital communications. In that perspective, a first set of functions to be embedded in such a system can be derived from the inspection of the relationship that holds between the modulating waveforms used in this area and the corresponding modulated RF signals to be propagated in the channel medium.
As a side effect, this approach enables us to understand how information that needs a complex baseband modulating signal to be represented can be carried by a simple real valued RF signal, thus leading to the key concept of the complex envelope. It is interesting to see that this concept allows us to define correctly classical quantities used to characterize RF signals and noise, in addition to its usefulness for performing analytical derivations. It is therefore used extensively throughout this book.
Finally, in this chapter we also review some particular modulation schemes that are representative of the different statistics that can be encountered in classical wireless standards. These schemes are then used as examples to illustrate subsequent derivations in this book.
1.1 Bandpass Signal Representation
1.1.1 RF Signal Complex Modulation
Digital modulating waveforms in their most general form are represented by a complex signal function of time in digital communications books . But, even if we understand that this complex signal allows us to increase the number of bits per second that can be transmitted by working on symbols using this two-dimensional space, a question remains. The final RF signal that carries the information, like the RF current or voltage generated at the transmitter (TX) output, is a real valued signal like any physical quantity that can be measured. Accordingly, we may wonder how the information that needs a complex signal to be represented can be carried by such an RF signal. Any RF engineer would respond by saying that an electromagnetic wave has an amplitude and a phase that can be modulated independently. Nevertheless, we can anticipate the discussion in Chapter 2, and in particular in Section 2.1.2, by saying that there is nothing in the electromagnetic theory that requires this particular structure for the time dependent part of the electromagnetic field. In fact, the right argument remains that this time dependent part, like any real valued signal, can be represented by two independent quantities that can be interpreted as its instantaneous amplitude and its instantaneous phase as long as it is a bandpass signal. Here, "bandpass signal" means that the spectral content of the signal has no low frequency component that spreads down to the zero frequency. In other words, the spectrum of the RF signal considered, whose positive and negative sidebands are assumed centered around ± c, must be non-vanishing only for angular frequencies in [ - u - c, - c + l] [ + c - l, + c + u], with c, l and u defined as positive quantitie