Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 29

Let us start from the unit of energy of a communications signal Φ_j(t), which can be defined as follows: If the receiver detects a 1 V signal across a 1 Ω resistor, the integration of the square value of signal voltage over a specific time period (T_g, T_f) is 1, that is, the receiver has detected one unit of energy.⁵ Notice the following:

1 The signal can be constructed in a multidimensional signal‐in‐space (SiS) as a vector.6

2 The time T = Tf − Tg is a critical factor in detecting the signal energy.7 If the signal is too weak, the integration of the square value of the signal voltage may need a long period of time to yield reliable energy detection.

The receiver of a communications signal detects a multicoefficient signal in N‐dimensional SiS and attempts to match the received signal with one of M signals.⁸ The energy detector cares only for the signal energy not the signal decoding.

The signal's ith dimension projected on the kth base can be expressed as follows:

(2.1)

where Φ_k(t) is the signal basis per coefficient.

Notice that both the signal receiver and the spectrum sensor performing energy detection need to carry similar steps to calculate the received signal energy. Chapter 3 covers how to use same‐channel in‐band sensing to hypothesize the presence of an interfering signal. This section is intended to show how to piggyback on the communications receiver's energy calculation to create same‐channel in‐band energy sensing.

Figure 2.4 shows how a communications receiver recovers a signal S_i(t) based on knowing its base per each dimension. The projection of the signal per each dimension is expressed in Equation (2.1). The baseband signal S_i(t) is then mapped to a point in the N‐dimensional SiS. The square distance from the origin to the projected point from Figure 2.4, , is simply the received signal energy. While the communications receiver takes further steps to decode the projected signal based on the squared intra‐signal distances, an outcome of the communications signal receiver,, is energy detection that can be utilized for same‐channel in‐band sensing. Calculating is the common process between signal decoding and signal energy detection. As will become clear in Chapter 3, the purpose of same‐channel in‐band sensing is to decide if the communications signal utilized frequency band suffers from an interference level that requires using a different frequency band or if the interference level is tolerable.

Figure 2.4 Leveraging signal receiver reconstruction of the received signal for same‐channel in‐band spectrum sensing.

A communication receiver also calculates SNIR. The SNIR calculation is based on the energy detection of the signal and the energy detection of the noise (known as the noise floor). Signal energy detection and noise floor energy detection are metrics that can be leveraged for same‐channel in‐band spectrum sensing because they are common between signal decoding and same‐channel in‐band sensing. The value of using SNIR in same‐channel in‐band sensing will become clear in Chapter 3.

While energy detection is a natural outcome of signal decoding, building specialized hardware for spectrum sensing can perform energy detection in different ways. This specialized hardware, which is sometimes referred to as an augmented sensor, may or may not have prior knowledge of the signal bases. The following subsections show different methods that can be utilized by the augmented sensors to perform energy detection.