Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 27

Although energy detection is the most common spectrum sensing technique, time, frequency, and power spectrum sensing are covered first in this chapter to emphasize the multidimensional aspects of spectrum sensing. The idea behind this spectrum sensing technique is to create multidimensional spectrum awareness. The simplest form of this spectrum sensing technique is a two‐dimensional spectrum sensing that uses the frequency and time dimensions. Figure 2.2 shows this two‐dimensional spectrum sensing where the spectrum sensor looks for occupancy of certain frequency bands at certain times.

Figure 2.2 Two‐dimensional spectrum sensing.

A secondary user can use this spectrum sensing technique where it can hop to a different frequency band once it detects another user on a frequency it is using. This technique does not consider the signal power and relies on DSA defining a cutoff RSSI level to consider a frequency band as occupied or can be opportunistically used. The cutoff RSSI level may also be an estimation of additive white Gaussian noise (AWGN) without the presence of any signal.

If a communications system is to consider underlay or overlay transmission, the two‐dimensional spectrum sensing in Figure 2.2 can be turned into three‐dimensional spectrum sensing as shown in Figure 2.3. In this case, the power dimension is added. In Figure 2.3, the unoccupied areas are often referred to as spectrum holes or white spaces.

Figure 2.3 Three‐Dimensional spectrum sensing.

Notice that more dimensions can be added to this multidimensional spectrum sensing technique. For example, the spreading code can be sensed and made a fourth dimension. Spreading code sensing can show opportunistic transmission based on using specific spreading codes. Spectrum sensing of spreading code is covered in Section 2.3.3 as a signal characteristic.