Эротические рассказы

Dynamic Spectrum Access Decisions. George F. ElmasryЧитать онлайн книгу.

Dynamic Spectrum Access Decisions - George F. Elmasry


Скачать книгу

      Notice the difference between time‐domain cyclic marks and frequency domain cyclic marks. OFDM signals use a frequency‐domain cyclic prefix, which protects the OFDM signals12 from inter‐symbol interference. This cyclic prefix can be utilized in a frequency‐domain based correlation technique to affirm the presence of the targeted signal. If the targeted signal uses a preamble of symbols, this preamble can be utilized to affirm the presence of the signal in time‐domain correlation. Both cyclic prefix and time‐domain preamble can help the autocorrelation capable sensor decrease the probability of misdetection and the probability of false alarm when making a decision regarding the presence of the targeted signal.

      2.4.3 Spreading Code Spectrum Sensing

      Spread spectrum is implemented differently in commercial signals than in military signals. Commercial signals tend to use direct sequence spread spectrum, which is easy to sense, while military signals tend to use frequency hopping spread spectrum, which is intentionally made hard to detect.

Schematic illustration of the direct sequence spread spectrum modulation. Schematic illustration of the frequency-hopping spread spectrum signal modulation.

      With Figure 2.11, a pseudorandom generator, clocked at the hopping rate Rc, feeds the frequency synthesizer. The frequency synthesizer generates the hopping carrier frequency for the modulator expressed as cos(ωit + θ). Notice that the binary stream Xnis modulated over the carrier with frequency ωi, which corresponds to the ith slot of the N slots available for hopping. The hopping rate is determined by Rc.

      When autocorrelation detection is used and the presence of the targeted signal is hypothesized to exist, the spectrum sensor can use a chip code generator to generate all the chip codes the targeted signal is known to use. One of the chip codes that is used by the targeted signal will result in the highest correlation with the sensed signal. The spectrum sensor may be able to help the local DSA decision fusion agent pinpoint which chip code is not used in a certain vicinity at a certain time, allowing for the opportunistic use13 of the sensed frequency band only with an unused chip code.

      2.4.4 Frequency Hopping Spectrum Sensing

Schematic illustration of the fast hopping where three hops occur during the modulation of one symbol.

      2.4.5 Orthogonality Based Spectrum Sensing

      This type of spectrum sensing is common among cooperative spectrum users of the same signal. With orthogonal spectrum sensing, the decision‐making entity can be distributed, centralized or hybrid, as introduced in Chapter 1. The centralized decision‐making entity is sometimes referred to as the decision fusion center (DFC). The decision‐making process attempts to exploit signal orthogonality for cooperative spectrum use while mitigating the effect of fading, shadowing, out‐of‐range, and other factors that can increase the probability of false alarm and the probability of misdetection.

Schematic illustration of the cooperative spectrum sensing with MIMO DFC.

      With this cooperative mode, the ROC decision‐making process explained in the next chapter is altered to a complementary receiving operating characteristics


Скачать книгу
Яндекс.Метрика