What technology is used in the digital high frequency modulation of IEEE802 15 4

Answer]: C Analysis: It is the first wireless LAN transportation network standard proposed by IEEE in 1997, the transmission rate can only reach 2Mbps, which is mainly used for data to let Hui access and answer, and the frequency used is 1-2Mbps bandwidth and 100M.

Yu Na lead answer]: Erect the eggplant posture d

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The three spread-spectrum techniques used are frequency hopping FH, direct sequence (DS or DSS), and orthogonal frequency division multiplexing (OFDM). Direct sequences (DS or DSS) are used in periods

and orthogonal frequency division multiplexing (OFDM) is used

IEEE is the abbreviation of the English Institute of Electrical and Electronics Engineers, and its Chinese translation is the Institute of Electrical and Electronics Engineers. Headquartered in the United States, the association develops data communication standards and other standards. The IEEE802 committee is responsible for drafting a draft of the LAN and sending it to the American National Standards Institute (ANSI) for approval and standardization in the United States.

The IEEE also sent the draft to the International Organization for Standardization (ISO). ISO refers to this 802 specification as the ISO 802 standard, and as such, many IEEE standards are also ISO standards. For example, a standard is an ISO standard.

The IEEE 802 specification defines how a network card can access the transmission medium (such as optical cable, twisted pair, wireless, etc.), how to transmit data on the transmission medium, and also defines the ways in which connections between network devices transmitting information are established, maintained, and removed. Products that comply with the IEEE 802 standard include network cards, bridges, routers, and other components used to establish local area networks.

Answer]: BIEEE802 is a series of local area transport and spring network standard implicit series, broadband LAN access control method side resistance and physical layer specification.

Answer]: B spread spectrum refers to a kind of signal modulation technology that broadens the signal bandwidth through a certain mechanism at the physical layer and turns the original narrowband digital signal into a broadband transmission signal. According to Shannon's formula c = w log2(1+s n), (where c channel state segment is good capacity, w band width, s n signal-to-noise ratio.

In the case of a certain capacity c of the signal-burning channel, increasing the frequency band width can transmit information under the condition of a lower signal-to-noise ratio, and can also effectively reduce the noise power spectral density, thereby improving the anti-dry-sail groundbreaking ability. FHSS and DSSS are the most common spread spectrum techniques. The basic operation process of the FHSS system is as follows:

The transmitter first modulates the information data into a baseband signal, and then enters the carrier frequency modulation stage. At this time, the carrier frequency is controlled by a pseudo-random code generator, and the bandwidth of the baseband is extended to the bandwidth of the transmitted signal in a given bandwidth much greater than that of the baseband signal. Next, the frequency hopping signal is sent by the antenna; After receiving the frequency hopping signal, the receiver first extracts the synchronization information from it, so that the pseudo-random sequence control frequency hopping of the local machine is synchronized with the received frequency hopping, so that the data carrier can be obtained, and the transmitter sends out the information after the carrier demodulation (i.e., spread spectrum demodulation). The DSSS spread-spectrum method is as follows: firstly, a composite code sequence is obtained by using the high-rate pseudo-noise (PN) code sequence and the information code sequence as modulo two addition (waveform multiplication) operation; This composite code sequence is then used to control the carrier phase to obtain the DSSS signal.

In this way, the original higher power and narrower frequency can be changed into a wide frequency and low power frequency, so that the system has better anti-noise interference ability. DSSS uses pseudo-noise code sequence rate that cannot be lower than the original data information sequence rate.