计算机科学论文代写 Direct Sequence Spread Spectrum

Benefits of Direct-sequence spread spectrum

Resistance to intended or unintended jamming

Sharing of a single channel among multiple users

Reduced signal/background-noise level hampers interception (stealth)

Determination of relative timing between transmitter and receiver

Uses of Direct-sequence spread spectrum

The United States GPS and European Galileo satellite navigation systems

DS-CDMA (Direct-Sequence Code Division Multiple Access) is a multiple access scheme based on DSSS, by spreading the signals from/to different users with different codes. It is the most widely used type of CDMA.

Cordless phones operating in the 900 MHz, 2.4 GHz and 5.8 GHz bands

IEEE 802.11b 2.4 GHz Wi-Fi, and its predecessor 802.11-1999. (Their successor 802.11g uses OFDM instead)

Automatic meter reading

IEEE 802.15.4 (PHY and MAC layer for ZigBee)

Multi-carrier code division multiple access

Multi-Carrier Code Division Multiple Access (MC-CDMA) is a multiple access scheme used in OFDM-based telecommunication systems, allowing the system to support multiple users at the same time.

MC-CDMA spreads each user symbol in the frequency domain. That is, each user symbol is carried over multiple parallel subcarriers, but it is phase shifted (typically 0 or 180 degrees) according to a code value. The code values differ per subcarrier and per user. The receiver combines all subcarrier signals, by weighing these to compensate varying signal strengths and undo the code shift. The receiver can separate signals of different users, because these have different (e.g. orthogonal) code values.

Since each data symbol occupies a much wider bandwidth (in hertz) than the data rate (in bit/s), a signal-to-noise-plus-interference ratio (if defined as signal power divided by total noise plus interference power in the entire transmission band) of less than 0 dB is feasible.

One way of interpreting MC-CDMA is to regard it as a direct-sequence CDMA signal (DS-CDMA) which is transmitted after it has been fed through an inverse FFT (Fast Fourier Transform)

Rationale of MC-CDMA

Wireless radio links suffer from frequency-selective channels. If the signal on one subcarrier experiences an outage, it can still be reconstructed from the energy received over other subcarriers.

Downlink of MC-CDMA

In the downlink (one base station transmitting to one or more terminals), MC-CDMA typically reduces to Multi-Carrier Code Division Multiplexing. All user signals can easily be synchronized, and all signals on one subcarrier experience the same radio channel properties. In such case a preferred system implementation is to take N user bits (possibly but not necessarily for different destinations), to transform these using a Walsh Hadamard Transform, followed by an I-FFT.

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