Published:2011/8/18 2:08:00 Author:Amy From:SeekIC
Stefan Tauschek
Spread-spectrum techniques
This problem can be solved relatively easily by taking a peek at the techniques used by (he military and copying their well-proven strategies. The magic word here is ’spread spectrum’, which amounts to modulating a narrow-band information signal in order to significantly increase its bandwidth while at the same time considerably reducing its level. An outstanding signal-to-noise OR gate along with a pseudo-noise<ode |PN code) signal before they reach the antenna (Figure 2). This causes the transmitted power level to drop below the noise limit, so the signal can only be picked up by receivers that use the same PN code. To give an example, if a 2-Mbit/s signal is spread using an eleven-bit PN code, (he resulting RF signal has a bandwidth of 22 MHz.
One example of an ISM transmitter IC using DS5S frequency spreading is the Micro linear type M12724 (Figure 3). This 32-pin IC, which operates from a 3.3-V supply voltage, works in the frequency range of ratio is obtained when the signal is recovered, even if the level of the spread-spectrum information signal is below the general noise floor before recovery. As o rule of thumb, the increase in the signal-to-noise ratio corresponds lo the ratio of the transmitted bandwidth to the bandwidth of the information signal. It’s hardly any wonder that the military are so fond of this technique: not only does it allow them to send messages undetected, since (he radio signals disappear into the noise of the airwaves, but it also enables them to obtain considerably better transmission quality than with conventional systems.
Two methods for generating spread spectrum signals have come lo prevail, and both of them are presently used commercially.
Direct Sequence Spread Spectrum (DSSS)
This technique for increasing the bandwidth occupied by the signal is essentially simple: the information signal is modulated by passing its data bits through on exclusive-2.400-2.485 GHz using simple FSK modulation, and it contains oil the functional elements (VCOs, mixer and data slicer) needed lo construe! a functional radio system using only a few additional components. Using DSSS, it achieves o sensitively of -90 dBm, which gives it a free-space range of significantly more than 100 meters in actual use. The transceiver is normally used without a transmission protocol, with the user data being simply fed serially into the Tx input and taken from the Rx output without any preconditioning or processing for data security. This is adequate for pumping data over the airwaves al up lo 1500 kbit/sec. Although the ML2724 is used in large quantities for game pads and wireless telephones, it also has more tricks up its sleeve. Its spread-spectrum capability makes it multiport-capable, which means that several MI2724 transceivers can communicate with each other concurrently. This capability is characteristic of all spread-spectrum techniques, and applications such as Bluetooth, WIAN and the like would not be possible without it.
Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/18/Wireless_Connectivity__ISM_WLAN_WMAN_Bluetooth_et_al_2.html
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