Published:2011/8/18 2:16:00 Author:Amy From:SeekIC
Stefan Tauschek
Code Division Multiple Access (CDMA)
With the DSSS technique, frequency spreading is achieved by modulation using a pseudo-noise signal. This signal is also called the ’PN code’, since it identifies an individual transmitter just like an identification code. The information signal can only be filtered out of the airwaves using the same PN code, with all other signals remaining below the noise limit. This makes il possible for several DSSS transceivers using different PN codes to shore the same frequency band and communicate with each other concurrently without causing any problems. This technique is called ’code division multiple access’ (CDMA), which essentially means ’multiple channels using code separation’.
Frequency hopping
The other important technique besides DSSS is frequency hopping |FH|. As the best-known representative of this technique, Bluetooth divides the ISM band between 2402 MHz and 2483.5 MHz into 79 channels, each with a width of I MH*. The signal hops (jumps) between channels in a cyclic pattern at the rate of 1600 hops per second. This means that typical narrow-band interference sources can cause only minor damage to the transmitted data. Figure 4 shows how several Bluetooth nodes can share the some frequency bond using different frequency-hopping sequences. In principle, the frequency-hopping sequence for the FH technique corresponds to the PN code for DS5S, since in each case mutual interference among the individual nodes is prevented by using a different spread-spectrum sequence for each node.
The FH technology used for Bluetooth demands quite precise channel frequency settings, with a tolerance of ±75 khz, and exact compliance with the scheme of hopping to a new frequency every 625 ns. Due to these technically demanding requirements, it took several years of intensive development work before the first single-chip Bluetooth transceiver reached the market at a price suitable for’ mass-consumption products.
Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/18/Wireless_Connectivity__ISM_WLAN_WMAN_Bluetooth_et_al_3.html
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