Listen-before-talk, a "polite" communication protocol, scans the channel for activity before initiating a transmission. Also called clear-channel-assessment (CCA), systems using it with frequency hopping have no duty-cycle limitations. Bandwidths of up to 7 MHz are available once listen-before-talk (LBT) or duty-cycle limits are met, as compared to the 2-MHz range available previously.
Wideband Modulation: DSSS
Besides FHSS, direct-sequence spread-spectrum (DSSS) is also addressed in the new European regulations. In a DSSS system, a narrow-band signal is multiplied by a high-speed sequence of pseudo-random numbers (PRNs) to generate a spread signal. Each PRN pulse is called a "chip," and the sequence rate is called the "chip rate." The extent to which the original narrow-band signal is spread is referred to as the processing gain; it is the ratio of the chip rate (Rc) to the narrow-band data symbol rate. Frequency spectra of FHSS and DSSS are compared in Figure 2.
2. Frequency spectra for FHSS and DSSS.
At the receiver, the incoming spread-spectrum signal is multiplied by the same PRN code to de-spread the signal, allowing the original narrow-band signal to be extracted. At the same time, any narrow-band interferers at the receiver are spread and appear to the demodulator as wideband noise. The allocation of different PRN codes to each user in the system allows isolation between users in the same frequency band. This is known as code-division multiple access (CDMA).
A few examples of systems using DSSS modulation include IEEE 802.15.4 (WPAN), IEEE 802.11 (WLAN), and GPS. The main advantages of DSSS are its interference-rejection capability, low power spectral density, resistance to jamming, and mitigation of multipath effects.
Other Wideband Modulation
An interesting aspect of the new European regulations is that they provide for other wideband spread-spectrum modulation schemes in addition to FHSS and DSSS. FSK/Gaussian frequency-shift-keying (GFSK) modulation, with an occupied bandwidth greater than 200 kHz, is considered wideband modulation under the European regulations.
Some wideband ISM-band transceiver ICs, such as the ADF7025, can use FSK modulation to take advantage of this new wideband standard. To operate in the 865-MHz to 870-MHz sub-band, the design must comply with the maximum occupied bandwidth (99%) and maximum power density limits. An edge-of-channel (or band) maximum power limit of -36 dBm is also specified. Using wideband modulation, a 384-kbps data rate allows transmission of audio and medium quality video (several frames per second) in the sub-1-GHz European ISM frequency bands.
3. Results for wideband-modulation using the ADF7025: (a) FSK modulated signal, 99%-occupied bandwidth measurement, (b)zoomed-in view of (a) to measure maximum power spectral density.
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The U.S. regulation (FCC Part 15.247) has an allocation similar to that of Europe, which provides for frequency-hopping systems operating in the 902 MHz to 928 MHz, 2400 MHz to 2483.5 MHz, and 5725 MHz to 5850 MHz bands, while also providing for "digitally-modulated" signals. This covers both spread-spectrum (DSSS) and other simpler forms of modulation (such as FSK, GFSK), and is thus similar to the "wideband modulation" specification in the European regulations.
The two main requirements are a 6-dB bandwidth of 500 kHz minimum, and a power spectral density conducted from the intentional radiator to the antenna of 8 dBm maximum in any 3 kHz band during any time interval of continuous transmission. A system other than FHSS would normally be limited to a field strength of 50 mV/m (-1.5 dBm ERP), but the maximum output power is 1 W when using "digital modulation," once the maximum power spectral density limit is met.
Using flexible transceivers, with "digital modulation" mode as defined in the U.S. standards and the "wideband modulation" mode as defined in the new European regulations, similar wideband modulated systems can now be employed in both the U.S. and Europe, thus simplifying the design of products intended for worldwide markets.