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Publication Number: 453039 Rule Parts: 15.247 Publication Date: 03/23/2007
Keyword: Bluetooth equipment, Spread Spectrum Digital Transmission System

First Category: Radio Service Rules
Second Category: Part 15 Intentional Radiators

Question:   Section15.247 (f) states; "...the digital modulation operation of the hybrid system, with the frequency hopping turned off, shall comply with the power density requirements of paragraph (d) of this section."  Do these requirements apply to Bluetooth equipment?

Answer:   A bluetooth device may operate under Section 15.247 as either a Digital transmission system, a Frequency hopping system or a Hybrid system, whichever provides an advantage to the grantee as long as all the requirements are met. Please note the following based on the requirements for Hybrids and clarification of spread spectrum systems regarding the recent changes to the spread spectrum rules.
Before the recent changes to the spread spectrum rules, there were two major types of spread spectrum systems. They were direct sequence and frequency hopping spread spectrum (FHSS) systems. These two distinct systems were required to comply with separate rule requirements. The new rules, however, allow manufacturers more flexibility and are not as limiting.
The new rules provide more flexibility for manufacturers by eliminating the requirement to employ direct sequence modulation techniques along with its associated requirement to comply with a minimum processing gain. Instead, manufacturers may now employ wideband digital modulation under the new rules for Digital Transmissions Systems (DTS).
It is possible for a device to be designed to operate as a DTS, as a FHSS system, or using a combination of these two modulation types. Because of this, the Commission has received several requests for interpretations by manufacturers that wish to produce transmission systems that employ both frequency hopping (or channel changing) techniques using digitally modulated channels. We believe that such systems fall under three possible combinations of standards, depending on the exact methods of modulation.
First example: We will allow a manufacturer of a combination DTS and FHSS system to demonstrate compliance with the rules required for DTS operation or for FHSS operation. There is no need to demonstrate compliance with both the FHSS standards and the DTS standards.
Second example: Systems may employ two mutually exclusive operational modes. One mode would be as a FHSS system and the other would be as a DTS. For example, a device may be operated as a FHSS system while transmitting data and as a DTS while in the acquisition mode. When operating in this manner, the device must fully comply with the rules for a FHSS system when operating in that mode and as a DTS when operating in that mode. The two types of operation must be distinct so that each mode of operation can be distinguished and separately demonstrated to comply with the pertinent standards.
Third example: The third method of authorizing a combination system is as a hybrid system under the provisions described in Section 15.247(f) of the rules. Before the new rules on DTS operation, a hybrid system consisted of a transmission system that employed a combination of both direct sequence and frequency hopping techniques. Such systems were required to show compliance with a 17 dB processing gain. This is no longer required since the processing gain requirement has been replaced by the DTS regulations. A hybrid system uses both digital modulation and frequency hopping techniques at the same time on the same carrier. This is similar to the combination DTS/FHSS system described above in the first example but the system is subject to slightly different standards. As shown in Section 15.247(f), a hybrid system must comply with the power density standard of 8 dBm in any 3 kHz band when the frequency hopping function is turned off. The transmission also must comply with a 0.4 second/channel maximum dwell time when the hopping function is turned on. There is no requirement for this type of hybrid system to comply with the 500 kHz minimum bandwidth normally associated with a DTS transmission; and, there is no minimum number of hopping channels associated with this type of hybrid system.
However, the hopping function must be a true hopping system, as described in Section 15.247(a)(1). The specific requirements in Section 15.247(a)(1) are: 1) a minimum channel separation; 2) pseudo-random hop sequence; 3) equal use of each frequency; 4) receiver matching bandwidth and Synchronization; The additional requirements in Section 15.247 for a hybrid transmitter include the requirements the 1 watt output limit and RF safety requirements in Section 15.247(b) and the spurious emission limits of Section 15.247(c).


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