Electronic Design

PAN Standard Uses Chirp Modulation For Location Systems

There’s now one more standard to muddy the waters for wireless system design. But on a more positive note, there’s one more standard to add to your already full portfolio of options to design short-range systems. The most familiar 802.15.4 method is what’s generally known as ZigBee. The new 802.15.4a version is part of the personal-area networking (PAN) category in IEEE’s wireless standards mix. It uses chirp modulation instead of the usual binary phase-shift keying (BPSK) modulated direct-sequence spread spectrum (DSSS) of 802.15.4. But chirp is a form of spread spectrum as well. Originally used in radar systems in the 1960s, it’s still widely used today. Chirp refers to varying the frequency of the carrier for a brief carrier pulse duration. In an up-chirp, the frequency starts low and is increased to some maximum. A down-chirp is the opposite, where the carrier pulse starts at a high frequency and is gradually decreased to some minimum. The frequency variation may be linear or geometric, with the linear variation being more common. One approach uses an up-chirp to represent a binary 1 and a down-chirp for a binary 0. The result is a spread-spectrum kind of system that can be used in a multiple-access mode to support multiple users in the same spectrum. The new standard operates in the 2.4-GHz ISM band and supports data rates up to 2 Mbits/s. The chirp pulses have a duration of 1 µs. At 2 Mbits/s, they use an 80-MHz bandwidth. The multiple-access system is known as multidimensional multiple access (MDMA). German company Nanotron Technologies completed work on the 802.15.4a chirp standard. While it can be used to transmit data in peer-to-peer, star, and mesh networks, it primarily target Real Time Location Systems (RTLS) that can be used to pinpoint the physical position of the radio in a network. RTLS locates the position of radio nodes. GPS is the best example, but it uses an expensive receiver and is overkill for many applications. A simpler RTLS would locate the node relative to some other fixed-location radio. The E911 location systems in GSM cell sites use a method called time difference of arrival (TDoA). Other systems use RFID tags and readers or Wi-Fi access points as reference points to locate other radios by a variety of methods. Nanotron developed a new method called symmetric double-sided two-way ranging (SDS-TWR), which is based on another RTLS method called time of flight (ToF). SDS-TWR measures the distance between two wireless nodes using chirp spread spectrum (CSS). It is accurate to within 1 m. Nanotron has developed a series of chips and modules to implement CSS and MDMA as well as SDS-TWR. As for more on the standard itself, go to the IEEE’s Standards Association Web site. Institute of Electrical and Electronic Engineers www.ieee802.org/15/standards.ieee.org Nanotron Technologies GmbH www.nanotron.com

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