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Differential Detection of a IEEE 802.15.4 Signal

IP.com Disclosure Number: IPCOM000021736D
Publication Date: 2004-Feb-04
Document File: 3 page(s) / 218K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method that reduces receiver complexity by detecting the offset QPSK (OQPSK) without the need for frequency and phase synchronization. Benefits include lower power consumption and cost savings.

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Differential Detection of a IEEE 802.15.4 Signal

Disclosed is a method that reduces receiver complexity by detecting the offset QPSK (OQPSK) without the need for frequency and phase synchronization. Benefits include lower power consumption and cost savings.

Background

The conventional method of detecting an OQPSK signal is by coherent demodulation, which requires accurate phase synchronization. Also, the frequency offset must be within roughly 2 kHz. With the 40 ppm crystals specified by the 802.15.4 standard, the frequency offset could be as high as 100 kHz, making AFC mandatory. While coherent demodulation is optimal for the AWGN channel, it requires significantly greater complexity for synchronization.

General Description

Two of the key design objectives for a wireless sensor network platform are low power and low cost. The disclosed method addresses these key issues in the design of the 802.15.4 receiver. Unlike the conventional method of coherent detection of the offset of the QPSK (OQPSK), the disclosed method eliminates the need for frequency and phase synchronization. The disclosed method is within 3 dB of the optimal receiver.

Figure 1 shows the mapping from the transmit bits to the modulated waveform. The binary bits entering the encoder are grouped into 4-bit blocks, which are mapped onto 16 possible symbols. Each symbol is mapped to a 32-chip long sequence, according to Table 1. The chip sequences in Table 1 were chosen to be mutually semi-orthogonal to each other, allowing the receiver to distinguish the symbols by correlation. In the final step of the encoding, the chip sequence is modulated with OQPSK waveforms.

Figure 2 shows the received signal on a polar plot. The markers indicate the constellation points at the optimum timing instants, and the lines are the transitional values of the waveform in between the optimum timing instants. The cross markers scattered around +/-1 represent the even index chips, and the circle markers scattered around +/-j represent the odd index chips.

Figure 3 is an overview of the entire disclosed method.

Advantages

The disclosed method performs approximately 3 dB better than the current differential decoder.

Fig. 1

Data Symbol (v)

Binary Bits...