Case Study (ZigBee): Phase IV

Transmitter & Receiver SimulationTransmitter & Receiver Simulation

The standard specifies the following four PHYs:1. An 868/915 MHz direct sequence spread spectrum

(DSSS) PHY employing binary phase-shift keying (BPSK) modulation

2. An 868/915 MHz DSSS PHY employing offset quadrature phase-shift keying (O-QPSK) modulation

3. An 868/915 MHz parallel sequence spread spectrum (PSSS) PHY employing BPSK and amplitude shift keying (ASK) modulation

4. A 2450 MHz DSSS PHY employing O-QPSK modulation

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2.4 GHz PHY – channels 11-265 MHz

2.4 GHz 2.4835 GHz

902 MHz 928 MHz

Channels 1-10

868.3 MHz

Channel 0

868 / 915 MHz PHY

2 MHz

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The functional block diagram below is provided as a reference for specifying the 868/915 MHz band BPSK PHY modulation and spreading functions. Each bit in the PPDU shall be processed through the differential encoding, bit-to-chipmapping and modulation functions in octet-wise order, beginning with the Preamble field and ending with the last octet of the PSDU.

Differential encoding is the modulo-2 addition (exclusive or) of a raw data bit with the previous encoded bit. This is performed by the transmitter and can be described by the following Equation:

En = Rn ⊕ En–1

where Rn is the raw data bit being encoded En is the corresponding differentially encoded bit En–1 is the previous differentially encoded bit

For each packet transmitted, R1 is the first raw data bit to be encoded and E0 is assumed to be zero.

Conversely, the decoding process, as performed at the receiver, can be described by:

Rn = En ⊕ En–1

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Each input bit shall be mapped into a 15-chip PN sequence as specified in the below Table:

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The chip sequences are modulated onto the carrier using BPSK with raised cosine pulse shaping (roll-off factor = 1) where a chip value of one corresponds to a positive pulse and a chip value of zero corresponds to a negative pulse. The chip rate is 300 kchip/s for the 868 MHz band and 600 kchip/s in the 915 MHz band.

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The raised cosine pulse shape (roll-off factor = 1) used to represent each baseband chip is described by:

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Discrete Multipath fading channel The channel parameters are:

1- Sampling time is 1/(20KHz * 15 bits/code * 16 samples/bit) = 2.1e-7 2- Maximum Doppler frequency:

▪ Indoor: λ= 3000000/868000000 = 0.3452 m let the speed indoor be 2 m/sec

so fd = 2/ λ = 6 Hz Outdoor:

let the speed outdoor be 100 km/hr = 28 m/sec so fd = 28/ λ = 80 Hz 3- Path delays: We can make multiple paths 4- Power for each path is given as pdp. Delay between paths = 8 samples = 0.5 *Ts

Signal Bandwidth (Lowpass equivalent) Bs = 10 kHz Symbol time, Ts = 1/Bs = 0.1 msec Data Rate = 20k sym/sec Sampling rate = 4.8M samples/sec Samples/symbol = 16

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First, random data is generated. The data is differentially encoded. DSSS (bit to code) mapping is done according to the table in the

standard. Then BPSK modulation is done. Data is upsampled by 16 (16 samples per symbol). Modulated data is filtered using pulse shaping filter of raised

cosine with rolloff factor = 1 The delay of because of the pulse shaping filter is 128 samples.

The pulse shaped data is passed through fading channel. AWGN noise is added. Data is downsampled. The delay for the faded channel and the

pulse shaping filter is compensated. Data is demodulated using BPSKDemodulation. Data is decoded for the differential encoding done at the

transmitter. BER is computed

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One-path FC three-path FCfive-path FC

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IEEE standard for 802.15.4 2006

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THANK YOU

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