11.2 HomeRF 1.0 Physical Layer Attributes

A HomeRF packet is created by attaching a preamble to the MAC frame and inserting a 4FSK filed between the MAC frame header and the MAC frame body if the 4FSK modulation is utilized. Data whitening, modulation, and frequency-hopping schemes are very similar to those of an IEEE 802.11 FHSS system. A HomeRF transceiver needs an additional voice/telephone interface. Receiver sensitivities are more relaxed resulting in channel capacities that are higher than those in the FHSS system.

11.2.1 Packet Format

The HomeRF packet format is shown in Figure 11.12. A HomeRF packet consists of a preamble, a PSDU1 (PHY Service Data Unit 1), followed by an optional 4FSK Symbol field, and a PSDU2. The HomeRF preamble has a Sync field of 80 bits and a Start Frame Delimiter of 16 bits.

Figure 11.12. Packet Format

graphics/11fig12.gif

The preamble Sync field contains an alternating zero-one pattern, starting with zero and ending with one, to be used by the receiver to detect a signal and to achieve frequency and timing synchronization with the rest of the received packet. The SFD consists of the binary pattern of 1111 0011 0100 0010 used for start-of-frame indication and transmitted with the leftmost bit first. The variable length PSDU1 field carries the MAC frame header and is transmitted using 2FSK. There is a 4FSK symbol field if the following PSDU2 field is transmitted using 4FSK modulation. The 4FSK symbol field consists of a 128-bit pattern specified by the HomeRF standards and can be used for receiver adjustment. The variable length PSDU2 field carries the MAC frame body including CRC. PSDU2 may be transmitted using either 2FSK or 4FSK modulation.

11.2.2 Data Whitening

To maintain an even DC level across the received signal, the physical layer uses a data-whitening algorithm, called Bias Suppression Encoding, similar to that used for FHSS wireless Ethernet. Both MAC frame header, PSDU1, and MAC frame body, PSDU2, are whitened using a pseudo-random sequence of length 127, which is created based on the generator polynomial S(x) = x7 + x4 + 1. The preamble and the 4FSK Symbol field are not whitened. The original MAC frame is exclusive-ORed with the repetition of this pseudo-random sequence to form the whitened MAC frame. In addition, a stuff symbol is inserted after the preamble and after every 32 whitened MAC frame symbols. The 4FSK Symbol field is bypassed if it exists and a stuff symbol is inserted after the 4FSK Symbol field and after every 32 whitened PSDU2 symbols.

The stuff symbol after the preamble is initialized to 0 or to 00 for 2FSK or 4FSK, respectively, after the 4FSK Symbol field. Meanwhile, an accumulation of bias is established by summarizing weights of each symbol for PSDU1 and PSDU2 separately only if the 4FSK Symbol field exists. For 2FSK, a 1 has a weight of 2 and a 0 has a weight of 2. For 4GFSK, weights are 3, 1, 1, and 3 for 10, 11, 01, and 00 bit combinations, respectively. Whenever the bias accumulation of a 32-symbol block is different than the previous total bias accumulation, the leading stuff symbol and following 32 symbols of the block are all negated for subsequent transmission and the inclusion in the total bias accumulation. The negation of 4FSK symbols is equivalent to reversing the most significant bit of these bit combinations. These negated symbols can be recovered at the receiver by recognizing a 1- or a 10-bit combination at the leading synchronization symbol.

11.2.3 Modulation

A Gaussian filter with a nominal bandwidth period product of 0.5 is used for both 2FSK and 4FSK modulations. For 2FSK modulation, a symbol for a 1 bit is represented by a positive frequency shift of 160 kHz from the carrier frequency and a symbol for a 0 bit is represented by a negative frequency shift of 160 kHz from the same carrier frequency. For 4FSK modulation, there are four possible frequency shifts of ±72 kHz and ±216 kHz. Bit combinations of 10, 11, 01, and 00 are represented by frequency shifts of 216, 72, 72, and 216 kHz, respectively. Bit rates of 1 and 2 Mbps are provided by 2FSK and 4FSK modulations, respectively. For the normal operation, the transmit power is defined to be between 16 and 20 dBm. For the low power operation, the transmit power is defined to be between 0 and 4 dBm. Averaged over an approximate bandwidth of 1.4 MHz accounting for both Gaussian filtering and modulation effects, PSD levels are between 45.5 and 49.5 dBm/Hz or 61.5 and 57.5 dBm/Hz for normal or low power operations, respectively. A HomeRF transceiver can operate on a received signal level of 76 and 62 dBm for 2FSK and 4FSK, respectively. The transceiver can also operate for received signals as strong as 20 dBm.

11.2.4 Frequency Hopping

Within the 2.4-GHz ISM band, 79 frequency channels, at 1 MHz apart, are defined starting with channel 0 of 2.402 GHz and ending with channel 79 of 2.48 GHz. Among these channels, 75 of them, channels 4 78, are used by HomeRF. A base-hopping sequence for HomeRF is defined by

Equation 11.4

graphics/11equ04.gif


where f(I) is the hopping channel number, x is the hop pattern, I is the hop index, and b(I) is the base-hopping sequence as defined by Figure 11.13. Different HomeRF home wireless networks can be established using different hop patterns. When the hop pattern x is zero, a HomeRF network hops following the base-hopping sequence.

Figure 11.13. The Base-Hopping Sequence

graphics/11fig13.jpg

The hopping frequency of HomeRF depends on the size of the superframe, which is defined in the Synchronization field of the type 3 header. The superframe is shared by voice channels and asynchronous data traffic. The HomeRF minimum hopping rate can be calculated based on the TDMA payload size and the transmission throughput of a voice channel. The TDMA payload size is 85 bytes or 680 bits. HomeRF voice channels are encoded with the ADPCM at a bit rate of 32 kbps. Therefore, we need at least 32 x 103/680 = 47.059 packets per second to carry these voice channels. Because a voice packet can only be presented once and repeated once during each hop, the minimum hop frequency is therefore 48 hops per second. The typical HomeRF hopping frequency is 50 hops per second. The typical duration of a HomeRF hop is 20 ms.

Figure 11.14 shows the general structure of a HomeRF transceiver, which consists of an RF front end, a MAC function, a Data/Host Interface, and/or a Voice/Telephone Interface depending on whether it is for an A-node, I-node, or AI-node. Requirements for the RF front end are almost the same as those for an 802.11 FHSS transceiver. The CSMA/CA part of MAC is also very similar to that of IEEE 802.11. The HomeRF MAC requires additional functions for TDMA voice connection management. The Data/Host Interface could be as simple as a serial interface or as advanced as a USB or a PC card interface. ADC, DAC, compander, and ADPCM codec can also be included in the Voice/Telephone Interface.

Figure 11.14. HomeRF Transceiver Structure

graphics/11fig14.gif

A HomeRF transceiver is required to operate on received signal levels of 76 and 62 dBm for 2FSK and 4FSK, respectively. On the other hand, the receiver front-end noise level Pnoise is calculated according to the input resistor thermal noise level, PR = kT = 174 dbm/Hz, the antenna and amplifier noise figure, NF = 14 dB, and signal bandwidth, B = 1.3 x 106 or B = 1.4 x 106. We have

Equation 11.5

graphics/11equ05.gif


Equation 11.6

graphics/11equ06.gif


At signal-to-noise levels of about 36.5 and 23 dB, channel capacities for the HomeRF environment are

Equation 11.7

graphics/11equ07.gif


Equation 11.8

graphics/11equ08.gif




Home Network Basis(c) Transmission Environments and Wired/Wireless Protocols
Home Networking Basis: Transmission Environments and Wired/Wireless Protocols
ISBN: 0130165115
EAN: 2147483647
Year: 2006
Pages: 97

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