Cambium Networks cnReach N500 handleiding

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cnReach N500 User Guide

Technical Reference

phn-4673_002v000 (March 2018)

3-8

Frequency Hopping (900 MHz ISM Only)

Frequency hopping is used with 900 MHz cnReach radios using the ISM unlicensed band. With

frequency hopping, the radio never dwells very long on a given channel but instead ‘hops’ on a pattern

defined in the configuration thereby avoiding interference. Note that for licensed channels

Note

Frequency hopping is ONLY used with 900 MHz Unlicensed/ISM bands. If the network is

deployed in a licensed band, it stays on the same frequency. The hopping features

described in this section are not relevant for licensed band operation.

Hop Channels

The occupied bandwidth of the RF signal is determined by the bit rate and modulation mode that is enabled. If

multiple modes are enabled, the occupied bandwidth is that of the highest-bandwidth mode.

The number of available hop channels is the number of whole “occupied bandwidth” channels that will fit in the band

allocated for hopping, avoiding any excluded band and FCC-mandated guard bands at the ISM band edges.

A hop channel is referred to by its center frequency, and occupies the band from (center frequency – occupied

bandwidth/2) to (center frequency + occupied bandwidth/2). It may also be referred to by its channel number,

where channel zero is the lowest frequency channel.

The radio displays the occupied bandwidth and number of available hop channels at the top of the RF and Hopping

Menu.

Pseudorandom Hop Pattern

When the hop pattern parameter is 1, the radio uses a linear feedback shift register to build a table of N

pseudorandom channel numbers between 0 and N-1, where N is the number of hop channels.

For example, if the number of hop channels is 20, the sequence of hop channel numbers in the table is:

19, 9, 4, 14, 16, 10, 18, 13, 6, 17, 8, 12, 15, 7, 3, 1, 0, 11, 5, 2.

Constant-Step Hop Pattern

When the hop pattern parameter is greater than 1, the radio builds a table of N channel numbers between 0 and N-1

that step by at least P channels for each hop, where N is the number of hop channels and P is the value given by the

hop pattern parameter. The radio makes the necessary adjustments so the hop pattern doesn’t repeat prematurely.

For example, if the number of hop channels is 20 and the hop pattern is 5, the sequence of hop channel numbers in

the table is:

0, 5, 10, 15, 1, 6, 11, 16, 2, 7, 12, 17, 3, 8, 13, 18, 4, 9, 14, 19.

Hop Processing

For either type of hop pattern:

• Each channel number appears in the table exactly once.

• The starting point of the hop sequence within the table is given by the hop offset parameter, where zero

indicates the first entry.

• The radio steps through the table, one step per hop, fetching the next channel number and converting it to

the corresponding center frequency.

• When it reaches the end of the table, the hop sequence returns to the beginning of the table.

• If multi-master sync is enabled, at each sync pulse the hop pattern restarts at the table location indicated by

the hop offset parameter.

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