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a question I'm asking (perhaps its stupid) is that, on BLE (Bluetooth Low Energy) why do we need 40 channels?.

37 of these channels are used for connection data and the last three channels (37, 38, and 39) are used as advertising channels to set up connections and send broadcast data.

nrf51 device of Nordic for example has only one receiver available in the chip, so it can listen only to one frequency at a time? so why 37??.

what's the purpose of multiple frequencies while we can do communication with one channel (frequency)

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If there were just two channels (one for uplink, one for downlink) you could only connect to one device at a time, and you could not operate multiple Bluetooth Low Energy devices simultaneously (unless some arbitration system was implemented where devices would take turns at transmitting at the expense of data rate).

Multiple channels not only permit multiple BLE links to operate concurrently in the same area, they also enable the use of frequency hopping, where all BLE devices rapidly change channels in a predetermined pattern (direct sequence spread spectrum). This rapid switching of channels makes it difficult for any single source of interference to jam any of the data links.

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    \$\begingroup\$ @MakhloufGharbi I already explained that too, to avoid interference. "Interference" is anything that modifies or overpowers the data while it is being transmitted to its destination. If you are trying to listen to somebody else speaking and I constantly yell "LALALA" to your ear, I'm interfering with your communication. \$\endgroup\$ – jms Feb 20 '16 at 12:09

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