很多小伙伴都搞不清楚RSSI、ED、LQI、CCA等概念，那我們就看看802.15.4標準文檔中的定義吧。

802.15.4標準文檔：
《IEEE Std 802.15.4?-2015, IEEE Standard for Low-Rate Wireless Networks》
CSDN下載地址（1積分）：https://download.csdn.net/download/gjh13/10453909
或者官網去下載，免費的。

10.2.5、Receiver ED
The receiver ED measurement is intended for use by a network layer as part of a channel selection algorithm. It is an estimate of the received signal power within the bandwidth of the channel. No attempt is made to identify or decode signals on the channel. The ED measurement time, to average over, shall be equal to 8 symbol periods.

The ED result shall be reported to the next higher layer using MLME-SCAN.confirm. The minimum ED value (zero) shall indicate received power less than 10 dB above the lowest specified receiver sensitivity, in dBm, for the PHY. The range of received power spanned by the ED values shall be at least 40 dB. Within this range, the mapping from the received power in decibels to ED value shall be linear with an accuracy of ± 6 dB.

10.2.6 Link quality indicator (LQI)
The LQI measurement is a characterization of the strength and/or quality of a received packet. The measurement may be implemented using receiver ED, a signal-to-noise ratio estimation, or a combination of these methods. The use of the LQI result by the network or application layers is not specified in this standard.

The LQI measurement shall be performed for each received packet. The minimum and maximum LQI values (0x00 and 0xff) should be associated with the lowest and highest quality compliant signals detectable by the receiver, and LQI values in between should be uniformly distributed between these two limits. At least eight unique values of LQI shall be used.

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