 to country, but typically the 5.15-5.35 GHz band is reserved for indoor use with a maximum EIRP of 200 mW, while the 5.47-5.725 GHz band has an EIRP limit of 1W and is reserved for outdoor use. Table 6-5: US FCC Specified U-NII Channels Used in the 802.11 a OFDM PHY RF Band Frequency Channel Centre frequency Maximum Range number (GHz) transmit power (GHz) U-NII lower 5.150-5.250 5.180 5.200 5.550 5.240 U-NII middle 5.250-5.350 5.260 5.280 5.300 5.320 U-NII upper 5.725-5.825 5.745 5.765 5.785 5.805 Wireless LAN Standards As part of the global spectrum harmonisation drive following the 2003 ITU World Radio Communication Conference, the 5.470-5.725 GHz spectrum has also been available in the US since November 2003, subject to the implementation of the 802.11h spectrum management mechanisms described in the Section "Spectrum Management at 5 GHz (802.11h), p. 160". Each of the 20 MHz wide channels accommodates 52 OFDM subcarriers, with a separation of 312.5 kHz (= 20 MHz/64) between centre frequencies. Four of the subcarriers are used as pilot tones, providing a reference to compensate for phase and frequency shifts, while the remaining 48 are used to carry data. Four different modulation methods are specified, as shown in Table 6-6, which result in a range of PHY layer data rates from 6 Mbps up to 54 Mbps. The coding rate indicates the error-correction overhead that is added to the input data stream and is equal to m/(m+n) where n is the number of error correction bits applied to a data block of length m bits. For example, with a coding rate of 3/4 every 8 transmitted bits includes 6 bits of user data and 2 error correction bits. The user data rate resulting from a given combination of modulation method and coding rate can be determined as follows, taking the Table 6-6: 802.11a OFDM Modulation Methods, Coding and Data Rate Modulation Code bits per Code bits per Coding rate Data bits per Data rate subcarrier (Mbps) symbol symbol 16-QAM 16-QAM 64-QAM 64-QAM Chapter Six 64-QAM, 3/4 coding rate line as an example. Durin