18 Power-Efficient Last Mile Access
as femtocells and BWA-FMDA have to limit their transmission power to meet
radio spectrum regulations and lower health concerns.
Taking microBS-6 as an example, we can extrapolate its spectral efficien-
cies at P
tx
=10
3
and P
tx
=10
4
mW/MHz per AE as 116.49 and 136.72 bps/Hz,
respectively. The corresponding energy consumption equals 87.54 and 141
watts. Thus, we can calculate the energy efficiency as 1.3307 bit/Joule when
P
tx
=10
3
mW/MHz and 0.9696 bit/Joule when P
tx
=10
4
mW/MHz. There-
fore, there exists a sudden drop on the micro BS energy efficiency for P
tx
larger than certain values in the range [10
3
,10
4
] mW/MHz. In practice, as
P
tx
becomes too large, the excessive micro BS transmit power will cause severe
interference to neighbor ...