Doing simple (and may be innacurate) maths based on https://sarwiki.informatik.hu-berlin.de/Packet_transmission_time_in_802.11, this is the expected performance on an ideal non TDD system (those which ACK each packet like airmax, they claim to be tdd but tdd is not what they do) and simplified for 1 CPE, for the best modulation MCS15, counting only DL data, no UL user layer (eg. tcp acks), etc
For example, for the mean download packet size (~800 bytes), and supposing same SIFS/DIFS times as 802.11a (for n they may be some uS lower):
Tx AP -> SM 800 bytes = 97 uS
TX SM -> AP ack = 57 uS
this is for distance=0 and continous transmit, then doing simple maths, if the raw transmit rate is 150 Mbit/s => the "real" transmit rate (counting, sifs, difs, etc, not the ip overhead) (96 uS for 800 bytes) =>
800*8bit/pkt*1pkt/(96+57)uS*1e6uS/1S*1Mbit/1e6bit => ~42 Mbit/s
if d=1km => add ~7us travel time => 40 Mbit/s
if d=10 km => add ~70 uS rtt => 28 Mbit/s !
if d=20 km => add ~140 uS rtt => 19 Mbit/s !!
these are IDEAL figures for 1 CPE, at max modulation both tx/ack, without retransmisions and WITHOUT the uplink data (eg.if the connection is TCP, there are TCP ACK messages coming from SM to AP which are not being considered right now; if so, the figures go down more quickly)...
If you count UL data (eg 1 tcp ack 64 bytes each DL data packet) => sum another data ul (for 64 bytes=>57uS) and ul ack (57uS) =>
DL + UL for d=0 => (800+64)*8bit/pkt*1pkt(96+57 (for dl) + 57 + 57 (for ul))*1e6Us/1S*1Mbit/1e6bit= 26 Mbit/s...
idem for d=1 km => 24 Mbit/s
for d=10 km => 17 Mbit/s
for d=20 km => 12 Mbit/s
these figures can improve with 802.11n aggregation (aggregation improves a lot PtP scenario but is of low impact on PtMP). As you can se the only way to increase the bandwidth on these equipment is to increase modulation ( 802.11ac) or increase the bandwidth (80, 160, etc.. mhz). Please note that when anybody say "I'm using 20 MHz channel", really you are using 40 MHz of spectrum due to both polarizations.. So when using 80 MHz really it means 160 MHz!!!
For any real TDD system, the performance doesn't degradate such a lot with distance, it is almost despreciable.
Regards
Antonio