Method and apparatus for rate adaptation in a wireless communication system
A method for selection of a particular modulation coding selection (MCS) from a plurality of available MCS selects a particular MCS after obtaining SNR_d, SNR_high, SNR_low, and if SNR_d>SNR_high, selecting the highest available MCS, and if SNR_d<SNR_low, selecting the lowest MCS, and if SNR_high>SNR_d>SNR_low, selecting MCS based on computing the function/table for each MCS according to the formula: f (MCS)=(1−PER(length_packet,MCS,SNR_ d ))/length_packet(packet_size,MCS), and thereafter selecting the MCS that maximizes the f(MCS) function. Further the neighboring MCS values are characterized for throughput in a throughput characterization step, and compared in throughput to the above determined throughput. In an embodiment of the invention, SNR_high, SNR_low are found by finding the SNR that satisfies: PER(length_packet,MCS_lowest,SNR_low)=PER_thresh PER(length_packet,MCS_highest,SNR_high)=PER_thresh Where PER_thresh is a design parameter, and MCS_lowest is the lowest MCS and MCS_highest is the highest available MCS.
1. A method for the selection of a particular Modulation and Coding Selection (MCS) from a plurality of available MCS in a wireless communications system, the method having the steps:
obtaining a derived signal to noise ratio (SNR_d);
establishing threshold values SNR_high, SNR_low;
if SNR_d>SNR_high, selecting the highest MCS;
if SNR_d<SNR_low, selecting the lowest MCS;
if SNR_high>SNR_d>SNR_low, selecting MCS based on:
obtaining a table of the product of f(MCS) and f_m corresponding to each MCS, where f(MCS) is an estimate of throughput in the absence of collisions or interference, and f_m is a correction factor for the effects of collisions or interference;
selecting the MCS that maximizes said product of f(MCS) and f_m;
said plurality of MCS including at least a modulation type and a coding rate;
and where said method steps are repeated when a change in SNR_d of more than 2 dB since a previous transmission occurs.
2. The method of claim 1 where
f
(
MCS
)
=
1
-
PER
(
length_packet
,
MCS
,
H_hat
,
noise_power
)
length_packet
(
packet_size
,
MCS
)
;
where f(MCS) is an estimated throughput for a particular MCS in the absence of collisions or interference where:
PER is a packet error rate;
length_packet is the length of the packet;
MCS is a particular Modulation and Coding Selection;
H_hat is an estimate of the channel matrix H;
noise_power is an estimated noise level in the absence of collisions or interference.
3. The method of claim 1 where SNR_d, SNR_high, SNR_low are derived from receive signal strength indication (RSSI).
4. The method of claim 3 where SNR_high is selected such that PER(length_packet, MCS_highest, SNR_high)=PER_thresh, where:
PER is the packet error rate of the communications channel and excludes the effect of collisions, and said PER is a function of said length_packet which is the time duration of a packet, said MCS_highest is an MCS corresponding to the highest data rate, and said PER_thresh is less than or equal to 1%.
5. The method of claim 3 where SNR_low is selected such that PER(length_packet, MCS_lowest, SNR_low)=PER_thresh, where:
PER is the packet error rate of the communications channel, and said PER is a function of said length_packet which is the time duration of a packet, said MCS_lowest is an MCS corresponding to the lowest data rate, and said PER_thresh has a value which is equal to or less than 1%.
6. The method of claim 1 where SNR_d is derived from a channel characterization matrix, and SNR_high and SNR_low are derived from a packet error rate.
7. The method of claim 6 or claim 5 where a packet error rate threshold (PER_thresh)=alpha* the ratio of collisions on a channel to total packets on a channel, and where alpha is less than or equal to 0.25.
8. The method of claim 1 where said length_packet is computed from said MCS, a guard interval, a payload length, and a preamble length.
9. The method of claim 1 where said length_packet is computed from said MCS, a guard interval, a payload length, a preamble length, and an inter-packet gap.
10. The method of claim 1 where said f(MCS) is derived from measurement of the number of transmitted packets that did not get acknowledged but were not the result of collisions.
11. The method of claim 1 where said f(MCS) is derived from measurement of packets in a communications channel which were not acknowledged and lost or not decoded due to thermal noise of said packet.
12. The method of claim 1 where said SNR_d is derived from an unsuccessfully decoded SIGNAL field of a packet header and successfully decoded SIGNAL field of other packet headers.
13. The method of claim 1 where said f(MCS) is multiplied by a f_m(MCS) factor derived from a peak information rate and packet length.
14. The method of claim 1 where said f(MCS) associated with a particular remote station is computed from information which includes retransmission requests from said particular remote station.
15. A method for selection of a Modulation and Coding Selection (MCS) from a plurality of available MCS in a wireless communications system, the method having the steps:
obtaining a derived signal to noise ratio (SNR_d);
establishing threshold values SNR_high, SNR_low;
if SNR_d>SNR_high, selecting the highest MCS;
if SNR_d<SNR_low, selecting the lowest MCS;
if SNR_high>SNR_d>SNR_low, selecting MCS based on:
obtaining a table of the product of f(MCS) and f_m corresponding to each MCS, where f(MCS) is an estimate of throughput in the absence of collisions or interference, and f_m is a correction factor for the effects of collisions or interference;
selecting the MCS that maximizes said product of f(MCS) and f_m;
and where said method of selection repeats when a change in SNR_d is detected.
16. A method for the selection of a particular Modulation and Coding Selection (MCS) from a plurality of available MCS in a wireless communications system, the method having the steps:
obtaining a derived signal to noise ratio (SNR_d);
establishing threshold values SNR_high, SNR_low;
if SNR_d>SNR_high, selecting the highest MCS;
if SNR_d<SNR_low, selecting the lowest MCS;
if SNR_high>SNR_d>SNR_low, selecting MCS based on:
obtaining a table of the product of f(MCS) and f_m corresponding to each MCS, where f(MCS) is an estimate of throughput in the absence of collisions or interference, and f_m is a correction factor for the effects of collisions or interference;
selecting the MCS that maximizes said product of f(MCS) and f_m;
said plurality of MCS including at least a modulation type and a coding rate where
f
(
MCS
)
=
1
-
PER
(
length
packet
,
MCS
,
H
hat
,
nosie
power
)
length_packet
(
packet
size
,
MCS
)
where f(MCS) is an estimated throughput for a particular MCS in the absence of collisions or interference and where:
PER is a packet error rate;
length_packet is the length of the packet;
MCS is a particular Modulation and Coding Selection;
H_hat is an estimate of channel matrix H;
Noise power is an estimated noise level in the absence of collisions or interference.
17. A method for the selection of a particular Modulation and Coding Selection (MCS) from a plurality of available MCS in a wireless communications system, the method having the steps:
obtaining a derived signal to noise ratio (SNR_d);
establishing threshold values SNR_high, SNR_low;
if SNR_d>SNR_high, selecting the highest MCS;
if SNR_d<SNR_low, selecting the lowest MCS;
if SNR_high>SNR_d>SNR_low, selecting MCS based on:
obtaining a table of the product of f(MCS) and f_m corresponding to each MCS, where f(MCS) is an estimate of throughput in the absence of collisions or interference, and f_m is a correction factor for the effects of collisions or interference;
selecting the MCS that maximizes said product of f(MCS) and f_m;
said plurality of MCS including at least a modulation type and a coding rate;
where SNR_d, SNR_high, SNR_low are derived from receive signal strength indication (RSSI)
where SNR_high is selected such that
PER(length_packet, MCS_highest, SNR_high)=PER_thresh, where:
PER is the packet error rate of the communications channel and excludes the effect of collisions, and said PER is a function of said length_packet which is the time duration of a packet, said MCS_highest is an MCS corresponding to the highest data rate, and said PER_thresh is less than or equal to 1%.
18. A method for the selection of a particular Modulation and Coding Selection (MCS) from a plurality of available MCS in a wireless communications system, the method having the steps:
obtaining a derived signal to noise ratio (SNR_d);
establishing threshold values SNR_high, SNR_low;
if SNR_d>SNR_high, selecting the highest MCS;
if SNR_d<SNR_low, selecting the lowest MCS;
if SNR_high>SNR_d>SNR_low, selecting MCS based on:
obtaining a table of the product of f(MCS) and f_m corresponding to each MCS, where f(MCS) is an estimate of throughput in the absence of collisions or interference, and f_m is a correction factor for the effects of collisions or interference;
selecting the MCS that maximizes said product of f(MCS) and f_m;
said plurality of MCS including at least a modulation type and a coding rate;
where SNR_d, SNR_high, SNR_low are derived from receive signal strength indication (RSSI)
where SNR_low is selected such that
PER(length_packet, MCS_lowest, SNR_low)=PER_thresh, where:
PER is the packet error rate of the communications channel and excludes the effect of collisions, and said PER is a function of said length_packet which is the time duration of a packet, said MCS_lowest is an MCS corresponding to the lowest data rate, and said PER_thresh is less than or equal to 1%.