Link adaptation in wireless networks for throughput maximization under retransmissions
The present invention implements a method and system for dynamically adapting the modulation and coding scheme for radio links in a wireless communications network based on a retransmission environment model in order to maximize throughput and most efficiently allocate bandwidth resources. The present invention encompasses a refined calculus and methodology for deriving the link adaptation thresholds in a retransmission environment using a complex model and analysis of the retransmission environment. The present invention holds particular application for wireless data communications as opposed to real time data services because it is based on a retransmission model applicable primarily for data services. A critical component of this new link adaptation system is a “no transmission” cutoff mode that is selected for SIR below a base threshold value. This new mode prevents system instability and misallocation of bandwidth in a wireless communication system.
1. A transmitter for wireless communication with a receiver, comprising:
a transceiver element,
a link adaptation module that is capable of implementing a plurality of modulation and encoding schemes, interposed between data and said transceiver element, and
a controller responsive to signal-to-interference ratio (SIR) measures provided by said receiver via said transceiver element, for managing said link adaptation module to effect a particular modulation and encoding scheme based on the received SIR and preselected SIR thresholds that incorporate a SIR margin that is based on a retransmission policy that is employed.
2. The transmitter of claim 1 where said controller manages said link adaptation module to transmit no data when said received SIR is below a given threshold.
3. The transmitter of claim 1 where said retransmission policy is one where portions of said data that are not received correctly are retransmitted until they are received correctly.
4. The transmitter of claim 1 where said SIR margin is related to increased load due to maximum number of allowed retransmissions pursuant to said retransmission policy.
5. The transmitter of claim 1 where said SIR margin is related to
∑
j
=
1
M
T
j
,
where
T
j
=
ρ
0
∑
n
∈
N
p
n
(
BLER
n
)
j
,
M is maximum number of allowed retransmissions pursuant to said retransmission policy, N is number of different modulation and channel coding schemes that are available for employing in the transmission of said data to said receiver, n is an index identifying a particular one of said schemes, p n is probability that scheme n is employed, BLER n is block error rate that is experienced when scheme n is employed, and ρ 0 load presented by said data.
6. The transmitter of claim 3 where said SIR margin is related to 10 log(1-BLER) where BLER is block error rate that is experienced when transmitting said data to said receiver.
7. The transmitter of claim 3 where said SIR margin is related to
-
10
log
(
∑
n
∈
N
p
n
(
1
-
BLER
n
)
)
,
where N is number of different modulation and channel coding schemes that are available for employing in the transmission of said data to said receiver, n is an index identifying a particular one of said schemes, p n is probability that scheme n is employed, and BLER n is block error rate that is experienced when scheme n is employed.
8. A method for performing wireless link adaptation in a retransmission environment comprising the steps of:
determining a plurality of signal-to-interference (SIR) threshold values for selecting different link adaptation modes for a transmitter to use in sending data to a receiver, said threshold value incorporating a SIR margin that is based on a selected retransmission policy that is employed by said transmitter; and
in response to a signal from said receiver that requests transmission of data, employing a link adaptation process that
selects a modulation and channel coding scheme based on said signal from said receiver, and
sends data to said receiver in accord with the selected scheme and the selected retransmission policy.
9. The method of claim 8 where said signal that is received from said receiver corresponds to a SIR measure that is experienced by said receiver.
10. The method of claim 8 where, when said received SIR signal is above a preselected threshold it is interpreted as said request to transmit data, and when said received SIR is not above said preselected threshold it is interpreted as a request to refrain from transmitting data.