LTE fast scheduling and link adaptation explain, CQI based and new strategy
In a
common approach to link adaptation for wireless communications, the transmitter
adjusts one or more transmission parameters responsive to
changes in the receivers channel quality. The receiver supports
link adaptation by the transmitter by sending channel quality information
as feedback to the transmitter. For
example, the receiver periodically measures channel quality and sends
corresponding
Channel Quality Indicators (CQIs) to the transmitter, which uses the reported CQIs to adjust the modulation and coding scheme used for transmitting to the receiver.
Channel Quality Indicators (CQIs) to the transmitter, which uses the reported CQIs to adjust the modulation and coding scheme used for transmitting to the receiver.
Ongoing
signal quality measurements at the receiver drive CQI generation and
feedback. For example, the receiver periodically measures received
signal quality as a signal-to-noise ratio (SNR), and maps
the measured SNRs in to a
defined table of CQI values, each value representing a range
of SNRs in dBs. CQI may be expressed in terms of transport format sizes which approximately follow an SNR dB
scale. Here, the receiver estimates the largest transport format
that can be received at a defined reliability or other
performance metric. In such embodiments, the CQI values quantize measured SNR and provide a more compact signaling format, which is desirable for high CQI reporting rates. Of course, CQIs can be based on measures other than an SNR scale. Regardless, higher CQI reporting rates are used in more sophisticated wireless communication networks to drive fast dynamic scheduling and link adaptation, which allows those systems to achieve high bit rates and high system throughput.
performance metric. In such embodiments, the CQI values quantize measured SNR and provide a more compact signaling format, which is desirable for high CQI reporting rates. Of course, CQIs can be based on measures other than an SNR scale. Regardless, higher CQI reporting rates are used in more sophisticated wireless communication networks to drive fast dynamic scheduling and link adaptation, which allows those systems to achieve high bit rates and high system throughput.
High
Speed Data Packet Access (HSDPA) services in Wideband CDMA, for example, rely on
high CQI reporting rates. Long Term Evolution (LTE)
networks also rely on high CQI reporting rates to support
the fast user scheduling and link adaptation used in such
networks to maintain high utilization of the communication
link i.e., to maintain high aggregate data throughput on the link.
Even with fast CQI
reporting, problems remain. For example, there is an “aging problem associated with the delay between the time a receiver measures its signal quality and the time that the correspondingly transmitted CQI is actually used at the transmitter for adapting the link with respect to that receiver. The age of a given CQI value includes delays between the receiver measuring and reporting signal quality, transmission link delays, and the delay between the transmitter receiving the CQI and using it for link adaptation. That last delay may include scheduling delays, where the transmitter schedules its downlink transmissions to different users. In HSDPA, such reporting delays are in the range of six milliseconds. That value in combination with a reporting period of eight milliseconds results in an overall delay that varies between eight and fourteen milliseconds. This magnitude of overall delay is tolerable, in terms of maintaining a desired tradeoff between latency and link throughput. That is, it is desirable to send data at the highest rate possible, while limiting the need for retransmissions. HSDPA targets a given Block Error Rate (BLER) at the receiver, e.g., a 10% BLER, and uses Hybrid Automatic Repeat Request (HARQ) for retransmitting data as needed. Even so, it is known in the art to mitigate the “aging problem”. For example, U.S. Pub. 2005/0181811 A1 teaches “correcting CQI feedback from a receiver according to an “offset” value. As this reference explains, a channel-dependent scheduler at a base station schedules the user or users
reporting the best channel conditions, but the actual channel qualities for those users may have deteriorated by the time the scheduled transmissions occur. The reference thus looks at
additional information that can be used to get a more accurate sense of channel quality. In one embodiment, ACK/NACK feedback from a receiver provides a basis for determining or otherwise updating an offset value that is used to correct CQI feedback from the receiver. In this manner, CQIs reported by the receiver can be discounted or otherwise reduced by a performance-based offset that is determined by monitoring one or more parameters indicative of reception performance. The approach is useful in that it helps prevent the selection of overly optimistic transmission parameter settings.
reporting, problems remain. For example, there is an “aging problem associated with the delay between the time a receiver measures its signal quality and the time that the correspondingly transmitted CQI is actually used at the transmitter for adapting the link with respect to that receiver. The age of a given CQI value includes delays between the receiver measuring and reporting signal quality, transmission link delays, and the delay between the transmitter receiving the CQI and using it for link adaptation. That last delay may include scheduling delays, where the transmitter schedules its downlink transmissions to different users. In HSDPA, such reporting delays are in the range of six milliseconds. That value in combination with a reporting period of eight milliseconds results in an overall delay that varies between eight and fourteen milliseconds. This magnitude of overall delay is tolerable, in terms of maintaining a desired tradeoff between latency and link throughput. That is, it is desirable to send data at the highest rate possible, while limiting the need for retransmissions. HSDPA targets a given Block Error Rate (BLER) at the receiver, e.g., a 10% BLER, and uses Hybrid Automatic Repeat Request (HARQ) for retransmitting data as needed. Even so, it is known in the art to mitigate the “aging problem”. For example, U.S. Pub. 2005/0181811 A1 teaches “correcting CQI feedback from a receiver according to an “offset” value. As this reference explains, a channel-dependent scheduler at a base station schedules the user or users
reporting the best channel conditions, but the actual channel qualities for those users may have deteriorated by the time the scheduled transmissions occur. The reference thus looks at
additional information that can be used to get a more accurate sense of channel quality. In one embodiment, ACK/NACK feedback from a receiver provides a basis for determining or otherwise updating an offset value that is used to correct CQI feedback from the receiver. In this manner, CQIs reported by the receiver can be discounted or otherwise reduced by a performance-based offset that is determined by monitoring one or more parameters indicative of reception performance. The approach is useful in that it helps prevent the selection of overly optimistic transmission parameter settings.
Another
known mitigation technique applies a similar type of offset to reported CQIs,
but bases the offset on CQI
age. The published patent application WO
2006/075208A1 provides an example of age-based CQI
compensation in the HSDPA
context. The
reference suggests that applying corrective back-off or offset values to
all CQIs is less preferable than applying an age-dependent offset, in the sense
that a relatively new CQI may well provide
an accurate sense of current channel conditions at the
reporting receiver. As such, the 208 reference teaches applying an
offset to reported CQIs, where the magnitude of the applied
offset is determined as a function of CQI age.
Neither
of the above approaches directly addresses the
challenges posed by some of the newer communication network
standards, such as Long Term Evolution (LTE). Like HSDPA
and other high-rate services, LTE relies on fast link adaptation
and dynamic user scheduling, to achieve high bit rates
and maintain high data throughput. For example, an LTE base
station, referred to as an eNodeB, may perform link adaptations
on a one millisecond basis. LTE receivers support such
operations by generating periodic CQI reports according to
measurements taken from common reference symbols
received in the downlink. The receivers send CQI reports on a physical uplink control channel (the PUCCH, for example), and also may send CQI reports on a physical uplink shared channel (the PUSCH, for example), responsive to receiving grants from an eNodeB.
received in the downlink. The receivers send CQI reports on a physical uplink control channel (the PUCCH, for example), and also may send CQI reports on a physical uplink shared channel (the PUSCH, for example), responsive to receiving grants from an eNodeB.
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