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July 2014
doc.: IEEE 802.11-14/0890r3
WLAN-3GPP Interworking Metric
Date: 2014-07-17
Authors:
Name
Company
Youhan Kim
Qualcomm
youhank@qca.qualcomm.com
Menzo Wentink
Qualcomm
mwentink@qti.qualcomm.com
Carlos Aldana
Qualcomm
caldana@qca.qualcomm.com
Submission
Address
Phone
Slide 1
email
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Revision History
Revision
Note
R0
Initial
R1
Fixed document number typo.
Minor editorial updates (e.g. fixed figures moving around).
Changed references from REVmc D3.0 to IEEE 802.11-2012
R2
Changed reference 14/0921r1 to 14/0921r2
R3
Changed reference 14/0921r2 to 14/0921r3
Submission
Slide 2
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Background
• 3GPP TSG RAN WG2 (RAN2) is developing a
mechanism for interworking (IW) between 3GPP
RATs (UMTS and LTE) and WLAN
– 3GPP had sent a liaison statement to IEEE 802.11 in April 2014
(IEEE 802.11-14/0519r0)
– IEEE 802.11 responded as in IEEE 802.11-14/0658r6
• See next slide
• Discouraged use of RCPI and RSNI
Submission
Slide 3
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Previous Liaisons
Questions in liaison statement from 3GPP
(IEEE 802.11-14/0519r0)
Response from IEEE 802.11 to 3GPP
(IEEE 802.11-14/0658r6)
Question 1: Does IEEE 802.11 WG consider
WLAN RCPI a suitable metric of WLAN
signal strength such that it can be compared to
thresholds as in the above described
mechanism?
We consider the RCPI value as defined in IEEE
802.11™-2012 a metric for signal strength.
Question 2: Does IEEE 802.11 WG consider
WLAN RSNI a suitable metric of WLAN
signal quality such that it can be compared to
thresholds as in the above described
mechanism?
We consider the RSNI value as defined in IEEE
802.11™-2012 a metric for signal quality in
downlink direction.
Submission
Slide 4
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Previous Liaisons (Cont’d)
Questions in liaison
statement from 3GPP
• 3GPP
TSG RAN
(IEEE
802.11-14/0519r0)
Response from IEEE 802.11 to 3GPP
(IEEE 802.11-14/0658r6)
WG2 (RAN2) is developing a
mechanism
for802.11
inter-working
between
3GPPofRATs
Question
3: Does IEEE
Understanding
that the objective
the mechanism is to select
WG(UMTS
consider any
other
theWLAN
network that provides the best match to the QoS and/or
and
LTE) and
WLAN signal metric more
throughput requirements of the system, the consideration of
–
3GPP
had
sent
a
liaison
statement istonot
IEEE
802.11
in April
suitable for the above
RNSI/RCPI
sufficient
on[1]
its own
to efficiently estimate
2014,
to which the IEEE
responded
in [2]
described
mechanism?
the802.11
available
throughputasand
QoS that will be experienced in the
IEEE 802.11 WLAN. Other metrics should be taken into
account, especially channel bandwidth, operating band, number
of spatial streams, BSS load, and WAN metrics, see also the
attached Table 1. Comparing only the RSNI/RCPI, as is, to
thresholds presents some risks of poor decisions. Ideally, a
single parameter, such as estimated available throughput, which
combines all of the above parameters, would be determined
inside of the WLAN modem and then delivered to the upper
layers.
Submission
Slide 5
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Further Background
•
3GPP RAN2 is still using RCPI and RSNI as the PHY metrics for
WLAN-3GPP IW
–
–
–
–
•
ftp://ftp.3gpp.org/tsg_ran/WG2_RL2/TSGR2_86/Docs/R2-142955.zip
ftp://ftp.3gpp.org/TSG_RAN/TSG_RAN/TSGR_64/Docs/RP-140556.zip
ftp://ftp.3gpp.org/TSG_RAN/TSG_RAN/TSGR_64/Docs/RP-141010.zip
ftp://ftp.3gpp.org/TSG_RAN/TSG_RAN/TSGR_64/Docs/RP-141011.zip
3GPP RAN2 had requested 3GPP SA2 to include RCPI and RSNI in
the requirements documents
– RAN2 is the technical group
– SA2 is the Architecture requirements group
•
During discussion in 3GPP SA2, additional issues were found for
RCPI and RSNI
– ftp://ftp.3gpp.org/tsg_sa/WG2_Arch/TSGS2_104_Dublin/Docs/S2-142942.zip
•
This contribution describes additional issues with using RCPI and
RSNI for WLAN-3GPP IW, and proposes way forward for WLAN3GPP IW
Submission
Slide 6
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
RSNI
Submission
Slide 7
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Definition of RSNI in IEEE 802.11
•
IEEE 802.11-2012
• P.7
• P.20
Submission
Slide 8
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Definition of RSNI in IEEE 802.11 (Cont’d)
• IEEE 802.11-2012
• 10.11.9.4. Noise Histogram report: P1069
Submission
Slide 9
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Example
• STA1 is DUT
• BSS1 and BSS2 have partially overlapping channel
– Also, AP2 close to STA1
– But BSS2 is lightly loaded. So, channel is clear for BSS1 most of
the time.
• Good candidate for 3GPPWLAN roaming for STA1
• Assume noise floor of -96 dBm/20 MHz at STA1 RX
Submission
Slide 10
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Example
Submission
Slide 11
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Example
• Case 1
Submission
Slide 12
Youhan Kim, Qualcomm Inc.
July 2014
• Case 2
Submission
doc.: IEEE 802.11-14/0890r3
Example
Slide 13
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Example
• Case 3
•
Before receiving the first green packet, STA1 RX likely listening to the entire 80 MHz
–
–
•
Don’t know the BW of the next packet coming in.
Need to perform CCA on 80 MHz
Then, should be ANPI for the first green packet be over 20 MHz or 80 MHz?
–
–
Submission
Of course, a ‘logical’ answer is that it should be over 20 MHz (-96 dBm), not 80 MHz (-90 dBm)
But the point here is that the standard does not define this clearly – yet another example of potential pitfall.
Slide 14
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Other Notes
• What if DUT has multiple RX antennas?
– How is ANPI defined?
• Average over RX chains? Summed over RX chains?
– Not defined clearly in the IEEE standard
 Interop issue expected w/o further clarification
• RSNI measurement is optional
• RSNI is NOT RSRQ in 3GPP
– RSRQ = Reference Signal Received Quality
• Roughly speaking, SINR
– RSNI does not measure the interference ‘present’ in the packet
– Rather, the interference is measured during ‘idle’ time some time before the
packet arrives
• That interference may or may not be present in the packets the DUT is receiving
Submission
Slide 15
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Summary – RSNI
• RSNI is currently broken
– RSNI definition fails (numerically cannot be computed) in some
cases
– Many ambiguities exists
• WLAN-3GPP interworking should not be based on a
metric which is broken
• RSNI is not RSRQ
Submission
Slide 16
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
RCPI AND RSSI
Submission
Slide 17
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
RCPI vs. RSSI
• RCPI and RSSI are similar in nature
– RCPI is the power measured during the data portion of the packet
– RSSI is the power measured during the preamble portion of the packet
• HT/VHT preamble in case of HT/VHT PPDUs
– Signal power between preamble and data are the same
– There is minor difference in noise bandwidth
• RCPI assumes noise equivalent BW 1.1 times greater than the channel BW
• No noise equivalent BW specified for RSSI
• Difference should be small
• Measurement accuracy
– Dominated by analog gain inaccuracy, not digital power measurement
inaccuracy
• RCPI and RSSI should be able to achieve similar accuracy
• Accuracy requirement in IEEE 802.11
– RCPI has a ±5 dB (95% confidence interval) accuracy requirement, while
RSSI does not.
• But as mentioned above, there is no reason why RSSI cannot achieve similar
accuracy as RCPI
Submission
Slide 18
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
RSSI is Mandatory
• RCPI is optional
• RSSI, on the other hand, is mandatory
– Used for CCA
• CCA is key functionality of CSMA, which all WLAN devices have to
implement
– IEEE 802.11-2012: P1811
Submission
Slide 19
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
RSSI Unit
• RSSI unit in IEEE 802.11 is a ‘relative’ dB, not an
‘absolute’ dBm
• But since RSSI is used for CCA
– CCA is in absolute dBm
– Thus, RSSI in absolute dBm is implicitly available
Submission
Slide 20
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Other Ambiguities
• Multiple RX chains
– RCPI for DSSS/CCK and NON_HT OFDM PPDUs does not
specify how to deal w/ multiple RX chains
• E.g. Average over RX chains? Summed over RX chains?
• HT/VHT PPDUs specify that the RCPI is averaged over RX chains
– RSSI also does not specify how to deal w/ multiple RX chains
Submission
Slide 21
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Summary – RSSI vs. RCPI
• RSSI and RCPI convey essentially the same
information
– RCPI is optional
– RSSI is mandatory
• Both RSSI and RCPI has some ambiguities to be
clarified
Submission
Slide 22
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
BEACON VS. DATA
Submission
Slide 23
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
What Packet Should be Used?
• Before WLAN has associated w/ an AP
– There is no traffic. Only Beacon RSSI is available.
• When performing active scan, Probe Response RSSI would be available instead of
Beacon RSSI
• But typically, Probe Response and Beacon are both sent in the lowest supported rate
• After WLAN has associated w/ an AP
– If device is using 3GPP link as the main data pipe, then WLAN will not have
much traffic
• WLAN will likely be in DTIM mode to save power
– Again, only beacon RSSI is available
• Even if data traffic is available
– Devices often change TX power as a function of MCS
• Several dB of TX power difference between the lowest rate and the highest rate is
not uncommon
– Hence, for the same path loss, data packet RSSI could vary considerably just
because of TX power
• Receiver has no information on what the TX power was
– Hence, data packet RSSI is not suitable for link quality assessment
Submission
Slide 24
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Beacon RSSI
• IEEE 802.11-2012
– P354
Submission
Slide 25
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Summary – Beacon vs. Data RSSI
• Beacon RSSI is always available
– Before association
– After association, w/ and w/o traffic
• Data packet RSSI is not as reliable for link quality
accessment
– Function of TX power, which is implementation specific (i.e. can
change drastically)
Submission
Slide 26
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
SUGGESTED WAY FORWARD
Submission
Slide 27
Youhan Kim, Qualcomm Inc.
July 2014
doc.: IEEE 802.11-14/0890r3
Suggested Way Forward
• Communicate the following to 3GPP
– Do not use RSNI for IW
• Fixing RSNI would take some effort
• Besides, RSNI does not necessarily reflect interference present in the
packet. Hence, benefit for WLAN-3GPP IW is not clear
– Replace RCPI w/ Beacon RSSI
• Need some clarifications in the IEEE 802.11
– Please see IEEE 802.11-14/0921r3 for details of the proposed changes
• For longer term, IEEE 802.11 should work on
enhancing metrics for WLAN-3GPP interworking
Submission
Slide 28
Youhan Kim, Qualcomm Inc.