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WLAN-3GPP Interworking Metric
July 2011 doc.: IEEE /0890r1 July 2014 WLAN-3GPP Interworking Metric Date: Authors: Youhan Kim, Qualcomm Inc. Youhan Kim, Qualcomm Inc.
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Revision History July 2014 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 Youhan Kim, Qualcomm Inc.
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July 2014 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 in April 2014 (IEEE /0519r0) IEEE responded as in IEEE /0658r6 See next slide Discouraged use of RCPI and RSNI Youhan Kim, Qualcomm Inc.
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Previous Liaisons July 2014 Questions in liaison statement from 3GPP
(IEEE /0519r0) Response from IEEE to 3GPP (IEEE /0658r6) Question 1: Does IEEE 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 ™-2012 a metric for signal strength. Question 2: Does IEEE 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 ™-2012 a metric for signal quality in downlink direction. Youhan Kim, Qualcomm Inc.
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Previous Liaisons (Cont’d)
July 2014 Previous Liaisons (Cont’d) Questions in liaison statement from 3GPP (IEEE /0519r0) Response from IEEE to 3GPP (IEEE /0658r6) Question 3: Does IEEE WG consider any other WLAN signal metric more suitable for the above described mechanism? Understanding that the objective of the mechanism is to select the network that provides the best match to the QoS and/or throughput requirements of the system, the consideration of RNSI/RCPI is not sufficient on its own to efficiently estimate the available throughput and QoS that will be experienced in the IEEE 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. 3GPP TSG RAN WG2 (RAN2) is developing a mechanism for inter-working between 3GPP RATs (UMTS and LTE) and WLAN 3GPP had sent a liaison statement to IEEE [1] in April 2014, to which the IEEE responded as in [2] Youhan Kim, Qualcomm Inc.
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July 2014 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/R zip ftp://ftp.3gpp.org/TSG_RAN/TSG_RAN/TSGR_64/Docs/RP zip ftp://ftp.3gpp.org/TSG_RAN/TSG_RAN/TSGR_64/Docs/RP zip ftp://ftp.3gpp.org/TSG_RAN/TSG_RAN/TSGR_64/Docs/RP 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/S zip This contribution describes additional issues with using RCPI and RSNI for WLAN-3GPP IW, and proposes way forward for WLAN-3GPP IW Youhan Kim, Qualcomm Inc.
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July 2014 RSNI Youhan Kim, Qualcomm Inc.
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Definition of RSNI in IEEE 802.11
July 2014 Definition of RSNI in IEEE IEEE P.7 P.20 Youhan Kim, Qualcomm Inc.
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Definition of RSNI in IEEE 802.11 (Cont’d)
July 2014 Definition of RSNI in IEEE (Cont’d) IEEE Noise Histogram report: P1069 Youhan Kim, Qualcomm Inc.
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Example STA1 is DUT BSS1 and BSS2 have partially overlapping channel
July 2014 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 3GPPWLAN roaming for STA1 Assume noise floor of -96 dBm/20 MHz at STA1 RX Youhan Kim, Qualcomm Inc.
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July 2014 Example Youhan Kim, Qualcomm Inc.
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July 2014 Example Case 1 Youhan Kim, Qualcomm Inc.
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July 2014 Example Case 2 Youhan Kim, Qualcomm Inc.
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July 2014 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? 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. Youhan Kim, Qualcomm Inc.
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Other Notes What if DUT has multiple RX antennas?
July 2014 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 Youhan Kim, Qualcomm Inc.
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Summary – RSNI RSNI is currently broken
July 2014 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 Youhan Kim, Qualcomm Inc.
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July 2014 RCPI and RSSI Youhan Kim, Qualcomm Inc.
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RCPI vs. RSSI RCPI and RSSI are similar in nature Measurement accuracy
July 2014 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 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 Youhan Kim, Qualcomm Inc.
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RSSI is Mandatory RCPI is optional
July 2014 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 : P1811 Youhan Kim, Qualcomm Inc.
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July 2014 RSSI Unit RSSI unit in IEEE 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 Youhan Kim, Qualcomm Inc.
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Other Ambiguities Multiple RX chains
July 2014 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 Youhan Kim, Qualcomm Inc.
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July 2014 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 Youhan Kim, Qualcomm Inc.
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July 2014 Beacon vs. Data Youhan Kim, Qualcomm Inc.
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What Packet Should be Used?
July 2014 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 Youhan Kim, Qualcomm Inc.
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July 2014 Beacon RSSI IEEE P354 Youhan Kim, Qualcomm Inc.
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Summary – Beacon vs. Data RSSI
July 2014 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) Youhan Kim, Qualcomm Inc.
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July 2014 Suggested way forward Youhan Kim, Qualcomm Inc.
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Suggested Way Forward Communicate the following to 3GPP
July 2014 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 Please see IEEE /0921r1 for details of the proposed changes For longer term, IEEE should work on enhancing metrics for WLAN-3GPP interworking Youhan Kim, Qualcomm Inc.
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