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Support WiFi and LTE

Co-existenceLili Qiu

Department of Computer ScienceThe University of Texas at Austin

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Introduction

• Great success of LTE • Lack of the bandwidth• Service providers demand more spectrum

• LTE in unlicensed spectrum• Proposed by major LTE manufacturers and

service providers (e.g., Huawei, Qualcomm, T-Mobile)

• Co-existence with WiFi is main issue• Solution – Dividing the spectrum in time-

domain

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Our Observations

• Mobile devices have LTE and WiFi antennas

• Theoretically feasible to decode concurrent LTE and WiFi signals

• Challenges• Decode signals from two heterogeneous PHY• Estimate channel without clean reference

signals• Carrier Sense• Synchronization

Galaxy S4 antenna location(http://www.s4gru.com)

LTE antenna

WiFi antenna

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Proposed idea

• Let LTE and WiFi send together without coordination

• Propose a receiver design that decodes WiFi-LTE overlapped signal

• Decode two overlapped OFDM symbols that are not aligned in time or frequency domain

• Estimate channel without clean reference symbol• Address practical issues

• Carrier sensing• Synchronization• Control-frame reception

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Related Work

• TIMO [1] – decode WiFi signal under unknown interference• Only decodes WiFi• Clean preamble based WiFi channel estimation• LTE-WiFi coexistence don’t have clean reference signals

• ZIMO [2] – decode WiFi and ZigBee signals• Exploit different bandwidth and power in WiFi and ZigBee• LTE and WiFi bandwidths and powers are similar

[1] S. Gollakota, F. Adib, D. Katabi, and S. Seshan. Clearing the RF smog: making 802.11 robust to cross-technology interference. In Proc. of ACM SIGCOMM, 2011.[2] Y. Yubo, Y. Panlong, L. Xiangyang, T. Yue, Z. Lan, and Y. Lizhao. ZIMO: building cross-technology MIMO to harmonize Zigbee smog with WiFi flash without intervention. In Proc. of ACM MobiCom, 2013.

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Outline

• Background• Decode when both channels are known• Decode when only channel is known• Decode when neither channel is known• Practical Issues

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LTE Background

• Frame transmission is continuous

• Reference symbols are transmitted periodically

• OFDM based transmission(FFT size- 2048)

• Actual data bandwidth: 18 MHz

SF0 SF1 SF4SF3SF2 SF5 SF6 SF7 SF9SF8

10ms

1ms

Reference symbolTime

Freq

uenc

y

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WiFi Background

• Preamble is transmitted only at the beginning of a frame

• OFDM based transmission (FFT size - 64)• Actual data bandwidth: 16.25 MHz

DATA ACK

Backoff interval

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Outline

• Background• Decode when both channels are known• Decode when only one channel is known• Decode when neither channel is known• Practical Issues

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WiFi and LTE overlapped signal

• LTE signal

• WiFi signal

• Received signals

𝑠𝑙

𝑠𝑤

𝑟1

𝑟2

, N=2048

, M=64

LTE data symbol

WiFi data symbol

𝒉𝟏𝒍

𝒉𝟐𝒍

𝒉𝟏𝒘

𝒉𝟐𝒘

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Decode when both channels are known • Decode LTE from the overlapped signal

• Perform FFT with respect to the FFT size of LTE (2048)

• With 2 signals from 2 antennas, decode

• Similarly, decode WiFi signal

Received signal in the k-th LTE subcarrier

Received signal in time domain

Undecodable WiFi signal

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Outline

• Background• Decode when both channels are known• Decode when only one channel is known• Decode when neither channel is known• Practical Issues

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Decode when only one channel is known• Received WiFi preamble overlapped with LTE signal

Not solvable due to 3 unknowns!

• Received LTE reference symbol overlapped with WiFi signal

Obtain WiFi channel ratio from the received LTE reference signal

Known WiFi preamble

Received symbol after removing LTE interference𝛼𝑘=

𝐻2𝑤 [𝑘 ]

𝐻1𝑤 [𝑘 ]

=𝑌 2 [𝑘 ]−𝐻2

𝑙 [𝑘 ] 𝑋𝑝𝑙 [𝑘 ]

𝑌 1 [𝑘 ]−𝐻2𝑙 [𝑘 ] 𝑋𝑝

𝑙 [𝑘 ]

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Decode when only one channel is known (cont.)

• With the channel ratio, we can decrease the number of unknowns and can solve the equation

• Similarly, we estimate LTE channel with known WiFi channel

𝐻2𝑤 [𝑘 ]=𝛼𝑘𝐻1

𝑤 [𝑘 ]

[𝐻1𝑤 [𝑘]𝐼𝑙 [𝑘 ] ]=[ 𝑋𝑝

𝑤 [𝑘 ] 𝐻1𝑙 [𝑘 ]

𝛼𝑘𝑋𝑝𝑤 [𝑘 ] 𝐻2

𝑙 [𝑘 ]]−1

[𝑌 1𝑤 [𝑘 ]

𝑌 2𝑤 [𝑘 ] ]

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Outline

• Background• Decode when both channels are known• Decode when only one channel is known• Decode when neither channel is known• Practical Issues

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Decode when neither channel is knownHow to estimate both LTE and WiFi channels?

• Exploit different bandwidth in LTE and WiFi• LTE has 18 MHz and WiFi has 16.25 MHz LTE has 1.75 MHz

uninterfered channel

• Estimate the remaining channel • Extrapolation inaccurate• Techniques to improve accuracy

• Joint WiFi and LTE channel estimation• Iterative channel estimation

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Joint LTE and WiFi channel estimation1. Estimate the boundary LTE

channel without interference

2. Estimate the first interfered LTE channel through extrapolation

3. Estimate the channel of WiFi subcarrier using known LTE channel

4. Estimate the LTE channels using known WiFi channel

5. Repeat 2-4 for the remaining part of the half channel

6. Repeat 1-5 for the other half

1 LTE Reference signalWiFi OFDM symbol

Time

Freq.

WiFi subcarrier

LTE subcarrier

4

23

5

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Iterative Decoding

• Estimate the channel using WiFi preamble and LTE reference symbol

• Decode WiFi data symbols and re-modulate them• Estimate the channel using the re-modulated symbols

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Practical considerations

• Decode WiFi MIMO signals• Our design is extendable to WiFi MIMO with multiple WiFi

antennas and additional LTE antenna

• Carrier sense only WiFi signals • Project the received signal onto the null space of the LTE

channel

• Synchronization for LTE• Use WiFi null DC subcarrier and LTE synchronization sequence

(PSS, SSS)

• Different channel widths• Nullify a few data subcarriers for channel estimation purpose

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Performance Evaluation

• USRP testbed• 10 MHz channel • USRP nodes at 8 locations with different channel

conditions• Use the best rate at each location

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Compare with TDMA

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Locations

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Our schemeTime division

1.87x throughput gain from time-division

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Compare different channel estimations

1 2 3 4 5 6 7 8

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10

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20

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Locations

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Our schemeknown LTE channels

known LTE and WiFi channelsExtrapolation Only

96% throughput compared to known LTE channel93% throughput compared to known LTE and WiFi channel

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Estimate channel using different # WiFi symbols

50 data symbols are sufficient for channel estimation

1 2 3 4 5 6 7 8

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Locations

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20 symbols50 symbols100 symbols150 symbolsKnown LTE and WiFi channels

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Conclusion

• A novel coexistence mechanism for LTE and WiFi in unlicensed band

• Decoding data symbol under cross-technology interference• Channel estimation without clean reference• Address practical issues

• In real channel experiment with USRP• 90% throughput improvement compared to time-division• 7% throughput loss compared to clean reference based

decoding

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Thank you!

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Decode WiFi MIMO

• In WiFi MIMO, one cannot get WiFi channel ratio due to LTE and WiFi interference

• We let only one of WiFi antennas send the signal during LTE reference symbol transmission

• In 802.11 n/ac, each TX antenna separately transmits preamble

• Using the interfered preamble and the channel ratio, we can apply the same method to estimate WiFi channel

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Carrier Sense Only WiFi Signals

• Due to continuous LTE transmissions, we should only carrier sense WiFi signals

• Projection based WiFi carrier sensing• Estimate LTE channel using LTE reference symbols• Project the received signal onto the null space of the LTE

channel • If only LTE signals exist, the projected signal is small• Otherwise, there is WiFi transmission and we need to defer• What if LTE channel estimation is wrong due to WiFi

interference? WiFi transmission detected!

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LTE synchronization under WiFi interference

• LTE transmits Primary Synchronization Sequence (PSS) and Secondary Synchronization Sequence (SSS) every 5 ms for synchronization

• 960 KHz bandwidth• One third of PSS overlaps with WiFi DC subcarrier where no

signal is transmitted

• With 30% of non-overlapped bandwidth, we can detect LTE signal timing even under -7dB SINR

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Wider channels• If WiFi chanenl > 20MHz, we cannot rely on guardband difference to get clean LTE reference symbols

• Solution• Nullify a few WiFi subcarriers and use them for LTE

channel estimation• Nullify 3 subcarriers every 10 MHz is sufficient

• 10% bandwidth loss for WiFi• in return allows concurrent LTE transmissions