Support WiFi and LTE Co-existence Lili Qiu Department of Computer Science The University of Texas at Austin

Introduction Great success of LTE LTE in unlicensed spectrum 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 LTE has been in great success over the last few years, and it is expected to keep growing fast. Because of the lack of the bandwidth, the cellular service providers are demanding more bandwidths, and recently, some LTE manufacturers such as Qualcomm and Huawei, and service providers such as T-mobile and NTT are proposing to use even unlicensed spectrum such as 5 GHz channel for LTE. In this case, the main challenge is how to co-exist with WiFi because most of the unlicensed bands are extensively used by WiFi. The main co-existence solution proposed by the manufacturers are dividing the spectrum in time domain so half of the time is allocated for WiFi and another half is allocated for LTE. This cannot be very efficient because we do not know the exact demand of LTE and WiFi traffics.

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 LTE antenna In this paper, we propose a different approach for LTE-WiFi co-existence. Our main observation is most of the smartphones have LTE and WiFi antennas, and they are separated enough to have independent channels. Then considering the theory of MIMO, it is possible to decode both of the signals if WiFi and LTE are transmitted together. However, it has many fundamental challenges. First, unlike MIMO that multiple streams are transmitted using the same baseband PHY technique, WiFi and LTE have totally different PHY structure and their signals are not aligned neither in time nor in frequency domain. Second, it is a big challenge how to estimate the channel coefficients without guarantee of the clean reference symbols. Third, there are a few practical issues such as carrier sensing and synchronization. WiFi antenna Galaxy S4 antenna location (http://www.s4gru.com)

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 This paper mainly focuses on resolving these challenges. We basically allow the LTE base station and WiFi AP transmit together without any coordination, and propose a new receiver design that decodes the WiFi and LTE overlapped signal. The receiver design consists of decoding two overlapped OFDM symbol that are not aligned both in time and frequency domain, channel estimation without clean reference symbol, and solving a few practical issues such as carrier sensing and synchronization. One of our important contributions is we use USRP testbed and perform the experiment in real channel when LTE and WiFi signals are transmitted together.

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.

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

LTE Background Frame transmission is continuous SF0 SF1 SF4 SF3 SF2 SF5 SF6 SF7 SF9 SF8 10ms 1ms Reference symbol Time Frequency Frame transmission is continuous Reference symbols are transmitted periodically OFDM based transmission (FFT size- 2048) Actual data bandwidth: 18 MHz Before describing our idea, let me briefly explain the key characteristics of LTE and WiFi. In LTE, the frame transmission is continuous and there is no idle time between frame transmissions. And the reference symbols that are used for the channel estimation are sent periodically every a few subcarriers and time slots. LTE uses OFDM to address ISI, and when the channel width is set to 20MHz, FFT size is 2048, and the actual data bandwidth excluding the guardband is 18 MHz

WiFi Background Preamble is transmitted only at the beginning of a frame OFDM based transmission (FFT size - 64) Actual data bandwidth: 16.25 MHz One of the main difference between WiFi and LTE is WiFi transmission is intermittent and distributed and the transmitters accesses channel based on CSMA. Also, reference symbol that is called preamble in WiFi standard is transmitted only in the beginning of the frame transmission. One common thing between LTE and WiFi is they both OFDM, but the FFT size and subcarrier width are very different. Given the channel width is 20MHz, the actual data bandwidth is 16.25 MHz. DATA ACK Backoff interval

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

WiFi and LTE overlapped signal LTE signal WiFi signal Received signals LTE data symbol 𝑠 𝑙 𝑛 = 1 𝑁 𝑘=0 𝑁−1 𝑋 𝑙 [𝑘] 𝑒 𝑗2𝜋𝑘𝑛/𝑁 , N=2048 𝑠 𝑙 𝑠 𝑤 𝑟 1 𝑟 2 𝒉 𝟏 𝒍 𝑠 𝑤 𝑛 = 1 𝑀 𝑘=0 𝑀−1 𝑋 𝑤 [𝑘] 𝑒 𝑗2𝜋𝑘𝑛/𝑀 , M=64 𝒉 𝟐 𝒍 WiFi data symbol 𝒉 𝟏 𝒘 𝒉 𝟐 𝒘 Now let’s talk about how to decode the data when LTE and WiFi signals are overlapped. In LTE and WiFi, because they use OFDM, the data symbols are transmitted in the frequency domain, and in time domain, signals are transmitted after IFFT. At the receiver side, the signals are convoluted with the channel coefficients and LTE and WiFi signals are linearly combined at the two receiver antennas. 𝑟 1 𝑛 = 𝒉 𝟏 𝒍 ∗ 𝑠 𝑙 𝑛 + 𝒉 𝟏 𝒘 ∗ 𝑠 𝑤 𝑛 𝑟 2 𝑛 = 𝒉 𝟐 𝒍 ∗ 𝑠 𝑙 𝑛 + 𝒉 𝟐 𝒘 ∗ 𝑠 𝑤 𝑛

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 𝑋 𝑙 𝑘 Similarly, decode WiFi signal 𝑌 𝑙 𝑘 = 1 𝑁 𝑖=0 𝑁−1 𝑟[𝑖] 𝑒 −𝑗2𝜋𝑖/𝑁 = 𝐻 𝑙 𝑘 𝑋 𝑙 𝑘 + 𝐻 𝑤 𝑘 𝐼 𝑤 [𝑘] Received signal in the k-th LTE subcarrier Received signal in time domain Undecodable WiFi signal Here I will explain how to decode the LTE and WiFi overlapped signals. let’s first assume all of the channel coefficients are known at the receiver. To decode LTE signal, what the receiver does is performing FFT with respect to the FFT size of the LTE. Then the received symbol at subcarrier k is the channel coefficient of LTE multiplied by LTE data symbol plus the channel coefficient of Wifi multiplied by some unknown WiFi interference. The WiFi data symbols are unorthogonalized because its FFT size is different from LTE FFT size. But what we are interested in here is LTE signal not WiFi signal. With two received signals from 2 antennas, we can get the LTE data symbol by solving two equations with two unknowns. By repeating the same process for WiFi symbol, we can also decode WiFi data symbol. Then the next question is how to estimate the channel coefficients. 𝑌 1 𝑙 𝑘 = 𝐻 1 𝑙 𝑘 𝑋 𝑙 𝑘 + 𝐻 1 𝑤 𝑘 𝐼 𝑤 𝑘 𝑌 2 𝑙 𝑘 = 𝐻 2 𝑙 𝑘 𝑋 𝑙 𝑘 + 𝐻 2 𝑤 𝑘 𝐼 𝑤 𝑘

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

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 𝑌 1 𝑤 𝑘 = 𝐻 1 𝑤 𝑘 𝑋 𝑝 𝑤 𝑘 + 𝐻 1 𝑙 𝑘 𝐼 𝑙 𝑘 𝑌 2 𝑤 𝑘 = 𝐻 2 𝑤 𝑘 𝑋 𝑝 𝑤 𝑘 + 𝐻 2 𝑙 𝑘 𝐼 𝑙 𝑘 Known WiFi preamble 𝑌 1 𝑙 𝑘 = 𝐻 1 𝑙 𝑘 𝑋 𝑝 𝑙 𝑘 + 𝐻 1 𝑤 𝑘 𝐼 𝑤 𝑘 𝑌 2 𝑙 𝑘 = 𝐻 2 𝑙 𝑘 𝑋 𝑝 𝑙 𝑘 + 𝐻 2 𝑤 𝑘 𝐼 𝑤 𝑘 𝑌 1 𝑙 𝑘 − 𝐻 1 𝑙 𝑘 𝑋 𝑝 𝑙 𝑘 = 𝐻 1 𝑤 𝑘 𝐼 𝑤 𝑘 𝑌 2 𝑙 𝑘 − 𝐻 2 𝑙 𝑘 𝑋 𝑝 𝑙 𝑘 = 𝐻 2 𝑤 𝑘 𝐼 𝑤 𝑘 Received symbol after removing LTE interference 𝛼 𝑘 = 𝐻 2 𝑤 𝑘 𝐻 1 𝑤 𝑘 = 𝑌 2 𝑘 − 𝐻 2 𝑙 𝑘 𝑋 𝑝 𝑙 𝑘 𝑌 1 𝑘 − 𝐻 2 𝑙 𝑘 𝑋 𝑝 𝑙 𝑘

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 𝑌 1 𝑤 𝑘 = 𝐻 1 𝑤 𝑘 𝑋 𝑝 𝑤 𝑘 + 𝐻 1 𝑙 𝑘 𝐼 𝑙 𝑘 𝑌 2 𝑤 𝑘 = 𝛼 𝑘 𝐻 1 𝑤 𝑘 𝑋 𝑝 𝑤 𝑘 + 𝐻 2 𝑙 𝑘 𝐼 𝑙 𝑘 𝑌 1 𝑤 𝑘 = 𝐻 1 𝑤 𝑘 𝑋 𝑝 𝑤 𝑘 + 𝐻 1 𝑙 𝑘 𝐼 𝑙 𝑘 𝑌 2 𝑤 𝑘 = 𝐻 2 𝑤 𝑘 𝑋 𝑝 𝑤 𝑘 + 𝐻 2 𝑙 𝑘 𝐼 𝑙 𝑘 𝐻 2 𝑤 𝑘 = 𝛼 𝑘 𝐻 1 𝑤 𝑘 𝐻 1 𝑤 [𝑘] 𝐼 𝑙 𝑘 = 𝑋 𝑝 𝑤 𝑘 𝐻 1 𝑙 𝑘 𝛼 𝑘 𝑋 𝑝 𝑤 𝑘 𝐻 2 𝑙 𝑘 −1 𝑌 1 𝑤 𝑘 𝑌 2 𝑤 𝑘 Given this alpha_k, we can reduce the number of unknowns of the previous equations from 3 to 2, so we can solve the equations and finally we can get the channel of WiFi.

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

Decode when neither channel is known How 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 In the previous slides, we showed that it’s feasible to get the clean LTE reference symbol and we can estimate LTE channel, and using the estimated LTE channel, we can also estimate WiFi channel. But the main difficulty in getting the LTE channel in advance is it requires us to always decode LTE signal and estimate LTE channel even if we have no data to receive, which can be energy inefficient. Here we introduce the way to jointly decode LTE and WiFi channel that does not require the LTE channel estimation in advance. The main idea is using the channel border that are not overlapped. When the channel width is set to 20MHz, the actual transmission bandwidth of LTE is 18 MHz and WiFi is 16.25MHz. Therefore, at one border of the channel, about 1MHz LTE channels are not overlapped with WiFi channel. Using it, we can estimate the LTE channel of the boundary channel without WiFi interference. However, still most of channels are overlapped with WiFi data. For that part, we use extrapolation and successive channel estimation. Also, to improve the channel estimation accuracy, we iteratively estimate the channel using the data symbol

Joint LTE and WiFi channel estimation Estimate the boundary LTE channel without interference Estimate the first interfered LTE channel through extrapolation Estimate the channel of WiFi subcarrier using known LTE channel Estimate the LTE channels using known WiFi channel Repeat 2-4 for the remaining part of the half channel Repeat 1-5 for the other half LTE Reference signal WiFi OFDM symbol Time Freq. WiFi subcarrier LTE subcarrier 1 2 3 4 5 Here I illustrate the joint channel estimation. In the beginning, we first estimate LTE channel in the spectrum boundary without WiFi interference. Second, we estimate the LTE channel of the closest subcarrier through extrapolation. This is feasible because the channel coefficients of the nearby subcarriers has very strong correlation. After that, using the estimated LTE channel, we estimate the channel coefficient for the first subcarrier of the WiFi channel. The method is exactly the same with the estimation of WiFi channel with known LTE channel that I just explained. Once we finish the WiFi channel estimation, using it, we estimate the LTE channel overlapped with the first WiFi subcarrier. Then we repeat the same process to the second WiFi subcarrier and LTE subcarriers overlapped with it, and we repeat it until we estimate the half of the channel. And then we repeat the same job from another side of the channel border.

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 The channel estimation using the interfered signals is pretty challenging, so base on our experimental result, that alone does not provide sufficient estimation performance. To improve the accuracy, we use data symbols for the channel estimation. Initially, we estimate the channel using WiFi preamble and LTE reference symbol. Using the initial channel estimation result, we demodulate WiFi symbols and decode them and remodulate them to find the original constellation points of the data symbols. After that, using the WiFi data symbols, we perform the channel estimation again. This is helpful improving the accuracy because WiFi has only 2 known OFDM symbols that can be used for the channel estimation. But if we use data symbols for the estimation, we can get a few hundreds of known symbols in addition for the channel estimation. But it can be a tradeoff because it increases the complexity. In the performance evaluation, we show how many data symbols are required to get the sufficient channel estimation accuracy.

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 In this talk, the main issue is decoding the interfered signal and the channel estimation. Besides it, there are several practical issues to enable LTE and WiFi coexistence in unlicensed band. I’m not going to introduce them in detail today, but the practical issues that we address in the paper are decoding wifi MIMO signals, carrier sensing, synchronization, and how to handle when LTE and WiFi channel width is not the same. You can find the detail in the paper.

Performance Evaluation USRP testbed 10 MHz channel USRP nodes at 8 locations with different channel conditions Use the best rate at each location Now I will introduce the performance evaluation result. For the experiment in real channel, we set up the testbed with USRP software radio. Because of the limitation of the processing power, we set the channel bandwidth to 10MHz. We evaluated the throughput performance and the accuracy of the channel estimation. the best modulation is used among BPSK, QPSK, 16QAM for WiFi, and QPSK and 16QAM for LTE

Compare with TDMA 1.87x throughput gain from time-division The first result is the comparison with time division based approach. In time division approach, half of the time is used for LTE and the other half is used for WiFi. In terms of the average throughput across all locations, our approach gets 87% higher throughput compared to the time division approach. It is slightly less than 2 because of the imperfect channel estimation. 1.87x throughput gain from time-division

Compare different channel estimations Here I compare the throughput with different channel estimation method. Compared to the channel estimation with clean LTE reference WiFi preamble, our scheme has slight throughput reduction. The throughput of our scheme is 93% of the throughput of known LTE and Wifi preamble. Here ‘extrapolation only’ is estimating the channel only using the boundary channel and only relies on the extrapolation to estimate the other interfered channels. Because the channel condition we performed the experiment was frequency-selective, the performance of the extrapolation is not satisfactory. 96% throughput compared to known LTE channel 93% throughput compared to known LTE and WiFi channel

Estimate channel using different # WiFi symbols Here we evaluate how many symbols are required for the channel estimation with sufficient accuracy. This graph shows the throughput in 8 locations with different number of data symbols for decoding. If you the result, you can see throughput is a little low when 20 symbols are used for the channel estimation. If more than 50 symbols are used, the throughput is not distinguishable. Therefore, we conclude 50 symbols are sufficient for the accurate channel estimation. 50 data symbols are sufficient for channel estimation

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

Thank you!

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

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!

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

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