Why Time-Reversal for Future 5G Wireless? K. J. Ray Liu Department of Electrical and Computer Engineering University of Maryland, College Park Acknowledgement: the Origin Wireless Team
What is Time-Reversal? Transmitter Receiver
What is Time-Reversal? Transmitter Receiver
A Fundamental Laws of Physics 6-10 db peak 1-2 cm resolution Describe a mutually stimulating resonances through the interaction with the surrounding environment.
Multipaths = Degrees of Freedom A large number of degrees of freedom existing in nature, ready for engineering applications How to harvest multipaths? Power and Bandwidth! The larger the power, the more observable multipaths. When the power is fixed, so as the maximum observable multipaths. The larger the bandwidth, the better the time resolution, therefore the more multipaths can be revealed.
System Theory Point of View Multi-paths create unique, location-specific signatures. time-reversed location-specific channel response signal in out A CHANNEL h(t) H(w) signal out in B The environment is the computer performing a perfect deconvolution, a matched filter!
A Brief History Compensating phase-delay distortion over telephone line/network [ 50 s] Designing noncausal recursive filter with zero phase shift [ 50 s] Phase conjugation in optics [ 70 s] Acoustics communication [ 90 s] Radar and imaging problems [ 00 s] Electromagnetic/RF radio [ 05 s] Information and Communications Technology [10 s] 7
Why Now? Advance of analog technologies Moore s Law ADC has been cost down dramatically 1G 30 khz; 2G 200 khz; 3G 2-5M Hz; 4G 20 MHz 5G? 200M-1G Hz? When the bandwidth is large enough Spectral efficiency becomes less significant Leveraged to reduce complexity, energy consumption, and offer other benefits
What 5G Envisions? Green low-power solution Massive MIMO effect for spectral efficiency Much larger bandwidth New modulation/multiple access Highly scalable for extreme densification Cloud-based radio for low cost devices D2D communications/iot More secure communications
Is Wireless Transmission Energy Efficient? fading due to multipath propagation Shadowing due to obstacles Path loss O(d -2 ) ~ O(d -6 ) 10
Green Wireless Communications How to qualify as Green? low energy consumption (environmental concerns) low radio pollution to others (health/interference concerns) Is less the answer for green? What are qualified to be called a new green technology? An order of magnitude less in energy consumption An order of magnitude less in radio pollution Much less complexity 11 B. Wang, Y. Wu, F. Han, Y.H. Yang, K.J.R. Liu, "Green Wireless Communications: A Time- Reversal Paradigm", IEEE Journal of Selected Areas in Communications, Sep 2011.
Advantages Time-Reversal is an ideal paradigm for green wireless communications inherent nature to fully harvest energy from the surrounding environment exploit all the multi-path propagation The environment is performing the deconvolution like a natural matched filter, e.g. the computer! As such Much less complexity no need of channel estimation Need much less energy to achieve the same performance Create much less interference to others But with an order of magnitude? 12
Numerical Result The expected ratio of energy needed for a TR system compared with Rake receivers For typical parameters, only 10~20% power is needed in a TR system. 13
Numerical Result The expected ratio of interference alleviation for a TR system compared with Rake receivers For typical parameters, interference can be reduced 30dB lower. 14
Why Massive MIMO for 5G? High spectral efficiency Simple spatial multiplexing/ de-multiplexing Massive MIMO at Lund University, Sweden Size: 0.8mx1.2mx1m Weight: 300kg Average power consumption: 2.5kW
Time-Reversal Prototype Size:5cmx15cmx20cm Weight: 400g Average power consumption: 0.1W
Achieving Massive MIMO using Virtual Antennas Time-reversal offers a simple, low complexity alternative to realize the massive MIMO effect. 17
Asymptotic Performance Where Kmax is the maximum observable multipaths When power and bandwidth are large enough, one can harvest enough multipaths to achieve massive MIMO effect using time-reversal.
Ideal for Large Bandwidth The larger the bandwidth, the higher degrees of freedom harvested! Want more? Easy, add one more antenna!
Using Bandwidth, Not Antennas! A single-antenna TR system with 1G Hz bandwidth can achieve comparable, if not better, rates with the genieaided massive MIMO system!
Optimal Waveform Design Non-orthogonal waveform leads to inter-user-interference (IUI) The large delay spread leads to severe inter-symbol interference (ISI)
Beyond Time-Reversal How to Control Virtual Antennas? The waveform can be designed such that the receive SINR is maximized, but no longer time-reversal! 22
Comparison with LTE-A The bandwidths of LTE, LTE-A and TRDMA are 20MHz, 100MHz, and 500MHz, respectively. Y. Chen, Y.H. Yang, F. Han, and K.J.R. Liu, "Time-Reversal Wideband Communications", IEEE Signal Processing Letters, vol 20, no 12, pp.1219-1222, Dec 2013.
Time-Reversal Division Multiple Access (TRDMA) Leveraging users location-specific signatures Unique multi-path profiles under rich-scattering environment TR spatial focusing effect maps such profiles into signatures These signatures can be exploited to separate intended signals, providing a novel wireless channel access method TRDMA F. Han, Y.H. Yang, B. Wang, Y. Wu, and K.J.R. Liu, "Time-Reversal Division Multiple Access over Multi-path Channels", IEEE Trans. on Communications, Vol 60, no 7, pp.1953-1965, July 2012. 24
TRDMA System Architecture Asymmetric communication architecture Signal separation solely depends on TRDMA Very low complexity at the client sides
Achievable Rate Region The high-resolution spatial focusing effect of TR mitigates the mutual interference, yielding a better achievable rate region than the conventional rake-receiver based scheme. 26
New 5G Indoor Technology Base-station technologies: FDMA: 1G 30k Hz TDMA: 2G 200k Hz CDMA: 3G 2-5 MHz OFDMA: 4G 20 MHz 5G 200 MHz 1 GHz? A new communication paradigm ideal for broadband indoor wireless communications, where severe interference/multipath is a killer of other technologies.
Extreme Wireless Densification More access points, users, devices,.. Traditional radios are range-based with near-far effect the notion of cellular High densification results in more smaller cells, thus more interference But interference management is a zero-sum game!
Strong-Weak Focusing Effect Time-reversal resonance depicts a strong-weak focusing effect, not near-far distance! It is highly scalable for extreme network densification More APs can be easily added without creating interference to others Self-configuration: newly added APs help pick up users and reuse the same entire spectrum 29
Self-Configuring without Cells
Aggregated Sum Rate TR system s sum rate increases as the number of APs increases, while 802.11 is a zero-sum game.
Graceful Degradation Each user s performance degradation of the 802.11 system is much more severe than the TR system.
Cloud-based Radio Access Networks (C-RAN) One key bottleneck of current C-RAN structures is the front-haul link capacity
A Time-Reversal Solution User 1 1 TR TX Symbols - User 1 0.8 Signature (Magnitude) - User 1 1 TR TX Signal - User 1 0.8 0.7 0.8 0.6 0.4 0.6 0.6 0.2 0.5 0.4 0 0.4 0.2-0.2-0.4 0.3 0-0.2-0.6 0.2-0.4-0.8 0.1-0.6 1 TR TX Signal - All Users -1 50 100 150 200 250 0 0 5 10 15 20 25 30-0.8 0.8 Data to be transmitted Signature -1 50 100 150 200 250 Data modulated with signature 0.6 0.4 0.2 0-0.2-0.4-0.6 1 TR TX Symbols - User 3 0.9 Signature (Magnitude) - User 3 1 TR TX Signal - User 3-0.8 0.8 0.6 0.4 0.2 0-0.2-0.4-0.6-0.8-1 50 100 150 200 250 User 2 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 5 10 15 20 25 30 0.8 0.6 0.4 0.2 0-0.2-0.4-0.6-0.8-1 50 100 150 200 250-1 50 100 150 200 250 Transmitted signals Waveforms have been separated in space-time!
Time-Reversal Tunneling Effect Increase linearly Keep constant
Stealth/Secure Wireless Communications Traditional security measures may become insufficient to protect wireless networks Broadcasting nature of wireless communications Known channel dynamics and distributed structure TR-based communications can be exploited to enhance PHY-layer security Unique location-specific signatures only known by the legitimate transmitter and receiver In other locations, signal is spread out and hidden below the noise floor 36
An Experiment (a) (b) The desired location: (a) Signal constellation, (b) data loss, and (c) data image encoded on a timereversed channel waveform at the Origin and received by the Bot in its original location. (c) (a) (b) Another location: (a) Signal constellation, (b) data loss, and (c) data image encoded on a timereversed channel waveform at the Origin and received by the Bot away from its intended location. (c)
Internet of Things Future Smart Home The Internet of Things allows people and things to be connected Anytime, Anyplace, with Anything and Anyone, ideally using Any path/network Any service.
D-2-D in IoT with Heterogeneous Bandwidth Y. Chen, F. Han, Y.H. Yang, H. Ma, Y. Han, C.X. Jiang, H.Q. Lai, D. Claffey, Z. Safar, and K.J.R. Liu, "Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview", IEEE Internet of Things Journal, Feb 2014.
Indoor Locationing Multipaths used to make indoor locationing very challenging about 3m accuracy! Time-reversal uses multipaths to create a very high resolution of focusing effect It is unique and location-specific An ill-posted problem becomes two routines: Create a logical location database in the multipath space Map the physical locations to the logical ones Z.H. Wu, Y. Han, Y. Chen, and K.J.R. Liu, "A Time-Reversal Paradigm for Indoor Positioning System", IEEE Transactions on Vehicular Technology, special section on Indoor localization, tracking, and mapping with heterogeneous technologies, April 2015.
3D Testing Platform
NLOS Localization Performance Perfect localization performance at the scale of 1-2 cm within a 0.9m-by-1m area!
Wireless Events Detection Multiple doors detection Single TX RX pair Multiple TX-RX locations: 2 Origin locations (Loc B and Loc C) 4 Bot locations (green dots)
Detection Rate & False Alarm Average Accuracy: LOC B (NLOS TX-RX location) LOC B TX 1 TX 2 TX 3 TX 4 Detection Rate (%) 99.12 99.5 99.67 99.81 False Alarm (%) 0.88 0.5 0.33 0.19 Average Accuracy: LOC C (LOS TX-RX location) LOC C TX 1 TX 2 TX 3 TX 4 Detection Rate (%) 99.09 99.28 99.31 99.35 False Alarm (%) 0.91 0.72 0.69 0.65
ROC Curves Performance comparison between TR and RSSI method. Fig. ROC curve for detecting between all doors open and D8 close under LOC B. Fig. ROC curve for detecting between D1 close and D8 close under LOC B.
Conclusions Time-reversal is energy efficient Low complexity means low cost A true 5G solution address all major issues envisioned by 5G technology Ideal for Internet of Things connect many devices, indoor locationing, events recognition Naturally stealth and secure And. Many more
Final Thoughts Time-Reversal is a fundamental physical principle, not a man-made phenomenon Multipaths are degrees of freedom existing in nature One can harvest multipaths from nature using power and bandwidth We start to understand its impact on communications and information technology (ICT) A time-reversal era is about to embark (back to the future? ) 48
Why Time-Reversal for Future 5G Wireless? K. J. Ray Liu Department of Electrical and Computer Engineering University of Maryland, College Park Acknowledgement: the Origin Wireless Team