Tian Hao, Ruogu Zhou, Guoliang Xing
This paper presents COBRA -- a visible light communication (VLC) system for off-the-shelf smartphones. COBRA encodes information into specially designed 2D color barcodes and streams them between screen and camera of smartphones. Due to the directionality and short range of visible light, COBRA can preserve user privacy and security in many near field communication scenarios such as opportunistic data exchange between smartphones. We develop a new 2D color barcode for COBRA that is optimized for streaming between small-size screen and low-speed camera of smartphones. COBRA adapts the size and layout of code blocks in streamed barcodes to deal with the significant image blur in mobile environments, and adopts new image processing techniques to achieve real-time barcode stream decoding. Our approach is evaluated through extensive experiments on Android smartphones.
Public Review uploaded by MarcoGruteser:
This public review has been prepared by Marco Gruteser. Over the last years there has been an increasing interest in exploiting visible light and cameras to communicate information. QR codes are one example that has already been widely adopted in practice. Research has been pushing the limits of this approach by using codes that change over time on LCD monitors or LED transmitters that are visible over longer ranges.
The novel contribution of this paper is that it explores this communication approach between two modern smartphones. One smartphone’s screen acts as a transmitter and the camera of the other phone as the receiver. While colored and time-varying barcodes have been considered before, it is, to our knowledge, the first to characterize the achievable throughput in this phone-to-phone scenario and to address the specific challenges of phone hardware. It also proposes a novel block size adaptation technique to deal with camera shaking.
As with most papers, there are several aspects that could be improved. It would have been interesting, for example, if the paper had considered more varied lighting conditions or if it had described experiments with bidirectional phone-to-phone communications. A particular concern with the paper, however, was that the motivational scenarios were not fully convincing. Many do not seem to require the higher throughput streaming method developed in this paper and for those that do, conventional radio frequency communication can provide even higher throughput. After all, the throughput of ~100kbps is rather limited compared to existing phone-to-phone communication systems such as Bluetooth. There are certainly security benefits to color barcode streaming, since an adversary would need line-of-sight to the smartphone screen to eavesdrop on the signal. Even here, however, a simple ‘low throughput’ key exchange through a screen could be followed by cryptographically secured high-throughput radio-frequency communication.
Overall, though, the program committee was excited by the idea of exploiting this novel communication channel, liked innovations such as adaptation of block size based on accelerometer readings and appreciated the thorough system evaluation. The committee also thought that this paper will stimulate discussion at the conference, which might help in identifying a killer application.
In this paper, we explore the possibilities of visible light communication on off-the-shelf smartphone platforms. By leveraging the directional and short-range nature of visible light, COBRA can preserve the security and privacy of communication. A new 2D barcode design is proposed for real-time streaming on smartpones. Several novel techniques are developed to address the chanllenges of low-quality cameras and significant dynamics in phone-to-phone communication. For example, in mobile scenarios, COBRA can adaptively change the size and layout of the code blocks to mitigate the image blur caused by motion. We are particularly excited by the experimental results that COBRA achieves high communication throughput and reliability under a range of realistic settings on light conditions, view angles, etc.
COBRA is mainly designed for one-way communication scenarios such as advertisement and wireless payments. In the future, we will extend COBRA to support feedback from the receiver by leveraging the front-facing cameras that are available on many modern smartphones. We have evaluated the performance of COBRA under different ambient lighting intensities ranging from 1 to 350 lux. In future work, we will conduct a more comprehensive evaluation on the impact of lighting conditions. However, the key idea of COBRA is to encode information by changing the colors of code blocks, which is much more resilient to ambient lighting conditions than existing approaches, such as PixNet, which modulate brightness of screens.
We believe the high-throughput visible light communication of COBRA is suitable for a range of applications like photo exchange between smartphone users and advertisements (e.g., displaying brochures and coupons in retail stores and museums). Although the alternative RF-based solutions can provide higher throughput, due to the broadcast nature of wireless signals, they are more subject to privacy and security breach. Moreover, compared with Bluetooth and Wi-Fi, COBRA offers a more intuitive, 'point and shoot' communication interface, without requiring a cumbersome authentication process.