1 An Interference Cancellation Scheme for The Multiuser TRDMA Uplink System Feng Han and K. J. Ray Liu, Department of ECE, University of Maryland, College Park, MD, 20742, USA Origin Wireless Communications, College Park, MD, 20740, USA Email: {hanf, kjrliu}@umd.edu Abstract—The concept of Time Reversal Division Multiple Access (TRDMA) has recently been proposed as a promising medium access technology for the multi-user wireless broad- band communications. Compared with the existing multi-carrier technology like OFDM/OFDMA, the TRDMA provides a cost- effective single-carrier alternative technology to combat the inter- symbol interference (ISI) for broadband communications, and at the same time leverages the degrees of freedom in a large number of multi-paths to form a unique high-resolution spatial focusing effect. Previous work on TRDMA mainly focus on the multi-user downlink system. In this paper, we first introduce a TRDMA-based multi-user uplink architecture and then propose a 2-dimensional (2D) parallel interference cancellation scheme to further enhance the system performance. The TRDMA uplink architecture keeps the cost of end-users at a minimum level, and reuses the processing power at the base station (BS) that has already been made available for the downlink. The proposed 2D parallel interference cancellation scheme utilizes the tentative decisions of detected symbols to effectively cancel the interference in both the time domain (i.e. ISI) and the user domain (i.e. inter- user interference (IUI)), which significantly improve the bit-error- rate performance in the high signal-to-noise-ratio (SNR) regime. Simulation results are provided and compared with the basic TRDMA system without interference cancellation. Index Terms—Time Reversal, TRDMA, interference cancella- tion I. I NTRODUCTION Very recently, the concept of time reversal division access (TRDMA) was introduced as a novel multi-user media ac- cess scheme for broadband communication systems [1]. The broadband communications over channels with large delay spread can be very challenging due to the severe inter-symbol interference (ISI). Conventionally, complicated multi-carrier techniques (like OFDM/OFDMA) are used to alleviate ISI [2]–[5]. Leveraging the unique temporal and spatial focusing effects of the time reversal (TR) phenomenon [6], [7], the TRDMA provides a cost-effective single-carrier alternative for broadband multi-user communications. The TRDMA scheme uses the multi-path channel profile associated with each us- er’s location as a location-specific signature for the user. In essence, each path of the multi-path channel is treated as a virtual antenna in the TRDMA, which collectively results in very high-resolution spatial focusing with “pin-point” accura- cy. Meanwhile, the temporal focusing effect effectively sup- presses ISI which significantly simplifies the terminal user’s complexity and gives rise to higher-speed data transmission. The authors in paper [1] focused on a broadband multi- user downlink system based on the TRDMA concept. In such a TRDMA downlink system, the base station (BS) transmit multiple simultaneous data streams to different users, each of which is associated with a unique multi-path profile of its channel in rich-scattering environments. The TRDMA downlink scheme exploits the spacial degrees of freedom of the environment, and focuses the useful signal power only at the intended locations. The time reversal mirrors (TRMs) [8], [9] at the BS first time-reverse 1 the channel impulse response (CIR) of each user’s channel as the user’s signature waveform, and then embed these signatures into the corresponding data streams. The transmitted signal from the BS in the TRDMA downlink is a mixed signal consisting of all the users’ data. When such a combined signal propagates to a certain user through the corresponding multi-path channel, the power of the useful signal component that carries this user’s data will automatically be boosted out of the combination thanks to the spatial focusing effect. Within the TRDMA framework, more sophisticated signature waveforms than the basic TR- waveform can be derived based on the multi-path channel responses to further improve the performance of the TRDMA downlink system, when additional computational complexity is affordable at the BS [10]. One very desirable feature of the TRDMA downlink scheme proposed in [1] is that most of the complexity can be shifted to the BS side, facilitating the extremely low complexity at the end-users. In line with the same design philosophy of minimizing the complexity of the terminal users, a TRDMA based uplink scheme can be developed. As one will see in this paper, the proposed TRDMA uplink scheme shares a strong duality in the mathematical structure with the downlink without increasing the complexity of the end-users. And as such, an equivalent spacial focusing effect (although not physically in the space domain) can be observed in the user’s signature domain at the BS. Similar to in the downlink scheme, the equivalent spacial focusing effect enables the BS to use the user’s TR signature waveform to extract the useful component out of the combined received signal, allowing multiple users access- ing the BS simultaneously. Additionally, unlike many other conventional communications paradigms that adopt symmetric architectures, the proposed uplink scheme shares the same processing power and channel knowledge at the BS with the 1 i.e. to rearrange the received waveform reversely over time. Globecom 2013 - Wireless Communications Symposium 978-1-4799-1353-4/13/$31.00 ©2013 IEEE 3583