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Two-Layered Superposition of Broadcast/Multicast and Unicast Signals in Multiuser OFDMA Systems

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Two-Layered Superposition of Broadcast/Multicast and Unicast Signals in Multiuser OFDMA Systems This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication. The final version of record is available at http://dx.doi.org/10.1109/TWC.2019.2950205 Two-Layered Superposition of Broadcast/Multicast and Unicast Signals in Multiuser OFDMA Systems David Vargas and Yong Jin Daniel Kim, Member, IEEE Abstract— We study optimal delivery strategies of one increasingly personal, interactive and immersive where the common and K independent messages from a source to multiple audio-visual content is changed according to the user users in wireless environments. In particular, two-layered requirements. Object-based Media, where the creation and superposition of broadcast/multicast and unicast signals is storage of the elements of a film, Radio or TV programme is considered in a downlink multiuser OFDMA system. In the literature and industry, the two-layer superposition is often done as media objects, allows content to be rendered at the considered as a pragmatic approach to make a compromise receivers in the form most suitable for a particular user (e.g. between the simple but suboptimal orthogonal multiplexing (OM) additional camera angles of an live sport event or changing the and the optimal but complex fully-layered non-orthogonal presenter for hearing-impaired viewers) [1]-[2]. multiplexing. In this work, we show that only two-layers are The efficient wireless delivery of these existing and new necessary to achieve the maximum sum-rate when the common experiences to multiple users require mobile broadband message has higher priority than the K individual unicast messages, and OM cannot be sum-rate optimal in general. We technologies with the capability to multiplex into the same develop an algorithm that finds the optimal power allocation over system broadcast/multicast signals (to convey common the two-layers and across the OFDMA radio resources in static information, e.g., live TV) and unicast signals (to convey channels and a class of fading channels. Two main use-cases are independent messages for personalisation). In this paper, considered: i) Multicast and unicast multiplexing when K users unicast refers to one-to-one communication between a with uplink capabilities request both common and independent transmitter and a receiver with uplink capability that can feed messages, and ii) broadcast and unicast multiplexing when the common message targets receive-only devices and K users with back the channel-state-information (CSI). Both multicast and uplink capabilities additionally request independent messages. broadcast refer to the transmission of a common message to Finally, we develop a transceiver design for broadcast/multicast multiple receivers, but in multicast the receivers have uplink and unicast superposition transmission based on LTE-A-Pro capabilities and can feedback to the transmitter CSI whereas in physical layer and show with numerical evaluations in mobile broadcast the receivers lack uplink capabilities and/or cannot environments with multipath propagation that the capacity feedback the CSI. The 3rd Generation Partnership Project improvements can be translated into significant practical performance gains compared to the orthogonal schemes in the (3GPP) introduced Multimedia Broadcast Multicast Services 3GPP specifications. The impact of real channel estimation is also (MBMS) in the third and fourth generation mobile broadband analyzed and it is shown that significant gains in terms of spectral specifications (3G and 4G), where broadcast/multicast and efficiency or increased coverage area are still available even in the unicast signals are allocated to separate time and frequency presence of estimation errors and imperfect interference resources, i.e., orthogonal multiplexing (OM), cf. Fig. 1a. cancellation for the two-layered superposition system. However, with the development of the fifth generation (5G) mobile broadband standards, there is an increasing interest in Index Terms— Broadcast, multicast, unicast, channel capacity, the industry in non-orthogonal multiplexing (NOM) OFDMA, non-orthogonal multiplexing, superposition coding, successive cancellation, LTE-Advanced Pro, 5G, New Radio. approaches that can increase the spectral efficiency (SE) by transmitting superimposed signals with different robustness I. INTRODUCTION requirements. In this paper, we study optimal delivery strategies of common and independent messages from one HE landscape of media consumption is radically changing source to multiple receivers in wireless environments 1 . thanks to the proliferation of smartphones and tablets with T Particularly, we consider a two-layered superposition of mobile internet connectivity. Users can now experience media broadcast/multicast and unicast signals in the downlink services in different environments, from in-home to on the multiuser scenario setup where different unicast signals are move where the consumption of live or on-demand TV allocated orthogonal resources, cf. Fig. 1b. programmes can be combined with the interaction of social media networks. Furthermore, the Media and Entertainment A. Related Work industry is evolving towards media experiences that are The theoretical analysis of the single-input single-output Gaussian downlink multiuser channel with K independent plus D. Vargas is with the BBC Research and Development, The Lighthouse, White City Place, 201 Wood Lane, London, W12 7TQ, U.K., (email: [email protected]). 1 The term broadcast channel, as used in information theory, often refers to Y.J.D. Kim is with the Department of Electrical and Computer Engineering, unicast transmission in downlink channel. In this paper, we give a broader Rose-Hulman Institute of Technology, Terre Haute, IN 47803 USA (e-mail: meaning to the term and use it to refer to a channel where a single source [email protected]). communicates any types of information simultaneously to multiple receivers. Copyright (c) 2019 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing [email protected]. This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication. The final version of record is available at http://dx.doi.org/10.1109/TWC.2019.2950205 M 0 M K M M 1 2 M0 M 2 M0 M 1 M 2 M1 frequency frequency M frequency 3 M M M K K 3 time time M0 time (a) Orthogonal Multiplexing (b) Two-layer Non-orthogonal Multiplexing (c) K+1 Layer Non-orthogonal Multiplexing Fig. 1. Comparison of different multiplexing methods on OFDMA: a) the conventional orthogonal multiplexing, b) the considered two-layer non-orthogonal multiplexing, and c) K+1 layer non-orthogonal multiplexing. one common messages to K receivers began in the 1970’s, different types of terminals where each type receives a common showing that the capacity is achieved by NOM of all messages message, and is generally different from the unicast in a K+1 layer architecture, cf. Fig. 1c, followed by multiuser transmission where each user requests independent message. decoding at each receiver [3]. Despite this seminal work, more The topic of joint broadcast/multicast and unicast than 30 years later the physical layer of 4G Long Term transmission is starting to get more attention in the recent Evolution - Advanced Pro (LTE-A-Pro) [4] and the recently research literature. In [25]-[27], the superposition of a multicast standardised 5G New Radio (NR) physical layer [5] use and a unicast message is considered with the users employing Orthogonal Frequency Division Multiple Access (OFDMA) as successive interference cancellation (SIC) to decode and the principal multiple access technology, motivated by the remove the multicast message before decoding the unicast reduced receiver complexity and flexible implementation of message. However, the scope is limited to transmission of a user scheduling algorithms. OFDMA assigns orthogonal time single unicast message targeting either one particular user in the and frequency resources to signals, and in LTE-A-Pro the network [27] or the user with the highest channel quality multiplexing of broadcast/multicast and unicast signals can only be realized orthogonally by evolved MBMS (eMBMS). [25]-[26]. Transmission of more than one unicast messages is Without a common message, the capacity region of downlink considered in [28]-[31]. In these works, after decoding and multiuser OFDMA with K unicast messages along with the subtracting the multicast signal, each user decodes the desired optimal power allocation are characterized in [6]. It is shown unicast signal while treating unicast signals that are intended that the maximal sum-rate is achieved by OM where one user for other users as additional noise. In order to minimize this with the best channel gain is assigned to each subcarrier and the interference between the unicast signals, linear beamforming in power is distributed across the subcarriers by a water-filling the spatial domain is proposed in [28] as well as cooperation policy [6]-[7]. In the presence of a common message, the between the base-stations in a multi-cell downlink in [29]-[31]. capacity region and the optimal power allocation are In [32], the broadcast message is placed in the null-space of the characterized for the case of two parallel channels in [8] and for channel collectively seen by the unicast users so that the more than two channels in [9], but for both references
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