Optimizing state-information feedback in downlink wireless systems [electronic resource]
Keywords: Electrical Engineering

Future wireless networks will be driven by various ubiquitous broadband services such as portable telephony, mobile Internet, Voice over IP (VoIP), and IPTV. A cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a base-station (BS). A cell is an area of radio coverage in a cellular network that serves multiple mobile users. To accommodate a single cell's large number of users, each running a different application on its mobile terminal, optimization of limited resources is critical. In a cellular network, 'downlink' is the transmission path from the BS to the mobile terminal. In the downlink, feedback of mobile user's condition to the BS is essential for optimal allocation of limited resources, however, it is a system design challenge. Mobile user's condition is comprised of its wireless channel condition, among multiple other such system metrics. Therefore, this thesis explores optimal resource allocation in the downlink, also known as broadcast channel (BC), with limited or partial knowledge of a mobile user's condition, referred to as state information, at the BS. This thesis first explores the case where both the BS and the users are equipped with a single antenna and only channel state information (CSI) constitutes the state-information. In this configuration, multiple forms of partial CSI feedback are studied. The ergodic rate region for a fading broadcast channel, when the transmitter knows only the channel priority (ordering) information of the users at any time instant, is derived. The ergodic rate region is the set of all long-term average data-rates achievable in a fading channel with arbitrarily small probability of error. Next the rate region for a 2-user fading BC, in which instantaneous channel state information at the transmitter (CSIT) is available for only one of the users, is studied. Then, a fading BC with only a quantized version of CSI at the transmitter, is analyzed. Thereby, this thesis explored partial CSIT's three possible definitions. Future wireless systems use multiple antennas both at the transmitter and receiver(s) to achieve a higher data rate. These systems are called multiple input multiple output (MIMO) systems. An important design concern for the MIMO BC is the complexity of the transmission technique to maximize total data-rate for the downlink users. The problem of finding the optimal transmit filter (or beamforming matrix) to maximize total data rate is intractable. This thesis derives a low complexity algorithm for finding a near-optimum transmit filter. Also, an opportunistic feedback (OF) protocol that achieves the highest possible total data rate, also called sum-capacity, in the fading MIMO BC with limited CSI feedback is proposed. A popular transmission technique for downlink is orthogonal frequency division multiple access (OFDMA) and this technique is now a part of almost every next-generation wireless standard. In an OFDMA system, each downlink user is transmitted to on orthogonal sub-carrier(s) or tones. Optimal tone allocation in downlink OFDMA networks is an NP-hard problem that requires extensive feedback for CSI. Two constant complexity limited-feedback algorithms are designed to achieve near-optimal performance. Using opportunistic feedback (OF), the proposed schemes are shown to reduce feedback overhead by requiring only users likely to be allocated resources to feed back. Additionally, the optimal allocation of feedback resources is derived for the case of fixed feedback rate. Specifically, the optimal number of tones grouped as a subchannel, the number of users that feed back for any subchannel and the number of bits used for quantization of CSI is found in this setting. The last part of this dissertation deals with the case when a downlink users's state information comprises both CSI and the number of packets in its receive buffer. With the advent of new applications, one of the primary applications supported in downlink cellular systems is streaming video. Quality of service (QoS) for streaming video entails data transmission to the mobile stations with limited latency and limited jitter, which maps to the number of yet-to-be-played packets in a mobile station's buffer, to always be above a certain minimum. This thesis solves the problem of optimal resource allocation in a downlink OFDMA system with this buffer constraint.