Stephen Kaminski

Throughput maximization under rate requirements for the OFDMA downlink channel with limited feedback

The purpose of this paper is to show the potential of UMTS long-term evolution using OFDM modulation by adopting a combined perspective on feedback channel design and resource allocation for OFDMA multiuser downlink channel. First, we provide an efficient feedback scheme that we call mobility-dependent successive refinement that enormously reduces the necessary feedback capacity demand. The main idea is not to report the complete frequency response all at once but in subsequent parts. Subsequent parts will be further refined in this process. After a predefined number of time slots, outdated parts are updated depending on the reported mobility class of the users. It is shown that this scheme requires very low feedback capacity and works even within the strict feedback capacity requirements of standard HSDPA. Then, by using this feedback scheme, we present a scheduling strategy which solves a weighted sum rate maximization problem for given rate requirements. This is a discrete optimization problem with nondifferentiable nonconvex objective due to the discrete properties of practical systems. In order to efficiently solve this problem, we present an algorithm which is motivated by a weight matching strategy stemming from a Lagrangian approach. We evaluate this algorithm and show that it outperforms a standard algorithm which is based on the well-known Hungarian algorithm both in achieved throughput, delay, and computational complexity.

Very Tight Coupling of Wireless LANs and UMTs Networks: A Technical Challenge and an Opportunity for Mobile Operators

Coupling of Wireless LANs like IEEE 802.11a/b or HIPERLAN/2 systems with Public Land Mobile Networks (PLMNs) like GSM or UMTS offers benefits for both operators and users. Coverage and capacity of an operator’s network can be extended with equipment expected to be considerably cheaper than for PLMN coverage. Thus, the operator can participate in the expected WLAN boom. Its customer experiences homogenous access to services with high data rate. PLMNs and WLANs should be considered as complementary systems: PLMNs provide universal coverage and high mobility support, while pico-cellular WLANs will be applied in hot spot areas, offering high data rates (up to about 45 Mbit/s/cell in HIPERLAN/2). Several levels of interworking are currently being defined by the relevant standardisation bodies. Tight and Very Tight Coupling integrate the WLAN into a unified Radio Access Network. But PLMNs and WLANs are based on different design philosophies. Chances, problems, and possible solutions are discussed in this paper.