Abstract-This paper proposes a new algorithm for downlink scheduling in OFDMA systems. The method maximizes the throughput, taking into account the amount of signaling needed to transmit scheduling maps to the users. A combinatorial problem is formulated and solved via a dynamic programming approach reminiscent of the Viterbi algorithm. The total computational complexity of the algorithm is upper bounded by ( 4 ) where is the number of users that are being considered for scheduling in a frame and is the number of resource blocks per frame. Index Terms-OFDMA, scheduling, control signaling, optimization. I. BACKGROUND FDMA is a common access method in many systems contemplated for the future, such as LTE [1] . An advantage of OFDMA is that users can be scheduled for transmission/reception precisely in the specific time/frequency slots where their channel is good. The resulting improvement in system capacity and robustness is often referred to as "multiuser diversity" [2] . The basic benefit of multiuser diversity is that the channel statistics of individual users become less important. What matters, instead, is that at any given point in time and frequency, there is at least one user who desires to transmit or receive data, and who has a good channel. There is a rather substantial literature on scheduling in OFDMA systems, see, e.g., [3] for a review. The basic philosophy behind the design of schedulers is to maximize the system sum-throughput, subject to some constraints. It is common, for example, to impose constraints on fairness [4] , or on the quality-of-service experienced on the application layer [5] . It is also common to design schedulers that explicitly take into account delay requirements of data packets [6] . A basic difficulty with OFDMA is that all receiving users have to be told in what time/frequency-slots their data are located. This requires signaling of control information, consisting of scheduling maps. The overhead associated with the transmission of such control information can be significant [7] -[9] and may constitute a limiting factor in some systems [10] . In this paper we design a scheduler for the OFDMA downlink that maximizes the throughput by taking into account the cost of conveying the scheduling decisions. To compress the scheduling information we use runlength encoding. We formulate the scheduling as a combinatorial problem that can be solved as a sequence of dynamic programming problems. As an integral component, the method contains a user selection mechanism. That is, herein "scheduling" means both choosing what users to transmit to, and assigning them time/frequency resources. The computational cost of our proposed algorithm is proportional to the number of subcarriers. The problem that we consider here has received very little attention previously. We are aware of one related sequence of work: [7] , [8] , that formulated quantitative models for the signaling cost and proposed some optimized schedulers. The main novelties over [7] , [8] are: (i) we formulate and solve the problem both for the case that the same scheduling map is broadcast to all users, and for the case that the users receive individual scheduling maps; (ii) we integrate user selection and scheduling, i.e., we do not assume that the set of users selected for transmission is fixed beforehand; (iii) we formulate the problem so that it can be computationally efficiently solved; and (iv) our formulation includes a single scalar parameter that models the relative cost of transmitting the scheduling information: by changing this parameter, more or less complex maps are obtained. A fundamental difference to [8] is that we do not force the scheduler to assign the same number of subcarriers to all users. This constraint was imposed to obtain "fair" solutions. By contrast, our formulation yields solutions where some users are allocated many resource blocks, some are allocated a few, and some are allocated none. We allow for fairness constraints via the per-resource-block capacity expressions that go into the optimization problem. We consider the downlink of an OFDMA system. We introduce the following terminology: • A frame is a set of consecutively transmitted OFDM symbols. Typically, the control signaling data that contain the scheduling information occupies parts of the first few OFDM symbol(s). • A resource block is an entity representing the smallest subdivision of the time/frequency domain that can be allocated to a user. We assume that the channel is approximately constant over one resource block. There are resource blocks in each frame. • is the largest number of users that can be scheduled in the frame. • ( ) is the "capacity" conveyed by resource block , if this block is assigned to user . With adaptive modulation, ( ) may be the modulation-constrained mutual 0090-6778/10$25.00 c ⃝ 2010 IEEE
In REF , a method for scheduling under a constraint on the control signaling complexity was proposed.
Optimal OFDMA Downlink Scheduling Under a Control Signaling Cost Constraint
IEEE Transactions on Communications
IEEE Transactions on Communications
Computer Science