A Dynamic Resource Allocation Scheme for Guaranteed Bit Rate Services in OFDMA Networks Chrysostomos Koutsimanis and G´abor Fodor Ericsson Research, SE-164 80 Stockholm, Sweden ICC 2008 1 1

Outline Introduction OFDM Resource Allocation for Narrow-band Services OFDM Resource Allocation for Elastic Services Numerical Results Conclusions 2 2

Introduction Several previous works have considered the problem of resource allocation in multi-cell OFDMA networks but only a few contributions have explicitly taken into account the elastic nature of data applications In this work, we consider two kinds of user requirements Guaranteed bit rate (GBR): with a minimum and maximum resource block (RB) requirement Peak bit rate (PBR) with a fixed RB requirement We propose a method that balances between maximizing the overall throughput and being feasible in real systems

Key Trade-offs Key trade-offs has not been proposed Coordination between BSs obviously increases the overall system throughput at the expense of backhaul communication and intra-node processing Limiting the use of some of the sub-carriers reduces inter-cell collisions at the expense of loosing some degree of multi-user diversity in a frequency selective environment Throughput maximization often leads to unfair allocation of resources which in turn may lead to QoS violations for GBR users

Dealing with the Trade-offs (1) In order to deal with the first and second trade-off above, we distinguish between two time scales Superframe level resource allocation for inter-cell resource coordination an OFDM superframe that consists of a number of consecutive frames including inter-cell collision avoidance and inter-cell power control based on the longer term channel conditions Frame level resource allocation for intra-cell scheduling and resource allocation allocating sub-carriers and power for the duration of the next scheduling interval (being typically at the millisecond level) based on the instantaneous channel gains of the users

Dealing with the Trade-offs (2) Finally, the third aspect is addressed by allowing a minimum and a maximum number of resource blocks to be associated with each user Requiring that the number of resource blocks allocated to each user must be between these values

Basic Considerations We first consider narrow-band (PBR requirement) services The corresponding RB requirement is ni = Nmin for each user i Environment The frequency resource is divided into N traffic channels The time resource is divided into frames and superframes The OFDMA system consists of L base stations (BS) users Ml denotes the number of users that are connected to BS-l l(m) denotes the BS that serves User-m Gm,l is the long term channel gain between User-m and BS-l ym,n take the value of 1 whenever RB-n is assigned to User-m Pl,n denote the transmission power employed by BS-l on RB-n

Total Number of Bits Carried over RB-n The long term SINR values experienced by User-i on RB-n where Ml is the set of users served by BS-l We assume that the system employs an adaptive modulation and coding scheme (AMC) that is characterized by the link adaptation function fLA The average number of bits transmitted (during a superframe) for User-i on RB-n can be expressed by assume the fLA function is given The total number of bits carried over RB-n in the multi-cell system is

Superframe Level Problem Formulation (1) The resource assignment problem at the superframe level can now be formulated as finding the Y and P matrices such that the overall multi-cell throughput is maximized

Superframe Level Problem Formulation (2) The key characteristics of the superframe level allocation is that it does not require the instantaneous channel conditions it does not require inter BS communication at the frame level Once Y and P are available and assuming that the instantaneous channel conditions are available, it is possible to take advantage of multi-user frequency diversity Let hm,n denote the instantaneous channel gain between User-m and BS-l(m) on RB-n

Frame Level Problem Formulation (1) When RB-n is assigned to User-i, the number of bits transmitted on that RB becomes where denotes the instantaneous SINR Let zi,n be an indicator taking the value of 1 when RB-n is assigned to User-i

Frame Level Problem Formulation (2) The frame level optimization task for each BS-l is formulated as follows building on the results from the superframe level

Resource Block Assignment (1) We assume that a central entity gathers intercell information and performs intercell channel assignment and power control according to the proposed Radio Network Controller (RNC) algorithm Resource Block Assignment Within the BS loop, it calculates the throughput contribution (Ω) of each served user on each resource block this calculation takes into account previous allocations to already examined BSs and treats the produced interference as background noise After calculating Ω for all the users of the particular BS over all resource blocks, a binary linear optimization problem maximizes the sum of these values by assigning RBs to users under the constraint that a user must not be assigned more than ni RBs

Resource Block Assignment (2) initialize variables calculate throughput contribution of user m on RB-n find optimal allocation for BS k

Power Control The power control algorithm attempts to further increase the overall throughput by decreasing the power that is allowed for each resource block decreasing the interference level The Jm element of the vector J specifies the index of the dominant interfering base station of User-m The frame level problem is an integer linear programming problem and can be solved with standard techniques

OFDM Resource Allocation for Elastic Services We now consider elastic services and the corresponding resource block requirement is ni,min = Nmin ≤ ni,max = Nmax for each user i For elastic services, the third constraint of superframe level allocation takes the form of The frame level allocation problem for elastic services is similar to that of narrow band services except that the exact number of resource blocks ni in the second constraint is an output of the superframe level problem solution rather than a predefined value

Radio Network Controller Algorithm for Elastic Services

Simulation Environment and System Parameters

Numerical Results for Narrow Band Services (1) Total number of users: 7 * 15 The number of required resource blocks per user: nmin = 2 average load per cell in terms of the used resource blocks: 37.5% F SF SF+F F SF SF+F

Numerical Results for Narrow Band Services (2) SF+F vs. F SF+F F SF

Numerical Results for Elastic Services Each session is associated with nmin = 3

Conclusions We considered a multi-cell OFDMA network that supports narrow band and elastic services It is appropriate to model these types of services as ones which have an associated minimum and maximum OFDM resource block requirement We formulated the resource block and power allocation problem as a throughput maximization problem We proposed a two level approach that separates the inter-cell coordination task appropriate at the OFDM superframe level from intra-cell scheduling appropriate at the frame level By means of system simulations we found that the two level approach seems feasible in a realistic model in terms of complexity and execution time