Partial Cached-FFT Algorithm for OFDMA

Communications

Chao-Ming Chen and Yuan-Hao Huang Institute of Communications Engineering and Department of E lectrical Engineering, National Tsing-Hua University

Hsinchu, 30013, Taiwan, R.O.C Email: yhhuang@ee. nthu. edu. tw

Ahstract- Orthogonal Frequency Division Multiplexing (OFDM) transmission has become a popular technique for modern wired and wireless communications. Next-generation mobile communication systems also plan to adopt orthogonal frequency division multiple access (OFDMA) technology for multi-user scenario, in which sub carriers can be grouped as resource blocks or subchannels and allocated to different users. Hence, only partial sub carriers in one OFDM symbol may be used by one user end (UE). In this situation, more redundant power consumption is produced by the complete FFT computation for OFDM demodulation in the UE. Among the available FFT architectures, cached-FFT has much regularity and lower power consumption than others. Thus, a partial FFT algorithm can be applied to the cached-FFT architecture for power reduction of OFDMA UE. In this paper, a partial cached-FFT algorithm is proposed to control the operation of the cached-FFT processor for different proposals of OFDMA resource block allocation schemes, thereby reducing the computing power.

I. INTRODUCTION

Orthogonal frequency division multiplexing (OFDM) trans­mission has played an important role in modern wired and wireless communication systems, and now it is going to be employed as orthogonal frequency division multiple ac­cess (OFDMA) in the next-generation mobile communication systems such as WiMAX and 3GPP-LTE standards [1][2]. Many users will share the same bandwidth within one base station, therefore many resource block allocation techniques for OFDMA are proposed to optimize the quality of service in the cell-based system. The resource block allocation issues of OFDMA system have a significant impact on the overall transmission quality, thus attract many discussions. However, from the viewpoint of user end (UE ) , usable and legal resource blocks of subcarriers are limited, and therefore the complete FFT computation for OFDM demodulation is not necessary especially under the low-power consideration of the battery­driven handsets. No related designs and concerns have been discussed and proposed yet. In the past, the partial FFT algorithms [3] [4] are used to extract the narrow band signals in a wide-band spectrum. Partial butterfly operations in the FFT algorithm are executed to reduce the complexity of FFT as depicted in Figure lea) . However, these partial FFT algorithms can only extract one group of the spectrum. In the case of multiple resource blocks in the OFDMA system, none of the present partial FFT algorithms can be applied to the OFDMA demodulation. To solve this problem, we

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(a) Partial FFT algorithm (b) Cached-FFT algorithm

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Fig. I. FFT algorithm

Fig. 2. (a) Localized physical resource block and (b) comb-distributed physical resource block.

propose to develop the partial FFT algorithm for accessing multiple resource blocks based on the cached-FFT architecture [5]. The cached FFT algorithm, as shown in Figure 1 (b) , is an architecture-level algorithm which inserts caches between butterfly processor and main memory so as to reduce the memory access power across FFT stages. Not only the power reduction due to cache property but also the reduction of but terfly operations can be further achieved to meet the harsh low-power requirement of the mobile devices.

II. RESOURCE ALLOCATION IN OFDMA

In the OFDMA systems, multiple users may share the sub­carrier resources within one OFDM symbol, i. e. the subcarriers in one OFDM symbol are divided, grouped, and allocated to different users. In the 3GPP LTE standard, resource block is defined as a unit for allocation, while subchannel is defined in the WiMAX system. Usually, the standard physical resource block (PRB ) and comb-distributed physical resource block (Comb-PRB ) , depicted in Figure 2, can be used as basic group unit for allocation in OFDM spectrum.

On the other hand, virtual resource block (VRB ) is defined as logical resource block, which can be mapped to PRB in different ways including localized resource block allocation