Adaptive Pilot Based Modulation Identification and Channel Estimation for OFDM systems Murat Karabacak Department of Electrical Engineering, Istanbul University, Avcilar 34320, Istanbul, Turkey Email: kmurat67@ogr.iu.edu.tr Hakan A. C ¸ ırpan Department of Electrical Engineering, Istanbul University, Avcilar 34320, Istanbul, Turkey Email: hcirpan@istanbul.edu.tr H¨ useyin Arslan Department of Electrical Engineering, University of South Florida, Tampa 33620, Florida, USA Email: arslan@usf.edu Abstract—Adaptive modulation is an important method for effective usage of channel capacity in Orthogonal Frequency Di- vision Multiplexing (OFDM) systems. Modulation identification algorithms unnecessitate the need of transferring the modulation information to the receiver, thereby these algorithms are very effective methods to increase the channel capacity. However, in practice, separate two sets of pilot symbols are used to identify the modulation type and to estimate the channel impulse response. In this paper, we propose one set of pilot symbols to get information about the channel and the modulation type, jointly. These pilots are named as “adaptive pilots” because they are related with the modulation type. Monte Carlo simulations are performed to observe the performance of the proposed adaptive pilot method and to compare with previously presented algorithm given in [1]. Simulation results indicate that the proposed adaptive pilots successfully help to identify correctly the modulation type without affecting the performance of the channel estimation. I. I NTRODUCTION Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme, which is also a well- accepted technique to avoid intersymbol interference (ISI) in multipath fading environments [2]. In OFDM systems, parallel subcarriers are used to transmit modulated data symbols. The subcarriers are formed by dividing the wide transmission spectrum into narrower bands. Therefore, symbol periods are increased by the number of subcarriers, which decrease the effects of ISI. Because of this advantage on ISI problem for multipath fading channels, OFDM has been applied to various wireless communication systems in the last decade. Specifically, multi-user OFDM is assumed to be used for the downlink air interface in Long Term Evolution (LTE) which is introduced in 3rd Generation Partnership Project (3GPP) Release 8 [3]. The channel capacity and the efficiency of the communica- tion system can be increased by adaptive modulation methods because the link quality between transmitter and receiver is continuously measured, and appropriate modulation type is selected to maximize the link capacity in the systems with adaptive modulation [4], [5]. In order to achieve this link capacity advantage, there are three additional steps that should be followed: channel estimation, modulation selection and modulation identification. The first two steps are performed by the transceiver. Modulation identification is performed by receiver because modulation information is necessary to detect the transmitted symbols and to estimate the channel. Blind modulation identification algorithms are proposed in [1], [6], [7], and [8]. Instead of modulation identification in the receiver, transmitter can send the modulation information through the channel but this process requires extra channel capacity. Therefore, if modulation type changes frequently, transmission rate of information symbols decreases signifi- cantly. Modulation identification algorithms unnecessitate this process and the decrease of the transmission rate. Thus, adaptive modulation methods are become more effective and applicable methods. In adaptive modulation, modulation order depends on chan- nel reliability. When the channel has high reliability, the mod- ulation order is increased to maximize the spectral efficiency and the throughput; when the transmission channel is unreli- able, the modulation order is decreased to provide acceptable Bit Error Rate (BER). Various modulation selection algorithms are proposed in literature, which use different criteria to decide modulation schemes taking the channel conditions into con- sideration. Depending on intended applications, modulation selection algorithms which are based on BER constraints, constant throughput, or both of them, are given in [9]. In general applications, pilot symbols are separated to subcarriers in OFDM symbol and they are used to estimate channel frequency response. In this paper, a new type of pilots with adaptive values is proposed. These adaptive pilots successfully help to identify the modulation type without affecting the performance of the channel estimation. Also, the modulation information is embedded into the pilot symbols without extending the number of pilot symbols. Therefore, adaptive pilots decrease error probability of the modulation identifier without sacrificing capacity of the transmission chan- nel or accuracy of the channel estimation. Furthermore, since information regarding the modulation type is inserted into the pilot symbols, the proposed method is likely to outperform any blind modulation identification algorithm available in the literature that assumes perfect knowledge of the communi- cation channel. The modulation information is embedded by assigning unique pilots to every possible modulation type. Possible adaptive pilot patterns with corresponding modulation types are given in Table I. 2010 IEEE 21st International Symposium on Personal Indoor and Mobile Radio Communications 978-1-4244-8016-6/10/$26.00 ©2010 IEEE 730