Two timing offset estimation methods for orthogonal frequency division multiplexing (OFDM) systems as modifications to Schmidl and Cox's method [6] are presented. The performances of the timing offset estimators in additive white Gaussian noise channel and intersymbol interference channel are compared in terms of estimator variance obtained by simulation. Both proposed methods have significantly smaller estimator variance in both channel conditions.

arconi's innovative perception of the electromagnetic waves and the air interface in 1897 was the first milestone on the important road to shared use of the radio spectrum. But only after almost a century later did mobile wireless communication 1 start to take off. Despite a series of disappointing false starts, the communication world in the late 1980s was rapidly becoming more mobile for a much broader segment of communication users than ever before. Historically, communication has been restricted primarily to voice traffic between two fixed locations rather than between two people. With the advent of wireless technology, a transition from point-to-point communication toward personto-person communication (i.e., independent of location) has begun. Testimony to this is the rapidly increasing penetration of cordless and cellular phones, not just in North America but all across the world. In anticipation of the growing consumer demands, the next generation of wireless systems endeavors to provide person-to-person communication of both circuit and packet multimedia data. Henceforth in this article we shall place greater emphasis on the advances in mobile cellular communication and paging, which represents the fastest growing segment of wireless technology. Initial AM mobile systems used for public safety in the United States since the 1930s were replaced with FM systems after World War II. However, the wireless system configuration remained a single-cell topology (similar to the broadcast model). Later, in the 1950s and 1960s, automatic channel trunking was introduced and implemented under the label Improved Mobile Telephone Service (IMTS) to increase spectrum efficiency. The current cellular configuration was also conceived during this time in response to the chronic problem of spectral congestion (the market gets quickly saturated even with IMTS systems) and poor service in the mobile telephone business. The government regulatory agencies could not make spectrum allocations in proportion to the increasing demand for mobile services. Therefore, it became imperative to restructure the radio telephone system to achieve high capacity with limited radio spectrum, while at the same time covering very large areas. However, this novel architecture was only launched in the 1980s when the first commercial mobile telephony was introduced. The first-generation cellular and cordless telephone networks, which were based on analog technology with FM modulation, have been successfully deployed throughout the world since the early and mid-1980s. A typical example of a first-generation cellular telephone system is the Advanced Mobile Phone Services (AMPS). Second-generation (2G) wireless systems employ digital modulation and advanced call processing capabilities. In view of the processing complexity required for these digital systems, two offered advantages are the possibility of using spectrally efficient radio transmission schemes (e.g., timedivision multiple access, TDMA, or code division multiple access, CDMA, in comparison to ...

Abstract-Adaptive automatic repeat request (ARQ) schemes are quite effective for throughput enhancement in time-varying mobile channel environments. In this paper, an ARQ scheme with adaptive error-correcting codes is considered. A robust channel sensing algorithm by making use of XOR-ing is proposed for link adaptation. With the proposed channel estimation algorithm, an adaptive scheme that adapts its error correcting codes according to channel conditions is presented. The efficiency of the proposed adaptive ARQ scheme is evaluated in terms of throughput performance by computer simulation for a time-varying mobile channel characterized by Rayleigh fading and log-normal shadowing. As an enhancement in counteracting long deep fades, the incorporation of frequency diversity into adaptive ARQ scheme is also investigated and the simulation results show that this scheme is quite efficient in counteracting long deep fades.Index Terms-Adaptive error control, adaptive automatic repeat request (ARQ), adaptive FH, channel sensing, frequency hopping (FH), time-varying mobile radio channel.

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