One of the major challenges, which prevents the coordinated multipoint (CoMP) communications concept from being widely deployed in new cellular systems, is timing synchronization. In order to achieve the gains promised by CoMP systems, the user equipment's signals in uplink (UL) or the base stations' signals in downlink should be synchronized, such that the time difference of arrivals do not exceed the cyclic prefix length of the transmitted signals; otherwise, the system suffers from unavoidable integer time offsets. These offsets lead to asynchronous interference in terms of inter-carrier, inter-block, and inter-symbol interference. This limits the percentage of area within a cell which can be covered by cooperation and imposes an upper bound on this area. In this paper, we investigate this problem in the UL, and by using geometrical and semi-analytical approaches, we define this upper bound. Also, by characterizing an accurate mathematical model for the asynchronous interference in Rayleigh fading channels, we were able to employ a standard MMSE-based receiver that mitigates this interference. Furthermore, a typical joint channel and delay estimation block is incorporated into the receiver to examine its performance with estimation errors.

This paper provides a detailed mathematical analysis of the impact of integer time offsets on the performance of OFDM systems. Although a number of approximate expressions for the probability of error in OFDM systems with integer time offset can be found in the literature, to the best of our knowledge, this work is the first that provides exact expressions. In particular, we derive exact closed-form expressions for the bit error rate (BER) and the symbol error rate (SER) of BPSK, QPSK, and 16-QAM modulation for transmission over both AWGN and Rayleigh fading channels. The effect of the fractional carrier frequency offset (CFO) is taken into consideration in the derivations. This makes the derived expressions more useful in evaluating the probability of error for OFDM systems with CFO only, especially in cases like 16-QAM modulation for transmission over Rayleigh fading channels where exact closed-form expressions are not available. For OFDM systems with a large number of subcarriers, an approximate method for evaluating the BER/SER is given. Finally, numerical results are included to demonstrate the exactness of the derived expressions and the accuracy of the approximate method.

To the best of our knowledge, the existing clustering schemes in coordinated multi-point (CoMP) systems do not take into consideration the timing synchronization requirements of the users' signals at the cooperating base stations. In this paper, a clustering scheme that can satisfy these requirements is proposed. With the aid of simulations, we compare this scheme to a clustering scheme that does not take these requirements into consideration and show that our scheme has better performance in high SNRs and it has less system complexity.

In this paper, we propose two novel methods for downlink preamble detection and initial coarse time synchronization for mobile WiMAX systems and other OFDM Systems that adopt similar preamble structures. The two methods are based on frequency domain processing. The two methods are compared to the conventional time domain method for frame detection in WiMAX and to another frequency domain algorithm in the literature. The proposed methods are less complex while providing similar or superior performance in the SNR range of interest. The performance measures are the detection and false alarm probabilities, in addition to the Mean Acquisition Time which is evaluated for all the considered methods. Since in the initial state the mobile station is in the state of searching for the downlink preamble, we take into account the interfering effect from other nearby Mobile Stations transmission in the uplink subframe with power larger than the received power from the Base Station in the downlink subframe. Simulation results show the robustness of the new proposed methods in different channel conditions and interference environments.