Licensed Assisted Access (LAA) represents a new paradigm for LTE, where unlicensed bands are used in conjunction with licensed bands to increase network capacity. The main challenge of LAA is ensuring that unlicensed bands are shared with WiFi systems in an efficient manner. In this paper, we study a coexistence solution based on channel selection. A centralized channel selection mechanism which increases network capacity of LTE in unlicensed bands is proposed. We also describe the implementation of our scheme using existing LTE signaling mechanisms. Simulation results show that channel selection can improve the performance of both LTE and WiFi by over 100\%.

In this paper, the multi-channel performance of large scale deployment of Long Term Evolution (LTE) in Unlicensed (LTE-U) band and WiFi using stochastic geometry is studied. LTE-U and WiFi cell placement is modeled as a Poisson Point Process (PPP), while transmission activity is modeled as hardcore simple sequential inhibition (SSI) point processes, which have been shown to accurately model Carrier sense multiple access (CSMA) access. This stochastic geometry based framework is used to analyze the co-existence of LTE-U and WiFi systems operating in the same set of available channels in unlicensed band. Several interesting results are derived. First, it is shown that fair coexistence can be achieved by LTE-U implementing a Listen-Before-Talk (LBT) protocol. Different variations of LBT are explored to improve the co-existence. Secondly, it is shown that the classical hidden node problem can severely affect performance, and that channel selection mechanisms can significantly alleviate the problem.

In multi-carrier CDMA (MC-CDMA) system, timing of the starting point of the desired signal must be estimated to demodulate the desired signal. Especially for asynchronous MC-CDMA systems, rather a complicated timing estimation is needed, due to the random delays of all users. Zong, Wang and Bar-Ness (see VTC2001)proposed a partial sampling MMSE (PS-MMSE) receiver which does not require timing estimation. However, it requires several times more filter's taps than the conventional MC-CDMA receiver. The number of taps is a multiple of, and depends on, the number of subcarriers (i.e., the processing gain). A reduced complexity partial sampling MMSE (RPS-MMSE) receiver is proposed, which can be adaptively implemented by using the least-mean-square (LMS) algorithm. The proposed structure allows one to choose the number of taps independent of the number of subcarriers, hence having more flexibility in choosing the number of taps than with PS-MMSE receiver. With this structure, we can use fewer taps than with the PS-MMSE receiver and still can obtain comparable performance.

Linear minimum mean square error (LMMSE) receivers for asynchronous multicarrier code division multiple access (MC-CDMA) system under frequency-selective Rayleigh fading channel is studied. The performance of this LMMSE receiver is evaluated and shown to be superior to that of the other two schemes, equal-gain combining (EQC) and maximum-ratio combining (MRC). However, a perfect timing estimation of the desired user is needed for these receivers, as a misaligned sampling interval of these receivers results in severely self intersymbol interference (ISI) and intercarrier interference (ICI) for the desired signal at the output. In order to remove the timing acquisition requirement of a receiver for an asynchronous MC-CDMA system, we proposed a novel partial sampling MMSE (PS-MMSE) receiver. Numerical result shows that the PS-MMSE receiver without timing knowledge provides significantly stronger interference suppression capability than the LMMSE receiver with known timing. Moreover, a so-called reduced complexity partial sampling MMSE (RPS-MMSE) receiver is proposed to make the number of the receiver's taps independent of the number of subcarriers. Results show that with a proper grouping parameter, a much less complicated RPS-MMSE receiver achieves almost the same performance as the PS-MMSE receiver. Thus, one is able to implement an MMSE receiver without a prior timing requirement to perform multiuser detection for the MC-CDMA system in an asynchronous scenario at the expense of a slight complexity increase.

We propose a novel dual-rate synchronous multi-carrier code division multiple access (MC-CDMA) system. In this system, all users are designed to have the same processing gain and share the same set of subcarriers. Two decorrelator-based receivers are implemented and analysed in this paper for the dual-rate MC-CDMA system; a high-rate decorrelator (HRD) matches high data rate and a low-rate decorrelator (LRD) matches low data rate. It has been proved that for low-rate users, HRD and LRD perform similarly, while for high-rate users, LRD outperforms HRD when the signature sequences of high-rate and low-rate users are not orthogonal. Furthermore, we propose a blind multiuser detection method, which can be implemented adaptively by applying the bootstrap algorithm. It is shown that the blind adaptive detector is near-far resistant and has performance similar to that of minimum mean square error (MMSE) detector.

Orthogonal frequency division multiplexing (OFDM) is believed to be an important technique for future wireline and wireless communications. One of the major disadvantages of such a multi-carrier modulation is the sensitivity of the performance to a frequency offset. The frequency offset can result from a Doppler shift due to mobile movement as well as from a carrier frequency synchronization error. Such a frequency offset causes a loss of the carriers' orthogonality, and hence inter-carrier interference (ICI). A previously proposed method to estimate and correct the frequency offset uses the retransmission of a data symbol, thus reducing the bandwidth efficiency. Another method, which merely reduces the effects of the ICI instead of completely correcting for effects of the frequency offset is based on repetition coding in the frequency domain, hence also reducing the throughput. In this paper, we propose a blind method using the correlation of the samples at the output of the discrete Fourier transform (DFT) to estimate the frequency offset. As no training sequence or symbol retransmission is required there is no loss in bandwidth efficiency. Additionally, we develop an adaptive algorithm for the frequency offset correction. The derived algorithm is basically a stochastic gradient algorithm which minimizes the mean squared frequency offset error.