Fatih Ozluturk

Cell search in 3GPP long term evolution systems

This paper reviews and presents the latest results in the cell search issue of the 3GPP Long Term Evolution (LTE) systems. Cell Search is a basic function of any cellular system, during which process time and frequency synchronization between the mobile terminal and the network is achieved. Such synchronization is especially important for 3GPP Long Term Evolution systems, which rely heavily on the orthogonality of the uplink and downlink transmission and reception to optimize the radio link performance. As in conventional cellular systems, the mobile terminal in an LTE system acquires time and frequency synchronization by processing the synchronization channel. Design of the synchronization channel is being developed within the standardization activities of 3GPP Long Term Evolution and is still evolving. In this paper, we present the design considerations and various new and promising design concepts for the synchronization channel. We evaluate some specific solutions and provide numerical performance results.

Performance bounds for DS/SSMA communications with complex signature sequences

A unified performance analysis of direct-sequence spread-spectrum multiple-access (DS/SSMA) communications with deterministic complex signature sequences is presented. The probability density function (PDF) of the multiple-user interference is determined. Using a round-down and round-up procedure on the PDF, arbitrarily tight lower and upper bounds on the probability of bit error (PBE) are obtained. Results based on the Gaussian approximation method are also presented. It is shown that complex sequences can yield better PBE performance than binary sequences. Using complex sequences, the number of signature sequences that have good auto- and cross-correlation properties is greatly enlarged. New users employing complex or binary signature sequences can be added to existing systems with graceful performance degradation. >

Effect of limiting the downlink power in CDMA systems with or without forward power control

This paper investigates the effects of limiting the downlink transmit power in cellular CDMA communications systems. The composite signal transmitted in downlink is the sum of all the signals intended for different users. The amplitude of the composite baseband signal depends on the data bits, the spreading waveforms of the users, and the forward power control algorithm, and it determines the transmit power. Two of the important constraints when designing the RF circuits are the average transmit power and the peak transmit power. These two parameters are often specified in terms of the peak-to-average power ratio. One way to reduce the peak-to-average power ratio is to clip the composite baseband signal before modulating it. The analysis in this paper investigates how the signal-to-noise ratio at the remote receiver changes as a result of this clipping. The results show that the peak-to-average power ratio can be made significantly smaller without notable signal-to-noise ratio degradation.

Performances of DS/SSMA communications with MPSK signaling and complex signature sequences

We study a direct-sequence spread-spectrum multiple-access (DS/SSMA) communication system with M-ary phase-shift-keyed (MPSK) signaling. Users in the system employ complex signature sequences, which are modeled as deterministic sequences. We present upper and lower bounds for the probability of bit error (p.b.e.) of a reference transmission. First, the conditional p.b.e., given the multiple-user interference (MUI), is obtained. Then, the conditioning is removed by averaging over the discretized probability density function (p.d.f.) of the MUI. The result is an upper (or lower) bound when the discretized p.d.f. is a shifted-high (or shifted-low) version of a sectioned p.d.f. This process requires the two-dimensional p.d.f. of the MUI, which is derived. Arbitrarily tight bounds can be obtained at the cost of increased computation. Numerical results are presented for complex FZC and near optimal 4-phase sequences, as well as real binary Gold sequences, for BPSK, 4-PSK, and 8-PSK. Gray code bit mapping is assumed, although the analysis can be applied to other bit mappings. Results show that complex sequences can yield better performance than binary sequences in MPSK signaling. Using complex sequences, the number of available signature sequences is greatly enlarged. >

Probability of bit error for DS/SSMA systems with MPSK signaling and complex signature sequences

The authors consider a direct-sequence spread-spectrum multiple-access (DS/SSMA) communication system with M-ary phase-shift keyed (MPSK) signaling. Users in the system use complex signature sequences. A bounding technique is used to obtain arbitrarily tight upper and lower bounds on the probability of bit error. Numerical results are presented for Frank-Zadoff-Chu (FZC) and near-optimal four-phase complex sequences, as well as binary Gold sequences, for binary phase shift keying, (BPSK), 4-PSK, and 8-PSK. Results show that complex sequences can yield better performance than binary sequences in MPSK signaling. Using complex sequences, the number of signature sequences with good correlation properties is greatly enlarged.<<ETX>>

Performances of DPSK and FSK CDMA systems with complex signature sequences

This paper investigates the performance of direct‐sequence code‐division multiple‐access (CDMA) communication systems in Gaussian noise channels using binary and M‐ary noncoherent signaling schemes with complex signature sequences. Differential‐phase‐shift‐keyed (DPSK) signaling with differentially coherent demodulation and frequency‐shift‐keyed (FSK) signaling with noncoherent demodulation are considered. Upper and lower bounds are presented for the probability of bit error (p.b.e.) of binary DPSK and binary FSK systems when the number of users is small. Simulation and Gaussian approximation are used to obtained the p.b.e. when the number of users is large. Two types of complex sequences (FZC and 4‐phase sequences) and one type of binary sequences (Gold sequences) are used for evaluating numerical results. Results show that complex sequences outperform binary sequence when the number of users is small. However, as the number of users gets large, all sequences have similar performances. These suggest that complex sequences should be used when the number of users is small, but binary sequences should be used when the number of users is large, as binary sequences are easier to generate.

Performance bounds for DS/SSMA systems with binary noncoherent signaling schemes

The authors investigate the performance of direct-sequence spread-spectrum (DS/SSMA) communications systems with binary noncoherent signaling schemes, using complex signature sequences. Upper and lower bounds on the probability of bit error (p.b.e.) are obtained for differential-phase-shift-keyed (DPSK) signaling with differentially coherent demodulation and frequency-shift-keyed (FSK) signaling with noncoherent demodulation. The probability density function (p.d.f.) of the multiple user interference (MUI) is obtained for deterministic complex signature sequences. Upper and lower bounds are obtained by utilizing appropriate shifting procedures on the p.d.f. Numerical results are presented for some complex signature sequences, as well as binary sequences, with binary DPSK and FSK data modulations. Results show that complex sequences have good performances in noncoherent signaling schemes.<<ETX>>