Number Of Orthogonal Sequences Research Articles

Generation of Orthogonal Sequence Sets for M-ary Spread Spectrum Communications with OFDM Modulation

The number of orthogonal sequences is a key point of M-ary spread spectrum (MSS) communication system since more sequences mean higher data rate. When orthogonal frequency division multiplexing (OFDM) modulation is used into MSS system, the additional low peak-to-average power ratio (PAPR) property is required due to high power efficiency. Based on the segmentation-shift operation, multiple orthogonal sequence sets with the same size and sequence length are generated in this paper. By combining these orthogonal sequence sets, the number of the generated orthogonal sequences is very large. For any two orthogonal sequence sets, there exists good cross-correlation performance between them at zero shift. In addition to large set size and good correlation property, the generated multiple orthogonal sequence sets possess low PAPR, which is very suitable for OFDM modulation. The generation algorithm of multiple orthogonal sequence sets is provided in this paper, and the corresponding simulation results show the effectiveness of the provide method.

Channel Sounding for Multi-User Massive MIMO in Distributed Antenna System Environment

We propose a generation scheme for a sounding reference signal (SRS) suitable for supporting a large number of users in massive multi-input multi-output (MIMO) system with a distributed antenna system (DAS) environment. The proposed SRS can alleviate the pilot contamination problem which occurs inherently in the multi-user system due to the limited number of orthogonal sequences. The proposed SRS sequence is generated by applying a well-chosen phase rotation to the conventional LTE/LTE-A SRS sequences without requiring an increased amount of resource usage. We also propose using the correlation-aided channel estimation algorithm as a supplemental scheme to obtain more reliable and refined channel estimation. It is shown that the proposed SRS sequence and the supplemental channel estimation scheme improve significantly the channel estimation performance in multi-user massive MIMO systems.

Large Sets of Orthogonal Sequences Suitable for Applications in CDMA Systems

In this paper, we employ the so-called semi-bent functions to achieve significant improvements over currently known methods regarding the number of orthogonal sequences per cell that can be assigned to a regular tessellation of hexagonal cells, typical for certain code-division multiple-access (CDMA) systems. Our initial design method generates a large family of orthogonal sets of sequences derived from vectorial semi-bent functions. A modification of the original approach is proposed to avoid a hard combinatorial problem of allocating several such orthogonal sets to a single cell of a regular hexagonal network, while preserving the orthogonality to adjacent cells. This modification increases the number of users per cell by starting from shorter codewords and then extending the length of these codewords to the desired length. The specification and assignment of these orthogonal sets to a regular tessellation of hexagonal cells have been solved regardless of the parity and size of $m$ (where $2^m$ is the length of the codewords). In particular, when the re-use distance is $D=4$ the number of users per cell is $2^{m-2}$ for almost all $m$, which is twice as many as can be obtained by the best known methods.

Peak Power Reduction for Pilot-Aided OFDM Systems with Semi-Blind Detection

Orthogonal Walsh-Hadamard sequences have been adopted for peak-to-average power ratio (PAPR) reduction without sending side information in pilot-aided orthogonal frequency division multiplexing systems. The PAPR reduction performance is determined by the number of orthogonal sequences, which is equal to the sequence length. However, increasing the length of pilot sequences significantly degrades bandwidth efficiency. The current study presents a novel set of pilot sequences obtained by sub-sampling the Zadoff-Chu sequences. The sub-sampled sequences are then equally partitioned into several groups, where sequences in the same group are orthogonal to one another and can therefore be blindly detected at the receiver. The number of pilot sequences can be doubled whenever one extra bit of side information is sent for a given pilot length. Therefore, the proposed scheme has the advantage of enhancing PAPR reduction performance with only marginal cost.