A Multi-Dimensional Code-Division-Multiplexed OFDMA Modem Using Cyclic Rotated Orthogonal Complete Complementary Codes

We present a multiple-input-multiple-output (MIMO) code-division-multiple-access (CDMA) communication system based on complete complementary (CC) codes (CC-CDMA) in a time-dispersive (i.e. frequency-selective fading) channel. This system utilizes a novel combination, which consists of orthogonal CC codes for data spreading and a suitably designed multiuser-detector (MUD) algorithm, in order to enhance the current 3rd generation (3G) mobile systems. The simulation results for the CC-CDMA system, obtained in a frequency-selective fading channel, reveal significant improvement in bit-error-rate (BER) performance, and consequently the achieved high data rates (HDR) compared with a conventional Walsh codes based MIMO CDMA (Walsh-CDMA) system.

Cellular communication systems need to support simultaneous transmission of multiple data rates over the same medium. A major aspect of design of cellular systems is simultaneous sharing of channel resources avoiding multiple access interference between users. For high quality communication this must be done without severe degradation in performance of the system. Orthogonal frequency division multiple access (OFDMA) is a multiple access technique that can accommodate many users with widely varying applications, data rates and quality of service requirements. This paper presents analysis of transmission of multiple data rates by using orthogonal variable spreading factor (OVSF) codes in an OFDMA system, which gives increase in spectral efficiency. The performance of the system is analyzed using bit error rate (BER), for additive white gaussian noise (AWGN) and Rayleigh fading channels.

In interleaved orthogonal frequency division multiple access (OFDMA) systems uplink, multiple access interference (MAI) occurs due to different carrier frequency offsets (CFOs) from different users. In this paper, a new low complexity scheme is proposed to compensate CFO at the OFDMA receiver. The proposed scheme maximizes signal to interference ratio (SIR). Simulation results show considerable improvement in comparison to the existing methods in the literature.

OFDM (Orthogonal Frequency Division Multiplexing) is a well-known multicarrier modulation technique that allows high-rate data transmissions over multipath broadband wireless channels. By usingOFDM, a high-rate data stream is split into a number of lower-rate streams that are simultaneously transmitted on different orthogonal subcarriers. Thus, the broadband channel is decomposed into a set of parallel frequency-flat subchannels; each one corresponding to an OFDM subcarrier. In a single user scenario, if the channel state is known at the transmitter, the system performance can be enhanced by adapting the power and data rates over each subcarrier. For example, the transmitter can allocate more transmit power and higher data rates to the subcarriers with better channels. By doing this, the total throughput can be significantly increased. In a multiuser scenario, different subcarriers can be allocated to different users, which constitutes an orthogonal multiple access method known as OFDMA (Orthogonal Frequency Division Multiple Access). OFDMA is one of the principal multiple access schemes for broadband wireless multiuser systems. It has being proposed for use in several broadband multiuser wireless standards like IEEE 802.20 (MBWA: http://grouper.ieee.org/groups/802/20/), IEEE 802.16 (WiMAX: http://www.ieee802.org/16/, 2011) and 3GPP-LTE (http://www.3gpp.org/). This chapter focuses on the OFDMA broadcast channel (also known as downlink channel), since this is typically where high data rates and reliability is needed in broadband wireless multiuser systems. In OFDMA downlink transmission, each subchannel is assigned to one user at most, allowing simultaneous orthogonal transmission to several users. Once a subchannel is assigned to a user, the transmitter allocates a fraction of the total available power as well as a modulation and coding (data rate). If the channel state is known at the transmitter, the system performance can be significantly enhanced by allocating the available resources (subchannels, transmit power and data rates) intelligently according to the users’ channels. The allocation of these resources determines the quality of service (QoS) provided by the system to each user. Since different users experience different channels, this scheme does not only exploit the frequency diversity of the channel, but also the inherent multiuser diversity of the system. In multiuser transmission schemes, like OFDMA, the information-theoretic system performance is usually characterized by the capacity region. It is defined as the set of rates 6

Cooperative communication is a promising technique to enhance the system performance by exploiting the spatial diversity provided by the relay use. Incremental relaying techniques are proposed to limit the resources use to only bad channel conditions. In this paper, we deal with AF and IAF performance optimization for uplink Orthogonal Frequency Division Multiple Access (OFDMA) communications, carried on a packet-per-packet basis. For OFDMA cellular system, we define a cost that reliably quantifies the additional multiple-access interference (MAI) inherent to the relaying use. This MAI cost expression depends highly on the adopted frequency reuse pattern. The optimization, in terms of Packet-Error-Rate (PER), is carried out while keeping this MAI cost constant, for regular and fractional frequency reuse patterns. For FFR, different frequency reuse factors are applied to cell-center and cell-edge areas. Simulation results are showing that IAF relaying outperforms the direct transmission scheme by more than 15 dB gain, in terms of the reduction of the MAI. Additionally, IAF seems to be usually more powerful than AF.

This paper presents an investigation into improving the channel estimation scheme and reducing the effects of symbol timing misalignment when the digital video broadcasting-return channel through terrestrial (DVB-RCT) is used as return path model. Under orthogonal frequency division multiple access (OFDMA) which uses the modulation scheme of DVB-RCT, appropriate symbol length of constant-amplitude zero-autocorrelation (CAZAC) sequences as a preamble could be utilized in accordance with the number of transmitting antenna and channel condition. However, because its capability of channel estimation is limited by the number of transmitting antennas and multipath components to maintain the orthogonality, we propose more flexible preamble design called extended-CAZAC (E-CAZAC). Also, taking into account the effect of multiple access interference (MAI) introduced by a symbol timing misalignment, the bit error rate (BER) and throughput performance are investigated for a typical OFDMA uplink scenario.

In the uplink of an orthogonal frequency division multiple access (OFDMA) system, because of multiple access interference (MAI), carrier frequency offset (CFO) estimation is much more difficult than that in single-user systems. This paper proposes a null subcarrier-based iterative CFO estimation method for the uplink of OFDMA system adopting sub-band based carrier assignment scheme. Rather than applying independent estimations for users separated by bandpass filters, we first obtain an inaccurate initial CFO estimate for each user. Then, we run an iterative scheme which could update the estimates and reduce errors iteration by iteration. The performance of the proposed method is evaluated by simulations.

Long Term Evolution (LTE) is one of the standards in 4G cellular technologies which offer different data rates for voice, video conference and other services. The Orthogonal Frequency Division Multiple Access (OFDMA) is a well suited multiple access scheme of LTE for supporting variable bit rate services. OFDMA-based Cellular communication network face serious challenges in providing good Quality of Service (QoS) to the users. In order to provide good QoS, network operators have to make the effective use of their available resources, which leads to effective network design and network planning. Erlang capacity is the dominant parameter used for effective network design and network planning. Co-channel Interference is an important limiting factor for Erlang capacity in OFDMA-based cellular systems. The capacity of a OFDMA cellular network is determined by the amount of CCI it can tolerate. In this paper a new method is proposed for the evaluation of Erlang capacity of a OFDMA system under CCI. The Erlang capacity of OFDMA-based cellular systems under CCI is evaluated and compared for different number of co-channel interferers, various spectral efficiencies, voice activity factors and various data rates.

In order to meet the 5G requirements on spectral efficiency and the number of connections, new multiple access (NMA) is becoming an important technology in 5G. Different from orthogonal multiple access (OMA), NMA technology is tolerant of symbol collisions in orthogonal channels, thereby significantly increasing the number of served users. In this paper, a unified framework of NMA is proposed for the next generation radio access networks. The evaluation results of several NMA schemes, such as sparse code multiple access (SCMA), pattern division multiple access (PDMA) and interleaver-grid multiple access (IGMA), are given. And the results show that NMA can obtain significant block error rate (BLER) performance gain, compared with orthogonal frequency division multiple access (OFDMA).

Orthogonal frequency division multiple access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA) are the two major multiple access schemes for 4G wireless communications. In long term evolution (LTE) the downlink multiple access scheme is based on OFDMA and the uplink multiple access scheme is based on SC-FDMA. In this letter, we derive the fundamental performance difference between OFDMA and SC-FDMA, and present the general performance comparison between them. Analytical results show that OFDMA performance upper bounds SC-FDMA in the sense of capacity. The conclusion is further confirmed by numerical results.

The Non-Orthogonal Multiple Access (NOMA) and the Orthogonal Frequency Division Multiple Access (OFDMA) are promising techniques for next-generation wireless systems. Although much attention in the literature considered these techniques, an effective combination between such systems as hybrid multiple access (HMA) needs more attention. For this reason, this paper devotes to proposing two different scenarios of HMA using multi-user OFDM (MU-OFDM) and power domain (PD) NOMA for downlink transmission to overcome the limitations of both Orthogonal Multiple Access (OMA) and NOMA. Due to the randomness of a wireless channel, different user grouping strategies and dynamic power allocation (DPA) strategies are employed to satisfy users’ requirements. The proposed systems give high flexibility utilizing bitrate allocation and user fairness. System results show superior performance to traditional OMA and NOMA systems. The achieved variety of fairness is helpful for the diversity of applications which is a principal requirement for beyond-fifth-generation (B5G) networks. The next step in this analysis is to enhance the proposed systems’ spectral efficiency (SE) by introducing beamforming for massive Multiple-Input Multiple-Output (MIMO) systems. The bit error rate (BER) result of the proposed system achieves almost the same error floor with a benefit of approximately 10 dB signalto-noise ratio (SNR) when 90 resource blocks (RBs) are used.

Complete complementary codes attract the attention of researchers thanks to their unique ideal correlation properties. Still, the boarder on maximum number of signatures is limiting. Quasi orthogonal complete complementary codes allow higher maximum number of signatures in exchange for a slight loss of some ideality of correlation properties. This article focuses on the analysis of these codes and identifies the shifts of broken ideal correlation properties as these vary for different element dimensions.

Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access technique that can accommodate many users with widely varying applications, data rates and Quality of Service requirements, with increased spectral efficiency. Peak to Average Power Ratio (PAPR) reduction for OFDMA is motivated by a desire to increase the mean transmit power and to improve the power amplifier efficiency. This paper presents use of chaotic sequences generated using tent map for multiple data rate spreading and PAPR reduction in OFDMA system. Increase in PAPR due to variable spreading could be reduced by spreading using chaotic sequences. Chaotic sequences gave constant PAPR values for different combinations of data rates. It was also noted that the sequences could be chosen irrespective of the data rates used. Bit error rate was found to be comparable with that for Orthogonal Variable Spreading Factor codes.

Orthogonal codes are majorly crucial for code division multiple access (CDMA) for suppressing the multiple access interference (MAI). The Walsh based orthogonal codes are used for data transmission in on-chip communication because of cross correction is zero between adjacent sequences. Though, the code utilization is less of Walsh based codes when compared with Pseudo-random noise (PN) sequences. To provide better code utilization with orthogonality, an orthogonal gold code is proposed in this paper thereby the orthogonal gold code is used for large size of on-chip communication architecture. These codes are suitable for CDMA and show better code utilization than the Walsh based orthogonal codes. For implementation, the Xilinx 14.7 software with Vertex-6 FPGA and is used and simulations of 32-bit are experimented in MATLAB thereby the crosscorrelation performance improved than Walsh based orthogonal codes.

Orthogonal frequency division multiple access (OFDMA) systems exploit multiuser diversity and frequency-selectivity to achieve high spectral efficiencies. However, they require considerable feedback for scheduling and rate adaptation, and are sensitive to feedback delays. We develop a comprehensive analysis of the OFDMA system throughput as a function of the feedback scheme, frequency-domain scheduler, and discrete rate adaptation rule in the presence of feedback delays. We analyze the popular best-n and threshold-based feedback schemes. We show that for both the greedy and round-robin schedulers, the throughput degradation, given a feedback delay, depends primarily on the fraction of feedback reduced by the feedback scheme and not the feedback scheme itself. Even small feedback delays at low vehicular speeds are shown to significantly degrade the throughput. We also show that optimizing the link adaptation thresholds as a function of the feedback delay can effectively counteract the detrimental effect of delays.