Cyclic Delay Diversity Research Articles

Transmit Diversity at the Cell Border Using Smart Base Stations

We address the problems at the most critical area in a cellular multicarrier code division multiple access (MC-CDMA) network, namely, the cell border. At a mobile terminal the diversity can be increased by using transmit diversity techniques such as cyclic delay diversity (CDD) and space-time coding like Alamouti. We transfer these transmit diversity techniques to a cellular environment. Therefore, the performance is enhanced at the cell border, intercellular interference is avoided, and soft handover procedures are simplified all together. By this, macrodiversity concepts are exchanged by transmit diversity concepts. These concepts also shift parts of the complexity from the mobile terminal to smart base stations.

Adaptive Bit Loading and Transmit Diversity for Iterative OFDM Receivers

In this paper, we consider a transmission system employing orthogonal frequency division multiplexing with bit-interleaved coded modulation and perfect channel state information at both transmitter and receiver. An adaptive bit loading scheme in combination with cyclic delay diversity and discontinuous Doppler diversity is proposed at the transmitter and iterative demapping and decoding at the receiver. The loading procedure minimizes the bit-error rate at the decoder output, and the transmit diversity schemes mitigate channel correlations. We analyze the iterative receiver with extrinsic information transfer charts and present the achievable gains.

Analysis of Cyclic Delay Diversity on DVB-H Systems over Spatially Correlated Channel

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The objective of this work is to research and analyse the performance of Cyclic Delay Diversity (CDD) with two transmit antenna on DVB-H systems operating in spatially correlated channel. It is shown in this paper that CDD can achieve desirable transmit diversity gain over uncorrelated channel with or without receiver diversity. However, in reality, the respective signal paths between spatially separated antennas and the mobile receiver is likely to be correlated because of insufficient antenna separation at the transmitter and the lack of scattering effect of the channel. Under this spatially correlated channel, it is apparent that CDD cannot achieve the same diversity gain as obtained under the uncorrelated channel. In this paper, a new upper bound on the pairwise error probability (PEP) of the CDD with spatial correlation of two transmit antennas is derived. The upper bound is used to study the CDD theoretical error performance and diversity gain losses over a generalized spatially correlated Rayleigh channel. This theoretical analysis is validated by the simulation of DVB-H systems with two transmit antennas and the CDD scheme. Both the theoretical and simulated results give the valuable insight that the CDD ability to perform well with a certain amount of channel correlation. </para>

Diversity Gain for DVB-H by Using Transmitter/Receiver Cyclic Delay Diversity

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The objective of this paper is to investigate different diversity techniques for broadcast networks that will minimize the complexity and improve received SNR of broadcast systems. Resultant digital broadcast networks would require fewer transmitter sites and thus be more cost-effective and have less environmental impact. The techniques can be applied to DVB-T, DVB-H and DAB systems that use Orthogonal Frequency Division Multplexing (OFDM). These are key radio broadcast network technologies, which are expected to complement emerging technologies such as WiMAX and future 4G networks for delivery of broadband content. Transmitter and receiver diversity technologies can increase the frequency and time selectivity of the resulting channel transfer function at the receiver. Diversity exploits the statistical nature of fading due to multipath and reduces the likelihood of deep fading by providing a diversity of transmission signals. Multiple signals are transmitted in such a way as to ensure that several signals reach the receiver each with uncorrelated fading. Transmit diversity is more practical than receive diversity due to the difficulty of locating two receive antennas far enough apart in a small mobile device. The schemes examined here comply with existing DVB standards and can be incorporated into existing systems without change. The diversity techniques introduced in this paper are applied to the DVB-H system. Bit error performance investigations were conducted by simulation for different DVB-H and diversity parameters. </para>

Performance of Cyclic Delay Diversity in DAB/DMB

Cyclic delay diversity (CDD) is a simple and elegant technique to exploit transmit diversity in a MIMO-OFDM system by artificially increasing the frequency-selectivity of the equivalent channel. In this paper, we study the application of CDD in digital audio broadcasting (DAB), which is well motivated by the mismatch analysis between the channel inherent diversity and the capability of the DAB channel coding and interleaving. Simulation of DAB with CDD in rural area and typical urban area is carried out; the results demonstrate the advantage of CDD in creating a more uniform coverage area compared to the system without CDD. We further devise a lower bound which allows justification of tradeoff between the design parameters and the performance improvement

Cyclic delay diversity based space-time/space-frequency coded MC-CDMA system

Spatial diversity techniques will inarguably play an important role in the realization of B3G wireless systems, which promise higher data transmission rates and affluent multi-media contents. In this article, we present a CDD based space time/space-frequency (ST/SF) block coded diversity transmitter for MC-CDMA system. Equipped with CDD a fraction of the total number of transmit antennas is used to extract the frequency (multipath) diversity while the remaining antennas employing ST/SF-block codes provide maximum spatial diversity. This hybrid architecture makes full use of the frequency domain spreading of MC-CDMA system and provides excellent spatial and frequency diversity which is otherwise impossible to realize with standalone CDD or block codes

Cyclic delay diversity with bit-interleaved coded modulation in orthogonal frequency division multiple access

We consider cyclic delay diversity in OFDMA. Cyclic delay diversity is an elegant way to obtain spatial diversity in an FEC coded OFDM system without exceeding the guard interval. We first address the problem of choosing the cyclic delays and propose a new robust design rule which enables to pick up the full spatial and frequency diversity which is inherent in a frequency-selective MIMO channel. Our choice of cyclic delays has consequences for the interleaving and multiple access scheme since the spatial diversity appears to be transformed into frequency diversity between neighbouring subcarriers. Therefore, a system with a conventional block frequency interleaver will fail to exploit the spatial diversity. We propose an interleaving and multiple access strategy which guarantees that all users obtain the maximum possible diversity advantage using FEC codes with a limited constraint length. Furthermore, we provide a performance comparison to transmit diversity from orthogonal designs

On time‐varying cyclic delay diversity

AbstractCyclic delay diversity (CDD) was introduced in 2001 to obtain spatial diversity in an orthogonal frequency division multiplexing (OFDM) based transmission system. In contrast to space‐time codes, CDD introduces no additional effort at the receiver side, since CDD changes only the channel impulse response seen by the receiver by increasing the frequency selectivity. Furthermore, CDD has the advantage that there is no rate loss even for more than two transmit antennas. In this paper, we analyse CDD for an orthogonal frequency division multiple access (OFDMA) based transmission system. To increase not only the frequency selectivity but also the time selectivity, we introduce time‐varying cyclic delay diversity (TV‐CDD), which changes the cyclic shift with each OFDM symbol. Especially in systems, where each user can only allocate a few subcarriers for data transmission, TV‐CDD improves the performance of the OFDMA‐based transmission system significantly. Copyright © 2006 AEIT

Differential modulation and cyclic delay diversity in orthogonal frequency-division multiplex

We propose a new scheme for differential modulation in orthogonal frequency-division multiplexing (OFDM) with cyclic delay diversity. Delay diversity is done in a cyclic manner in order not to exceed the guard interval. However, the increased frequency selectivity, and consequently, reduced coherence bandwidth, causes problems for noncoherent detection of differential modulation in frequency direction. Our proposal is able to cope with the increased frequency selectivity,and enables picking up the spatial diversity in frequency-selective channels with unknown delay spread in combination with standard differential modulation techniques. The new scheme is less complex than differential unitary space-time modulation. The overhead due to reference symbols is minimized and the detection delay is reduced, compared with differential modulation in time direction.

Modulation, coding and signal processing for wireless communications - Performance comparison of space-time block coded and cyclic delay diversity MC-CDMA systems

Spatial diversity is a widely applied technique for enhancing wireless system performance since it greatly reduces the detrimental effects of multipath fading. Space-time block codes have been considered the best choice for transmit diversity in narrowband environments, but their use in broadband channels is questionable due to their inability to pick up multipath diversity. However, when used in conjunction with an MC-CDMA system, they achieve not only full spatial but also variable multipath diversity depending on the employed spreading. In comparison, cyclic delay diversity is an attractive approach to achieve spatial and multipath diversity. Its simplicity and conformability with current standards makes it desirable for multicarrier systems. Previous studies suggest that CDD is only advantageous with an outer channel code for OFDM systems. In this article, we compare STBCs and CDD applied to an MC-CDMA system in terms of complexity and performance. It is shown that for an MC-CDMA system, CDD benefits from spreading and channel coding that makes it very competitive with STBCs, particularly since it is applicable to any number of transmit antennas with no loss in rate.

Spectrally efficient transmit diversity scheme for differentially modulated multicarrier transmissions

Cyclic delay diversity is a simple, yet effective, transmit diversity scheme for multicarrier based transmissions employing coherent digital linear modulation schemes. It is shown that, for satisfactory operation, the scheme requires additional channel estimation overhead compared to single antenna and traditional space-time coded transmissions owing to the inherent increase in frequency selective fading. The authors analyse the additional channel estimation overhead requirement for a Hiperlan #2 style system with two transmit antennas operating in a NLOS indoor environment. The analysis shows that an additional overhead of 500% is required for the candidate system compared to a single antenna system. It is also shown that by employing differential modulation the channel estimation overhead can be eliminated with significant performance improvement compared to a system employing a practical channel estimation scheme. This novel combination, termed 'differentially modulated cyclic delay diversity', is shown to yield a highly spectral efficient, yet simple transmit diversity solution for multi-carrier transmissions.

On space-frequency coding using cyclic delay diversity for OFDM-based transmission systems

We present a space-frequency coding scheme using cyclic delay diversity (CDD) for an orthogonal frequency division multiplexing (OFDM)-based transmission system. CDD is a simple multiple transmit antenna approach for increasing the frequency selectivity of the channel seen at a single antenna receiver. This is due to the insertion of virtual echoes caused by antenna specific cyclic delays in the time domain at the transmitter. On each antenna a shifted version of the signal is sent, with these shifts being performed in the frequency domain. This allows the receiver to apply optimum demodulation, and thus exploit the diversity introduced by CDD. We optimize the selection of the time domain cyclic delays in the CDD so as to get full spatial diversity with the proposed space-frequency codes with CDD (SFC-CDDs) scheme. Moreover, we show simulation results for SFC-CDD, which confirm theoretical results with respect to the achieved spatial diversity, and present a comparison with the well-known Alamouti scheme implemented as SFC.

On space-frequency coding using cyclic delay diversity for OFDM-based transmission systems

On space-frequency coding using cyclic delay diversity for OFDM-based transmission systems