Stefan Kaiser

OFDM-CDMA versus DS-CDMA: performance evaluation for fading channels

In code division multiple access (CDMA) systems direct sequence (DS) applying maximum ratio combining (MRC) at the receiver has achieved high interest for mobile communications. A second and rather novel technique is orthogonal frequency division multiplexing (OFDM) applying maximum-likelihood detection (MLD) at the receiver. In this paper the performance of both techniques is compared considering the mobile radio environment. The analytical results for both DS-CDMA and OFDM-CDMA are confirmed by Monte Carlo simulations. The analysis enables a performance comparison between the DS-CDMA system and the OFDM-CDMA system with respect to the demands of low complexity receivers which is important for system design. The results show that OFDM-CDMA outperforms DS-CDMA in terms of spectral efficiency.

A Unified Analysis of Diversity Exploitation in Multicarrier CDMA

Since the introduction of multicarrier code-division multiple access (MC-CDMA), several publications have presented various detection concepts for efficient and robust multiuser communications in the up- and downlink. System concepts, which either exploit channel knowledge at the transmitter or at the receiver, have been investigated. The approach, which exploits channel knowledge at the transmitter and at the receiver simultaneously, referred to as combined-equalization, is a new approach that targets a common optimization of data transmission and reception. In this paper, we introduce a unified analysis for optimally exploiting diversity. We apply this analysis exemplarily to MC-CDMA systems exploiting any of the aforementioned concepts. It should be noted that this analysis could be applied to numerous other systems that exploit diversity. The presented approach analytically derives the theoretical lower performance bounds for pre-, post-, and combined-equalization achievable with MC-CDMA in up- and downlink. This fundamental investigation of lower bounds will serve as a benchmark for future optimization of MC-CDMA in multiuser scenarios. Moreover, we exemplarily present multiuser performance results with combined-equalization for the uplink which approach the lower bound derived in this paper.

Spatial Pre-Coding with Phase Flipping for Wireless Communications

The performance of spatial pre-coding schemes depends on the amount of channel state information available at the transmitter. A novel approach is spatial pre-coding with phase flipping investigated in this paper. The scheme is referred to as spatial phase coding (SPQ). Two flipping criteria, the angle criterion (AC) and the absolute value criterion (AVC), are proposed which change the phase relation of the signals between multiple transmit antennas such that the probability of constructive superposition of the signals at the receiver antenna is increased. Compared to known pre-coding schemes, SPC AVC does not need knowledge about the channel from each transmit antenna for pre-coding. Instead, it requires only information about the absolute value of the superimposed channel. This paper shows that SPC can achieve an array gain additional to the diversity gain. This is not possible with schemes like transmitter antenna selection diversity (SD). The lower bounds for SPC are defined by equal gain transmission (EGT) and maximum ration transmission (MRT).

Performance of Spatial Phase Coding (SPC) in Broadband OFDM Systems

In this paper a spatial phase coding (SPC) scheme is investigated for its application in OFDM systems. With SPC the phase relation of the signals between multiple transmit antennas is modified such that the probability of constructive superposition of the signals at the receiver antenna is increased. The principle is based on selective phase flipping. The promising feature of this scheme is that only the superimposed channel has to be estimated at the receiver and not all channels from the different transmit antennas. Moreover, it requires a very small amount of feedback information for pre-coding compared to existing solutions. The performance of SPC is compared to that of spatial diversity and pre-coding schemes like phase flip diversity (PFD), cyclic delay diversity (CDD) and maximum ratio transmission (MRT) for typical indoor as well as outdoor environments.

Rate Adaptation with CDM versus Adaptive Coding for Next Generation OFDM Systems

This paper proposes data rate adaptation based on code division multiplexing (CDM). Under investigation are OFDM systems with cyclic delay diversity (CDD) and with spatial phase coding (SPC). These schemes are promising candidates for future broadband wireless systems. As lower bound, maximum ratio transmission (MRT) is chosen as reference. Imperfections due to channel estimation are taken into account in the investigations. The comparison of adaptive CDM versus adaptive coding shows that adaptive CDM outperforms adaptive coding, especially when spatial transmit diversity schemes are applied. This result is of interest for the design of future broadband adaptive transmission systems like IMT-Advanced.

Effects of Channel Estimation Errors on Spatial Pre-Coding Schemes with Phase Flipping

In this paper the effects of channel estimation errors on spatial pre-coding schemes are analyzed. Under investigation is a novel pre-coding scheme with phase flipping, referred to as spatial phase coding (SPQ. The sensitivity of SPC to channel estimation errors in an OFDM system is compared to that of maximum ratio transmission (MRT), equal gain transmission (EGT) and transmit antenna selection diversity (SD). These diversity and pre-coding schemes have different requirements on the channel estimation and the feedback. Results are presented for a broadband OFDM system with two-dimensional pilot-symbol-aided channel estimation in a typical frequency- and time-selective propagation environment. It is shown that SPC is more robust to channel estimation errors than other spatial pre-coding schemes.

Potential of Code Division Multiplexing (CDM) and Spatial Diversity in Future Broadband OFDM Systems

Code division multiplexing (CDM) is an efficient coding scheme without rate loss. The combination of robust and spectrally efficient orthogonal frequency division multiplexing (OFDM) with CDM results in an orthogonal frequency code division multiplexing (OFCDM) scheme. This paper investigates the potential of OFCDM for future broadband mobile radio schemes such as IMT-Advanced. The focus is on the performance analysis and comparison of OFDM and OFCDM. The investigations take into account channel coding and different spatial diversity and pre-coding schemes like cyclic delay diversity (CDD), spatial phase coding (SPC) and maximum ratio transmission (MRT). The results show that OFCDM outperforms OFDM with respect to bit error rate performance.

Misallocation-Averse Policy for Decentralized Resource Allocation in Spectrum Sharing Systems

It is becoming increasingly difficult to satisfy the growing demands on spectrum with the conventional policy of fixed spectrum allocation. To overcome this problem, flexible and dynamic spectrum sharing methods that can significantly improve utilization of the spectrum have gained increasing interest recently. In this paper, resource allocation in decentralized spectrum sharing systems applying the existing policy rule referred to as inequality-averse policy is focused on. The problem related to the impact of inaccurate estimation of future traffic demands on the performance of such systems is addressed. To alleviate the problem, a novel policy termed misallocation-averse policy, which introduces adjusting factors and is resistant to such estimation errors, is proposed. The effectiveness of the proposed policy rule is verified through computer simulations.

OFDM‐CDM with spatial pre‐coding in fading channels

This paper investigates the potential of orthogonal frequency division multiplexing-code division multiplexing (OFDM-CDM) for its application in future broadband mobile radio systems. The focus is on the performance analysis of OFDM-CDM with spatial pre-coding and its comparison with OFDM. Code division multiplexing (CDM) is an efficient coding scheme without rate loss. The investigations take into account the spatial diversity and pre-coding schemes cyclic delay diversity (CDD), spatial phase coding (SPC), equal gain transmission (EGT) and maximum ratio transmission (MRT). SPC can closely approach the performance of optimum pre-coding schemes, whereas SPC requires less complexity and overhead with respect to channel estimation and feedback information. Imperfections due to channel estimation are taken into account in the analysis. It can be shown that OFDM-CDM with spatial pre-coding applying single symbol detection as well as multi-symbol detection outperforms OFDM with spatial pre-coding. Copyright

Downlink Performance of MC-CDMA Systems with Spatial Phase Codes in Fading Channels

Spatial phase coding (SPC) can closely approach the performance of optimum pre-coding schemes, whereas SPC requires less complexity and overhead with respect to channel estimation and feedback. This paper analyses the effects of SPC in MC-CDMA multi-user systems. The downlink performance and complexity is evaluated and compared to that of known pre-coding schemes. It can be shown that MC-CDMA with SPC applying single as well as multi-user detection can outperform OFDM(A) with SPC.