Tcw Tim Schenk

Implementation of a MIMO OFDM-based wireless LAN system

The combination of multiple-input multiple-output (MIMO) signal processing with orthogonal frequency division multiplexing (OFDM) is regarded as a promising solution for enhancing the data rates of next-generation wireless communication systems operating in frequency-selective fading environments. To realize this extension of OFDM with MIMO, a number of changes are required in the baseband signal processing. An overview is given of the necessary changes, including time and frequency synchronization, channel estimation, synchronization tracking, and MIMO detection. As a test case, the OFDM-based wireless local area network (WLAN) standard IEEE 802.11a is considered, but the results are applicable more generally. The complete MIMO OFDM processing is implemented in a system with three transmit and three receive antennas, and its performance is evaluated with both simulations and experimental test results. Results from measurements with this MIMO OFDM system in a typical office environment show, on average, a doubling of the system throughput, compared with a single antenna OFDM system. An average expected tripling of the throughput was most likely not achieved due to coupling between the transmitter and receiver branches.

Frequency synchronization for MIMO OFDM wireless LAN systems

The paper proposes an accurate and time-efficient technique for frequency synchronization of multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The technique uses a preamble and is thus especially suitable for burst mode communication. The preamble consists of training sequences simultaneously transmitted from the various transmit antennas. From analysis, it is shown that the accuracy of frequency synchronization is close to the Cramer-Rao lower bound and increases for increasing RMS delay spreads and number of receive antennas. Furthermore, application of the proposed algorithm in MIMO OFDM wireless local-area-network (WLAN) systems leads to a BER that is only slightly higher than that of a perfectly synchronized system, making it highly applicable.

On the influence of phase noise induced ICI in MIMO OFDM systems

The influence of transmitter and receiver phase noise (PN) on the performance of a multiple-input multiple-output orthogonal frequency division multiplexing (MIMO OFDM) based communication system is analyzed. It is shown that in the case of frequency flat Rayleigh fading, the influence of receiver and transmitter PN is equal. In the case of independent Rayleigh fading, however, the impact of the receiver PN is shown to depend on the ratio between the number of transmit and receive branches.

Estimation and Compensation of Frequency Selective TX/RX IQ Imbalance in MIMO OFDM systems

The influence, digital estimation and compensation of IQ mismatch at both transmitter (TX) and receiver (RX) side of a direct-conversion based multiple antenna OFDM system are presented in this paper. The resulting IQ imbalance can be split into a frequency independent and a frequency selective part. For estimation and compensation a two-step approach is presented, combining a data-aided and decision directed approach. First the frequency independent IQ imbalance is estimated using a preamble and, subsequently, the frequency selective IQ imbalance is removed using adaptive filtering. A numerical performance analysis reveals that the presented approach effectively mitigates the influence of IQ mismatch in OFDM-based space division multiplexing systems experiencing both TX and RX IQ imbalance.

Estimation and compensation of TX and RX IQ imbalance in OFDM-based MIMO systems

This paper studies the influence, estimation and digital compensation of IQ imbalance at both transmitter and receiver side of a multiple-input multiple-output (MIMO) OFDM system. Hereto a preamble is designed, which enables simultaneous estimation of the channel and imbalance parameters. New estimation approaches for TX, RX and joint TX and RX IQ imbalance in MIMO OFDM systems are proposed and evaluated. Results from a numerical study show that all three proposed compensation approaches are able to significantly suppress the influence of IQ mismatch.

Distribution of the ICI Term in Phase Noise Impaired OFDM Systems

Orthogonality between the subcarriers of an orthogonal frequency division multiplexing (OFDM) system is affected by phase noise, which causes inter-carrier interference (ICI). The distribution of this interference term is studied in this paper. The distribution of the ICI for large number of carriers is derived and it is shown that the complex Gaussian approximation, generally applied in previous literature, is not valid and that the ICI term exhibits thicker tails. An analysis of the tail probabilities confirms these finding and shows that bit-error probabilities are severely underestimated when the Gaussian approximation for the ICI term is used, leading to too optimistic design criteria. Results from a simulation study confirm the analytical findings and show the validity of the limit distribution, obtained under the assumption of a large number of subcarriers, already for a modest number of subcarriers.

The Application of Spatial Shifting for Peak-to-Average Power Ratio Reduction in MIMO OFDM Systems

This paper investigates the application of spatial shifting (SS) of partial transmit sequences (PTSs) in multiple-input multiple-output OFDM. The technique rearranges the transmit (TX) vector in such a way that subparts are transmitted on those TX branches that result in the lowest overall peak-to-average power ratio. The application of different subcarrier grouping schemes and the combination of SS with phase shifting (PS) of the PTSs is investigated. Furthermore, a transparent extension of the techniques, without extra signalling overhead, is proposed. Numerical results prove the effectiveness of the SS and the combined SS/PS approach

Performance impact of IQ mismatch in direct-conversion MIMO OFDM transceivers

This contribution analytically studies the influence of transmitter (TX) as well as receiver (RX) IQ mismatch on the performance of multiple-antenna OFDM systems based on direct-conversion. Analytical expressions are derived for the probability of error for MIMO OFDM systems in both nonfaded and Rayleigh faded channels. The results can be used to derive matching specifications for the TX- and RX-branches. It is concluded that in fading channels RX IQ imbalance is on average more destructive than TX IQ imbalance. Additionally, it is concluded that the addition of extra RX antennas is beneficial to reduce RX IQ imbalance dependence, but increases the TX IQ imbalance impact

Impact of nonlinearities in multiple-antenna OFDM transceivers

This paper investigates the performance impact of nonlinearities in the transmitter (TX) as well as the receiver (RX) of multiple-antenna OFDM systems. First different, previously proposed, nonlinearity models are reviewed and their equivalence is shown. Subsequently, analytical expressions are derived for the probability of error of multiple-input multiple-output (MIMO) OFDM systems in Rayleigh-faded channels, which show good agreement with results from a simulation study. It can be concluded from the results that for high SNR values the TX nonlinearities cause performance floors, which are independent of the MIMO configuration. For RX nonlinearities, however, the performance in this high SNR region does depend on the MIMO configuration. The results can be used to derive limits for the permissible nonlinearities in MIMO OFDM systems

Symbol timing for multiple antenna OFDM systems

Accurate symbol timing in multiple-input multiple-output (MIMO) OFDM systems allows for guard-intervals (GIs), which are short compared to the channels rms delay spread. These short GIs improve the effective throughput of these kind of systems. This paper proposes different timing approaches for MIMO OFDM systems, which apply knowledge about the channel impulse response. The techniques differ in complexity and performance. From a numerical performance study it is concluded that for single-cluster channels and moderate delay spreads the low-complex algorithms can be applied without considerable loss in performance. For high delay spreads and multiple clusters, however, the proposed algorithm, which attempts to maximize the signal-to-inference ratio, is shown to be preferable