Victor Stolpman

A reduced CSI feedback approach for precoded MIMO-OFDM systems

To obtain the closed-loop capacity of a multiple-input and multiple-output (MlMO) orthogonal frequency division multiplexing (OFDM) system, channel state information (CSI) is required at the transmitter. To reduce the data rate of CSI feedback, preceding matrix approach has been proposed for MIMO systems in flat fading channels. In this paper, we develop a novel approach for MIMO-OFDM systems in frequency-selective fading channels. The proposed approach exploits the correlation of frequency responses at different subchannels in MIMO-OFDM systems to reduce CSI feedback. It is not only flexible to multiple data stream transmission but also has better performance than the existing approaches

Pipelined Block-Serial Decoder Architecture for Structured Ldpc Codes

We present a pipelined block-serial decoder architecture for structured LDPC codes, implementing the layered-mode belief-propagation. We introduce the concept of LLR-update and mirror memory to enforce a pipelined decoding schedule. The pipelined architecture improves the latency of the LDPC decoder by about 2x-3x and has negligible performance loss when implemented with clever layer scheduling. We also present a low-complexity check-node architecture suitable for block-serial processing and utilize the properties of the min approximation to significantly reduce the memory requirement. The proposed architecture is suitable for mobile devices with data-rates of tens of mbps

Multiple users adaptive modulation schemes for MC-CDMA

Multi-carrier CDMA (MC-CDMA) unites CDMA and OFDM together and potentially offers the advantages of both technologies. Under a frequency-selective fading channel, the subchannel frequency functions have different amplitude and phase. While the frequency domain spreading process gains some advantage from the frequency diversity available, it does not make the most of the different sub-channel conditions. Instead, the subcarriers should be adaptively loaded according to their states. This would improve the performance in a way similar to what adaptive bit loading offers in OFDM. The paper investigates this topic and proposes a configuration that not only permits multiple users to employ adaptive modulation, but also leads to an equivalent sub-carrier concept that enables a group of sub-carriers to be represented by an equivalent sub-carrier of a conventional OFDM modem, thereby allowing various powerful bit/power loading schemes, originally developed for OFDM, to be directly deployed to MC-CDMA.

An adaptive learning approach to adaptive OFDM

Orthogonal frequency division multiplex (OFDM) technique has been successfully deployed in indoor wireless LAN and outdoor broadcasting applications. It is well-known that under a frequency-selective fading channel the resultant sub-channel frequency functions are frequency-variant and may also be time-variant, i.e. the channel magnitude may be highly fluctuating across the sub-carriers and varies from symbol to symbol. Hence adaptive modulation may be appropriately applied to improve the error performance and data throughput in an OFDM modem by assigning different modulation and coding schemes to different sub-carriers. One fundamental issue in deploying adaptive modulation is to determine what modulation and coding scheme to use. Virtually all past investigations were either based on heuristic methods or analytical means. This paper proposes a novel adaptive learning control approach that is capable of dynamically adjusting the switching thresholds to improve or maximize the data throughput.

Modified Min-Sum Algorithm for LDPC Decoders in UWB Communications

Several variations of the sum-product algorithm (SPA) have been proposed for the decoding of the low density parity check (LDPC) codes. Among these, the log domain belief propagation algorithm (log-BPA or simply BPA) is widely adopted in most of the proposed LDPC decoder architectures. The check node unit (CNU), which computes the check-to-variable (C2V) messages, is computationally the most complex part of the decoder employing the belief propagation algorithm. The BPA can be approximated using a sub-optimal min-sum algorithm; however, it suffers from the performance loss. The min-sum algorithm can be easily implemented with comparators and multiplexers, thereby reducing the area and the power consumption of the decoder, critical for the high data-rate applications such as ultra wide-band (UWB) communications system. Due to the performance degradation, the decoder needs to perform more decoding iterations to reach the desired performance. This, in turn, increases the decoder latency and reduces the decoder throughput. One approach to improve the decoder throughput is to layout more parallel CNUs inside a decoder, but this offsets the advantage of the min-sum implementation. On the other hand, the performance of the min-sum algorithm can be improved by scaling the node-to-check messages and it is possible to find optimal scaling factor for a given channel. In this paper, we propose an improvement to the min-sum algorithm based on the analysis of the error term in the min-sum algorithm. The proposed algorithm achieves the performance similar to the scaled min-sum (SMS) approach with optimal scaling, in the additive white Gaussian noise (AWGN) channel. We also show min-sum utilized with different fading channels and LDPC codes without any further adjustments. Thus, the proposed MMS algorithm provides flexibility in real applications, which need to support multiple code rates, block sizes and have to operate in unknown fading channels. The performance of the proposed algorithm is evaluated for the MB-OFDM UWB systems with short distance indoor channel models. To meet the speed requirement of the of the UWB systems, a bit-level pipelined comparator is developed, which significantly reduces the latency of the critical path in the comparison circuit

Performance Analysis of the Structured Irregular LDPC Coded MIMO-OFDM System with Iterative Channel Estimator

In this paper, we evaluate the performance of the receiver employing an iterative RLS-based data detection and channel estimation for the structured irregular LDPC coded MIMO-OFDM system. Using the EXIT chart analysis, the performance of the detector with various approximate decoding algorithms is analyzed

Performance analysis of the detector for structured irregular LDPC coded MIMO-OFDM system

In this paper, we evaluate the performance of the MIMO data detector with various decoding architectures in the structured irregular LDPC coded MIMO-OFDM system. Using the EXIT chart analysis, the performance of the data detector in the iterative MIMO-OFDM system with various approximate decoding algorithms is analyzed.

Structured Interleavers and Decoder Architectures for Zigzag Codes

We propose structured interleaver design for parallel concatenated Zigzag codes. While the proposed design performs as good as or better than random interleavers for various block-sizes, it improves the error floor of the Zigzag codes and offers a lot of parallelism suitable for high data-rate applications. The interleaver can be specified with only a few parameters and can be efficiently implemented in both hardware and software. We also evaluate semi-parallel Zigzag decoder architecture that exploits the parallelism of the proposed interleavers to improve the throughput. We also evaluate the performance of an efficient decoding schedule for semi-parallel Zigzag decoder that provides better throughput and performance trade-offs compared to the fully parallel and serial decoder schedule. The proposed interleaver scheme and architecture are suitable for high throughput Ultra Wideband communications that demand data-rates up to several hundred mbps.

A decentralized adaptive learning scheme for adaptive OFDM

Orthogonal Frequency Division Multiplex (OFDM) technique has been successfully deployed in indoor wireless LAN and outdoor broadcasting applications. It is well-known that under a frequency-selective fading channel the resultant sub-channel frequency functions are frequency-variant and may also be time-variant. Hence adaptive modulation may be appropriately applied to improve the error performance and data throughput in an OFDM modem by assigning different modulation and coding schemes to different sub-carriers. One fundamental issue in deploying adaptive modulation is to determine what modulation and coding scheme to use according to the sub-channel SNR. Virtually all past investigations were either based on heuristic methods or analytical means. This paper proposes a novel decentralized adaptive learning scheme that is capable of dynamically adjusting the switching thresholds to improve or maximize the data throughput.