Stephan Pfletschinger

An efficient waterfilling algorithm for multiple access OFDM

In this paper we present a novel and computationally efficient waterfilling algorithm for multiuser OFDM. This algorithm is based on the multiuser waterfilling theorem and determines the subcarrier allocation for a multiple access OFDM system. This approach maximizes the total bitrate under the constraints of user-individual power budgets. Once the subcarrier allocation has been established, the bit and power allocation for each user can be determined with a single-user bitloading algorithm, for which several implementations can be found in the literature. The presented iterative algorithm performs well, also with a high number of subchannels.

Error Floor Removal for Bit-Interleaved Coded Modulation with Iterative Detection

The performance of bit-interleaved coded modulation with iterative detection (BICM-ID) depends strongly on the chosen constellation mapping. In this paper, we present an EXIT chart analysis of a number of mappings that have been designed especially for BICM-ID. This analysis reveals that all these mappings inevitably yield a bit error rate floor, which cannot be overcome by increasing the number of iterations or other measures taken at the receiver side. In order to remove this error floor, we introduce an inner code based on differential encoding with code doping. This coding scheme is of low complexity and introduces no additional redundancy. Two detection schemes of different complexity and similar performance are presented. This enhanced BICM scheme is applied to V-BLAST and a new concept to iteratively mitigate the multi-stream interference is presented

Key technologies for IMT-advanced mobile communication systems

WINNER is an ambitious research project aiming at identification, development, and assessment of key technologies for IMT-advanced mobile communication systems. WINNER has devised an OFDMA-based system concept with excellent system-level performance for flexible deployments in a wide variety of operating conditions. The WINNER system provides a significant step forward from current 3G systems. Key innovations integrated into the system concept include flexible spectrum usage and relaying, adaptive advanced antenna schemes and pilot design, close to optimal link adaptation, hierarchical control signaling, and a highly flexible multiple access scheme. The end-to-end performance assessment results demonstrate that the WINNER concept meets the IMT-advanced requirements.

Getting Closer to MIMO Capacity with Non-Binary Codes and Spatial Multiplexing

In this paper, we discuss the combination of non-binary channel coding with higher-order modulation and MIMO transmission in the form of spatial multiplexing. In addition to the benefits of non-binary LDPC codes on the AWGN channel, we identify an inherent advantage for non-binary coding in multiple antenna schemes. By comparing binary and q-ary information processing at the receiver, we highlight the intrinsic advantages of non-binary coding. A performance comparison based on simulation results shows that, at similar system complexity, non-binary information processing can actually outperform the Shannon limit of its binary counterpart.

Optimized impulses for multicarrier offset-QAM

The design of optimum pulses for digital multicarrier offset-QAM (MC-OQAM) is presented. The impulses are designed as finite duration pulses with maximum signal energy in one subcarriers frequency band. The intersymbol and intercarrier interference is minimized. The solution is obtained by expanding the impulses into finite-length discrete prolate spheroidal sequences. The design method, optimum impulses and simulation results are presented. With this technique, more than 95% of the signal energy can be concentrated in the frequency band of one subcarrier and the out-of-band spectral parts fall off very rapidly in comparison with rectangular pulses used by conventional OFDM systems. Thus, the presented multicarrier system provides much less spectral overlap between modulated subcarriers and is therefore much less susceptible against frequency-selective interference.

Adaptive HARQ With Non-Binary Repetition Coding

We consider Incremental Redundancy Hybrid Automatic Repeat reQuest (IR-HARQ) in which the code rate and modulation of the initial transmission and all retransmissions are adjusted based on average channel statistics. In the absence of instantaneous channel state information at the transmitter (CSIT), we present a method which computes, prior to transmission, the optimum code rates and modulations and explicitly considers a given maximum number of retransmissions. For the case that additional feedback on CSI of previous transmission attempts is available, we present two heuristic schemes which exploit this knowledge and offer increased throughput at the cost of higher computational complexity. We employ a rate-adaptive non-binary LDPC coding scheme which makes use of non-binary repetitions. While this coding scheme is particularly well-suited for adaptive IR-HARQ, we note that the presented analysis can be applied to any other channel code which employs soft decoding.

Non-binary coding for vector channels

We consider the application of non-binary coding schemes to vector channels, in particular we apply q-ary LDPC codes to multiple-antenna transmission schemes. Based on the principle of providing a good equivalent channel to the coding scheme, we directly map the code symbols to the transmit signal and find some suitable multi-dimensional mappings. The performance is assessed by simulations and we find that non-binary coding with direct mapping provides a significant advantage over its binary counterpart and over some space-time diversity schemes.

Energy-efficient data collection in WSN with network coding

We consider the application of a network coding scheme in WSN (wireless sensor networks) for robustness. The use of network coding in WSN is evaluated in terms of reliability improvement, energy efficiency and resilience to network protocol failures. We have concentrated on the evaluation of coding schemes that take advantage of the spatial diversity inherent in different layers of the communication protocol. In particular, we evaluate the effect of overhearing (opportunistic listening) on the network coding performance and investigate different schemes for energy efficiency improvement. One of the main challenges is to determine the amount of overhearing, which enables to exploit the spatial diversity without consuming too much energy.

Joint decoding of multiple non-binary LDPC codewords

We develop a belief-propagation (BP) decoder for the joint decoding of multiple codewords which belong to the same non-binary LDPC code. Decoding is based on soft information in form of joint channel-posterior probabilities of all codeword symbols. We extend the BP algorithm for q-ary LDPC codes such that the FFT-based check node processing is preserved and the complexity remains manageable. This joint decoding is useful in settings in which multiple codewords are transmitted in a non-orthogonal way over the same channel, including multiple-access with packet collisions, physical-layer network coding and multi-resolution broadcasting. We show in an example that joint decoding can be far superior to separate decoding.

Decoding options for trellis codes in the two-way relay channel

We study the application of trellis codes to physical-layer network coding in the two-way relay channel. In particular, we consider quaternary decoding on a combined trellis as an alternative to binary decoding and evaluate its gain in several scenarios, including asymmetric channels and the combination of different codes. We focus on the multiple-access phase, in which both terminals transmit their messages simultaneously, while in the second phase the relay retransmits the network-coded combination of both messages. We also evaluate the performance of both options when decoding the individual message of each terminal.