Browsing by Author "Vehkalahti, Roope"

Filter results by typing the first few letters
Now showing 1 - 12 of 12
  • Code Design Principles for Ultra-Reliable Random Access with Preassigned Patterns
    (2019-07-01) Boyd, Christopher; Vehkalahti, Roope; Tirkkonen, Olav; Laaksonen, Antti
     A4 Artikkeli konferenssijulkaisussa
    We study medium access control layer random access under the assumption that the receiver can perform successive interference cancellation, without feedback. During recent years, a number of protocols with impressive error performance have been suggested for this channel model. However, the random nature of these protocols causes an error floor which limits their usability when targeting ultra-reliable communications. In very recent works by Paolini et al. and Boyd et. al., it was shown that if each user employs predetermined combinatorial access patterns, this error floor disappears. In this paper, we develop code design criteria for deterministic random access protocols in the ultra-reliability region, and build codes based on these principles. The suggested design methods are supported by simulations.
  • Combinatorial code designs for ultra-reliable IoT Random access
    (2018-02-14) Boyd, Christopher; Vehkalahti, Roope; Tirkkonen, Olav
     A4 Artikkeli konferenssijulkaisussa
    We consider Combinatorial Code Designs (CCD) for ensuring ultra-reliability in the random access channel. By constructing user-specific repetition patterns to be utilised over a synchronised uplink frame consisting of a number of access slots, we guarantee successive reception up to a given number of simultaneously active users. Employing advanced receivers capable of Successive Interference Cancellation (SIC) further improves reliability. As an example, we consider a system with access frames of 24 bundled slots, repetition factor 3, and reliability target 99.999%. When compared to slotted repetition ALOHA, SIC provides a 30% gain in the tolerated user activity; CCD a 30% gain; whereas CCD combined with SIC provides a gain of more than 700%. These gains come at the cost of a strict limit on the supported user population. In the given example, the system can support a total of 2024 users.
  • CSI Quantization for FDD Massive MIMO Communication
    (2021-06-15) Vehkalahti, Roope; Liao, Jialing; Pllaha, Tefjol; Han, Wei; Tirkkonen, Olav
     A4 Artikkeli konferenssijulkaisussa
    We consider high-dimensional multiuser MIMO transmissions in Frequency Division Duplexing systems. For precoding, the frequency selective channel has to be measured, quantized and fed back to the base station by the users. In 5G New Radio (NR), a modular quantization approach has been applied for this, where first a low-dimensional subspace is identified for the whole frequency selective channel, and then subband channels are linearly mapped to this subspace and quantized. We analyze how the components in such a modular scheme contribute to the overall quantization distortion. Based on this analysis we improve the technology components in the modular approach. We compare the improved quantization scheme to the 5G NR standardized version by simulation in a scenario with a realistic spatial channel model. The improvements lead to a more than 25% improvement in spectral efficiency.
  • Fast-decodable asymmetric space-time codes from division algebras
    (2012) Vehkalahti, Roope; Hollanti, Camilla; Oggier, Frédérique
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Grant-Free Access in URLLC with Combinatorial Codes and Interference Cancellation
    (2018) Boyd, Christopher; Vehkalahti, Roope; Tirkkonen, Olav
     A4 Artikkeli konferenssijulkaisussa
    When targeting ultra-reliability for randomly accessing uplink users, an irreducible error floor exists when employing random repetition coding in synchronised frames. Combinatorial Code Designs (CCDs) that produce deterministic access patterns have been shown to overcome this performance limit in MAC layer channel models, where the physical layer has been abstracted to a collision model. In this work, we demonstrate that CCDs remain a promising option for URLLC for a properly modelled physical layer with fading and collision recovery through diversity reception. We investigate uplink users arriving according to a Poisson process, attempting to directly transmit data packets in a grant-free manner. We measure the achievable rate using a system outage capacity formulation which takes into account the access intensity, the size of the data packets transmitted, and the packet error rate, with the packet loss rate constrained to be less than a given URLLC target. We compare random and deterministic access patterns by system simulation in a factory environment. The results show that considerable gains in system throughput can be achieved by using deterministic access patterns. Unlike random patterns, patterns based on CCDs have the potential to support considerable communication rates with very high success probability. With ideal channel estimation, CCDs enable an aggregate system spectral efficiency of 2 bps/Hz in a grant-free URLLC access scenario.
  • Interference Cancelling Codes for Ultra-Reliable Random Access
    (2018-12) Boyd, Christopher; Vehkalahti, Roope; Tirkkonen, Olav
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Combinatorial code designs (CCDs) are proposed as a means for achieving ultra-reliability in the random access channel. In contrast to traditional access protocols that use random repetition coding, we show that by uniquely allocating repetition patterns to users, successful reception may be guaranteed up to a number of simultaneously active users in small frame sizes. Such codes are particularly robust in the low activity region where mission-critical machine-type communication is expected to operate. We also present deterministic codes designed to work in conjunction with successive interference cancellation to further improve reliability. The optimal IC code for frames of 5 access slots is given. Unlike slotted ALOHA, it is shown to limit packet losses to well below the ultra-reliability threshold (10−5). These error performance gains come at the cost of a strict limitation on the supported user population (11 users in the case of 5 slots). We therefore consider larger frames of 24 slots, and analyse heuristic, low-complexity CCDs with fixed repetition factors that support up to 2024 users. While these are sub-optimal IC codes, significant gains are still observed compared to random codes.
  • Random Access -protokollat
    (2020-05-17) Niska, Rune
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Random Access Esineiden Internetissä
    (2021-05-15) Kemppainen, Roope
     Sähkötekniikan korkeakoulu | Bachelor's thesis
  • Secret Keys from Parity Bits in the Satellite Setting
    (2022) Lietzén, Jari; Tirkkonen, Olav; Vehkalahti, Roope
     A4 Artikkeli konferenssijulkaisussa
    We consider a two-way secret key distribution protocol in the satellite setting, where Alice, Bob and Eve each decode bits from noisy signals received from a source in their environment. Alice and Bob perform advantage distillation to find a secret key. We apply a Two-way Protocol with Parity bit Reconciliation (TPPR) where secret keys are collected from parity bits in course of advantage distillation, not only from the final distilled bits. We analyze the mutual information acquired by Eve from exploiting the original eavesdropped information together with the information leaked during the distillation protocol, as well as TPPR secret key rate. Comparing to the ParityCheck Protocol (PCP) known in the literature, TPPR provides complementary performance. In operation regions where PCP fare badly as compared to one-way protocols, TPPR provides gains in key rate.
  • Signature Code Design for Fast Fading Channels
    (2021-09-01) Vehkalahti, Roope; Pllaha, Tefjol; Tirkkonen, Olav
     A4 Artikkeli konferenssijulkaisussa
    We address the problem of codebook design for sparse user detection in fast fading channels, where the fading realization changes from channel use to next. In this scenario, codebook design criteria based on quasi-static fading, and/or channel state information at the receiver, become ineffective. In this paper we suggest new code design principles for signature coding in fast fading channels and provide examples of codes that are built using these methods.
  • Towards Ultra-Reliable Signature Coding With Multiple Transmit Antennas
    (2021-06-15) Vehkalahti, Roope; Pllaha, Tefjol; Tirkkonen, Olav
     A4 Artikkeli konferenssijulkaisussa
    We consider sparse user detection in fading channels. With Rayleigh flat fading, deep fades occur with relatively high probability and it becomes challenging to provide highly reliable user detection, irrespective of the chosen multiuser detection algorithm. It has been proven that with a large number of receive antennas, this problem can be overcome and both the reliability and number of detectable users can be increased. In this paper, we show that similar improvements can be achieved by moderately increasing the number of transmit antennas at the user terminals. With multiple transmit antennas, code design becomes a problem. We provide a design criterion and show that the detection probability can be considerably improved by using the resulting well-balanced MIMO signature codes, especially in the high-reliability regime.
  • A Two-way QKD Protocol Outperforming One-way Protocols at Low QBER
    (2020-06) Lietzen, Jari; Vehkalahti, Roope; Tirkkonen, Olav
     A4 Artikkeli konferenssijulkaisussa
    Two-way quantum key distribution (QKD) protocols can provide positive secret key rates for considerably higher quantum bit error rates (QBER) than one-way protocols. However, when QBER is low, only modest key rate gains have been achieved. This is one of the major obstacles for using two-way protocols. In this paper we introduce a new two-way QKD protocol which is a step towards overcoming this shortcoming. Under the assumption that the eavesdropper can only perform individual symmetric quantum attacks, our protocol performs quantum key distribution with a secret key rate that is higher than the information theoretical bound limiting the performance of any one-way protocol. This holds true also for very low QBER values.