Filtreler

2003 - 20083
Filtreleri Sıfırla

Ayarlar

Yazar: Aygölü, Hasan Ümit × Konu: Trellis codes ×

Arama Sonuçları

Listeleniyor 1 - 3 / 3
  • Küçük Resim
    Yayın
    Hızlı sönümlemeli kanallarda yeni bir uzay-zaman-frekans kodlamalı OFDM sistem tasarımı
    (IEEE, 2004) Oğuz, Onur; Aygölü, Hasan Ümit; Panayırcı, Erdal
    Bu çalışmada, frekans seçici hızlı sönümlemeli kanallarda telsiz iletişim için bir uzay-zaman-frekans çeşitlemesi yöntemi önerilmiştir. Önerilen yöntem, OFDM tekniğini kullanarak frekans seçici kanalı uygun hale getirdikten sonra sırasıyla dik uzay-frekans ve dik uzay-zaman blok kodlama uygulayarak uzay-zaman-frekans çeşitlemesi sağlamaktadır. Elde edilen sistemin başarımını arttırmak üzere sisteme uygun bu kafes kodlama tekniği araştırılmış ve uygun kafes yapısının oluşturulması için yeni kriterler ortaya çıkarılmıştır. Oluşan sistemin başarımını, bilgisayar benzetimleriyle incelenmiş ve var olan yöntemlerle karşılaştırılmıştır.
  • Küçük Resim
    Yayın
    EM-Based sequence estimation for wireless systems with orthogonal transmit diversity
    (IEEE, 2003) Panayırcı, Erdal; Aygölü, Hasan Ümit; Pusane, Ali Emre
    In this paper, an optimum sequence estimation algorithm for wireless systems with Alamouti's two transmitter diversity in the presence of multipath fading is proposed. The algorithm is based on a jointly iterative channel and sequence estimation according to the maximum likelihood (ML) criterion, using the Expectation-Maximization (EM) algorithm employing M-PSK modulation scheme with additive Gaussian noise. The discrete multipath channel is represented in terms of the channel gains from each transmit antenna to the receive antenna. EM algorithm derived estimates jointly the complex channel parameters of each channel And the data sequence transmitted, iteratively, which converges to the true ML solution. The channel estimation is achieved in a simple way through the iterative equations by decoupling of the signals transmitted from different antennas. The algorithm is applied to the trellis coded modulation systems and efficiency of the algorithm proposed has been shown by the computer simulations. Simulation results show that the EM algorithm converges quickly for fast fading channels. The performance of the EM-based decoder approaches that of the ML receiver which has perfect knowledge of the channel.
  • Küçük Resim
    Yayın
    Design and performance analysis of a novel trellis coded space-time-frequency OFDM transmission scheme
    (Wiley-Blackwell, 2008-01) Oğuz, Onur; Aygölü, Hasan Ümit; Panayırcı, Erdal
    In this paper a new transmit diversity technique is proposed for wireless communications over frequency selective fading channels. The proposed technique utilises orthogonal frequency division multiplexing (OFDM) to transform a frequency selective fading channel into multiple flat fading subchannels on which first space-frequency coding and then additional space-time coding are applied resulting in a space-time-frequency OFDM (STF-OFDM) scheme. This scheme provides a quasi-orthogonal 4 x 4 transmission matrix but relax the constraint on channel fading parameters to be constant over four time or frequency slots into two time and two frequency slots. The pairwise error probability of the new scheme is evaluated and new code design criteria are obtained to improve the error performance. 64-, 128- and 256-state 4-PSK trellis codes for STF transmission scheme are generated based on these criteria and their frame error performances in an OFDM system are evaluated by computer simulations.