Achieving High Frequency Diversity with Subcarrier Allocation in OFDMA Systems

Abstract

OFDM can provide frequency diversity gain for point-to-point communications over frequency-selective slow fading channel. Recent works show that OFDM may also form a flexible and efficient multiple access method, which is often referred to OFDMA. However, the user outage probability and the optimal frequency diversity gain in OFDMA systems are not known. In this paper, random bipartite graph is used to model and analyse the multi-user subcarrier allocation problem over frequency-selective slow fading channels. Our aim is to minimize the user outage probability as well as guarantee fairness by dynamic allocating various subcarrier to each user. An optimal subcarrier allocation algorithm, which we shall refer to as the Hungarian method with random vertices rotation, is introduced to achieve these objectives. A simple but effective approximation equation for user outage probability is then derived. It is shown that the optimal frequency diversity gain in OFDMA system is the same as the point-to-point OFDM system. In particular, the frequency diversity gain does not decay as the number of users increases.

An Efficient Diversity Exploitation in Multiuser Time-Varying Frequency-Selective Fading Channels

“…However, there are numerous ways to have band-type sub-channels (since the first subchannel can start at any subcarrier index) and this imposes unnecessarily large control overhead to inform users of the sub-channel format. A practical solution is to use a fixed band-type sub-channel (i.e., the first subchannel starts at a fixed subcarrier index) in the first symbol (hence overcoming the overhead issue), and fortunately based on our simulation (due to space limitation, the figure is not included), incorporating this additional constraint into (4) with (8) and (10) does not change the result. Fig.…”

“…Fig. 2 shows the optimal resource structures obtained from a simulation based approach to (8) and (10) for OFDMA with = B = 16, SC = 1, = 4, and = 14, in a 4-tap Rayleigh fading channel at the speed of 120 km/h and 350 km/h. Here, we take…”

“…The existing literature on frequency diversity exploitation in OFDMA includes bit-interleaved coded modulation (BICM) [5], precoding with subcarrier selection [6], block spreading [7], and subcarriers and bit allocation [8], [9]. The multiuser diversity exploitation is addressed by resource allocation (e.g., [10]- [15]) or scheduling (e.g., [16], [17]) according to the channel state information (CSI) of the users.…”

Joint power and subcarrier allocation in OFDMA systems

“…Son yıllarda OFDMA sistemlerinde alt-taşıyıcı atama yöntemi olarak grafik tabanlı algoritmalar kullanılmaya başlanmıştır [10], [11], [12], [13]. OFDMA kanallar için alttaşıyıcı atama problemi rastgele ikili diyagram (random bipartite graph) modeli yardımıyla tanımlanmıştır [10].…”

“…OFDMA kanallar için alttaşıyıcı atama problemi rastgele ikili diyagram (random bipartite graph) modeli yardımıyla tanımlanmıştır [10]. Kullanılan maksimum eşleme algoritması ile OFDMA sistemlerindeki frekans çeşitlemesi kazancının noktadan noktaya OFDM sistemine eşit oldugu gösterilmiştir ancak frekans çogullaması konusuna deginilmemiştir.…”