Delay Analysis of OFDMA-Aloha

Abstract

Abstract-OFDMA is the basis of future broadband access, due to its many inherent advantages such as scalability and fine granularity for multi-user access. OFDMA-Aloha combines the flexibility of OFDMA with basic Aloha's collision resolution mechanism over sub-carriers, in an attempt to reduce packet collisions and achieve faster retransmission. However, this comes at the expense of a larger slot size, due to lower channel rates per subcarrier. The above gives rise to a fundamental question: whether to use a single wide-band Aloha channel and retransmit via random back-off in next K time slots, or to retransmit immediately in one of K narrow-band sub-channels which are each 1/K slower (OFDMA-Aloha)? We answer this question, by analyzing the two protocols: Aloha and OFDMA-Aloha under the same total bandwidth and load conditions. We first derive the exact distribution of the packet access delay of OFDMAAloha in the saturated case. Then, we extend the analysis to the unsaturated case and derive the mean queue length and packet delay by decomposing the system of interfering queues into multiple independent queues utilizing the symmetry in our system. Our results show that if the network is already saturated, channelization does not bring substantial reduction in the collision rate to the point where it outweighs the effect of expanded slot size. In this case the single channel Aloha performs better than OFDMA-Aloha especially when the gap between the number of channels and the number users is large. On other hand, when the network is lightly loaded, OFDMA-Aloha enjoys smaller packet delays, but not for long as it saturates faster than the single channel Aloha.

End-to-End Delay Bound for VR Services in 6G Terahertz Networks with Heterogeneous Traffic and Different Scheduling Policies

“…Considering the time as divided into slots of duration equal to the packet transmission time, the slots are assigned to the uVR user in an exclusive (no-contention) mode, as well as for the traffic G. Differently, the elements of each device community, C and D, access the channel following the well-known contention based Aloha scheme as detailed later on. Furthermore, the analysis of the machine devices' traffic is performed assuming a saturation condition, that is, each device always has a new packet ready to be transmitted whenever an access attempt is successfully accomplished [24]. According to [25,26], we have assumed that each device is located at a fixed distance d 0 from the tagged SBS.…”

On Fast Retrial for Two-Step Random Access in MTC

“…For re-transmissions in 2-step approaches, in this paper, fast retrial [21] is used to lower access delay [22]. With fast retrial in 2-step approaches, a device experiencing collision immediately re-transmit with another randomly selected preamble.…”

“…With fast retrial in 2-step approaches, a device experiencing collision immediately re-transmit with another randomly selected preamble. Note that as there are multiple preambles, the resulting random access becomes multichannel ALOHA as in [21] [22]. Throughout the paper, it is assumed that the two steps can be carried out within a time slot and t is used for the index of time slots, where t ∈ {0, 1, .…”

RADB: Random Access with Differentiated Barring for Latency‐Constrained Applications in NB‐IoT Network

“…Most projects of current research focus on computing of mean value and variance for random access latency; there are few projects of research that focus on probability density function (PDF) [23][24][25] of random access latency [26][27][28]. With quantity of waiting users and channel busy/idle as state variables, the moment generating function (MGF) for PDF of random access latency is deduced based on Markov process in [25,29,30]. But the problem of high computation complexity remains; it is even unsolvable when the quantity of users is too large.…”