PHY, MAC, and RLC Layer Based Estimation of Optimal Cyclic Prefix Length

Adriana Lipovac,Vlatko Lipovac,Borivoj Modlic

doi:10.3390/s21144796

Adriana Lipovac, Vlatko Lipovac + Show 1 more

Open Access

https://doi.org/10.3390/s21144796

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Abstract

This work is motivated by growing evidence that the standard Cyclic Prefix (CP) length, adopted in the Long Term Evolution (LTE) physical layer (PHY) specifications, is oversized in propagation environments ranging from indoor to typical urban. Although this ostensibly seems to be addressed by 5G New Radio (NR) numerology, its scalable CP length reduction is proportionally tracked by the OFDM symbol length, which preserves the relative CP overhead of LTE. Furthermore, some simple means to optimize fixed or introduce adaptive CP length arose from either simulations or models taking into account only the bit-oriented PHY transmission performance. On the contrary, in the novel crosslayer analytical model proposed here, the closed-form expression for the optimal CP length is derived such as to minimize the effective average codeblock length, by also considering the error recovery retransmissions through the layers above PHY—the Medium Access Control (MAC) and the Radio Link Control (RLC), in particular. It turns out that, for given protective coding, the optimal CP length is determined by the appropriate rms delay spread of the channel power delay profile part remaining outside the CP span. The optimal CP length values are found to be significantly lower than the corresponding industry-standard ones, which unveils the potential for improving the net throughput.

Highlights

The role of Cyclic Prefix (CP)—the copy of Orthogonal Frequency-Division Multiplexing (OFDM) symbol waveform tail inserted at the beginning is to mitigate multipath channel delay spreading and consequent intersymbol interference (ISI)
It is well known that, in Long Term Evolution (LTE) systems, both Incremental-Redundancy HARQ (IR-HARQ) and ARQ functionalities are used. The former is run by both physical layer (PHY) and Medium Access Control (MAC) layers; at the transmitter, during each transmit time interval (TTI), the transport block (TB) that PHY receives from MAC as
Up to four increasing-redundancy and rate-matched IR-HARQ redundancy versions (RV0 to RV3 ) of the codeblock can be sent until the codeblock cyclic redundancy check (CRC) at the receiver indicates error-free transmission

Summary

Introduction

The role of Cyclic Prefix (CP)—the copy of Orthogonal Frequency-Division Multiplexing (OFDM) symbol waveform tail inserted at the beginning is to mitigate multipath channel delay spreading and consequent intersymbol interference (ISI). With appropriate time sampling of the received signal and the CP length at least slightly larger than the longest expected channel delay spread, the ISI and the Intercarrier. The price for that is paid in terms of spectral and power efficiency loss, as CP insertion reduces information throughput for the ratio of the CP length to the OFDM symbol period. This equals 7% for the so-called normal standard CP length of 4.69 μs, adopted in the Long Term Evolution (LTE). Physical layer (PHY) specifications [3,4], wasting the transmitter energy, degrading the Signal-to-Noise Ratio (SNR), and shortening the mobile terminal battery life.

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