# Degrees of Freedom and Achievable Rate of Wide-Band Multi-cell Multiple Access Channels With No CSIT

## TL;DR

This paper analyzes the degrees-of-freedom and achievable rate of wide-band multi-cell multiple access channels without CSIT, showing interference management via delay spread differences enables near interference-free capacity as user numbers grow.

## Contribution

It introduces a blind interference management technique exploiting delay spread differences to achieve interference-free DoF without CSIT in multi-cell channels.

## Key findings

- Sum DoF is (β-1)/β * K with large user numbers
- Interference-free DoF per cell approaches 1 as β→∞
- Closed-form expression for achievable sum rate

## Abstract

This paper considers a $K$-cell multiple access channel with inter-symbol interference. The primary finding of this paper is that, without instantaneous channel state information at the transmitters (CSIT), the sum degrees-of-freedom (DoF) of the considered channel is $\frac{\beta -1}{\beta}K$ with $\beta \geq 2$ when the number of users per cell is sufficiently large, where $\beta$ is the ratio of the maximum channel-impulse-response (CIR) length of desired links to that of interfering links in each cell. Our finding implies that even without instantaneous CSIT, \textit{interference-free DoF per cell} is achievable as $\beta$ approaches infinity with a sufficiently large number of users per cell. This achievability is shown by a blind interference management method that exploits the relativity in delay spreads between desired and interfering links. In this method, all inter-cell-interference signals are aligned to the same direction by using a discrete-Fourier-transform-based precoding with cyclic prefix that only depends on the number of CIR taps. Using this method, we also characterize the achievable sum rate of the considered channel, in a closed-form expression.