[Paper Review] Directional Cell Discovery in Millimeter Wave Cellular Networks

C. Nicolas Barati, S. Amir Hosseini+5 more|arXiv (Cornell University)|Apr 20, 2014

Millimeter-Wave Propagation and Modeling22 references4 citations

TL;DR

This paper proposes a directional cell discovery procedure for mmWave cellular networks using periodic synchronization signals transmitted in time-varying random directions or omnidirectionally. It derives GLRT-based detectors under various signal and receiver models and shows through realistic 28 GHz NYC measurements that digital beamforming significantly outperforms analog beamforming even with low quantization, and omnidirectional transmission generally outperforms random scanning due to consistent signal power.

ABSTRACT

The acute disparity between increasing bandwidth demand and available spectrum, has brought millimeter wave (mmW) bands to the forefront of candidate solutions for the next-generation cellular networks. Highly directional transmissions are essential for cellular communication in these frequencies to compensate for high isotropic path loss. This reliance on directional beamforming, however, complicates initial cell search since the mobile and base station must jointly search over a potentially large angular directional space to locate a suitable path to initiate communication. To address this problem, this paper proposes a directional cell discovery procedure where base stations periodically transmit synchronization signals, potentially in time-varying random directions, to scan the angular space. Detectors for these signals are derived based on a Generalized Likelihood Ratio Test (GLRT) under various signal and receiver assumptions. The detectors are then simulated under realistic design parameters and channels based on actual experimental measurements at 28~GHz in New York City. The study reveals two key findings: (i) digital beamforming can significantly outperform analog beamforming even when the digital beamforming uses very low quantization to compensate for the additional power requirements; and (ii) omni-directional transmissions of the synchronization signals from the base station generally outperforms random directional scanning.

Motivation & Objective

• Address the challenge of initial cell search in mmWave networks, where high path loss and directional beamforming complicate base station detection.
• Investigate whether base stations should transmit synchronization signals omnidirectionally or in randomly varying beam directions.
• Evaluate the performance trade-offs between analog and digital beamforming at the mobile user equipment under realistic mmWave channel conditions.
• Provide a detection framework based on Generalized Likelihood Ratio Test (GLRT) for directional cell search under practical signal and receiver assumptions.

Proposed method

• Proposes a directional cell discovery procedure where base stations periodically transmit synchronization signals in either randomly varying beam directions or omnidirectional patterns.
• Derives GLRT-based detectors for signal detection under different assumptions, including known and unknown signal powers, and various receiver architectures.
• Models the mobile receiver with either analog beamforming (one beam at a time) or digital beamforming (full digital access to all antenna elements).
• Incorporates realistic system parameters: 28 GHz carrier frequency, 10 MHz signal bandwidth, 5 ms frame duration, and 3 PSS signals per frame.
• Uses a standard AWGN model for quantization noise and derives effective SNR degradation due to low-bit quantization using a scalar uniform quantizer model.
• Simulates detection performance using measured mmWave channel data from New York City, with 8×8 BS and 4×4 UE arrays spaced at λ/2.

Experimental results

Research questions

• RQ1 Does omnidirectional transmission of synchronization signals outperform random directional scanning in mmWave cell discovery?
• RQ2 How does digital beamforming compare to analog beamforming in terms of detection performance under practical quantization constraints?
• RQ3 What is the impact of low-resolution ADCs on detection performance in mmWave beamformed systems?
• RQ4 How do frequency offset and timing uncertainty affect cell search reliability in mmWave networks?
• RQ5 Can GLRT-based detection achieve reliable cell discovery under realistic mmWave propagation conditions and hardware constraints?

Key findings

• Digital beamforming significantly outperforms analog beamforming even with only 3-bit quantization, due to superior spatial diversity and coherent combining gain.
• Omnidirectional transmission of synchronization signals generally outperforms random directional scanning because it provides consistent signal power at the receiver, avoiding deep fades.
• The required false alarm rate for initial access is extremely low at approximately 1.45 × 10⁻⁸, necessitating careful hypothesis testing over delay, Doppler, and beam angle space.
• With 3-bit quantization, the effective SNR is reduced by about 14.5 dB relative to ideal sampling, but digital beamforming can still achieve reliable detection.
• The system achieves reliable detection with a target false alarm rate of 1.45 × 10⁻⁸ using 10⁴ delay hypotheses, 3 PSS signals, and 23 frequency offset hypotheses.
• The use of 4×4 and 8×8 uniform planar arrays at the UE and BS, respectively, provides sufficient beamforming gain to compensate for free-space path loss at 28 GHz in urban environments.