  Fifth-generation (5G) systems will extensively employ radio access network
(RAN) softwarization. This key innovation enables the instantiation of "virtual
cellular networks" running on different slices of the shared physical
infrastructure. In this paper, we propose the concept of Private Cellular
Connectivity as a Service (PCCaaS), where infrastructure providers deploy
covert network slices known only to a subset of users. We then present SteaLTE
as the first realization of a PCCaaS-enabling system for cellular networks. At
its core, SteaLTE utilizes wireless steganography to disguise data as noise to
adversarial receivers. Differently from previous work, however, it takes a
full-stack approach to steganography, contributing an LTE-compliant
steganographic protocol stack for PCCaaS-based communications, and packet
schedulers and operations to embed covert data streams on top of traditional
cellular traffic (primary traffic). SteaLTE balances undetectability and
performance by mimicking channel impairments so that covert data waveforms are
almost indistinguishable from noise. We evaluate the performance of SteaLTE on
an indoor LTE-compliant testbed under different traffic profiles, distance and
mobility patterns. We further test it on the outdoor PAWR POWDER platform over
long-range cellular links. Results show that in most experiments SteaLTE
imposes little loss of primary traffic throughput in presence of covert data
transmissions (< 6%), making it suitable for undetectable PCCaaS networking.
