Spectrum Engineering

We describe a major effort to quantify the speech intelligibility associated with a range of narrowband, wideband, and fullband digital audio coding algorithms in various acoustic noise environments. The work emphasizes the relationship between these intelligibility results and analogous ones for an analog FM land-mobile radio reference. The initial phase of this project includes 54 noise environments and 83 audio codec modes. We use an objective intelligibility estimator to narrow the scope and then design a practically sized modified rhyme test (MRT) covering 6 challenging yet relevant noise environments and 28 codec modes for a total of 168 conditions. The MRT used 36 subjects to produce 432 trials for each condition. Results show that intelligibility depends strongly on noise environment, data rate, and audio bandwidth. For each noise environment we identify codec modes that produce MRT intelligibility values that meet or exceed those of analog FM. We expect that these results can inform some of the design and provisioning decisions required in the development of mission-critical voice applications for LTE.

Keywords: background noise; speech coding; modified rhyme test (MRT); speech intelligibility; audio coding; acoustic noise; ABC-MRT

• TR-15-520

A spectrum sharing scheme is considered in which ship-based radar stations are operating in the same spectrum band as on-shore communication transmitters, and in which the communication transmitters will cause interference to the radar receivers when interference, I, to noise, N, ratios in the radar receivers exceed a given level (e.g., I/N >= -6 dB). The problem is that on-shore environmental sensing capability (ESC) monitors need to determine whether interference is occurring at off-shore radar receivers based only on information from the radars’ transmitters, with no information available from the victim radar receivers themselves. We describe an on-shore monitoring approach in which the principle of reciprocal propagation between the directions of radar-to-ESC and ESC-to-radar provides a simple go/no-go (single-bit) output from the ESCs to an associated Spectrum Access System (SAS) controlling the communication network, to perform on-shore channel changes for protection of the off-shore radar receivers. The ESC station outputs are based on a power-detection threshold of radar signals at the ESCs (e.g., -64 dBm peak-detected power in 1 MHz bandwidth). Examples are provided in which ship-based radar receivers are protected by a simple algorithm applied to a group of on-shore ESCs and a SAS controller for the terrestrial communication network channel frequencies.

Keywords: radar; radio propagation; antenna gain; spectrum sharing; spectrum access system (SAS); Citizens Broadband Radio Service Devices (CBSD); environmental sensing capability (ESC); interference monitoring

• TR-16-521

We describe the design, implementation, and analysis of a speech intelligibility test. The test included five codec modes, four frame-erasure rates, and two background noise environments, for a total of 40 conditions. The test protocol required twenty listeners to repeat all words that they heard in short messages with median length of seven words. Each condition was tested using approximately 1100 words total. Listeners’ responses were scored against the original message transcripts to produce a count of words correctly repeated and thus a measure of speech intelligibility. We present results that show exactly how this measure of speech intelligibility drops as frame-erasure rate increases for three of the five codec modes. The remaining two codec modes did not produce valid results due to defects in the reference software provided to us.

Keywords: background noise; speech coding; packet loss; speech intelligibility; audio coding; frame erasures; acoustic noise

• TR-17-522