Compensation of Multiple Access Interference Effects in CDMA-Based Acoustic Positioning Systems

Recently developed acoustic positioning systems operate in a code division multiple access (CDMA) configuration, in which the ranging signals between the nodes are digitally modulated orthogonal codes with the same carrier frequency and overlapping in time. CDMA permits higher position update rate than the alternative time division multiple access, but suffers from multiple access interference (MAI) effects, leading to outliers in the estimated ranges, and potentially large errors in position estimation. In this communication, we present and demonstrate experimentally a subtractive parallel interference cancelation (PIC) method, which achieves a high degree of resistance to MAI effects, and also permits us to compensate the intersymbol interference (ISI) caused by the limited frequency range of acoustic transducers. When evaluated empirically in an acoustic positioning system, the PIC algorithm obtains nearly total outlier cancelation for four operating beacons, and 2/3 reduction of outliers for a seven beacon setup with 32 bits long codes. Outliers are further reduced (down to 2%) by the modified PIC algorithm with ISI compensation. The method outperforms alternative outlier reduction techniques like doubling or quadrupling the signal length, or using power control to adjust the relative amplitudes of the beacon signals, and permits that the system is available for positioning over 95% of the time.