A receiver system for extracting data from a plurality of electromagnetic data signals (111, 121, 122, 131, 132, 133, 134) collectively encoding the data, the system comprising: a phase modulator for receiving an electromagnetic beam, and generating an electromagnetic primary reference beam with a defined phase; one or more mixing units (500, 510, 520), each mixing unit (500, 510, 520) being arranged to receive a respective one of the data signals (111, 121, 122) and a reference beam, and to generate two mixed signals (501, 502, 511, 512, 521, 522), the one or more mixing units (500, 510, 520) including a first mixing unit for which the reference beam is the electromagnetic primary reference beam; a plurality of detection units (611, 612, 621, 622), each detection unit (611, 612, 621, 622) being arranged to receive a respective one of the data signals (131, 132, 133, 134) and a respective mixed signal (511, 512, 521, 522) from one of the mixing units (500, 510, 520), and to obtain a difference measurement (711, 712, 721, 722) indicative of a difference between the respective data signal (131, 132, 133, 134) and the respective mixed signal (511, 512, 521, 522); and a summation unit, arranged to receive the difference measurements (711, 712, 721, 722) generated by the detection units (611, 612, 621, 622), and to obtain a summed difference value indicative of the sum of the difference measurements (711, 712, 721, 722); there being a plurality of mixing units (500, 510, 520), the mixing units (500, 510, 520) being composed of the first mixing unit and one or more additional mixing units (510, 520), the additional mixing units (510, 520) being arranged to receive, in addition to the respective data signal (121, 122), a mixed signal (501, 502) generated by another of the mixing units (500, 510, 520): the mixing units being arranged in a hierarchical array having at least two successive layers (50, 51), the first layer comprising the first mixing unit, and each other layer of the array comprising, for each mixing unit of the preceding layer, a corresponding pair of mixing units (510, 520) which each receive a respective mixed signal generated by that mixing unit of the preceding layer. a phase modulator for receiving an electromagnetic beam, and generating an electromagnetic primary reference beam with a defined phase; one or more mixing units (500, 510, 520), each mixing unit (500, 510, 520) being arranged to receive a respective one of the data signals (111, 121, 122) and a reference beam, and to generate two mixed signals (501, 502, 511, 512, 521, 522), the one or more mixing units (500, 510, 520) including a first mixing unit for which the reference beam is the electromagnetic primary reference beam; a plurality of detection units (611, 612, 621, 622), each detection unit (611, 612, 621, 622) being arranged to receive a respective one of the data signals (131, 132, 133, 134) and a respective mixed signal (511, 512, 521, 522) from one of the mixing units (500, 510, 520), and to obtain a difference measurement (711, 712, 721, 722) indicative of a difference between the respective data signal (131, 132, 133, 134) and the respective mixed signal (511, 512, 521, 522); and a summation unit, arranged to receive the difference measurements (711, 712, 721, 722) generated by the detection units (611, 612, 621, 622), and to obtain a summed difference value indicative of the sum of the difference measurements (711, 712, 721, 722); there being a plurality of mixing units (500, 510, 520), the mixing units (500, 510, 520) being composed of the first mixing unit and one or more additional mixing units (510, 520), the additional mixing units (510, 520) being arranged to receive, in addition to the respective data signal (121, 122), a mixed signal (501, 502) generated by another of the mixing units (500, 510, 520): the mixing units being arranged in a hierarchical array having at least two successive layers (50, 51), the first layer comprising the first mixing unit, and the mixing units being arranged in a hierarchical array having at least two successive layers (50, 51), the first layer comprising the first mixing unit, and each other layer of the array comprising, for each mixing unit of the preceding layer, a corresponding pair of mixing units (510, 520) which each receive a respective mixed signal generated by that mixing unit of the preceding layer.