M-ary Orthogonal Modulation
Definition: A data modulation scheme which uses M, a power of 2, orthogonal signaling waveforms in which each of the M signaling waveforms is used to send a particular pattern of log2M binary symbols.
Note: Suppose M equals 2N. There are exactly 2N different blocks or patterns of binary symbols taken N at a time. Hence, a unique signaling waveform can be assigned to each possible symbol pattern. The transmission of that signaling waveform conveys the binary symbols to the receiver. See Example.
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© 1999 DR. ERNEST SIMO - SPACE2000
M-ary Orthogonal Modulation
Application: IS-95 uses a 64-ary (26) orthogonal modulation scheme on the reverse links. The 64 orthogonal signaling waveforms are the 64 Walsh words . The binary symbols occurring at the 28.8 ksym/sec rate are blocked or partitioned into groups of 6. There are exactly 64 different binary-symbol patterns of length 6 possible. Each different pattern is assigned to a particular Walsh word. In IS-95 the assignment rule is very straightforward. The 6-binary-symbol block is viewed as a 6-bit word with the right-most bit the least significant. The decimal equivalent of the 6-bit word is then determined and the Walsh word with that decimal number is transmitted to convey the 6-bit pattern to the receiver. At the base station, a Fast Walsh/Hadamard Trans-form, similar to a Fast Fourier Transform, is performed which is able to detect which of the 64 possible Walsh words was sent. Noting the number of the received Walsh word, the corresponding binary-symbol pattern is reconstructed at the receiver.
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© 1999 DR. ERNEST SIMO - SPACE2000
M-ary Orthogonal Modulation
Example: Consider an M-ary orthogonal modulation scheme in which M equals 8. The 8 orthogonal signaling waveforms are assumed to be the 8 Walsh words of 8 Walsh chips. The 8 Walsh words will be denoted by Wn, n equal 0 to 7. Since 8 equals 23, N in the above discussions equals 3. This means that the binary symbols will be partitioned into groups of 3 binary symbols and each group or pattern will be assigned a particular Walsh word. The 8 different groups or patterns and the associated Walsh word are shown below.
| Binary Pattern | Walsh Word | Binary Pattern | Walsh Word
| | 000 | W0 | 100 | W4
| | 001 | W1 | 101 | W5 | | 010 | W2 | 110 | W6 | | 011 | W3 | 111 | W7 |
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