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Scalable voip mobility intedration and deployment- P17

Scalable voip mobility intedration and deployment- P17: The term voice mobility can mean a number of different things to different people. Two words that can be quite trendy by themselves, but stuck together as if forgotten at a bus station long past the last ride of the night, the phrase rings a number of different, and at times discordant, bells. | 160 Chapter 5 180 quadrature axis Q 90 x x x x x x x x x x x x x x x x -90 16-QAM 0 in-phase axis I 180 quadrature axis Q 90 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x -90 64-QAM 0 in-phase axis I Figure 5.21 16-QAM and 64-QAM www.newnespress.com Introduction to Wi-Fi 161 source bits and record whether that sum is even or odd as one additional bit. If one bit is lost the sum of the surviving bits can be compared in evenness to the sum bit therefore recovering the original data. This is called a parity check code and conceptually introduces the concept of using arithmetic on some or all of the source bits to produce the extra bits. 802.11ag uses a convolutional encoder to expand the source bits in this case producing twice the number of bits but unlike the doubler doing so more intelligently to avoid some loss patterns of equal bit losses being worse than others. This expansion still produces only one data rate at a 12 -coding rate per modulation. To produce the other coding rates 802.11ag just uses the property that the error-correcting code can tolerate loss and goes ahead and starts tossing bits to get to a higher coding rate. This process called puncturing sounds inefficient by reducing redundancy that was just added why not just not add as many extra bits in the first place but ends up saving on the complexity of the coding hardware the radio. The overall picture then for 802.11ag encoding is for the data bits to be scrambled as with 802.11b then expanded with the error-correcting code then split among the subcarriers and then modulated. Because of the larger number of things going on in the signal there is more risk of losing data if the receiver s timing goes off a bit from the sender s. To compensate timing is maintained by the addition of four pilot subcarriers. These carry a known signal pattern like the preamble but do so for the length of the transmission. If the .