Aside from video, cellular and wide-area wireless communication technologies will also have a large impact on architectures and form factors deployed in residential IP systems. Leading the way are fixed-mobile convergence (FMC) applications that are beginning to emerge, offering greater convenience and cost-efficiency to consumers. For example, dual-mode phones are coming to market that automatically and seamlessly switch between a homeowner's Wi-Fi wireless local area network (WLAN) and the provider's cellular network. When connected to the WiFi network, the phone makes use of the VoIP calling capabilities in the home's VoIP gateway system to avoid cellular charges. In another application, the cell phone could function as another extension to the home's residential phone number, increasing the flexibility and access to the home's phone system.

The microphone element constitutes a purely capacitive load, so no current is drawn from the biasing reference. In an extended version of the amplifier system an on-die charge pump is included, thus addressing this problem by providing the necessary biasing for the microphone MEMS element.

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Part IPart II

WiMAX Physical Layer The WiMAX physical layer is based on orthogonal frequency division multiplexing. OFDM is the transmission scheme of choice to enable high-speed data, video, and multimedia communications and is used by a variety of commercial broadband systems, including DSL, Wi-Fi, Digital Video Broadcast-Handheld (DVB-H), and MediaFLO, besides WiMAX. OFDM is an elegant and efficient scheme for high data rate transmission in a non-line-of-sight or multipath radio environment. In this section, we cover the basics of OFDM and provide an overview of the WiMAX physical layer. Chapter 8 provides a more detailed discussion of the WiMAX PHY.

*OFDM Basics* OFDM belongs to a family of transmission schemes called multicarrier modulation, which is based on the idea of dividing a given high-bit-rate data stream into several–parallel lower bit-rate streams and modulating each stream on separate carriers–often called subcarriers, or tones. Multicarrier modulation schemes eliminate or minimize intersymbol interference (I–I) by making the symbol time large enough so that the channe—induced delays–delay spread being a good measure of this in wireless channels–are an insignificant (typically, <10 percent) fraction of the symbol duration. Therefore, in high-data-rate systems in which the symbol duration is small, being inversely proportional to the data rate, splitting the data stream into many parallel streams increases the symbol duration of each stream such that the delay spread is only a small fraction of the symbol duration.

OFDM is a spectrally efficient version of multicarrier modulation, where the subcarriers are selected such that they are all orthogonal to one another over the symbol duration, thereby avoiding the need to have nonoverlapping subcarrier channels to eliminate intercarrier interference. Choosing the first subcarrier to have a frequency such that it has an integer number of cycles in a symbol period, and setting the spacing between adjacent subcarriers (subcarrier bandwidth) to be BSC = B/L, where B is the nominal bandwidth (equal to data rate), and L is the number of subcarriers, ensures that all tones are orthogonal to one another over the symbol period. It can be shown that the OFDM signal is equivalent to the inverse discrete Fourier transform (IDFT) of the data sequence block taken L at a time. This makes it extremely easy to implement OFDM transmitters and receivers in discrete time using IFFT (inverse fast Fourier) and FFT, respectively.

In order to completely eliminate ISI, guard intervals are used between OFDM symbols. By making the guard interval larger than the expected multipath delay spread, ISI can be completely eliminated. Adding a guard interval, however, implies power wastage and a decrease in bandwidth efficiency. The amount of power wasted depends on how large a fraction of the OFDM symbol duration the guard time is. Therefore, the larger the symbol period–for a given data rate, this means more subcarriers–the smaller the loss of power and bandwidth efficiency.

The size of the FFT in an OFDM design should be chosen carefully as a balance between protection against multipath, Doppler shift, and design cost/complexity. For a given bandwidth, selecting a large FFT size would reduce the subcarrier spacing and increase the symbol time. This makes it easier to protect against multipath delay spread. A reduced subcarrier spacing, however, also makes the system more vulnerable to intercarrier interference owing to Doppler spread in mobile applications. The competing influences of delay and Doppler spread in an OFDM design require careful balancing. Chapter 4 provides a more detailed and rigorous treatment of OFDM.

*OFDM Pros and Cons* OFDM enjoys several advantages over other solutions for high-speed transmission.

6.3 Transient response In the perception of music, the leading edge of a waveform is highly significant for the recognition of instruments. With the leading edges removed it can be difficult to tell the sound of a guitar from the sound of a violin, for example. It follows that the subtle differences between different guitars and different violins – and most other instruments for that matter – can be very dependent upon the accuracy of the transient waveform of the onset of the note.

Apart from the need to supply adequate current when a bass drum is played loudly through the loudspeakers, slew rate and frequency response are also important factors. The former is measured in volts per microsecond, and is a measure of the speed with which the output of an amplifier can respond to the input signal. Forty volts per microsecond would be the typical slew rate capability of a good monitoring amplifier, but this same rate needs to be maintained at high levels, and not just at low levels, or slew limiting will occur, which can produce some very odd waveform distortion.

The transient response is also greatly affected by the frequency response of the amplifier. An electrical step function is shown in Figure 6.2. This waveform is also known as a Heaviside function, named after Oliver Heaviside. (Discoverer of the Heaviside layer which surrounds the Earth.) The generalised function is defined by

H(x) = 0 for x < 0="">

*U(x)* = 1 for x > 0