1.3 Wireless communications  (Page 2/5)

Power consumption

Wireless devices are battery-operated and designed to consume a minimum amount of power. This allows longer battery life and fewer recharge cycles. In limited energy scenarios, power-efficient modulation schemes are used, along with control hardware, to regulate power consumption during the transmission and reception of packets. Control hardware puts radios in low-power modes during inactive periods.

Spectrum utilization

Frequency spectrum is a natural resource; its efficient use is stipulated by regulatory authorities. A radio service provider can buy or lease a portion of the spectrum, where it can have complete control over its operations. Alternatively, a portion of the spectrum can be allocated to a service, such as a cordless phone, where users can then set up hardware for that service without needing a license. The industrial, scientific and medical (ISM) bands are license-free bands where proprietary communication techniques can be used to implement wireless services.

In multiple user environments, multiple access schemes are employed. These can be implemented in the following ways. Frequency-division multiple access (FDMA) allocates bandwidth to users in a frequency spectrum, where they can operate in specific frequency bands. Time-division multiple access (TDMA) is based on time-slicing the occupied spectrum and creating a schedule of when each pair of radios is on the air. Code-division multiple access (CDMA) is a spread-spectrum technique where each user is provided with a unique uncorrelated pseudo-random noise (PN) sequence (code) that can be used to despread the received signal before demodulation. The main advantage of CDMA over either FDMA or TDMA is that the frequency reuse factor is 100 percent. This means that the entire allocated frequency band can be used for transmission. Frequency reuse in FDMA and TDMA systems depends on the isolation between areas of operation, depending on the path loss of the radio channel. CDMA can reuse the allocated spectrum for all areas.

Blocks of a wireless communication system

To simplify the drawings, let's split a typical transceiver into its two major components, the receiver and transmitter, and describe them individually.

Block diagram of a receiver

Figure 1 is a system block diagram of a direct conversion receiver and transmitter. An antenna is followed by a bandpass filter, used as a band select filter. This eliminates out-of-band noise and presents the signal to the low-noise amplifier (LNA). The LNA then amplifies the desired signal, adding a minimum amount of inherent noise. The signal processed by the LNA is then down-converted to the desired IF frequency by a set of mixers operating in quadrature. These mixers are often image-reject mixers and have some gain as well. After down-conversion, the low-frequency IF signal is lowpass-filtered to remove aliasing components and converted to digital samples by the analog-to-digital converter. In the digital domain, more filtering is applied for channel selection, and plenty of signal processing is performed to remove any channel effect before the detection stage.