Chapter 10. Wireless Ethernet

IEEE 802.11 Standards, also known as wireless Ethernet, have been developed mainly for enterprise LAN applications. However, the convenience of wireless connections and the affordability of transceiver technologies make it an attractive option for home networking. A wireless Ethernet can be initiated by installing an 802.11 hub, which is linked to a backbone wired network such as a conventional Ethernet, an ADSL, or a cable modem based broadband access network, in an office or a home environment. Wireless Ethernet NICs also need to be installed inside other computing or communication devices to share these wireless connections. A laptop PC equipped with a wireless Ethernet NIC can enjoy wireless connectivities at home, office, or even some public area such as an airport or a hotel with 802.11 hubs.

Initial versions of RF wireless Ethernet are based on spread spectrum technologies. Spread spectrum technology was originally developed during World War II for secret RF communication applications. A spread spectrum communication system can be implemented using either frequency hopping or direct sequence technologies. For the Frequency-Hopping Spread Spectrum (FHSS) technology, the carrier hops on a set of frequencies in a particular sequence. For the Direct Sequence Spread Spectrum (DSSS) technology, the energy of a signal is spread to a broader bandwidth by a particular higher rate time domain sequence. These frequency hopping or time domain sequences are known only by the intended partners to maintain communications secrecy. On the other hand, the use of spread spectrum technology can also provide a Signal-to-Noise Ratio enhancement in a nonregulated RF environment. For SNR enhancement, both FHSS and DSSS technologies have been attempted for wireless LAN applications before the finalization of the IEEE 802.11 standards.

The original IEEE 802.11 wireless Ethernet standards released during June 1997 included three versions of physical layers: one for Infrared and the other two for Radio Frequency in the ISM band of 2.4 GHz. Only RF versions of wireless Ethernet are discussed in this book. The FHSS version of wireless Ethernet defined by the IEEE 802.11 standards have two transmission throughputs of 1 and 2 Mbps using Gaussian frequency keying modulation. At a minimum of 2.5 hops per second and at least 6 MHz per hop, up to 78 different frequencies can be used depending on system parameters initialized and maintained by a wireless Ethernet access point. The DSSS version of wireless Ethernet defined by the IEEE 802.11 standards also has transmission throughputs of 1 and 2 Mbps. The bandwidth of every bit or every pair of bits is expanded to 11 MHz by a spreading process based on an 11-chip Barker code running at a chip rate of 11 MHz. There are also 11 carriers, with 5-MHz separation between adjacent carriers, allocated for use by DSSS wireless Ethernet. Two DSSS wireless Ethernets can be established in the same location without much interference.

A common MAC protocol is defined for all wireless Ethernet physical layers. Modulation method dependent packet formats are used by different physical layers to carry MAC frames. Because of the dynamic nature of received signals, the collision detection in the RF environment is sometimes not guaranteed. The wireless Ethernet uses CSMA/CA, where every reception of a long packet is acknowledged, instead of CSMA/CD as defined for conventional Ethernet. In addition, a Wired Equivalent Privacy (WEP) encryption procedure is defined for protection against eavesdropping over the open air. The IEEE 802.11b standard for a high-throughput extension to the DSSS wireless Ethernet was later released during 1999. The High Rate Direct Sequence Spread Spectrum (HRDSSS) wireless Ethernet uses a Complementary Code Keying modulation method to carry 4 or 8 data bits on each signaling symbol consisting of 8 chips. The chip rate of HRDSSS is also 11 MHz. HRDSSS can have transmission throughputs of 5.5 and 11 Mbps. An IEEE 802.11a standard for an Orthogonal Frequency Division Multiplex (OFDM) wireless Ethernet was also released during 1999. OFDM wireless Ethernet operates in 5-GHz ISM bands and provides transmission throughputs of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.

The implementation of a wireless Ethernet transceiver involves some RF circuits and some digital circuits. Early realizations usually require a few RF semiconductor components including a transmit Power Amplifier (PA), a receive Low Noise Amplifier (LNA), a Radio Frequency and Intermediate Frequency (RF/IF) convertor, an IF demodulator, and a few digital chips such as a baseband processor and a MAC processor including the host interface function. In most recent implementations, these functions have been integrated into a single RF chip and a single digital chip for minimal power consumption enabling the wireless Ethernet adoption into some portable electronic devices such as a Personal Digital Assistant (PDA). Functions of RF circuits are different, but similar, for different wireless Ethernet physical layers. Not much digital processing is required for FHSS wireless Ethernet other than shifting the carrier frequency to encode data bits and detecting frequency shifts to recover them. Analog to Digital Converters (ADC) and Digital to Analog Converters (DAC) are required in a DSSS transceiver for signal shaping and detection as well as automatic gain control. The received signal, after digitalization by ADCs, also needs to be correlated to the Barker code for data symbol identification. Digital matched filters, fast Walsh transform operation, and decision feedback channel equalization techniques are necessary inside a HRDSSS transceiver to achieve the required transmission performance. An OFDM wireless Ethernet transceiver uses IFFT/FFT, cyclic prefix, convolution encoding, interleaving, and Viterbi decoding digital processing techniques instead to maintain its high-transmission performance.

We start this chapter with frame structure and MAC protocol which are common features for all versions of wireless Ethernet. We then look into each RF wireless Ethernet version individually in the sequence of FHSS, DSSS, HRDSSS, and OFDM. For each version, we highlight features of the standards, examine its typical transceiver structure, and study corresponding transmission performance.