1. Field of the Invention
The present invention relates to communication equipment and, more specifically, to equipment for wireless local area networks (WLANs).
2. Description of the Related Art
Reliable and efficient transmission of information signals over imperfect communication channels is essential for wireless communication systems. One successful approach to achieving such transmission is multi-carrier modulation (MCM). The principle of MCM is to divide a communication channel into a number of sub-carriers (also called tones or bins), each independently modulated. Information is modulated onto a tone by varying the tone's phase, amplitude, or both.
Orthogonal frequency division multiplexing (OFDM) is a form of MCM, in which tone spacing is selected such that each tone is orthogonal to all other tones. OFDM WLAN systems are typically designed to conform to either a contention-based wireless medium access standard such as IEEE 802.11 or a scheduled time-division duplex (TDD) wireless medium access standard such as ETSI HIPERLAN/2. In a WLAN system conforming to a contention-based standard, OFDM stations compete for access to the wireless medium using “fair contention” medium-sharing mechanisms specified in the standard. In contrast, medium access in a scheduled TDD conforming WLAN system is controlled by a single designated station, which schedules medium access for all other participating transceivers.
IEEE Standard 802.11 and its extensions 802.11a/b/g specify the physical layers and medium access control procedures for OFDM WLAN systems. For example, an 802.11a-compliant system operates in the 5-GHz radio-frequency band and provides data communication capabilities of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s. The system uses 52 tones (numbered from −26 to 26, excluding 0) that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM), or 64-QAM. In addition, the system employs forward error correction (convolutional) coding with a coding rate of ½, ⅔, or ¾.
FIG. 1 is a block diagram of a representative OFDM transceiver 100 of the prior art that can be configured, for example, as an access point (AP) or a client terminal (CLT) in a WLAN system. A typical WLAN system has an AP that provides access to the backbone, wired network for one or more wireless CLTs. Transceiver 100 has a receive path 102 and a transmit path 104, both coupled, at one end, to a medium access controller (MAC) 106 and, at the other end, to an antenna 124 via switch 126. Depending on the mode of operation, switch 126 connects antenna 124 to either transmit path 104 or receive path 102.
In transmit path 104, information bits received via MAC 106 are encoded and interleaved by a convolutional encoder 108 and interleaver 110, respectively. The interleaved data are then converted from the binary format into, e.g., QAM values using a mapping converter 112. To facilitate coherent reception, four pilot values are added to each 48 data values to form an OFDM symbol having 52 QAM values. The QAM values are demultiplexed in a serial-to-parallel (S/P) converter 114 and modulated onto 52 tones using an inverse fast Fourier transform (IFFT) element 116, which tones are then combined in a parallel-to-serial (P/S) converter 118. A cyclic prefix (CP) is added in a CP adder 120 to reduce inter-symbol interference due to the multi-path delay spread (signal dispersion) in the communication channel. The resulting OFDM symbol is applied to a radio-frequency (RF) transmitter 122, where it is converted to an analog signal, up-converted to the 5-GHz band, and transmitted through antenna 124.
Receive path 102 is designed to perform the reverse operations of transmit path 104 as well as additional training functions. In particular, RF signals are received through antenna 124 by an RF receiver 128, which first estimates frequency offset and symbol timing using special training symbols in the preamble of each OFDM data packet. Receiver 128 divides the received RF signals into OFDM symbols, which are then frequency down-shifted and digitized. A CP-removing circuit 130 strips each symbol of the cyclic prefix and applies the result to an S/P converter 132. A fast Fourier transform (FFT) element 134 then recovers QAM values corresponding to the 52 tones. The training symbols and pilot tones are used to correct for the communication channel response as well as phase drift. The recovered QAM values are then multiplexed, de-mapped, and de-interleaved using a P/S converter 136, de-mapping converter 138, and de-interleaver 140, respectively, to recover the corresponding binary data. The information bits are decoded from the binary data in a convolutional (e.g., Viterbi) decoder 142 and then output from transceiver 100 via MAC 106.
One problem with transceiver 100 is related to the reliability of operation in relatively high-scattering environments, such as homes, offices, and/or production facilities. In particular, high-rate transmission/reception (e.g., at rates over 20 Mbit/s) is very sensitive to the quality of the communication channel. In addition, RF signals in the 5-GHz band intended for such high-rate transmission/reception are subjected to a higher propagation loss than those in, for example, a 2.4-GHz band. As a result, operation at high rates may be limited to a relatively short range. Outside that range, lower fall-back rates (e.g., 6 Mbit/s) may have to be utilized. This limits information throughput and may cause, for example, a WLAN system employing transceiver 100 as an access point to operate at a fraction of its potential capacity.