The wireless local area network or WLAN is expanding rapidly into a wide variety of implementations and applications. This rapid growth is attributable to several factors, including improved accuracy (lower BER), increased bandwidth and falling costs. While wireless networking was considered fairly exotic and certainly expensive just a few years ago, it is rapidly gaining users in all markets. Commercial enterprises can avoid considerable wiring costs by deploying wireless networks in their buildings and campuses. Even families and home office workers now enjoy the benefits of wireless networks, as the cost of an access point has fallen below $100 at retail and an individual WLAN “station” costs around $50. (A station or “client” has a MAC and a PHY, and connects to the wireless medium. Common implementations include standard PC-type circuit boards and PCM cards.)
High manufacturing volumes, lower costs, and technical improvements to a large degree can all be traced to standardization. The IEEE established the 802.11 Working Group in 1990, and has promulgated various standards in this area in recent years. In 1999, the IEEE-SA Standards Board approved the 2.4 GHz, 11 Mbps 802.11b and the 5 GHz, 54 Mbps 802.11a high-rate PHY extensions. These two standards have relatively identical MAC layers, but different PHY layers. The IEEE Standard 802.11 (1999), the IEEE Standard 802.11a (1999) and 802.11b (1999) high-rate PHY Supplements, and the 802.11g (draft 2002) high-rate PHY Supplement are incorporated herein fully by reference.
The lower bandwidth 802.11b standard was first to be widely implemented, fueling considerable growth and competition in the WLAN industry. That system has several drawbacks, however. The 2.4 GHz (unlicensed) band is very congested, hosting everything from wireless telephones to Bluetooth transceivers to microwave ovens. (Although the maximum packet length in the 802.11 protocol was designed to operate between the 8 msec pulses of the microwave oven magnetron.) The 2.4 Gz system provides limited numbers of channels (three non-overlapping) and bandwidth limited to 11 Mbps best case.
The 5 GHz (“Wi-Fi5”) band is relatively clean. The 802.11a extension provides for 8 non-overlapping channels (plus 4 channels in the upper band), and improved bandwidth up to 54 Mbps. Systems can provide higher bandwidths, and still be compliant, as long as they implement at least the mandatory data rates, namely 6, 12, and 24 Mbps. Some manufacturers are offering “dual band” systems that implement both 802.11b and 802.11a standards. Because of differences in the PHY layer, particularly different frequency radios and modulation schemes, dual band systems essentially have to implement both types of systems in one package.
Another extension of the basic 802.11 architecture is the 802.11g proposal (not yet a standard), which is something of a hybrid. It operates in the 2.4 GHz band, like 802.11b, but uses orthogonal frequency division multiplexing OFDM in the PHY layer, like 802.11a, to achieve data rates in excess of 20 Mbps. (802.11g also employs Complementary Code Keying (CCK) like 802.11b for data rates below 20 Mbps. In fact it has a variety of operating modes). Final ratification of the 802.11g protocol is expected in 2003. It is attractive in part because the 2.4 GHz band is largely available world-wide. Some predict that combination 802.11a/g designs will soon supplant 802.11a/b. Indeed, the present invention is applicable to 802.11a/g combination designs (as well as each of them separately).
As noted, both 802.11a and 802.11g protocols employ OFDM encoding. Briefly, OFDM works by dividing one high-speed data carrier into multiple low-speed sub-carriers which are used for transmission of data, in parallel. Put another way, the data stream is divided into multiple parallel bit streams, each transmitted over a different sub-carrier having a lower bit rate. In the case of 802.11a/g there are 52 such sub-carriers, 48 of which are used to carry data. (The other four are pilot sub-carriers, discussed later.) These sub-carriers are orthogonal to each other, so they can be spaced closer together than in conventional frequency-division multiplexing. Mathematically, the integral of the product of any two orthogonal sub-carriers is zero. This property allows separating the sub-carriers at the receiver without interference from other sub-carriers. OFDM systems and circuits are described in greater detail in, for example, U.S. Pat. Nos. 5,345,440; 5,889,759 and 5,608,764 each incorporated herein fully by this reference.
In OFDM systems, carrier frequency offset and noise can produce large degradations of the Bit Error Rate (BER) performance. Indeed they not only produce extra noise due to Inter-Carrier Interference (ICI) but also a parasitic rotation of the symbols which also increases the BER as further discussed later. Carrier frequency offset means any difference in frequency between the carrier frequency generators in the transmitting and receiving circuitry.
U.S. Pat. No. 6,198,782 to De Courville et al. describes methods and apparatus for carrier and clock frequency offset compensation. De Courville obtains a ML estimate jointly for carrier and clock frequency offsets. However, the approach in the 782 patent first removes the channel using equalization for coherent modulation systems (col. 5, line 5). The equalization leads to different noise variance on each subcarrier (i.e. non-white noise). This factor apparently is not considered in the derivation of the ML estimator in the patent (col. 7, line 7).
Several methods are known for estimating and compensating a carrier frequency offset. See for example, U.S. Pat. No. 5,450,456 to Mueller. These techniques are useful, but they do not address sample phase noise or “jitter”, in other words discrepancies between the sample rate in a transmitter and a receiver.