Timing synchronization in OFDM communications receivers

Details Timing synchronization in OFDM communications receivers Timing synchronization in OFDM communications receivers

2000-09-29

2004-06-22

Patel, Ajit (Department: 2664)

Multiplex communications

Generalized orthogonal or special mathematical techniques

Particular set of orthogonal functions

C370S503000, C370S343000, C375S354000

Reexamination Certificate

active

06754170

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The apparatus and methods consistent with the present invention generally relate to orthogonal frequency division multiplexing (OFDM) communications systems and, more particularly, to achieving faster timing synchronization in an OFDM receiver.
2 Description of the Prior Art
A. Wireless Networks
A wireless local area network (LAN) typically uses infrared (IR) or radio frequency (RF) communications channels to communicate between portable or mobile computer terminals and stationary access points or base stations. These access points are, in turn, connected by a wired or wireless communications channel to a network infrastructure which connects groups of access points together to form the LAN, including, optionally, one or more host computer systems.
Wireless IR and RF protocols are known which support the logical interconnections of such portable roaming terminals having a variety of types of communication capabilities to host computers. The logical interconnections are based upon an infrastructure in which at least some of the terminals are capable of communicating with at least two of the access points when located within a predetermined range therefrom, each terminal being normally associated, and in communication, with a single one of such access points. Based on the overall spatial layout, response time, and loading requirements of the network, different networking schemes and communication protocols have been designed so as to most efficiently regulate the communications.
One such protocol is described in U.S. Pat. Nos. 5,029,183; 5,142,550; 5,280,498; and 5,668,803, each assigned to the assignee of this application and incorporated herein by reference. Still another protocol is set forth in the IEEE Standard 802.11 entitled “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications” available from the IEEE Standards Department, Piscataway, N.J. (hereinafter, the “IEEE 802.11 Standard”).
IEEE Project 802 is concerned with network architecture for LANs. The IEEE 802.11 Standard is directed to wireless LANs, and in particular specifies the MAC and the PHY layers. These layers are intended to correspond closely to the two lowest layers of the ISO Basic Reference Model of OSI, i.e., the data link layer and the physical layer.
The IEEE 802.11 Standard permits either IR or RF communications at 1 Mbps, 2 Mbps and higher data rates, a medium access technique similar to carrier sense multiple access/collision avoidance (CSMA/CA), a power-save mode for battery-operated mobile stations, seamless roaming in a full cellular network, high throughput operation, diverse antenna systems designed to eliminate “dead spots”, and an easy interface to existing network infrastructures.
In Europe, the European Telecommunications Standards Institute (ETSI) has been working on HIPERLAN (European HIgh PERformance LAN), the next generation of high speed wireless systems. The frequency spectrum for HIPERLAN in the 5 GHz and 17 GHz bands has been allocated by the European Conference of Postal and Telecommunications Administrations (CEPT), with a data rate of over 20 Mbit/sec.
B. Spread Spectrum Modulation Techniques
The current implementations of commercial wireless LANs utilize a radio operating in the 2.4 to 2.4835 GHz spread spectrum band which is the industrial, scientific, and medical (ISM) band allocated for unlicenced use by the Federal Communications Commission (FCC). The current systems utilize one of two basic types of spread spectrum modulation: direct-sequence and frequency-hopping. In the description that follows, the specific modulation parameters specified by the IEEE 802.11 Standard shall be used to illustrate the different modulation techniques.
In a direct-sequence spread spectrum (DSSS) system, each binary bit of data in a data signal is spread over each of 11 discrete frequency channels at the same time, i.e., an 11-bit pseudo-random noise (PN) code. The data of each user is coded using a different PN code so that the signals of different users are orthogonal to each other. Thus, another user's signal is merely interpreted as noise. The IEEE 802.11 Standard provides two modulation formats and data rates in the DSSS system—a basic access rate using differential binary phase shift keying (DBPSK) modulation operating at 1 Mbps, and an enhanced access rate using differential quadrature phase shift keying (DQPSK) modulation operating at 2 Mbps.
In a frequency-hopping spread spectrum (FHSS) system, each binary bit of data in the data signal is associated with a group of distinct “chips”, or discrete signal frequency output, in different parts of a frequency band, with a minimum hop of at least 6 MHZ (in North America/Europe). The chipping pattern or hopping sequence is a pseudo-random sequence uniformly distributed throughout the band and set forth in the IEEE 802.11 Standard. Each access point executes a unique hopping pattern across 79 non-overlapping frequencies at a rate of one hop every 100 milliseconds. There are three sets of hopping patterns specified in the IEEE 802.11 Standard for North American/European operations, with each set containing 26 sequences. The sets are selected to minimize the possibility of interference. The RF modulation technique used in the FHSS system is 2-level or 4-level Gaussian filtered frequency shift keying (GFSK). Frequency-hopping spread spectrum systems are currently preferred over direct-sequence for most applications by the majority of users as they allow increased capacity and decreased interference.
The IEEE 802.11 FHSS systems hop over channels with an effective raw data rate of 1 Mbps or 2 Mbps. Current commercial systems can typically cover from an area of 25,000 to 70,000 square feet with a process gain of 10 dB. The relatively low power output used in such systems is a consequence of limits placed by regulatory agencies. Power output standards currently in effect limit the power output to either 100 mW, 230 mW, or 500 mW depending on the country.
In a spread spectrum system, one can multiplex users by assigning them different spreading keys. Such a system is called a code division multiple access (CDMA) system. Most wireless LANs are not CDMA systems since users belonging to the same wireless LAN utilize the same spreading key. Instead, as noted above, the MAC set forth in the IEEE 802.11 Standard provides that use access to the channel is multiplexed in time using nearly the same Carrier Sense Multiple Access (CSMA) protocol as in the Ethernet.
The CDMA modulation technique is one of several techniques for facilitating communications in which a large number of system users is present. The use of CDMA in a digital cellular spread spectrum communications system was adopted by the Telecommunication Industry Association in 1993 as standard IS-95. Other multiple access communications system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known in the art. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307.
C. OFDM Communications Systems
The IEEE 802.1a Standard also specifies the PHY layer operating in the 5 GHz band, which is open to unlicensed devices in the U.S. The IEEE 802.11a Standard is based on orthogonal frequency division multiplexing (OFDM) to modulate the data. Digital data is divided among a large number of adjacent carriers so that a relatively small amount of data is carried on each carrier. Adjacent carriers are mathematically orthogonal. Their sidebands may overlap but signals can be received without adjacent carrier interference. The main benefit of OFDM modulation is its robustness to multipath echoes, which are encountered in the indoor and mobile environments. Each OFDM symbol is composed of fifty-two non-zero subcarriers of which forty-eight are data subcarriers and the remaining four are carrier pilot subcarriers. The PHY specifications encompass data

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