Fixed ofdm wireless man utilizing cpe having internal antenna

IS 2 298 162 T3

DESCRIPTION

Fixed OFDM wireless MAN using CPE with internal antenna.

Field of the invention

The present invention relates generally to the field of wireless data communication systems. More specifically, the present invention relates to a fixed wireless metropolitan area network (MAN) utilizing orthogonal frequency division multiplexing (OFDM) carrier access modulation configured to allow Consumer Premise Equipment (CPE) uses an antenna deployed internally within the customer's facility, rather than requiring an externally accessible antenna that has a line-of-sight transmission path to a base station.

Background of the invention

Wireless data communication systems that use radio frequency (RF) signals to transmit and receive data are widely known for example from WO 98 / 26520A which discloses a system for broadband radio access that integrates micro and millimeter wavelength systems. This application discloses the features set forth in the pre-characterizing part of the independent claim. US-A-5 867485 discloses a coherent synchronous orthogonal frequency division multiplexing system with SC-OF DM transmitters and receivers. Document WO-A-96/27962 discloses the characteristics set forth in the pre-characterizing part of claim 1. In general, wireless data communication technology has been applied to high-performance long-distance communication systems such as satellite communications or microwave tower telecommunications, or to local area network (LAN) communication systems. ) long distance, such as a wireless LAN within a home or office environment. In the case of long-distance communication systems, a point-to-point antenna system is required and there must be a transmission path in the line of sight to the transmitter and receiver. In the case of short distance wireless LAN communication, an omni-directional antenna system can be used and a transmission path is not required in line of sight because distances are generally less than one mile. The reason for this difference is due to the fact that RF signals lose power rapidly over long distances or when transmitted through obstacles such as buildings or walls.

A metropolitan area network (MAN) is a network that can communicate over mid-range distances of between approximately 1 to 40 miles as you would normally find when providing coverage in an entire metropolitan area. Digital Subscriber Loops (DSL) services are a good example of a cable MAN system that uses telephone cables as the communication medium. Cable modem systems are another example of a cable MAN system that uses coaxial cable as the communication medium. One of the main advantages of a MAN system is that it allows higher speed data communications compared to the speeds of conventional telephone modems. The main problem with such MAN cable systems is the cost of installing and maintaining the high quality telephone or coaxial cable communication medium. A fixed wireless MAN system has the obvious advantage of eliminating the costs associated with installing and maintaining a wired communication medium.

Another advantage of a fixed wireless MAN system is that the wireless communication medium can be designed to provide higher data communication speeds than conventional wired MAN systems. This advantage has made the fixed wireless MAN systems that have been deployed to date designed for extremely high performance and relatively expensive dedicated networks. The market for these fixed wireless MAN systems was a small number of customers with high speed data communication needs that can justify the expense and complicated installation of such systems on an individual level. As a consequence of the limited customer base and the need for extraordinarily high performance, existing fixed wireless MAN systems designs have been developed rather in the vein of high performance long distance wireless communication systems.

Although there are numerous factors to consider when designing RF communication systems, some of the most important factors to consider when designing a fixed wireless MAN system are the assigned frequency, signal modulation, and access modulation. to the carrier. Assigned frequency refers to the range of frequencies or oscillations of the radio signal that are available for use by the system. An example is the assigned band for AM radio signals that operates between 500 KHz and 1600 KHz. Signal modulation refers to the way information or data is encoded in the RF signal. An example is the difference between amplitude modulation (AM) radio signals and frequency modulation (FM) radio signals. Carrier access modulation refers to the way the assigned carrier frequencies are used to carry the RF signal. An example is the difference between using a single wide channel or multiple narrow channels over the same assigned frequency bandwidth.

For the purposes of this invention, the design of a fixed wireless MAN system focuses on the frequency range below 10GHz. Other medium-distance wireless communication systems have been developed, such as the local multipoint distribution system (LMDS, local multipoint distribution system) that operates in ranges of

ES 2 298 162 T3 much higher frequencies, such as 28GHz to 31 GHz. These higher frequencies are subject to different technical problems and require larger external antenna systems that provide transmission paths in line of sight from the top of a building to another.

Due to the desire for higher data rates, all existing fixed wireless MAN systems have used more complicated schemes for signal modulation. To support higher speed downstream transmissions, these systems typically use 16-bit Quadrature Amplitude Modulation (QAM) or 64-bit QAM to transmit downstream from the base station to the CPEs at a data rate. of at least 10 Mbps.

Unlike the many fixed wireless LAN systems that have been developed for short-distance communications and that use a spread spectrum form of carrier access modulation that spreads a signal across the assigned frequency bandwidth, the relatively few Fixed wireless MAN systems that have been developed to date have used multicarrier modulation as their carrier access modulation. In multicarrier modulation, the signal is divided into several parallel data streams and these parallel data streams are sent simultaneously along different lower speed channels and then rejoined at the receiver to produce a higher effective transmission rate. . The multi-carrier modulation scheme that has been designed by the IEEE standards committee for use as an extension of the 802.11 wireless LAN standard for high-speed wireless data communications is known as orthogonal frequency division multiplexing (OFDM). The OFDM modulation scheme makes more efficient use of the allocated bandwidth and improves the ability to receive higher speed transmissions.

All of these more complicated modulation schemes for existing fixed wireless MAN systems generally require more expensive equipment and more transmit power at each base station. To realize the increased investment associated with each base station, existing fixed wireless MAN systems have been designed to minimize the number of base stations required to provide coverage for a given area. The radius of a typical coverage area for existing wireless MAN systems varies between 10 and 30 miles.

Larger coverage areas are also used to minimize the need to reuse the same frequency channels in adjacent coverage areas. Since higher transmission powers are used to transmit at higher data rates in all existing fixed wireless MAN systems, higher power signals prevent reuse of the same frequency channels in adjacent coverage areas and may even rule out reuse of the same frequency channels at distances of up to three to five times the radius of the coverage area. Consequently, larger coverage areas reduce the impact of problems caused by the inability to reuse frequencies in adjacent coverage areas.

The most significant disadvantage of larger sizes for coverage area for each base station is the greater potential for signal loss or attenuation between the base station and the CPEs. To counteract this potential signal loss over longer distances and improve reception at higher power, higher transmission rates, all existing fixed wireless MAN systems use a point-to-point antenna system that requires a transmission path on the line. of vision between the base station and an externally accessible antenna that is connected to the CPEs. For example, see the prior art fixed wireless MAN system configuration of Figure 1 in which CPEs within a single user environment, eg, a home, are connected to an antenna that is outside the environment of single user and where within a multi-user environment, for example a small office, each CPE is connected to its own antenna that is located outside the multi-user environment.

Given the relatively limited customer base and the need for extraordinarily high performance that is dictated by the development of existing fixed wireless MAN systems, the use of an externally accessible antenna that provides a transmission path in the line of sight is necessary and understandable. However, it will be desirable to provide a fixed wireless MAN system that does not require the use of an externally accessible antenna and that could be more widely deployed to provide higher data rates more efficiently to a larger number of customers.

Summary of the invention

The needs described above are largely met by a fixed OFDM wireless MAN system of the present invention. The fixed wireless access system generally comprises a client installation equipment (CPE) unit that is connected via an Ethernet interface to a home office / small office personal computer or a local area network, and a base station that is connected through an Ethernet interface to a network. The CPE unit is located in a home or small office facility, has an antenna that is deployed internally within that facility and can be easily installed by the user. The base station unit is preferably tower mounted at a distance of 1 to 5 miles from the CPE unit. The CPE unit preferably incorporates an internal, built-in data transceiver / switch that allows it to receive a digital signal from a computer or network, transform that signal into an analog format, and transmit the analog signal via radio frequency technology, preferably by operating in the range of 2.5 to 2.686 GHz, to one base station unit. The base station unit preferably incorporates an integrated data transceiver / switch. Upon receiving the signal, the base station unit transforms the analog signal back into a digital signal and

ES 2 298 162 T3 passes that signal through the Ethernet connection to the personal computer, LAN, and / or network. Orthogonal frequency division multiplexing is used in uplink and downlink transmissions between CPE units and base station units.

The fixed wireless access system transmits using OFDM signals incorporating OFDM symbols. OFDM symbols are presented without a training symbol and are detected symbol by symbol.

The fixed wireless access system uses a framed downlink transmission and an unframed uplink transmission.

Brief description of the drawings

Figure 1 provides an overview of a prior art fixed wireless MAN system using external antennas.

Figure 2 provides an overview of a fixed OFDM wireless MAN system of the present invention using internal antennas.

Figure 3 illustrates an overview of a single sector configuration within a cell of a fixed wireless access system of the present invention.

Figure 4 illustrates a cellular system of the present invention.

Figure 5 illustrates a standard cell reuse pattern above.

Figure 6 illustrates a prior art cell reuse pattern using TDMA.

Figure 7 illustrates the preferred cell reuse pattern of the present invention.

Figure 8 illustrates the arrangement of the uplink and downlink transmission slots used with the system of the present invention as well as the arrangement of the message packets contained within the slots.

Figure 9 illustrates, in block diagram format, the processing of a bit stream of a data packet that is transmitted or received by radio frequency within the fixed wireless access system of the present invention.

Detailed description of the preferred embodiments

An overview of a fixed OFDM wireless metropolitan area network (MAN) with computer installation equipment (CPE) using internal antennas of the present invention is shown in Figure 2. As illustrated, the fixed wireless access system 10 of the present invention can be configured for a single-user environment or a multi-user environment, eg, a local area network. System 10 operates to transfer data to and from users of system 10 through the use of highly reliable radio transmission technology. The system 10 can be especially applied to the residential and small office / home office (SOHO) markets.

Referring now to Figure 3, an overview of a single sector configuration within a cell of the fixed wireless access system 10 is shown. As shown in Figure 3, system 10 generally comprises one or more hosts, for example one or more central computers 12 and / or one or more local area network servers 13, that are connected to one or more units. 14 Customer Installation Equipment (CPE) via an Ethernet 16 connection. Each CPE unit 14 communicates with one or more base station units 18 within system 10 by radio frequency. Each base station unit 18 is connected via an Ethernet interface 19 to one or more of several types of networks 19, or switching fabrics, eg, asynchronous transfer modes (ATM).

I. System components, component layout, and component recognition

Each CPE unit 14 incorporates hardware necessary to implement Ethernet communication with a user personal computer 12 or LAN server, as well as radio frequency communication with base station units 18. That hardware is preferably implemented, at least in part, using Field Programmable Gate Array (FPGA) technology, or ASIC technology, and is preferably designed for a maximum power consumption of about 10 watts. More specifically, each CPE unit 14 preferably incorporates an integrated data transceiver / switch and one or more Ethernet connectors, for example, a 10Base-T RJ45 connector (10BASE-T is an IEEE 802.3 specified transmission medium that carries information at rates of up to 10Mbps in baseband form using twisted pair conductors, also called unshielded twisted pair (UTP) wire. Regarding the integrated data transceiver / switch, it is to be noted that individual components can be used without departing from the spirit or scope of the invention.

IS 2 298 162 T3

To facilitate the installation of the CPE unit 14, the integrated data switch / transceiver preferably incorporates an integral directional antenna that allows the CPE unit 14 to be installed by a customer near an associated host computer 12 and within the installation of client. The use of a standard Ethernet connector 22 further enhances the ease of installation of the CPE unit 14 and allows the CPE unit 14 to be easily installed by the user for communication with their host computer 12 or local area network server 13 . The CPE unit 14 is preferably sized and shaped such that it can be placed and / or mounted on a desk, which again further facilitates installation by the user.

Base station unit 18 incorporates hardware necessary to implement Ethernet communication with one or more of several types of networks 19, or switching fabrics, for example, asynchronous transfer modes (ATM), as well as radio frequency communication with units 14 of CPE. That hardware is preferably implemented, at least in part, using FPGA technology or ASIC technology and is preferably designed for a maximum power consumption of about 100 watts. More specifically, each base station unit 18, similar to each CPE unit 14, preferably incorporates an integrated data switch / transceiver and one or more Ethernet connectors, eg, a 10Base-T RJ45 connector. Regarding the integrated data transceiver / switch, it is to be noted that individual components can be used. Base station unit 18 is preferably further equipped with a global positioning system (GPS) receiver to provide a time reference, for system resolution and accuracy. A GPS time pulse is preferably used by system 10 to provide synchronization to geographically distributed base station units 18 to avoid interference between base station units 18. Regarding the integrated data transceiver / switch, it should be noted that individual components can be used.

Referring to Figure 3, the base station unit 18 is preferably tower mounted to facilitate a long communication radius, out of line of sight. The high system gain provided by the transmit levels, antenna gains, and receiver sensitivity allow for out of line of sight operation of the base station unit 18. If the base station unit 18 is mounted at the bottom of the tower, an extensive length of coaxial cable is required between the base station unit 18 and its antenna. The longer the length of the coaxial cable, the less loss in system gain and the operating distance will be reduced for a given level of out-of-line coverage.

Each base station unit 18 is positioned by a distributed cellular system 30, see Figure 4, in which each cell 32 preferably includes one or more sectors 34, and each sector 34 preferably includes a base station unit 18. Figure 4 is a diagram of an exemplary distributed cellular system 30 in which each cell 32 has six sectors 34. Each cell 32 preferably has a communication radius of about 1 to 5 miles, with a typical radius of 3 miles. However, the use of a sector-based cellular base station unit 18 deployment does not restrict the use of a single omnidirectional base station unit 18. More specifically, it is not necessary to have a cell with multiple sensors to operate as a single cell operation. In the case of a small geographic area, for example less than a three mile radius, where the potential user base is small and a single base station unit 18 could achieve data processing capacity, a single station base could be installed with a high gain omni-directional antenna.

Once each CPE unit 14 and each base station unit 18 are properly installed, each can transmit and receive communication signals to and / or from the other. Simply stated, the combined effect of radio frequency communication between CPE unit 14 and base station unit 18 is that of a standard Ethernet switch, with certain added enhancements. For example, radio frequency communication is facilitated between units 14 and 18 due to the fact that each CPE unit 14 and each base station unit 18 have been assigned a unique address, similar to a radio switching system. Ethernet. Furthermore, radio frequency communication between units 14 and 18 preferably occurs in the form of a data packet that includes a source and / or destination address indicating which CPE unit 14 or base station unit 18 is coming from. communication and / or to which one is directed, respectively, which again is similar to an Ethernet switching system. Broadcast traffic, eg, traffic sent to all units within system 10, can also communicate between base station units 18 and CPE units 14, similar to an Ethernet switching system.

Thus, just as an Ethernet switch improves the performance of an Ethernet system, the switching configuration provided by the CPE unit 14 and the base station unit 18 operates to increase the performance of the system 10 by allowing it to just roam essential traffic between CPE units 14 and base station units 18; data packets are filtered or forwarded based on their source and / or destination addresses without intervention by intermediate base station units 18, ie, distributed switching. Also, like an Ethernet system, CPE unit 14 and base station unit 18 preferably implement a Dynamic Central Control Protocol (CDP), a protocol that is observed by CPE unit 14. and the base station unit 18 for dynamically discovering the low-level physical network hardware address that corresponds to the Internet Protocol (IP, Internet Protocol) of the central computers 12 associated with a CPE unit 14.

More specifically, when a CPE unit goes online for the first time, it begins to monitor the signals from the base station unit 18 using its transceiver. When the CPE unit 14 detects a signal from the base station unit 18 of sufficient quality, the CPE unit 14 registers with the base station unit 18. Unit

The base station ES 2 298 162 T3 uses an authentication server within the network 20 to determine if the CPE unit 14 is authorized and to determine how many central computers 12 can be associated with the CPE unit 14. The base station unit 18 then either denies or recognizes the CPE unit 14 with the number of central computers 12 allowed. Once registered with one of the base station units 18, the CPE unit 14 enters a learning phase in which the CPE unit 14 operates to learn the level 3 address and the Ethernet physical layer address by observing the traffic. The traffic observed is that of one of the central computers 12 requesting a level 3 address from a server in the data communication network, that is, LAN 13, and that of the server's response, which is preferably in DHCP.

After observing the traffic, the CPE unit 14 creates a table of the IP, level 3 address of the associated host computer (s) and the associated low-level Ethernet physical network hardware address. . By creating this table, the CPE unit 14 can guarantee that it will not transmit messages over the air link to the base station unit 18 that has a level 3 address destination that corresponds to a central computer 12 that is already associated with the unit. 14 of CPE through 13 LAN interface. Similar to CPE unit 14, base station unit 18 operates to observe traffic and creates a level 3 IP address table of the associated host computer (s), the address of associated Ethernet low-level physical network hardware, and the associated over-the-air hardware address of the CPE unit 14. By creating this table the base station unit 18 can ensure that it will not transmit messages over the air link when the message includes a level 3 address destination that is not in the base station unit 18 address table.

In addition, like an Ethernet system, CPE unit 14 and base station unit 18 preferably implement Address Resolution Protocol (ARP), a protocol that is used by end devices, central computers, and others. other computers associated with the network, to dynamically discover the Ethernet low-level physical network hardware address of an associated host 12 that corresponds to the associated IP address of said host 12.

However, unlike standard Ethernet systems, the fixed wireless access system 10 provides an ARP proxy in which one of the base station units 18 can respond to ARP requests directed to a central computer 12 associated with a unit 14. of CPE. By acting on behalf of a CPE unit 14, the intercepting base station unit 18 accepts responsibility for the routed data packet and may respond to it, for example, the base station unit 18 may return the actual Ethernet MAC address of CPE Unit 14. Of course, different and / or additional proxy protocols can be used without departing from the spirit or scope of the invention. Using ARP and proxy ARP, channel capacity can be maintained and system efficiency increased

10, that is, airborne broadcast traffic is reduced. Additionally, the CPE unit 14 observes the data traffic of the central computer (s) 12 that are associated with the CPE unit 14. If the traffic is directed to another central computer 12 that is also associated with the CPE unit 14, then the CPE unit 14 does not transmit that traffic to the base station unit 18, therefore the channel capacity can be maintained and increased. the effectiveness of the system 10.

The CPE unit 14 preferably incorporates a roaming function that allows the CPE unit 14 to move from a facility within range of one base station unit 18 to a facility within range of another, or to switch base stations 18 if one must. stop working through the air. The CPE unit 14 monitors the quality of all signals from the base station unit 18 and registers with a different base station unit 18 when the signal from the current base station unit 18 degrades below that of another unit. 18 base station. As with the original base station unit 18, when a change occurs, the CPE unit 14 registers with the new base station unit 18 and additionally passes the level 3 address and physical layer address table of Ethernet of those central computers 12 connected to the CPE unit 14 to the new base station unit 18 to enable proper synchronization of the tables between the CPE unit and the new base station unit 18. The new base station 18 then performs free ARPs to cause the table update of the first base station unit 18 in order to speed up the process of the base station units 18 by suitably switching traffic to the CPE unit 14 for their purposes. 12 associated central computers.

Furthermore, a central computer 12 can be disconnected from one CPE unit 14 and connected to a different CPE unit 14. The new CPE unit 14 can then observe, through the traffic, that another central computer 12 is active on its LAN interface 13. The new CPE unit 14 then performs a registration with the added host 12 by adding the level 3 address and the Ethernet physical layer address of the added host 12 to its table. The base station unit 18 associated with the new CPE unit 14 then recognizes that a new host 12 has been added and operates to create a new entry in the base station unit address table for the new host 12. Base station unit 18 additionally performs free ARP to update other base station units 18.

eleven. System data transmission

The fixed wireless access system 10 preferably operates in the 2.5 to 2.686 GHz frequency range of instructional television fixed service / multipoint distribution service (ITFS / MDS). The FCC (Federal Communications Commission) licenses these frequencies in the form of 31 channels, each with a 6 MHz bandwidth for bidirectional digital communication. In a recent order, the FCC determined that channel licenses would be given a blank license by removing

ES 2 298 162 T3 thus the need for each user to register their CPE unit 14 and eliminating the need for each base station unit 18 to register individually.

As noted above, system 10 is preferably a cellular system 30 in which each cell 32 in the system is divided into one or more sectors 34. A 6 MHz channel can be used to support an entire system using a reuse combination of cellular frequencies and a time division multiplexing method. Alternatively, more than one 6 MHz channel can be used; adding more channels increases the capacity of the system 10 in terms of radio frequency communication capacity and performance.

A preferred system 10, as shown in Figure 4, uses a cellular system 30 in which each cell 32 is divided into six sectors 34 and is provided with six channels such that one sector 34 can use one channel all the time. . In this preferred configuration, system 10 provides a 1: 1 reuse pattern, a transmission rate of 9 Mbps per sector (54 Mbps per cell), and a data processing rate of 3 Mbps per sector (18 Mbps per cell). The preferred system 10 can support approximately 300 simultaneous active users per sector (1800 per cell) and approximately 1000 to 1500 subscribers per sector (6000 to 9000 per cell). At a minimum, system 10 is designed to support at least 250 concurrent active users per sector.

Prior art wireless systems generally require at least one ring of spacer cells for frequency reuse. For example, referring to prior art Figure 5, three frequencies are being used within cells 32, as indicated by three different shading. In the configuration of Figure 5, the cellular system operates to separate each cell that shares the same set of channels by at least one cell 32 in order to minimize interference while allowing the use of the same frequencies elsewhere. of the system. In another prior art, time division multiple access (TDMA) of a wireless system is used to decrease frequency interference between cells. For example, referring to prior art Figure 6 each cell 32 is divided into sectors 34, each sector 34 having its own frequency channel, the channels repeating in the next next cell 32. To allow for this frequency reuse, TDMA is used to give each user a unique time slot within the channel. As such, in the lower cell 32, sector 1, a user transmits according to the indicated staggered time signal, in the right adjacent cell 32, a user transmits according to the indicated staggered time signal, that is, after it has transmitted the lower cell 32, and in the upper adjacent cell 32, a user transmits according to the indicated staggered time signal, that is, after the right adjacent cell 32 has transmitted, and so on, so that each sector 1 in each cell transmits at a different time. However, according to the present invention by using quadrature phase-shift keying (QPSK) and the reduced diameter of each cell, further described below, neither cell separation 32 nor cell separation is necessary. Inter-cell TDMA, see Figure 7.

In alternative embodiments of the present invention, each cell 32 may be provided with three sectors 34 whereby the time division multiplexing method used within that cell is based on a two cell (six sector) pattern. When the two-cell pattern is provided with a single 6 MHz channel, transmission takes place one sixth of the time in each sector, when the two-cell pattern is provided with two 6 MHz channels, the transmission takes place a third part of the time in each sector, and when the two-cell pattern is equipped with three 6Mhz channels, transmission takes place half the time. Changing cell and sector patterns has, of course, an effect on transmission rates, data processing rates, and the number of users the system can support 10. However, time-sharing capacity eg 1: 1, 1: 2, 1: 3, 1: 4, 1: 6, etc., allows the deployment of a system 10 with a low number of frequencies for a given area to be covered. It is to be noted that other cell, sector and channel configurations can be used within system 10 without departing from the spirit or scope of the invention. However, it is also to be noted that increasing the number of sectors increases the overall cost of the base station unit 18 by increasing the number of separate antennas that are then required for each base station unit 18.

Regardless of the exact cell arrangement and time division multiplexing duty factor between cells, each sector 34 preferably uses its intended channel for data packet transmissions during time increments called frames. System 10 preferably uses time division duplexing (TDD) to support bidirectional communication in each sector 34. Each frame is divided into two main parts, a downlink transmission time and an uplink transmission time. The downlink transmission time preferably allows base unit 18 to transmit in one of a plurality of downlink channel slots 100, see FIG. 8. Likewise, the uplink transmission time preferably allows the CPE units 14 to transmit in one of a plurality of uplink channel slots 102. There is preferably a variable ratio of downlink channel slots 100 relative to uplink channel slots 102 to allow adaptation of system data processing rates of the given type of communication traffic. The ratio is a preferably configurable parameter but can be changed during operation.

Each downlink and uplink channel slot preferably contains the transmission of a single OFDM signal containing a data packet (OFDM is preferred to digital spread spectrum since digital spread spectrum does not provide enough power for each symbol that is transmitted for all frequency; increasing power to support longer transmission distances results in a dispersion of signal power beyond the allocated bandwidth). The timing of the total frame duration

ES 2 298 162 T3 can preferably be set to a standard preferred length of time. However, the duration of each frame can vary in length from one frame to the next and can vary between cells and sectors. Note that to provide signaling and a time / frequency reference for uplink operation, the downlink of a given sector 34 preferably transmits for the duration of the downlink transmission time, even though there is no data to send on the downlink. for a given frame or part of a frame.

Referring to FIG. 8, each downlink transmission preferably contains a downlink message packet 104, comprising a continuous byte stream that has been generated by the host computer 12 or network 19. Each byte stream begins and ends with a label 106, eg 1 or 2 bytes, to mark the start and end of the message packet. Among the tags 106, each byte stream preferably includes a 4-byte destination address 108, a 2-byte length / type field 110, up to 2k data bytes 112, and a cyclic redundancy code 114 (CRC, cyclic redundancy code) of 4 bytes, which covers the address field 108, the length / type field 110, and the data 112.

Additionally, the downlink transmission portion is framed using an air link MAC protocol and preferably contains a frame header (FH) field 116 and a plurality of uplink channel status fields 118 ( UCS, uplink channel status), the UCS fields 118 appearing at intervals of one downlink slot time in the downlink transmission. In addition, each OFDM downlink symbol begins with an eight-bit Symbol Sequence Flag (SSF) label 119, which indicates whether a downlink symbol contains a frame header field 116. As such, each OFDM symbol contains a packet of data and detection aid information sufficient to demodulate the symbol; separate OFDM symbols containing known, fixed information are not used for learning, ie data that is embedded in the symbol to allow the receiver to acquire and capture a transmission.

The frame header field 116 contains the over-the-air address of the base station unit 18 and other information that is specific to the given base station unit 18 for the overall operation of the base station unit and the (s ) CPE unit (s) 14 which is (are) using the given base station unit 18. The preferred configuration of the frame header field 116 provides a total of eight bytes that include: (1) various labels (1 bit each) for the start of a super frame, the end of a super frame, and idle symbol; (2) system identifier, 4 bits; (3) transmit power level, 4 bits; (4) sector / cell base station unit address, 4 bytes; (5) an offset number indicating the number of OFDM symbols in the downlink part of the frame, 4 bits; (6) time division multiplexing reuse factor (eg 1: 1, 1: 2, 1: 3, etc.), 4 bits; and (7) cyclic redundancy code (CRC), 1 byte.

Uplink channel status (UCS) field 118 contains information on whether an uplink channel slot 102 is in use. As such, there is a 118 UCS field in each of the first "n" downlink OFDM symbols, where "n" is the number of uplink slots in the frame. If slot 102 is in use, UCS 118 contains: (1) the direction through the air CPE unit 14 that is using the specific uplink channel slot 102; (2) if the uplink channel slot 102 is reserved, and for which CPE unit 14; and (3) other information pertinent to control of the given uplink channel slot 102. A preferred configuration of UCS field 118 provides a total of six bytes including: (1) mobile address, 4 bytes; (2) slot in use, 1 bit; (3) acknowledgment, 1 bit; (4) preferred, 1 bit; (5) reserved, 2 bits; (6) quality of service (QoS), 3 bits; and (7) cyclic redundancy code (CRC), 1 byte.

The mobile address of the UCS field 118 generally refers to the CPE unit 14 that used the slot 102 given in the previous frame. However, it may refer to a CPE unit 14 that will use slot 102 in the uplink transmission portion of the current / next frame but may not have used slot 102 previously. "Slot in use" refers to whether the given slot 102 will be available for random access in the transmitting portion of the CPE unit 14 of the current frame. "Acknowledgment" refers to the results of the uplink transmission in the slot 102 given in the previous frame. A CPE unit 14 must retransmit any bad blocks before transmitting a new block. "Preferred" means that slot 102 is reserved for a "new" CPE unit 14 in the transmitting portion of the CPE unit of the next frame. The "spare" bits are not used. "Quality of Service" (QoS) refers to the priority of slot 102 in the transmission part of the CPE unit of the current frame, that is, only users of specific priority or higher will be allowed to transmit access bursts random in the slot 102 given in the uplink transmission portion of the current frame. The CRC is the same polynomial as used in frame header field 116 and covers all other fields in UCS field 118.

The downlink provides media access control (MAC) by the CPE unit (s) 14 for transmission on the uplink through the UCS field 118. The MAC provided by the downlink preferably uses an air link MAC protocol. This MAC preferably acts as an access to the slotted aloha medium, providing users with on-demand access to the overhead link between the CPE unit 14 and the base station unit 18, with additional implicit slot reservation for transmission of long messages from a CPE unit 14. Quality of service (QoS) is preferably provided in UCS fields 118 to control which services are allowed to access.

IS 2 298 162 T3

The byte stream is conditioned for transmission by CPE unit 14 or base station unit 18 at the bottom level of the block diagram of Figure 9. As shown, the byte stream is first encoded. Forward error correction, such as provided by a Reed / Solomon block encoder 40, and a convolutional encoder 42. The Reed / Solomon block encoder 40 operates to add Reed / Solomon parity bytes, e.g., ten parity bytes, to the byte stream, in which a certain number of byte errors, e.g., five byte errors, can be corrected. bytes. After the Reed / Solomon block encoder 40, the byte stream is applied as a serial bit stream to the convolution encoder 42. The convolutional encoder 42 is preferably a half rate convolutional encoder that operates to add redundancy to the bit stream. Note that the Reed / Solomon codeword is preferably input to the convolutional encoder 42 with a clipping length of 7, a depth of 35, and a code rate of 0.5. Of course, other limiting code lengths and rates can be used.

In the preferred embodiment, the byte stream is encoded with the Reed / Solomon block encoder 40 and the convolutional encoder 42 at half the rate to utilize 672 carriers. More specifically, these 672 carriers, which carry data information, are modulated with two bits providing 1344 bits of data that are transmitted per symbol. These 1,344 data bits are convolutionally encoded at half the rate for random errors leaving 672 data bits when received and convolutionally decoded by the receiver. The 672 bits comprise 84 bytes of data that are separated into 74 bytes of payload data to be transferred and 10 bytes of error correction using Reed / Solomon encoding. When all 84 bytes of data are received, Reed / Solomon decoding error correction (as described below) is performed to correct up to five bytes of data that may be incorrect, which corrects burst errors that are received .

The bit stream left by the convolutional encoder 42 is provided to a signal mapper 44 which is preferably composed of interleaver block 46 and "bits block 48 for QPSK symbols. The signal mapper 44 operates to interleave the output bits of the Convolutional encoder with a specific spacing and depth, for example, 32 and 42, respectively. Bit values ​​of 1/0 are then encoded to -1/1 and unmodulated dibits, for example three unmodulated (0,0) dibits, are then inserted into the center of the bit sequence to form a sequence total of 675 information dibits, each of which modulates a quadrature phase shift keying (QPSK) subsymbol. Nullification, or no modulation, of the three center carriers eliminates the need to preserve DC and low-frequency content in the modulated signal, easing the design and implementation limitations of a transmitter and receiver.

The use of QPSK modulation on the information carriers allows an optimized cellular system. More specifically, the use of QPSK modulation on the carriers provides an optimal carrier-to-interference ratio for a given data throughput rate. This optimal carrier-to-interference ratio allows for a cellular deployment style that uses a 1: 1 frequency reuse pattern. This allows each cell to use the same six frequencies in a six sector cell. Higher modulation levels require a higher carrier-to-interference ratio and therefore require more frequencies, that is, three times or more, than a system with QPSK modulation.

For further explanation, reference is made to Figure 10 which is a diagram showing the interference for a 1: 1 repeating pattern of a cell having six sectors of 60 ° with an offset of 30 °, the reference being for the distance of 1 one vertex of a sector, R. In this diagram, point X is the main transmission point. The subscribers that would be interfered with are A, B, and C. The points that would interfere would be T and U. Cells below and to the right of T and U would also add to the interference but to a much lesser extent than T and U. The interference levels then are as follows (the propagation loss factor A 1 / R⁴ is used for the following analysis):

1. “A” would be interfered with by T and U. The interference level is approximately - 14.84 dB.

2. "B" would be interfered with by T and U. The interference level is approximately - 14.84 dB.

3. "C" would be interfered with by T and U. The interference level is approximately - 13.9 dB.

Additional protection of 2 to 4 dB is available when radiation patterns from directional antennas are taken into account.

OFDM signaling using QPSK requires only 5 dB of SNR (signal-to-noise ratio) protection to achieve a bit error rate of 10 ⁶. The six sector cell provides at least 8 dB additional interference protection. Higher level modulations require a higher SNR compared to QPSK for the same symbol error rate. The following table shows the modulation level and additional protection required for higher level modulations in relation to QPSK.

IS 2 298 162 T3

<td>Modulation</td><td>Bits / s / Hz</td><td>Cup of transmission</td><td>Added protection required</td><td>Reuse</td><td>Ef.</td>

<td>BPSK</td><td> 1</td><td>2.5 Mbps</td><td>0.0 dB</td><td> 1:1</td><td> 0,50</td>

<td>QPSK</td><td> 2</td><td>5 Mbps</td><td>0.0 dB</td><td> 1:1</td><td> 1,00</td>

<td>16 QAM</td><td> 4</td><td>10 Mbps</td><td>7.0 dB</td><td> 3:1</td><td> 0,66</td>

<td>64 QAM</td><td> 6</td><td>15 Mbps</td><td>13.2 dB</td><td> 5:1</td><td> 0,60</td>

<td>256 QAM</td><td> 8</td><td>20 Mbps</td><td>19.3 dB</td><td> 7:1</td><td> 0,57</td>

The transmission rate is an example of a transmission rate for comparison between modulations. The added protection is the additional amount of SNR required for the higher modulation to achieve the same symbol rate error as QPSK. This added protection remains true for interference at home channel points. The levels of added protection that are required are close to or exceed the available range of a 1: 1 six sector cell pattern as described above. The reuse factor is the number of channel sets that are required to create a reuse pattern that can provide the required protection. The fact of the matter is that every time the modulation level is doubled, there is a 3dB increase needed for additional protection. This 3 dB increase in power translates into an increase in propagation distance resulting in the inability to achieve a one-to-one frequency reuse ratio between adjacent cells.

An efficiency factor can then be calculated as bits / s / Hz / area relative to QPSK. The present invention maximizes this efficiency factor to create a highly efficient cellular system for a fixed OFDM wireless MAN. The present invention recognizes that higher level modulations have a lower efficiency factor when considering a complete cellular network. Therefore, QPSK is the optimal modulation for a cell-divided system that uses a minimal amount of spectrum over a given area in a cellular network. It is also to be noted that higher level modulations require signal levels for higher fading margins due to multipath conditions.

Next, continuing the topic of signal conditioning and referring back to FIG. 9, modulation, preferably orthogonal frequency division modulation (OFDM), is carried out on subsymbol output signal mapper 44 QPSK. OFDM 50, as indicated in Figure 9, preferably includes the following steps. First, pilot subsymbols with modulation dibit value (1,1) are inserted uniformly between the information dibits, unmodulated guard subsymbols are inserted at the top and bottom of the 6 MHz channel, and out-of-band subsymbols are added to realize a desired total sequence length of subsymbols, eg, 1024 subsymbols, per OFDM symbol, see block 52. Next, a signal bit scrambler is applied to the subsymbols, see block 54. More specifically, the subsymbol sequence is preferably multiplied by a pseudrandom noise (PRN) sequence to eliminate amplitude peaks due to the non-random nature of the data + pilot + guard + out-of-band subsymbols.

The next step in OFDM preferably comprises performing an inverse fast Fourier transform on the now scrambled sequence of subsymbols, see block 56. After the transform is complete, a cyclic prefix / suffix is ​​inserted at the start of the downlink symbol, see block 58. With modulation now complete, the digital sequence is preferably sent to a low-pass filter and, if necessary, interpolated at a higher frequency rate before input to a digital-to-analog converter, see block 60. Finally, the sequence it is sent to a digital-to-analog converter 62 and transmitted from CPE unit 14 or base station unit 18 via analog radio circuitry.

OFDM operates, at least in part, to combat the effects of, for example, constructive and destructive interference, and multipath signal phase shift. Multipath is a propagation phenomenon that results in radio signals reaching a receiving antenna over two or more paths.

Referring back to FIG. 8, each uplink transmission preferably contains an uplink message packet 120, comprising a continuous byte stream that has been generated by a computer 12 or network 19. Each byte stream preferably includes a 4-byte destination address 122, a 4-byte source address 124, a 2-byte type / length field 126, 60 bytes 128 of data, and a cyclic redundancy code 130 (CRC ) 32-bit, which covers both address fields 122 and 124, length / type field 126, and data 128. Note that with an uplink transmission, the message packet 120 is not framed, as with downlink transmission, however a fixed number, for example six, of

ES 2 298 162 T3 uplink channel slots 102. System 10 may be configured to allow any given CPE unit 14 to transmit in only one uplink channel slot 102 of a given frame. However, the system 10 may alternatively be configured to allow a plurality of uplink messages from a single CPE unit 14 to be processed simultaneously, up to the number of uplink slots 102 per frame. Thus, subject to MAC layer control, an individual CPE unit 14 can increase its uplink performance by using two or more uplink slots 102 in each frame if desired, up to the total number of link slots 102. ascending in the plot.

The byte stream is conditioned for reception by the CPE unit 14 or the base station unit 18 by the upper level of the block diagram of Figure 9. As indicated, an analog signal is received by the unit 14 of CPE or base station unit 18 via analog radio circuits. The analog signal is then sent to an analog-to-digital converter 70. The output of the analog-to-digital converter is sampled and provided as feedback to an automatic gain control loop so that the analog-to-digital converter is kept in a linear operating range, see block 72. The output of the analog-to-digital converter is also sent to block 74 "decimator digital low passfilter (LPF)" whereby the digital output is shifted to the DSP preferably using programmable field gate array technology ( FPGA) or Application Specific Integrated Circuil (ASIC), and low-pass is filtered. The signal is now in the form of an OFDM symbol.

Acting on the OFDM symbol, the next step to complete the reception is to remove the cyclic prefix and suffix from the OFDM symbol, see block 76. A fast Fourier transform is then carried out on the received OFDM symbol, see block 78 A signal bit descrambler is then implemented, see block 80. Coarse sync / coarse frequency and fine sync / fine frequency of the OFDM symbol are provided by blocks 82 and 84, respectively.

Approximate timing is preferably achieved by correlating the cyclic prefix of a given OFDM symbol with the content of the symbol. More specifically, the cyclic prefix, which is a repetition of a part of the symbol, allows the receiver to perform an autocorrelation function to determine if the start of a symbol is found in time within several samples. The receiver can detect symbol by symbol once approximate timing has been acquired by observing various symbols (these symbols do not have to be training symbols, with fixed data content). The approximate frequency is preferably acquired by pilot correlation. More specifically, the receiver performs frequency domain autocorrelation based on pilots to determine the receiver carrier frequency.

Accurate timing of the OFDM symbol is preferably achieved by evaluating the phase of the pilots. The pilots are transmitted to a known phase thus allowing the receiver to use its known information to determine precisely where the start of a symbol is, rather than a fractional part of a sample. The precise frequency of the OFDM symbol is preferably acquired from the cyclic prefix. The cyclic prefix is ​​used to tune the carrier frequency precisely with respect to the transmitter carrier. Once the receiver has acquired the precise timing and precise frequency, then each OFDM symbol is tuned to the precise timing and precise frequency allowing for improved symbol detection, reception with improved sensitivity, and improved error performance by the receiver. .

The OFDM symbol is then sent for demodulation including channel equalization through pilot processing, see block 86. With the OFDM signal now demodulated, the pilot, guard, and out-of-band subsymbols are extracted leaving a total sequence of information dibits, each of which modulated a quadrature phase shift keying (QPSK) subsymbol, see block 88. The QPSK symbols are then preferably sent to a signal demapper 90, which comprises block 92, in which the QPSK symbols are returned to bit values ​​of 1/0, and block 94, in which the bits are deinterleaved. The signal demapper 90 operates efficiently to arrange the bits in the same order as the original signal to be transmitted. The output of the signal demapper 90 is a serial bit data stream that is preferably sent to a Viterbi decoder 96 in which the bit rate of the serial bit data stream is halved to correct errors. The output of the Viterbi decoder 96 is then preferably sent to a Reed / Solomon block decoder 98 which operates to correct for residual errors in the sent data stream.

The uplink data stream is then sent to a cyclic redundancy code (CRC) check at base station unit 18. CRC checking is a data communication error detection technique used to ensure that a data packet has been faithfully transferred. The CRC is the result of a calculation on the set of transmitted bits that the transmitter, eg, CPE unit 14, attached to the data packet, as described above with respect to uplink transmission. At the receiver, eg, base station unit 18, the calculation is repeated and the results are compared to the encoded value. The calculations are chosen to optimize error detection. If the CRC check is good, the data packet is processed. If the CRC check is bad, then the data packet is rejected from further processing, as if the packet was not received at all by the base station unit 18.

In view of the above, it can be seen that the fixed wireless access system 10 of the present invention can provide maximum performance of multichannel multipoint distribution service (MMDS) operators and user capacity per assigned spectrum with a network deployment

ES 2 298 162 T3 simple both at the base station and at the client sides. More specifically, the system 10 can support a higher efficient throughput, which is defined as the density of clients multiplied by the data processing rate per client, than other existing wireless systems. With respect to the client side, the CPE unit 14 can be fully user installed using a simple Ethernet connector and does not require registration with the FCC. In addition, the segmented and cell-divided structure of the base station unit 18 design allows full frequency reuse of the allocated channel set allowing for ease of network planning, and the ability to vary cell sizes. consistent with the density of subscribers, that is, a high subscriber density is preferably addressed with a plurality of adjacent smaller cells 32 rather than a single larger cell.

With respect to a retail implementation of the fixed wireless access system 10 the following preferably occurs: (1) a potential end user of the system 10 goes to a retail electronics store to purchase a CPE unit 14; (2) the end user receives from the retailer a contract with the service provider in the area in which they are providing the fixed wireless access system 10; (3) the end user contacts the service provider and provides the service provider with the information necessary to allow the service provider to enable the end user specific CPE unit 14; and (4) the end user installs the CPE unit 14 using its internal antenna, as described above, allowing interaction with the system 10. The service provider does not have to send service personnel to the end user facility to install the CPE unit 14. Of course, other forms of retail implementation can be used without departing from the spirit or scope of the invention.

Applications of the fixed wireless access system 10 include, but are not limited to: (1) high speed data applications, for example, Internet access (DSL speeds), hosting remote access email, WAN / lAn extension, services remote support MIS; (2) telephony, for example, Internet telephony, voice over Internet protocol (VoIP, voice over IP); and (3) video, eg video conferencing, video streaming, remote video camera surveillance, distance teaching, telemedicine.