HK1118139B - Method for performing point-to-multipoint digital radio frequency transport and digital radio frequency transport system - Google Patents

Description

Method for performing multipoint-to-point digital radio frequency transmission and digital radio frequency transmission system

The present application is a divisional application of chinese application 01815499.9 entitled "point-to-multipoint communication method using digital radio frequency transmission" filed on 7/3/2001.

Technical Field

The present invention relates to a high capacity mobile communication system, and more particularly, to a point-to-multipoint digital microcellular communication system.

Background information

Due to the widespread use of wireless technology, additional signal coverage is required in both urban and suburban areas. One obstacle to providing complete coverage in these areas is steel structural buildings. Inside these Tall and Shiny Buildings (TSBs), the signals emitted by the wireless base stations are significantly attenuated, thus significantly affecting the ability to communicate with the wireless telephones within the buildings. In some buildings, very low power overhead transmitters are installed in corridors and conference rooms in the building to distribute signals throughout the building. The signal is typically fed from a single point and then split to feed the signal to different points within the building.

To provide coverage, a single Radio Frequency (RF) source needs to feed multiple antenna devices simultaneously, each antenna device providing coverage for a different part of a building, for example. While bi-directional RF distribution often involves splitting the signal in the forward path (toward the antenna) and combining the signal in the reverse path (out of the antenna). This can currently be done directly at RF frequencies using passive splitters and combiners to feed into a coaxial cable distribution network. In passive RF distribution systems, the signal shunted in the forward path is significantly limited due to the inherent insertion loss associated with passive devices. Each shunt reduces the signal level distributed in the building, making reception of, for example, a cellular phone more difficult. In addition, the high insertion loss of coaxial cable at RF frequencies severely limits the maximum distance over which RF signals can be distributed. In addition, the system lacks any means of compensating for the difference in insertion loss in the various paths.

Another solution for distributing RF signals in a TSB is to convert RF signals from an amplifier or base station to a lower frequency and distribute them to remote antenna devices via Cat 5(LAN) or coaxial cable lines. In the remote antenna device, the signal is up-converted and transmitted. While down-conversion reduces insertion loss, the signal is still sensitive to noise and is in a limited dynamic range. Furthermore, each path in the distribution network requires a separate gain adjustment in order to compensate for the insertion loss in that path.

In another approach, fiber optic cables are used to distribute signals to antennas within a building. In this method, an RF signal is received from a bi-directional amplifier or base station. The RF signal directly modulates the optical signal and transmits it as an analog modulated optical signal through the optical cable throughout the building. Unfortunately, conventional systems employing analog optical modulation transmission over optical fibers require very complex linear lasers to achieve adequate performance. Furthermore, the distance over which the analog optical system can transmit signals within a building is limited. Typically, this limitation is made worse by the general use of multimode optical fibers within buildings. Multimode fibers are wider than single mode fibers and support multiple different reflection modes, so the signal tends to exhibit dispersion at the receiving end of the fiber. In addition, analog installation typically involves effective balancing when building the system. In addition, the RF level in the system needs to be balanced with the optical level. If there is optical attenuation, the RF level needs to be readjusted. Additionally, the RF level may change if the connector is not cleaned well or properly protected.

Digitizing the RF spectrum prior to transmission solves many of these problems. The level and dynamic range of the digitally transmitted RF remain unaffected over a wide range of path losses. This allows much larger distances to be covered and eliminates the path loss compensation problem. However, this is a strict point-to-point architecture. A disadvantage of digital transport RF in a point-to-point architecture is the equipment and cost requirements. A host RF to digital interface device needs to be provided for each remote antenna unit. In particular, the number of RF to digital interface devices and the optical fibers connecting these devices is burdensome for use in buildings or complex building complexes. For example, in a building having 20 floors, the requirements may include 20 host RF-to-digital interface devices for 20 remote antenna devices, one for each floor. In some applications, it may be desirable to provide more than one remote antenna device per floor. As a result, there is a need in the art for improved techniques to distribute RF signals in TSBs, incorporating the advantages of digital RF transport into point-to-multipoint architectures.

Summary of The Invention

The above-mentioned problems with distributing RF signals within buildings and other problems are addressed by the present invention, which will be understood by reading and studying the following specification.

In one embodiment, a digital radio frequency transport system is provided. The transmission system includes a digital host unit and at least two digital remote units connected to the digital host unit. The digital host unit includes shared circuitry for bi-directional simultaneous digital radio frequency distribution between the digital host unit and at least two digital remote units.

According to one aspect of the present invention, there is provided a digital radio frequency transmission system comprising:

a digital host unit; and

at least two digital remote units connected to said digital host unit, wherein said digital host unit includes shared circuitry that performs bi-directional simultaneous digital radio frequency distribution between said digital host unit and said at least two digital remote units.

According to the present invention, there is also provided a digital radio frequency transmission system comprising:

a digital host unit;

at least one digital expansion unit connected to the digital host unit; and

at least two digital remote units, each digital remote unit connected to one of the digital host unit and the digital extension unit, the digital host unit including a shared circuit for bi-directional simultaneous digital radio frequency distribution between the digital host unit and the at least two digital remote units.

According to the present invention, there is also provided a digital radio frequency transmission system comprising:

a digital host unit; and

at least two digital remote units connected to the digital host unit, wherein the at least two digital remote units each comprise:

a primary radio frequency antenna;

a duplexer connected to said main rf antenna for receiving rf signals in a reverse path and transmitting rf signals in a forward path;

a radio frequency-to-digital converter connected to the duplexer in a reverse path;

a digital-to-radio frequency converter connected in a forward path to the duplexer;

a multiplexer chipset coupled to the RF-to-DAC in the reverse path and the DAC in the forward path;

the optical transmitter is connected with the output end of the multiplexing chipset; and

an optical receiver connected to an input of the multiplexer chipset,

wherein the digital host unit includes shared circuitry for bi-directional simultaneous digital radio frequency distribution between the digital host unit and the at least two digital remote units.

According to the present invention, there is also provided a digital radio frequency transmission system comprising:

a digital host unit, the digital host unit comprising:

a radio frequency-to-digital converter;

the multiplexer is connected with the output end of the radio frequency-digital converter;

a plurality of optical transmitters connected to the output of the multiplexer;

a first local oscillator connected to an input of the radio frequency-to-digital converter;

a reference oscillator coupled to an input of the first local oscillator;

a second local oscillator connected to an output of the reference oscillator;

a digital-to-radio frequency converter connected to an output of the second local oscillator;

the channel adder is connected with the input end of the digital-radio frequency converter;

a plurality of demultiplexers coupled to the channel summer;

a plurality of clock and bit recovery circuits, each of said plurality of clock and bit recovery circuits being coupled to one of said plurality of demultiplexers; and

a plurality of optical receivers, each of the plurality of optical receivers coupled to one of the plurality of always and bit recovery circuits,

at least two digital remote units, each digital remote unit coupled to one of the plurality of optical receivers, wherein the digital host unit includes shared circuitry for bi-directional simultaneous digital radio frequency distribution between the digital host unit and the at least two digital remote units.

According to the present invention, there is also provided a digital radio frequency transmission system comprising:

a digital host unit comprising a channel adder;

at least one digital expansion unit connected to the digital host unit; and

at least two digital remote units coupled to the digital host unit, wherein the digital host unit includes shared circuitry for bi-directional simultaneous digital radio frequency distribution between the digital host unit and the at least two digital remote units.

In another embodiment, a digital radio frequency transport system is provided. The transmission system includes a digital host unit and at least one digital expansion unit coupled to the digital host unit. The transmission system further includes at least two digital remote units, each digital remote unit being connected to one of the digital host unit and the digital extension unit. The digital host unit includes shared circuitry for bi-directional simultaneous digital radio frequency distribution between the digital host unit and at least two digital remote units.

In another embodiment, a method of performing point-to-multipoint radio frequency transmission is provided. The method includes receiving a radio frequency signal in a digital host unit and converting the radio frequency signal to a digitized radio frequency spectrum. The method also includes optically transmitting the digitized spectrum of radio frequencies to a plurality of digital remote units. The method also includes receiving the digitized radio frequency spectrum in the plurality of digital remote units, converting the digitized radio frequency spectrum to an analog radio frequency signal, and transmitting the analog radio frequency signal through the main radio frequency antenna in each of the plurality of digital remote units.

Brief description of the drawings

Fig. 1 is an illustration of an embodiment of a point-to-multipoint communication system according to the present invention.

Fig. 2 is a block diagram of one embodiment of a communication system according to the present invention.

Fig. 3 is a block diagram of another embodiment of a communication system according to the present invention.

Figure 4 is a block diagram for one embodiment of a digital host unit according to the present invention.

Figure 5 is a block diagram for one embodiment of a digital remote unit in accordance with the present invention.

Fig. 6 is a block diagram for one embodiment of a digital expansion unit according to the present invention.

Fig. 7 is a block diagram of one embodiment of a microcellular base station in accordance with the present invention.

Fig. 8 is an illustration of one embodiment of an overflow algorithm for a channel summer according to the present invention.

DETAILED DESCRIPTIONS

In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

Fig. 1 is an illustration of an exemplary embodiment of a point-to-multipoint digital transmission system, designated generally by the numeral 100, constructed in accordance with the teachings of the present invention. The point-to-multipoint digital transmission system 100 is shown distributed within a Tall Shiny Building (TSB)2 population. While the system 100 is shown within the population of TSBs 2, it is to be understood that the system 100 is not limited to this embodiment. Rather, the system 100 in other embodiments may be used to distribute signals in individual buildings or other suitable structures or indoor or outdoor locations that exhibit high attenuation of RF signals. Advantageously, the system 100 employs a digital sum of digitized RF signals from multiple antennas to improve signal coverage in structures such as TSBs.

The point-to-multipoint digital transmission of RF signals is accomplished by remote antenna arrangements or networks of digital remote units 40 and 40' and digital host unit 20, digital host unit 20 interfacing with a wireless network 5 connected to a Public Switched Telephone Network (PSTN) or a Mobile Telecommunications Switching Office (MTSO) or other switching office/network. The system 100 operates by digitally transmitting RF signals over fiber optic cables. Signals received at the DHU 20 are distributed to a plurality of DRUs 40 and 40' to provide coverage throughout the building complex. In addition, the signals received at the respective DRUs 40 and 40' are added together in the DHU 20 for interfacing with the wireless network.

In one embodiment, the digital extension unit DEU30 is located between the DHU 20 and one or more DRUs. In the forward path, the DEU30 expands the coverage area by splitting the signals received from the DHU 20 into multiple DRUs 40'. In the reverse path, DEU30 receives signals from multiple DRUs 40', digitally adds the signals together and transmits them to DHU 20 or another DEU, such as DEU 30. This system allows for the use of the DEU30 to transfer signals sequentially and to extend the coverage area to multiple DRUs 40 and 40'. This system provides an efficient method of providing signal coverage for wireless communications without increasing the attenuation losses and distance limitations present in analog systems. By using the DEU30, the antenna can be placed further away from the DHU 20 without adversely affecting signal strength, since shorter fiber optic cables can be used.

The digital transmission system 100 includes a Wireless Interface Device (WID)10 that provides an interface to a wireless network. In one embodiment, WID 10 includes conventional transmitter and receiver or all digital transmitter and receiver equipment and interface circuitry to a Mobile Telecommunications Switching Office (MTSO). In one embodiment, the wireless interface device 10 is connected to the MTSO via a T1 line and receives and transmits signals between the MTSO and the DHU 20. In another embodiment, the wireless interface device 10 is connected to a Public Switched Telephone Network (PSTN). In one embodiment, the WID 10 includes a base station and is connected directly to the DHU 20 via a coaxial cable. In another embodiment, the WID 10 includes a base station and is wirelessly connected to the DHU 20 via a bi-directional amplifier connected to an antenna. In one embodiment, the antenna is an outdoor antenna.

WID 10 communicates signals between the wireless device and the wireless network through digital remote units DRU40 and 40'. The WID 10 is connected to the DHU 20. The DHU 20 is connected to at least one digital expansion unit DEU30 and a plurality of DRUs 40. In addition, the DEU30 is connected to a plurality of DRUs 40'. The DHU 20 receives the RF signals from the WID 10 and converts the RF signals to digital RF signals. The DHU 20 also optically transmits digital RF signals to a plurality of DRUs 40, either directly or through one or more DEUs 30.

The DRUs 40 and 40' are connected by an optical cable (or alternatively another high bandwidth carrier) to carry digital RF signals to one of the DHU 20 or DEU 30. In one embodiment, the optical cable comprises pairs of multimode optical fibers connected between DRUs 40 and DHUs 20, DRUs 40 and 40' and DEU30, and DEU30 and DHU 20. In one embodiment, the DEU30 is connected to the DHU 20 via a single mode fiber and the DEU30 is connected to the DRU 40' via a multimode fiber pair. Although the transmission system 100 is described in terms of a fiber optic cable, other carriers, such as coaxial cables, may be used.

In another embodiment, the DHU 20 is connected to the DRUs 40 by dc power cables to provide power to each DRU 40. In one embodiment, the DC power cable delivers 48 volts DC to each DRU40 connected to the DHU 20. In another embodiment, DEU30 is connected to DRUs 40 'by dc power cables to provide power to each DRU 40'. In one embodiment, the DC power cable delivers 48 volts DC to each DRU 40' connected to DEU 30. In an alternative embodiment, DRUs 40 and 40' are directly connected to a power source. In one embodiment, the power supply provides direct current to the DRUs 40 and 40'. In an alternative embodiment, the power supply provides alternating current to the DRUs 40 and 40'. In one embodiment, DRUs 40 and 40' each include an ac/dc power converter.

The DHU 20 and DEU30 each split the signal in the forward path and add the signal in the reverse path. In order to accurately sum the digital signals in the DHU 20 or DEU30, the data needs to enter the DHU 20 or DEU30 at exactly the same rate. As a result, all DRUs 40 and 40' need to be synchronized so that their digital sample rates are all locked together. Synchronizing the signals in time is achieved by locking all of these signals to the bit rate on the fiber. In one embodiment, the DHU 20 emits a digital bit stream that is detected by an optical receiver in the DEU30 or DHU 40 and its clock is locked to the bit stream. In one embodiment, this is accomplished by multiplexing the chipset and the local oscillator, as described below. Splitting and combining signals in the digital state avoids combining and splitting losses present in analog systems. In addition, transmitting digital signals over multimode optical fiber can result in a low cost transmission system that does not suffer much degradation.

The down-and up-conversion of the RF signal is achieved by mixing the signal with a Local Oscillator (LO) in the DRU and DHU. In order to recover the original frequency of the RF signal, the signal must be upconverted with an LO that is exactly the same frequency as the LO used for the downconversion. Any difference in LO frequency will translate into a comparable end-to-end frequency offset. In the described embodiment, the down-conversion and up-conversion LOs are located far apart from each other. Thus, in a preferred embodiment, the frequency coherence between local and remote LOs is established as follows: at the DHU end a 142MHz reference oscillator is provided which results in a bit rate of 1.42GHz on the fiber. This reference oscillator also generates a 17.75MHz reference clock that is used as a reference for LO locking at the DHU end.

In each DRU there is provided another 17.75MHz clock which is recovered from the optical bit stream by means of a clock and bit recovery circuit. Since this clock is recovered from the bit stream generated in the host, it is coherent with the reference oscillator frequency in the host. A 17.75MHz reference clock is then generated for use as a reference for the remote and local oscillators. Since the remotely recovered bit clock is frequency coherent with the host's master clock, the host and remote reference clocks, and any LOs locked to these clocks, are also frequency coherent, thus ensuring that the LOs of the DHU and DRU are frequency locked. It will be appreciated that in other embodiments, the bit rate on the fiber may vary, as may the clock frequency.

Figure 2 is a block diagram of one embodiment of a communication system, generally indicated at 200, constructed in accordance with the teachings of the present invention. In this embodiment, a Digital Host Unit (DHU)220 is connected to a bidirectional amplifier (BDA) 211. The BDA 211 receives communication signals from a Wireless Interface Device (WID) and transmits the communication signals to the DHU220 as RF signals, and receives RF signals from the DHU220 and transmits the RF signals to the WID. The DHU220 receives RF signals from the BDA 211, digitizes the RF signals, and optically transmits the digital RF signals to a plurality of DRUs over transmission lines 214-1 to 214-N. The DHU220 also receives digitized RF signals from the plurality of DRUs, either directly or indirectly via the DEUs, over the transmission lines 216-1 to 216-N, reconstructs corresponding analog RF signals, and adds them to the BDA 211. In one embodiment, the DHU220 receives signals directly from the N DRUs. The signals are digitally summed and then converted to an analog signal and sent to the BDA 211. In another embodiment, the DHU220 receives signals directly from one or more DEUs and one or more DRUs. Likewise, the signals are all digitally summed and then converted to analog signals and sent to the BDA 211. Signals received via transmission lines 216-1 to 216-N may be received directly from the DRU, or the signals may be received by the DEU and summed together and then transmitted via 216-1 to 216-N to the DHU220 for additional summing and transformation for transmission to the BDA 211. The DEU provides a way to expand the coverage area and digitally add signals received from DRUs or other DEUs for transmission in the reverse path to other DEUs or DHUs 220. In one embodiment, transmission lines 214-1 through 214-N and 216-1 through 216-N comprise pairs of multi-mode optical fibers. In an alternative embodiment, each fiber pair is replaced by a single fiber, carrying the bidirectional optical signals by employing Wavelength Division Multiplexing (WDM). In an alternative embodiment, transmission lines 214-1 through 214-N and 216-1 through 216-N comprise single mode optical fibers. In one embodiment, N is equal to 6. In an alternative embodiment, the number of transmission lines in the forward path directions 214-1 through 214-N is not equal to the number of transmission lines in the reverse path directions 216-1 through 216-N.

Figure 3 is a block diagram of an alternative embodiment of a communication system, generally designated 300, constructed in accordance with the teachings of the present invention. The communication system 300 includes a base station 310 connected to a DHU 320. The base station 310 includes a conventional transmitter 323 and receiver 328, and conventional radio controller or interface circuitry 322 coupled to an MTSO or a telephone switching network. The DHU 320 is connected to the base station 310. DHU 320 is also connected to transmission lines 314-1 through 314-M that transmit in the forward path direction and transmission lines 316-1 through 316-M that transmit in the reverse path direction.

The DHU 320 essentially converts the RF spectrum to digital in the forward path and from digital to analog in the reverse path. In the forward path, the DHU 320 receives the combined RF signal from the transmitter 323, digitizes the combined signal, and transmits it in a digital format over the optical fibers 314-1 to 314-M, with the optical fibers 314-1 to 314-M being connected directly to multiple DRUs or indirectly to one or more DRUs via one or more DEUs.

In one embodiment, the DHU 320 receives signals directly from M DRUs. The signals are digitally summed and then converted to analog signals and transmitted to the base station 310. In another embodiment, the DHU 320 receives signals directly from one or more DEUs and one or more DRUs. Likewise, the signals are all added digitally, then converted to analog signals, and transmitted to the base station 310. Signals received via transmission lines 316-1 to 316-M may be received directly from the DRU or the signals may be received by the DEU and summed together and then transmitted via 316-1 to 316-M to the DHU 320 for additional summing and transformation for transmission to the base station 210. The DEU provides a method of expanding the coverage area by splitting signals in the forward path and digitally summing signals received from a DRU or other DEU in the reverse path for upstream transmission to other DEUs or DHUs. In the reverse path, the DHU 320 also receives digitized RF signals from multiple DRUs over the optical fibers 316-1 through 316-M, either directly or indirectly via the DEU, reconstructs the corresponding analog RF signals, and uses them in the receiver 328.

In one embodiment, transmission lines 314-1 through 314-M and 316-1 through 316-M comprise pairs of multi-mode optical fibers. In an alternative embodiment, each fiber pair is replaced by a single fiber, carrying the bidirectional optical signals by employing Wavelength Division Multiplexing (WDM). In an alternative embodiment, transmission lines 314-1 through 314-M and 316-1 through 316-M comprise single mode optical fibers. In one embodiment, M is equal to 6. In an alternative embodiment, the number of transmission lines in the forward path directions 314-1 through 314-M is not equal to the number of transmission lines in the reverse path directions 316-1 through 316-N.

Referring now to FIG. 4, one embodiment of a DHU 420 constructed in accordance with the present invention is shown. The DHU 420 includes an RF-to-digital converter 491 that receives a combined RF signal from a wireless interface device, such as a base station, BDA, or the like. RF-to-digital converter 491 provides a digitized traffic stream that is sent to multiplexer 466. Multiplexer 466 converts the parallel output of the analog-to-digital converter to a framed serial bit stream. At the output of the multiplexer there is a 1 to P fan-out buffer 407 that divides the digital signal into P paths. P optical transmitters 431-1 to 431-P are provided, each transmitter feeding a signal to each of the P optical transmission lines 414-1 to 414-P. The digital signals are applied to optical fibers 414-1 to 414-P for transmission to the corresponding DRUs either directly or via the DEUs. In one embodiment, P is equal to 6.

In one embodiment, the DHU 420 includes an amplifier 450 that receives the combined RF signal from a wireless interface device, such as a base station or BDA. The combined RF signal is amplified and then mixed by mixer 452 with a signal from local oscillator 468. Local oscillator 468 is coupled to reference oscillator 415. In one embodiment, the local oscillator is coupled to a frequency divider circuit 470, and the frequency divider circuit 470 is coupled to the reference oscillator 415. The local oscillator is locked to a reference oscillator 415, which is the master clock, so the down-conversion of the RF signal is the same as the up-conversion. The result is end-to-end, i.e., from DHU to DRU or DHU to one or more DEUs to DRU, with no frequency offset in the received and transmitted signals. Local oscillator 463 is also connected to synthesizer circuit 476.

The output signal of the mixer 452 is provided to an amplifier 454 for amplification and then filtered by an Intermediate Frequency (IF) filter 456. The resulting signal is a combined RF signal that is downconverted to an IF signal. The IF signal is mixed by mixer 460 with another signal from reference oscillator 415. The output of the mixer 460 is summed 462 with a signal produced by a Field Programmable Gate Array (FPGA) 467. This output is then converted from an analog signal to a digital signal by an analog-to-digital (a/D) converter 464, which, after conversion, applies a digital RF signal to a multiplexer 466. In one embodiment, analog-to-digital converter 464 is a 14-bit converter that processes 14-bit signals. In other embodiments, the analog-to-digital converter 464 may be any size to accommodate the appropriate signal. In one embodiment, the input signal from FPGA 467 is a dither signal from dither circuit 462, and dither circuit 462 adds limited out-of-band noise to improve the dynamic range of the RF signal.

In one embodiment, the DHU 420 includes an ac-dc distribution circuit 6, the distribution circuit 6 providing dc power to DRUs connected to the DHU 420.

The DHU 420 also includes a plurality of digital optical receivers 418-1 to 418-P in the reverse path. Each receiver 418-1 to 418-P outputs an electronic digital signal which is applied to a clock and bit recovery circuit 445-1 to 445-P for clock and bit recovery of the electronic signal, respectively. The signals are then applied to demultiplexers 441-1 through 441-P, respectively, which extract the digitized signals generated in the DRU as described in detail below. The demultiplexers 441-1 through 441-P also extract alarm (monitoring) and voice information framed with the digitized signal. The digital signal outputs from the demultiplexers 441-1 through 441-P are then applied to the FPGA 467 where the signals are summed together and applied to the digital-to-RF converter 495. Converter 495 operates on the sum of the digitized signals extracted by demultiplexers 441-1 through 441-P and reconstructs baseband replicas of the RF signals received at all digital remote units. The baseband replicas are then up-converted to their original radio frequency by mixing with a local oscillator 482 and filtering to remove the image frequency. Local oscillator 482 is connected to synthesizer 476 and a reference oscillator as discussed above with respect to local oscillator 468.

In one embodiment, digital-to-RF converter 495 comprises a digital-to-analog (D/a) converter 484 coupled to the output of FPGA 467, and the digitized RF signal is converted to an analog RF signal and then mixed with the signal from reference oscillator 415 by mixer 492. The signal is then filtered by IF filter 490 and amplified by amplifier 488. The resulting signal is then mixed with a signal from a local oscillator 482 and then applied to an RF filter 484, amplifier 480, and RF filter 478 for transmission over a wireless interface device, such as a BDA or base station.

In one embodiment, FPGA 467 includes alarm/control circuitry 474 that extracts additional bits from the DRU to monitor for error and alarm information. In one embodiment, FPGA 467 includes summer 498 that mathematically sums together the digital RF signals received from fibers 416-1 through 416-P. In another embodiment, the FPGA includes an overflow algorithm circuit 486 connected to the output of adder 486. Algorithm circuit 496 allows the summed digital RF signal to saturate and keep the summed signal within a predetermined number of bits. In one embodiment, the arithmetic circuit includes a limiter. In one embodiment, the RF signal is a 14-bit signal that when summed and limited by summer 498 and overflow circuit 496 results in a 14-bit output signal.

For example, in one embodiment, each digital RF signal received from optical fibers 416-1 through 416-P (where P equals 6) includes a 14-bit input. All of the 6 different 14-bit inputs are then provided to adder 498. To account for overflow, a resolution of at least 17 bits is required in adder 498 to handle the worst case when all 6 14-bit inputs are at maximum scaling at the same time. In this embodiment, a 17-bit wide adder 498 is employed to handle this dynamic range. The 14-bit signal into the reverse path needs to come out of the adder 498. In one embodiment, an algorithm circuit 496 for managing overflow is implemented. In one embodiment, adder 498 and overflow algorithm 496 are included in FPGA 467. In one embodiment, overflow algorithm 496 acts as a slicer and allows the sum to saturate and keep the summed signal within 14 bits. In an alternative embodiment, overflow algorithm circuit 496 controls the gain and dynamically scales the signal to handle overflow conditions.

Fig. 8 illustrates one embodiment of an algorithm circuit 863 for a channel adder 865 to limit the sum of input signals 0 through 5 to 14 bits. In this embodiment, input signals 0 through 5 comprise 6 signals that are added together by adder 865. The sum of the input signals 0 to 5 is reduced by the algorithm 863 to a signal having 14 bits or less. It will be appreciated that the algorithm 865 is an example only and is not meant to limit the type of algorithm used to limit the sum of the input signals 0 through 5 to 14 bits or less.

For example, when the sum of 6 input signals 0 to 5 is greater than or equal to 13FFBh, the sum is divided by 6 to obtain a signal of 14 bits or less. When the sum of 6 input signals 0 to 5 is greater than 13FFBh but less than or equal to FFFCh, then the sum is divided by 5 to obtain a signal of 14 bits or less. When the sum of 6 input signals 0 to 5 is greater than FFFCh but less than BFFDh, then the sum is divided by 4 to get a signal of 14 bits or less. When the sum of 6 input signals 0 to 5 is greater than BFFDh but less than 7FFEh, then the sum is divided by 3 to get a signal of 14 bits or less. Finally, when the sum of 6 input signals 0 to 5 is greater than 7FFEh but less than or equal to 3FFFh, the sum is divided by 2 to obtain a signal of 14 bits or less.

Figure 5 is a block diagram of one embodiment of a Digital Remote Unit (DRU)540 constructed in accordance with the teachings of the present invention. The digital optical receiver 501 receives the optical digital data stream transmitted by the DHU either directly or via the DEU. The receiver 501 converts the optical data stream into a corresponding series of electrical pulses. The electrical pulses are applied to a clock and bit recovery circuit 503. The series of electrical pulses is then applied to the demultiplexer 505. Demultiplexer 505 extracts the digital traffic signal and converts the signal from serial to parallel. The output parallel signal is then applied to a digital-to-RF converter 595 for conversion to RF and transmission to the duplexer 547. The RF converter 595 is connected to the main antenna 599 through the duplexer 547. Thus, radio frequency signals from the wireless interface device are transmitted from the main antenna 547.

In one embodiment, digital-to-RF converter 595 includes a digital-to-analog (D/a) converter 509 that reconstructs an analog RF signal and applies it to IF 504 and amplifier 506. The analog signal is mixed by mixer 502 with the output signal from reference oscillator 515. The output of amplifier 506 is mixed with a signal from local oscillator 519, and local oscillator 519 synchronizes the RF signal with the digital signal returned via reference oscillator 515 connected to local oscillator 519. In one embodiment, the reference oscillator is coupled to a frequency divider 517, and the frequency divider 517 is coupled to local oscillators 519 and 529. Local oscillators 519 and 529 are also coupled to a synthesizer 521, and synthesizer 521 is coupled to a programmable logic device 525.

The RF signal received at the main antenna 599 is transferred to the RF-to-digital converter 593 through the duplexer 547. The RF signal is converted to a digital signal and then applied to a multiplexer 536, converted from parallel to serial, and optically transmitted to the DEU or DHU via an optical transmitter 532.

In one embodiment, the RF-to-digital converter 593 includes a first amplifier 543 that receives the RF signal from the duplexer 547, amplifies the signal and sends it to a digital attenuator 539. In one embodiment, the amplifier 543 is a low noise amplifier. The digital attenuator 539 receives the amplified signals and digitally attenuates them to control the level in the event of an overflow. The RF-to-digital converter 593 further comprises a second amplifier 537 which receives the attenuated signal, amplifies this signal and applies the amplified signal to the mixer 535. The mixer 535 mixes the amplified signal with a signal from a local oscillator 529. The resulting signal is sequentially applied to a third amplifier 533, an IF filter 548 and a fourth amplifier 546, thereby being down-converted to an IF signal. The IF signal is then mixed with a signal from a reference oscillator 515, and the mixed signal is added to a signal from a dither circuit 527. The resulting signal is applied to an analog-to-digital converter 538 and converted into a digital signal. The output digital signal is then applied to multiplexer 536. In one embodiment, multiplexer 536 multiplexes the signal with some additional bits to form the frame and control information. In one embodiment, multiplexer 536, clock and bit recovery circuit, and demultiplexer 505 comprise a set of multiplexer chips.

Programmable logic circuit 525 programs synthesizer 521 for up and down conversion of the reference oscillator and local oscillators 519 and 529. Programmable logic 525 looks for error conditions and oscillator out-of-lock conditions and reports error patterns and looks for overflow conditions in analog-to-digital converter 538. If an overflow condition occurs, programmable logic 525 indicates saturation and adds some additional attenuation in digital attenuator 539 to reduce the RF signal level from RF antenna 599 and protect the system from overload.

In one embodiment, DRU 540 includes an internal dc distribution system 5. In one embodiment, the distribution system receives 48 volts direct current and internally distributes the three outputs of +3.8V, +5.5V, and + 8V.

FIG. 6 is a block diagram of one embodiment of a Digital Expansion Unit (DEU)630 constructed in accordance with the teachings of the present invention. The DEU 630 is designed to receive optical signals and transmit optical signals. The optical receiver 651 receives the digitized RF signals and sends them to a clock and bit recovery circuit 653, which performs clock and bit recovery to lock the local clock and clear signals. The signal is then split into X RF digital signals by 1-to-X fan-out buffer 607. The signal is then transmitted via optical transmitters 655-1 to 655-X to X receiving devices, such as DEUs or DRUs. The X receiving devices may be any combination of DEUs or DRUs. In one embodiment, X is equal to 6.

DEU 630 also includes optical receivers 669-1 through 669-X, which receive digitized RF signals directly from the DRU or indirectly via the DEU. In operation, signals are received, applied to clock and bit recovery circuits 673-1 through 673-X, respectively, to lock the local clock and clear the signals, and then applied to demultiplexers 671-1 through 671-X. Each demultiplexer 671-1 to 671-X extracts the digitized traffic and applies the samples to a field programmable gate array 661. The signals are digitally summed together and sent to a multiplexer 657, and the multiplexer 657 multiplexes the signals with some additional bits to form the frame and control information. In addition, multiplexer 657 converts the signals from parallel to serial. The signal is then applied to an optical transmitter 659 for further transmission. In one embodiment, the signal is sent to the DHU directly or indirectly via one or more additional DEUs.

In one embodiment, FPGA 661 includes an adder 665 that mathematically adds together the digital RF signals received from demultiplexers 671-1 through 671-X. In another embodiment, FPGA 661 includes an overflow algorithm circuit 663 connected to the output of adder 665. The algorithm circuit 663 allows the summed digital RF signal to saturate and keep the summed signal within a predetermined number of bits. In one embodiment, the arithmetic circuit includes a limiter. In one embodiment, the RF signal is a 14-bit signal that when summed and limited by summer 665 and overflow algorithm 663 results in a 14-bit output signal.

In one embodiment, DEU 630 includes AC-DC distribution circuit 7, which provides DC power to the DRUs connected to DEU 630.

In an alternative embodiment, the Digital Host Unit (DHU) and the Wireless Interface Device (WID) are located at a distance from the building being serviced. The DHU inside the building is replaced by a DEU, the link between which and the remotely located DHU is via single mode optical fibre. Fig. 7 is a block diagram of this embodiment. The microcellular base station, generally designated by the numeral 700, includes a conventional transmitter 723 and receiver 728, as well as a conventional radio controller or interface circuit 722. In the forward path, the DHU 767 receives the combined RF signal from the transmitter 723, digitizes the combined signal, and sends it in a digital format over a single mode fiber to the DEU. In the reverse path, the DHU 767 receives the digitized RF signal from the DEU, reconstructs the corresponding analog RF signal, and applies it to the receiver 728.

In another alternative embodiment, the Wireless Interface Device (WID) is a software defined base station, and the interface between the DHU and WID is done digitally, eliminating the need for RF-to-digital conversion circuitry in the DHU.

Conclusion

A digital radio frequency transport system is described above. The transmission system includes a digital host unit and at least two digital remote units connected to the digital host unit. The digital host unit includes shared circuitry that performs bi-directional simultaneous digital radio frequency distribution between the digital host unit and at least two digital remote units.

In addition, a digital radio frequency transmission system is also described. The transmission system includes a digital host unit and at least one digital extension unit coupled to the digital host unit. The transmission system further includes at least two digital remote units, each digital remote unit being connected to one of the digital host unit and the digital extension unit. The digital host unit includes shared circuitry that performs bi-directional simultaneous digital radio frequency distribution between the digital host unit and at least two digital remote units.

Furthermore, a method of performing point-to-multipoint radio frequency transmission is described. The method includes receiving an analog radio frequency signal in a digital host unit and converting the analog radio frequency signal to a digitized radio frequency signal. The method also includes splitting the digitized radio frequency signal into a plurality of digital radio frequency signals and optically transmitting the digital radio frequency signals to a plurality of digital remote units. The method also includes receiving the digital radio frequency signal in the plurality of digital remote units, converting the digital radio frequency signal to an analog radio frequency signal, and transmitting the signal through the primary radio frequency antenna in each of the plurality of digital remote units.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. For example, a digital remote unit is not limited to receiving, summing, splitting, and transmitting digital radio frequency signals. In other embodiments, the digital host unit can receive and add analog radio frequency signals in addition to or instead of receiving and adding digital radio frequency signals. Also, the digital host unit can separate and transmit analog radio frequency signals in addition to or instead of separating and transmitting digital radio frequency signals. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.

Claims (1)

1. A method of performing multipoint-to-point digital radio frequency transmission, the method comprising:

receiving analog radio frequency signals in a plurality of digital remote units;

converting the analog radio frequency signal to a digital radio frequency signal in each of the digital remote units;

optically transmitting said digital radio frequency signals from each of said digital remote units to a digital host unit;

receiving the plurality of digital radio frequency signals in the digital host unit;

digitally summing the plurality of digital radio frequency signals together, wherein the plurality of digital radio frequency signals have been received from the plurality of digital remote units, and wherein an overflow algorithm is used to keep the sum of the plurality of digital radio frequency signals within a predetermined number of resolution bits; and

the digitally summed digital radio frequency signals are converted back into analog radio frequency signals and these signals are sent to the radio interface device for further transmission to the switched telephone network.

2. The method of claim 1, wherein the converting of the analog radio frequency signal to a digital radio frequency signal comprises amplifying the analog radio frequency signal.

3. The method of claim 1, wherein the converting of the analog radio frequency signal to the digital radio frequency signal comprises synchronizing a reverse path local oscillator with a master clock to reduce end-to-end frequency translation.

4. A digital radio frequency transport system comprising a digital host unit and at least two digital remote units communicatively coupled to said digital host unit, wherein said digital host unit comprises circuitry for performing bi-directional simultaneous digital radio frequency distribution of digital radio frequency signals between said digital host unit and said at least two digital remote units; wherein the digital host unit digitally sums the digital radio frequency signals received at the digital host unit from the at least two digital remote units, wherein an overflow algorithm is used to keep the sum of the plurality of digital radio frequency signals within a predetermined number of resolution bits.

5. A digital radio frequency transport system comprising a digital host unit, a digital extension unit communicatively coupled to said digital host unit, and at least two digital remote units communicatively coupled to said digital extension unit, wherein at least one of said digital host unit and digital extension unit includes circuitry for performing bi-directional simultaneous digital radio frequency distribution of digital radio frequency signals between said digital host unit or digital extension unit and said at least two digital remote units; wherein at least one of the digital host unit and digital extension unit digitally sums the digital radio frequency signals received at the digital host unit or digital extension unit from the at least two digital remote units, wherein an overflow algorithm is used to keep the sum of the plurality of digital radio frequency signals within a predetermined number of resolution bits.