WO2022048751A1

WO2022048751A1 - Distributed agc system to overcome dynamic range limitations in fronthaul optical analog interfaces

Info

Publication number: WO2022048751A1
Application number: PCT/EP2020/074566
Authority: WO
Inventors: Rafael PUERTA; Filippo Ponzini; Luca Giorgi
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2020-09-03
Filing date: 2020-09-03
Publication date: 2022-03-10
Anticipated expiration: 2023-03-03

Abstract

Automatic gain control is used to improve the dynamic range performance of a distributed wireless system comprising a Central Unit (CU) coupled to a Distributed Unit (DU) via an optical fiber connection. The DU determines how much to amplify an uplink signal, generates a modulated uplink wavelength carrying the amplified uplink signal, generates a modulated gain wavelength carrying the amplification information, and couples these modulated wavelengths into an optical fiber to convey the amount of applied amplification along with the amplified uplink signal to the central unit. The CU uses the amplification information in the received uplink signal to compensate the gain applied to a digital version of the uplink signal. In so doing, the solution presented herein compensates for the dynamic range limitations of the distributed unit.

Description

DISTRIBUTED AGC SYSTEM TO OVERCOME DYNAMIC RANGE LIMITATIONS IN FRONTHAUL OPTICAL ANALOG INTERFACES

TECHNICAL FIELD

The solution presented herein relates generally to Distributed Multiple-Input, Multiple- Output (D-MIMO) systems, and more particularly to dynamic range limitations of such D-MIMO systems.

BACKGROUND

A Cloud Radio Access Network (C-RAN) architecture provides an attractive alternative for implementing Distributed Multiple-Input Multiple-Output (D-MIMO) systems, which have the potential to increase the performance relative to mobile networks using traditional co-located massive MIMO systems. However, most of the C-RAN current solutions use optical digital interfaces for the fronthaul (FH) links, e.g., enhanced Common Public Radio Interface (eCPRI) or Open Base Station Architecture Initiative (OBSAI) interfaces. Thus, the Distributed Units (DUs) for the current C-RAN solutions use a digital interface termination and include all the Digital Signal Processing (DSP) of a Digital Front-End (DFE), which increases the complexity, power consumption, and size of the DUs, and thus reduces its scalability capabilities. In addition, given the limited capacity of the optical digital FH interfaces and the limited processing capabilities of the DFEs (e.g., Radio Frequency (RF) transceiver digital serial interfaces), the baseband bandwidth (BW) that each DU can support is limited to up to few hundreds of MHz. Thus, a FH optical digital interface is not suitable for systems having extreme BWs, e.g., baseband BW greater than 1GHz, and/or extreme capacity requirements.

In some cases, the C-RAN architecture may use FH optical analog interfaces, where all the DSP occur in the Central Units (CUs), which overcomes some of the above-noted limitations of the digital interfaces. However, the link dynamic range (of such FH optical analog interfaces, the difference between the maximum signal level (e.g., the largest peak power the link can support with an acceptable signal distortion) and the minimum detectable signal level (e.g., generally just above the system noise floor) the link can support while maintaining a specific transmission performance, e.g., Bit Error Rate (BER), is limited. Unlike with the gain or noise figure of a system, amplification will not enhance the dynamic range

Technical specifications define the minimum RF characteristics and minimum performance requirements of Base Stations (BSs). Generally, blocking requirements, which measures the receiver’s capability to receive a wanted signal in its assigned channel in the presence of an unwanted interferer, impacts the dynamic range of BSs the most. 3GPP TS 38.104, v15.9.0 “New Radio (NR) BS radio transmission and reception,” referred to herein as “TS 38.104,” defines that the highest value of the interfering signal mean power may be up to -35 dBm for in-band blocking. The dynamic range of a receiver therefore needs to be capable to handle signals both at the sensitivity level (i.e., close to the noise floor) and at the maximum specified interferer blocking level. For example, for a signal bandwidth of 20 MHz, the corresponding noise floor power is given by:

P_NF = -174 + 101og₁₀ (20*10⁶) = -101 dBm .

Thus, for a maximum interferer blocking level of -35 dBm, the dynamic range of the BS receiver must be at least:

DR = -35 dBm-(-101 dBm) = 66 dB , as shown in Figure 1 . To ensure the uplink FH optical analog interfaces do not distort the received signals, the dynamic range of such uplink FH optical analog interfaces must therefore be large enough to fulfill the BS’s receiver dynamic range requirements specified in TS 38.104. To date, such dynamic range performance has been difficult, if not impossible, to achieve with uplink FH optical analog interfaces. Even when coherent receivers, which are limited by shot noise, are used instead of direct-detection receivers, which are limited by thermal noise, current dynamic range requirements cannot be met. Thus, there remains a need for improved uplink FH optical analog interfaces solutions.

SUMMARY

The solution presented herein uses automatic gain control to improve the dynamic range performance of a distributed wireless system. More particularly, a distributed unit determines how much to amplify an uplink signal, generates a modulated uplink wavelength carrying the amplified uplink signal, generates a modulated gain wavelength carrying the amplification information, and couples these modulated wavelengths into an optical fiber to convey the amount of applied amplification along with the amplified uplink signal to the central unit. The central unit uses the amplification information to compensate the gain applied to a digital version of the uplink signal. In so doing, the solution presented herein compensates for the dynamic range limitations of the distributed unit.

One exemplary embodiment comprises a method, implemented by a distributed unit, of compensating for dynamic range limitations of a distributed wireless system comprising a central unit coupled to the distributed unit via at least one optical fiber. The method comprises determining a first value indicative of an amplification level for a variable gain amplifier from a relationship between an output of the variable gain amplifier and a second value derived based on a dynamic range of the distributed unit. The method further comprises controlling the variable gain amplifier using the amplification level indicated by the determined first value to amplify an uplink signal to generate an amplified uplink signal. The method further comprises converting the amplified uplink signal to an analog optical uplink signal on a first wavelength dedicated to uplink signals, and encoding the determined first value indicative of the amplification level onto an analog optical state signal on a second wavelength dedicated to state signals. The method further comprises optically coupling the analog optical uplink signal and the analog optical state signal into an optical fiber for transmission to the central unit.

In exemplary embodiments, the second value is derived based on the dynamic range of the distributed unit and at least one of a saturation level of an electrical-to-optical converter in the distributed unit and one or more characteristics of the optical fiber.

In exemplary embodiments, the determining the first value indicative of the amplification level for the variable gain amplifier comprises selecting the first value from a plurality of different first values according to the relationship between the output of the variable gain amplifier and the second value, where each of the plurality of different first values corresponding to a different one of a plurality of amplification levels.

In exemplary embodiments, the number of the plurality of different first values represents the number of amplification levels for the range between a system dynamic range requirement and the dynamic range of the distributed unit.

In exemplary embodiments, the encoding the determined first value indicative of the amplification level comprises encoding the determined first value to a corresponding one of a plurality of different M-ary Pulse-Amplitude Modulation (PAM) levels, where each PAM level representing a different one of the plurality of amplification levels.

In exemplary embodiments, the encoding the determined first value indicative of the amplification level comprises encoding the determined first value to a corresponding one of a plurality of different return-to-zero pulses, each of the plurality of different return-to-zero pulses representing a different one of the plurality of amplification levels.

In exemplary embodiments, the encoding the determined first value indicative of the amplification level comprises driving a first laser with the determined first value to encode the determined first value onto the analog optical state signal on the second wavelength.

In exemplary embodiments, the converting the amplified uplink signal comprises driving a second laser with the amplified uplink signal to generate the analog optical uplink signal on the first wavelength.

In exemplary embodiments, the converting the amplified uplink signal comprises extracting the first wavelength from a downlink signal, and modulating the amplified uplink signal onto the first wavelength using an intensity modulator to generate the analog optical uplink signal.

In exemplary embodiments, the analog optical state signal on the second wavelength does not overlap the analog optical uplink signal on the first wavelength in the frequency domain when coupled into the optical fiber.

In exemplary embodiments, the variable gain amplifier comprises a fixed amplifier coupled to a variable attenuator, and the controlling the variable gain amplifier comprises controlling the variable attenuator using the amplification level indicated by the determined first value.

In exemplary embodiments, the determining the first value indicative of the amplification level for the variable gain amplifier comprises continuously comparing the output of the variable gain amplifier to the second value, and continuously determining the first value indicative of the amplification level from a result of the comparison.

In exemplary embodiments, the optical fiber comprises a unidirectional fiber configured to convey the coupled analog optical uplink and state signals from the distributed unit to the central unit.

In exemplary embodiments, the optical fiber comprises a bidirectional fiber configured to convey the coupled analog optical uplink and state signals in a first direction from the distributed unit to the central unit, and to convey analog optical downlink signals in a second direction from the central unit to the distributed unit.

One exemplary embodiment comprises a distributed unit for use in a distributed wireless system comprising a central unit coupled to each of one or more distributed units via at least one corresponding optical fiber. The distributed unit comprises a variable gain amplifier, a gain control circuit, a first electrical-to-optical converter, a second electrical-to-optical converter, and an optical coupler. The gain control circuit is configured to determine a first value indicative of an amplification level for the variable gain amplifier from a relationship between an output of the variable gain amplifier and a second value derived based on a dynamic range of the distributed unit. The amplification level indicated by the determined first value controls the variable gain amplifier to amplify an uplink signal to generate an amplified uplink signal. The first electrical-to- optical converter is configured to convert the amplified uplink signal to an analog optical uplink signal on a first wavelength dedicated to uplink signals. The second electrical-to-optical converter configured to encode the determined first value indicative of the amplification level onto an analog optical state signal on a second wavelength dedicated to state signals. The optical coupler configured to optically couple the analog optical uplink signal and the analog optical state signal into an optical fiber for transmission to the central unit.

In exemplary embodiments, the gain control circuit determines the first value indicative of the amplification level for the variable gain amplifier by selecting the first value from a plurality of different first values according to the relationship between the output of the variable gain amplifier and the second value, each of the plurality of different first values corresponding to a different one of a plurality of amplification levels. In exemplary embodiments, the number of the plurality of different first values represents the number of amplification levels for a range between a system dynamic range requirement and the dynamic range of the distributed unit.

In exemplary embodiments, the second electrical-to-optical converter is configured to encode the determined first value indicative of the amplification level by encoding the determined first value to a corresponding one of a plurality of different M-ary Pulse-Amplitude Modulation (PAM) levels, each PAM level representing a different one of the plurality of amplification levels.

In exemplary embodiments, the second electrical-to-optical converter is configured to encode the determined first value indicative of the amplification level by encoding the determined first value to a corresponding one of a plurality of different return-to-zero pulses, each of the plurality of different return-to-zero pulses representing a different one of the plurality of amplification levels.

In exemplary embodiments, the second electrical-to-optical converter comprises a first directly modulated laser, and wherein the determined first value indicative of the amplification level drives the first directly modulated laser to generate the analog optical state signal on the second wavelength.

In exemplary embodiments, the first electrical-to-optical converter comprises a second directly modulated laser, and the amplified uplink signal drives the second directly modulated laser to generate the analog optical uplink signal on the first wavelength.

In exemplary embodiments, the first electrical-to-optical converter comprises an intensity modulator configured to receive the first wavelength extracted from a downlink signal path of the distributed unit, and modulate the amplified uplink signal onto the first wavelength to generate the analog optical uplink signal on the first wavelength.

In exemplary embodiments, the variable gain amplifier comprises a fixed amplifier and a variable attenuator. The fixed amplifier is configured to amplify the uplink signal by a fixed gain to generate a first amplified signal. The variable attenuator is controlled by the amplification level indicated by the determined first value to attenuate the first amplified signal to generate the amplified uplink signal.

In exemplary embodiments, the gain control circuit determines the first value indicative of the amplification level for the variable gain amplifier by continuously comparing the output of the variable gain amplifier to the second value, and continuously determining the first value indicative of the amplification level from a result of the comparison. In exemplary embodiments, the optical fiber comprises a unidirectional fiber configured to convey the coupled analog optical uplink and state signals from the distributed unit to the central unit.

One exemplary embodiment comprises method, implemented by a central unit, of compensating for dynamic range limitations of a distributed wireless system comprising the central unit coupled to at least one distributed unit via at least one optical fiber. For each of the at least one distributed units, the method comprises receiving, from the corresponding distributed unit via a corresponding optical fiber, an analog optical signal comprising an optical uplink signal on a first wavelength dedicated to uplink signals and an optical state signal on a second wavelength dedicated to state signals, the optical state signal representing a first value indicative of an amplification level applied to a variable gain amplifier of the corresponding distributed unit. The method further comprises, for each of the at least one distributed units, decoupling the optical uplink signal from the optical state signal, converting the optical uplink signal to a digital uplink signal, and converting the optical state signal to a digital state signal. The method further comprises, for each of the at least one distributed units, compensating a gain of the digital uplink signal using the digital state signal to compensate for dynamic range limitations of the corresponding distributed unit.

In exemplary embodiments, the digital state signal comprises an M-ary Pulse-Amplitude Modulation (PAM) signal comprising one or more of a plurality of different PAM levels, each PAM level representing a different amplification level for the variable gain amplifier of the corresponding distributed unit.

In exemplary embodiments, the digital state signal comprises a pulse signal comprising one or more of a plurality of different return-to-zero pulses, each return-to-zero pulse representing a different amplification level for the variable gain amplifier of the corresponding distributed unit.

In exemplary embodiments, the compensating the gain of the digital uplink signal produces a digitally compensated uplink signal, where the method further comprises demodulating the digitally compensated uplink signal.

In exemplary embodiments, the converting the optical uplink signal comprises detecting the optical uplink signal using a first photodetector to generate an electrical uplink signal, mixing the electrical uplink signal with a local oscillator frequency to downconvert the electrical uplink signal to a baseband uplink signal, and converting the baseband uplink signal to the digital uplink signal using a first analog-to-digital converter. In exemplary embodiments, the converting the optical state signal comprises detecting the optical state signal using a second photodetector to generate an electrical state signal, and converting the electrical state signal to the digital state signal using a second analog-to-digital converter.

In exemplary embodiments, the method further comprises, for each of the at least one distributed units, filtering the digital uplink signal to reduce an amplitude of one or more interfering signals, wherein the compensating the gain of the digital uplink signal comprises compensating the gain of the filtered digital uplink signal using the digital state signal to compensate for the dynamic range limitations of the corresponding distributed unit.

One exemplary embodiment comprises central unit for use in a distributed wireless system comprising the central unit coupled to one or more distributed units via at least one optical fiber. The central unit comprises for each corresponding distributed unit, a demultiplexer, a first conversion circuit, a second conversion circuit, and a digital compensation processing circuit. The demultiplexer is configured to receive, from the corresponding distributed unit via a corresponding optical fiber, an analog optical signal comprising an optical uplink signal on a first wavelength dedicated to uplink signals and an optical state signal on a second wavelength dedicated to state signals. The optical state signal represents a first value indicative of an amplification level applied to a variable gain amplifier of the corresponding distributed unit. The demultiplexer is further configured to decouple the optical uplink signal from the optical state signal. The first conversion is circuit configured to convert the optical uplink signal to a digital uplink signal. The second conversion circuit is configured to convert the optical state signal to a digital state signal. The digital compensation processing circuit is configured to compensate a gain of the digital uplink signal using the digital state signal to compensate for dynamic range limitations of the corresponding distributed unit.

In exemplary embodiments, the digital state signal comprises an M-ary Pulse-Amplitude Modulation (PAM) signal comprising one or more of a plurality of PAM levels, each PAM level representing a different amplification level for the variable gain amplifier of the corresponding distributed unit (200).

In exemplary embodiments, the digital state signal comprises a pulse signal comprising one or more of a plurality of return-to-zero pulses, each of the plurality of different return-to-zero pulses representing a different amplification level for the variable gain amplifier of the corresponding distributed unit.

In exemplary embodiments, the digital compensation processing circuit produces a digitally compensated uplink signal, the central unit further comprising a demodulator configured to demodulate the digitally compensated uplink signal.

In exemplary embodiments, the first conversion circuit comprises a first photodetector, a mixer, and a first analog-to-digital converter. The first photodetector is configured to convert the optical uplink signal to an electrical uplink signal. The mixer is configured to mix the electrical uplink signal with a local oscillator frequency to downconvert the analog uplink signal to a baseband uplink signal. The first analog-to-digital converter is configured to convert the baseband uplink signal to the digital uplink signal.

In exemplary embodiments, the second conversion circuit comprises a second photodetector and a second analog-to-digital converter. The second photodetector is configured to convert the optical state signal to an electrical state signal. The second analog-to- digital converter is configured to convert the electrical state signal to the digital state signal.

In exemplary embodiments, the central unit further comprises a digital filter configured to filter the digital uplink signal to reduce an amplitude of one or more interfering signals. The digital compensation processing circuit compensates the gain of the digital uplink signal by compensating the gain of the filtered digital uplink signal using the digital state signal to compensate for the dynamic range limitations of the distributed unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an exemplary dynamic range for a DU.

Figure 2 shows a block diagram of a distributed wireless system according to exemplary embodiments of the solution presented herein.

Figures 3(A)-3(B) show a block diagram of fiber connections for the a distributed wireless system according to exemplary embodiments of the solution presented herein.

Figure 4 shows dynamic range compensation method for a DU according to exemplary embodiments of the solution presented herein.

Figure 5 shows a block diagram of a DU according to exemplary embodiments of the solution presented herein. Figure 6 shows a block diagram of another DU according to exemplary embodiments of the solution presented herein.

Figure 7 shows a block diagram of a VGA according to exemplary embodiments of the solution presented herein.

Figures 8(A)-8(B) show exemplary formats for values indicating amplification levels according to exemplary embodiments of the solution presented herein.

Figures 9(A)-9(B) demonstrate the effect of the automatic gain control according to exemplary embodiments of the solution presented herein.

Figure 10 shows a dynamic range compensation method for a CU according to exemplary embodiments of the solution presented herein.

Figure 11 shows a block diagram of a CU according to exemplary embodiments of the solution presented herein.

Figures 12(A)-12(C) show an exemplary signal progression through the CU according to exemplary embodiments of the solution presented herein.

DETAILED DESCRIPTION

Figure 2 shows an exemplary distributed wireless system 10 comprising a plurality of Distributed Units (DUs) 200 connected to a Central Unit (CU) 100, each via an optical fiber connection 50. The CU 100 provides downlink signals to the corresponding DU 200 to enable the DU 200 to wirelessly transmit to a corresponding remote device. DU 200 also receives wireless uplink signals from one or more remote devices, and processes and conveys these uplink signals to the CU 100 for further transmission and/or processing. The optical fiber connection 50 may comprise one or more optical fibers. Figure 3(A) shows one example, where the optical fiber connection 50 comprises two unidirectional fibers, e.g., a downlink unidirectional fiber 51 conveying signals from the CU 100 to the DU and an uplink unidirectional fiber 52 conveying signals from the DU 200 to the CU 100. In other exemplary embodiments, the optical fiber connection 50 may be achieved using a bidirectional fiber. For example, as shown in Figure 3(B), the fiber connection 50 may comprise a bidirectional fiber 53, circulator 54, downlink fiber 54, and uplink fiber 56. In this example, downlink signals travel from the CU 100 through fiber 53 to the circulator 54, which couples the downlink signals to the downlink fiber 55 for reception at the DU 200, while fiber 56 conveys uplink signals from the DU 200 to the circulator 54, which couples the uplink signals to the bidirectional fiber 53 for transmission to the CU 100. For longer distances between the CU 100 and DU 200, the dual fiber solution of Figure 3(A) may be more suitable because it avoids Rayleigh scattering and optical circulator leakage that may occur with the bidirectional fiber implementation.

System-level dynamic range requirements for the distributed wireless system 10 may exceed the dynamic range capabilities of the DUs 200. The solution presented herein compensates for this dynamic range deficiency of the DUs 200 so that the system dynamic range requirements are still met. More particularly, the solution presented herein enhances the dynamic range of uplink fronthaul (FH) optical analog interfaces by using Automatic Gain Control (AGC) in the DUs 200 and forwarding information regarding the applied AGC to the CU 100, e.g., by introducing a feedback channel (e.g., a dedicated AGC modulated wavelength) from the DU 200 (where the AGC resides) to the CU 100 (where the digital signal processing (DSP) functions are performed). In so doing, the solution presented here enables the uplink optical analog interfaces to fulfill the stringent system dynamic range requirements, e.g., as specified in TS 38.104

Figure 4 shows an exemplary dynamic range compensation method 300 implemented by a DU 200 of the distributed wireless system 10 according to the solution presented herein, with reference to the DU 200 of Figure 5. The method 300 comprises determining a first value indicative of an amplification level for a Variable Gain Amplifier (VGA) 222 from a relationship between an output of the VGA 222 and a second value derived based on a dynamic range of the DU 200 (block 310). The method 300 further comprises controlling the VGA 222 using the amplification level indicated by the determined first value to amplify an uplink signal (block 320). The method 300 further comprises converting the amplified uplink signal to an analog optical uplink signal

on a first wavelength J_JL dedicated to uplink signals (block 330), and encoding the determined first value indicative of the amplification level onto an analog optical state signal S_A on a second wavelength _AGC dedicated to state signals (block 340). The method 300 further comprises optically coupling the analog optical uplink signal A'_{/; ;} and the analog optical state signal S_A into an optical fiber 52, 53, 54, 56 for transmission to the CU 100 (block 350). Because both the analog optical uplink signal

and the analog optical state signal S are transmitted by the same source (e.g., DU 100) through the same fiber connection 50, the synchronization between the analog optical uplink signal

and the analog optical state signal S_z is maintained. Further, because the length of a typical fiber connection 50 between a CU 100 and DU 200 is no more than 15 km, chromatic dispersion (CD) is negligible and will not impact the synchronization of the signals.

It will be appreciated that each optical signal coupled into the fiber 52, 53, 54, 56 is a modulated wavelength that carries the corresponding signal. For example, the optical uplink signal represents a modulated wavelength carrying the amplified uplink signal, and the optical state signal S_z represents a different modulated wavelength carrying the determined first value indicative of the amplification level. For simplicity, however, these modulated wavelengths are referred to herein as optical signals.

Figure 5 shows an exemplary DU 200 comprising a transceiver 210, an uplink path 220 and a downlink path 240, where the uplink path 220 is configured to implement the method 300 of Figure 4. The downlink path 240 comprises a photodetector 242, amplifier 244, and filter 246, collectively configured to generate a downlink RF signal for wireless transmission to a remote device (not shown). To that end, photodetector 242 detects an optical downlink signal to convert the optical downlink signal

to an electrical downlink signal. The amplifier 244 amplifies the electrical downlink signal, which is subsequently filtered in filter 246 to prepare it for wireless transmission to a remote device via transceiver 210.

The uplink path 220 comprises the VGA 222, an AGO circuit 224, electrical-to-optical converters, e.g., Directly Modulated Lasers (DMLs) 226, 228, and optical coupler 230. AGC 224 determines a first value indicative of an amplification level for the VGA 222 from a relationship between an output of the VGA 222 and a second value derived based on a dynamic range of the DU 200, and uses the amplification level indicated by the determined first value to control the VGA 222 to amplify the uplink signal. DML 226 encodes the determined first value indicative of the amplification level in an analog optical state signal S by modulating the determined first value onto wavelength _AGC dedicated to amplifier control signals. DML 228 converts the amplified uplink signal to an analog optical uplink signal

by modulating the amplified uplink signal onto an uplink wavelength

dedicated to uplink signals. The optical coupler 230 multiplexes, or optically couples, the analog optical uplink signal A'_{/; ;} and the analog optical state signal S into the optical fiber 52, 53, 54, 56 for transmission to the CU 100. To prevent the analog optical state signal S from interfering with the analog optical uplink signal 5^ in the optical fiber 52, 53, 54, 56, the wavelengths onto which the corresponding signals are modulated are sufficiently separated so as to prevent the analog optical state signal S from overlapping the analog optical uplink signal

in the frequency domain when coupled into the optical fiber 52, 53, 54, 56.

While not required, the uplink path 220 may include additional circuitry, e.g., filter 232 and/or mixer 234 to further prepare the uplink signal for transmission to the CU 100. For example, filter 232 may filter out some of the noise in the uplink signal. Further, if the carrier frequency of the uplink signal received by transceiver 210 is higher than the bandwidth of the DML 228, mixer 234 may be included to downconvert the uplink signal, e.g., to an intermediate frequency within the bandwidth of the DML 228. When mixer 234 is included, it will be appreciated that the AGC 224 may use the output of the VGA 222 or the output of the mixer 234 (e.g., input to DML 228) to determine the first value indicative of the amplification level.

While Figure 5 shows a DML 228 for converting the amplified uplink signal to the analog optical uplink signal

, other electrical-to-optical converters may be used. For example, an Intensity Modulator (IM) 236, e.g., an electro-absorption modulator, may alternatively be used, as shown in Figure 6. In this exemplary embodiment, the CU 100 multiplexes an uplink wavelength with the downlink optical signal. The downlink path 240 includes a demultiplexer 248 to extract this uplink wavelength ^_JL from the incoming downlink optical signal. Exemplary demultiplexers include, but are not limited to, a narrow optical filter, a fiber Bragg grating in combination with an optical circulator, etc. IM 236 modulates the amplified uplink signal onto this extracted uplink wavelength ^_JL to generate the analog optical uplink signal 5^ . Because the CU 100 generates both the downlink and uplink optical wavelengths, the DU 200 of Figure 6 may be more resilient against temperature changes in the DU 200 as compared to the DU 200 of Figure 5.

The VGA 222 amplifies the uplink signal to facilitate the extraction of the uplink signal from the noise. In some exemplary embodiments, the VGA 222 comprises a variable amplifier with a controllable gain, where the AGC 224 controls the amplification of the uplink signal by controlling the amplification level of the variable amplifier. In some embodiments, such a variable amplifier may be achieved using a set of fixed gain amplifiers ranging from low to high amplification levels, where each fixed amplification corresponds to a specific one of the amplification levels. In other exemplary embodiments, as shown in Figure 7, VGA 222 may comprise a fixed gain amplifier 222A coupled to a variable attenuator 222B. In this embodiment, the fixed gain amplifier 222A provides the maximum gain desired for the VGA 222, and the AGC 224 controls the overall amplification provided by VGA 222 by controlling the attenuation applied to the fixed amplifier output. While Figure 7 shows a single variable attenuator 222B, it will be appreciated that the variable attenuator 222B may comprise a set of fixed value attenuators ranging from low to high attenuation values, where each fixed attenuation corresponds to an attenuation required to achieve a specific one of the amplification levels.

The VGA 222 and AGC 224 collectively work together to not only amplify the uplink signal, but also to ensure the amplified uplink signal does not exceed the dynamic range limitations of the DU 200. The dynamic range of the DU 200 is limited by the component in the DU 200 having the smallest dynamic range, which is generally the electrical-to-optical converter 228, 236. Thus, the AGC 224 controls the gain of the VGA 222 to prevent the amplified uplink signal from saturating the electrical-to-optical converter 228, 236. In some embodiments, the AGC 224 may also consider the power of the signal being coupled into the fiber when controlling the gain of the VGA 222 so as to prevent nonlinearities/distortions within the fiber.

To ensure the amplified uplink signal does not saturate the electrical-to-optical converter 228, 236, and thus does not exceed the dynamic range limitations of the DU 200, the AGC 224 measures the output of the VGA 222, e.g., by measuring an output power of the VGA 222, a magnitude of the output of the VGA 222, etc. As shown in Figures 5 and 6, this measurement is a measurement of the power or magnitude of the amplified uplink signal, and thus may alternatively be achieved by measuring the input to the electrical-to-optical converter 228, 236. AGC 224 then determines how the measured output relates to a second value derived from the dynamic range of the DU 200. From this relationship, the AGC 224 determines how to adjust the gain of the VGA 222. In one exemplary embodiment, the second value comprises a threshold where the AGC 224 determines the relationship between the measured output and the threshold by continuously comparing the VGA output to the threshold, where the AGC 224 then continuously determines the first value indicative of the amplification level for the VGA 222 from the results of the continuous comparisons.

The threshold used by the AGC 224 may be set to any second value just below the nominal dynamic range of the DU 200 (e.g., just below the dynamic range of the DML 228 or IM 236 in the DU 200). The second value or threshold may further be adjusted to prevent fiber non-linearities/distortions. In some embodiments, the AGC 224 may rely on multiple thresholds, e.g., a rising threshold for when the output is too high and needs to be reduced, and a falling threshold for when the output is too low and needs to be increased. Typically, the rising threshold is selected to be higher than the falling threshold to avoid noise amplification, i.e., AGC hysteresis (not shown).

The AGC 224 may use any number of techniques to determine the first value indicative of the amplification level for the VGA 222. For example, a number of amplification levels between the system dynamic range requirement and the dynamic range of the DU 200 may be defined for the VGA 222, where each amplification level is indicated by a different first value. In some embodiments, the difference between each amplification level is the same, whereas the difference between amplification levels for other embodiments may be different, e.g., to enable varying amplification changes. In any event, AGC 224 determines (or otherwise selects) one of these first values from the relationship between the VGA output and the threshold, and controls the VGA 222 according to the amplification level indicated by the determined first value to amplify the uplink signal by the corresponding amplification level.

It will be appreciated that the first values indicative of the amplification levels may be represented using any known format, including but not limited to M-ary Pulse Amplitude Modulation (PAM), Return-to-Zero (RZ) pulses, etc. Consider an example where there are four equally-spaced amplification levels defined between the system dynamic range requirement and the dynamic range of the DU 200, where each of these amplification levels is represented by one of four distinct first values. When M-ary PAM is used for this example, each of the four amplification levels may be represented using PAM4, which has four different two-bit values, each of which indicates a different amplification level. When RZ is used for this example, each of the four amplification levels may be represented using one of four different RZ pulses, each having a different amplitude. As shown in Figures 8(A)-8(B), the PAM4 (Figure 8(A)) or RZ pulse (Figure 8(B)) changes as the desired amplification level changes in response to the relationship between the measured VGA output and the threshold. Both formats have benefits and drawbacks. For example, using PAM provides a more reliable solution because the first value indicative of the amplification level is always active/available. However, PAM also has a higher power consumption relative to RZ because the PAM value indicative of the amplification level is always available/active.

Figures 9(A)-9(B) demonstrate the effect of the VGA 222 on the uplink signal when controlled by the AGO 224 as disclosed herein. Figure 9(A) shows an exemplary uplink signal including a wanted signal and a dominant interferer. When the VGA 222 amplifies the uplink signal, the VGA 222 necessarily amplifies both the wanted signal and the interferer, which will eventually saturate the electrical-to-optical converter 228, 236. When the threshold is chosen to prevent the amplified uplink signal from exceeding the dynamic range limitations of the DU 200, the AGC 224 prevents this saturation by controlling the gain of the VGA 222 to produce an amplified uplink signal, e.g., as shown in Figure 9(B), where SO, S1 , S2, and S3 represent the first values indicative of the amplification level for the VGA 222 in this example. As the amplitude of the uplink signal increases, e.g., due to the interferer, AGC 224 selects a lower amplification level each time the measured output hits the threshold, and then after passing the peak of the interferer, the AGC 224 selects a higher amplification level each time the measured output hits the threshold (which in some cases is a different threshold). Figure 9(B) shows the envelope of the signal after AGC assuming four AGC first values, each representing the same change in attenuation/amplification between states. The signal envelope has discontinuities, as shown in Figure 9(B), due to change in the AGC amplification levels between SO and S3, e.g., SO to S1 . After amplification of the uplink signal as dictated by the AGC 224, the amplified uplink signal drives the DML 228 or IM 236 to send it to the CU 100 for further digital signal processing (DSP). Depending on the change rate of the signal that triggers the AGC mechanism, a multi-state change may be allowed to reduce the number of signal discontinuities, e.g., directly from SO to S2 instead of stepping first from SO to S1 , and then S1 to S2. In any event, by conveying the first values indicative of the applied amplification levels (and how these amplification levels change with time) to the CU 100, CU 100 is able to compensate for these amplification changes to recreate the original uplink signal of the corresponding DU 200, as discussed further herein.

Figure 10 shows an exemplary method 400 implemented for each of the DUs 200 by the CU 100 of compensating for the dynamic range limitations of the distributed wireless system 10. The method 400 comprises receiving, from the corresponding DU 200 via a corresponding optical fiber connection 50, an analog optical signal comprising the optical uplink signal

and the optical state signal S (block 410), where the optical state signal S represents a first value indicative of an amplification level applied to the VGA 222 of the corresponding DU 200. The method 400 further comprises decoupling the optical uplink signal 5^ from the optical state signal (block 420), converting the optical uplink signal 5^ to a digital uplink signal (block 430), and converting the optical state signal S to a digital state signal (block 440). Further, the method 400 comprises compensating a gain of the digital uplink signal using the digital state signal to compensate for dynamic range limitations of the corresponding DU 200 (block 450).

Figure 11 shows an exemplary CU 100 comprising downlink path 110 and an uplink path 120, where the uplink path 120 is configured to execute the method 400 of Figure 11. It will be appreciated that the CU 100 will comprise a downlink path 110 and an uplink path 120 for each of the DUs 200 connected to the CU 100. The downlink path 110 comprises modulation and downlink signal generation circuitry 112 and an amplifier 114. The modulation and downlink signal generation circuitry 112 generates a downlink signal, and amplifier 114 amplifies the downlink signal for transmission to the corresponding DU 200 via the fiber connection 50. When the corresponding DU 200 uses an IM 236 to implement the solution presented herein, the generated downlink signal conveyed to the DU 200 via the fiber connection 50 also includes the unmodulated uplink wavelength _UL.

The uplink path 120 of CU 100 comprises a demultiplexer 122, conversion circuits 124, 130, and a digital compensation processing circuit 140. Demultiplexer 122 decouples the optical uplink signal

from the optical state signal S . A first conversion circuit 130 converts the optical uplink signal

to a digital uplink signal, and a second conversion circuit 124 converts the optical state signal

to a digital state signal. The digital compensation processing circuit 140 compensates a gain of the digital uplink signal using the digital state signal to compensate for dynamic range limitations of the corresponding DU 200, as discussed further herein. While not critical to the solution presented herein, CU 100 further comprises a demodulator 142 that demodulates the digitally compensated uplink signal. Further, while not required, the digital uplink signal may first be filtered in filter 148 to remove or reduce an amplitude of one or more out-of-band signals from the digital signal before compensation is applied by the digital compensation processing circuit 140. It will be appreciated that one or more of the digital components of the CU, e.g., digital compensation processing circuit 140, demodulator 142, and/or filter 148, may be implemented as part of a digital signal processor 150, as shown in Figure 11 .

The first conversion circuit 130 comprises a photodetector 132, mixer 134, and Analog- to-Digital Converter (ADC) 136. Photodetector 132 converts the optical uplink signal

to an electrical uplink signal. Mixer 134 mixes the electrical uplink signal with a local oscillator frequency to downconvert the analog uplink signal to a baseband uplink signal, and the ADC 136 converts the baseband uplink signal to the digital uplink signal. The first conversion circuit 130 may also include a filter 138 before analog-to-digital conversion to prevent aliasing components from being sampled. The second conversion circuit 124 comprises a photodetector 126 and an ADC 128.

The photodetector 126 converts the optical state signal S to an electrical state signal, and the ADC 128 converts the electrical state signal to the digital state signal.

Figures 12(A)-12(C) show an example of the signal processing progression through the CU 100 according to one exemplary embodiment. Figure 12(A) shows the uplink signal envelope before filtering, including both the wanted signal and an interferer, which as expected, looks similar to the signal envelope of Figure 9(B). Figure 12(B) shows the wanted signal after filtering the interferer, e.g., using filter 148, where the different first values indicative of the different amplification levels triggered by the AGC are evident. The upper part of Figure 12(B) also shows the AGC first value information, e.g., depicted as PAM4 signal corresponding to the AGC control applied in Figure 9(B). Figure 12(C) shows the results of the digital gain compensation applied by the digital compensation processing circuit 140, which used the first values indicated in the optical state signal S to compensate the different gain levels of the uplink signal, thus providing a constant gain to the output signal. Demodulator 142 may then use this output to retrieve the wanted signal.

The solution presented herein provides several advantages over conventional optical fronthaul solutions. First, the solution presented herein allows the CU 100 to correctly recover the original signal, without distortions, over a wider dynamic range than would be achievable without the AGC. Additional advantages include:

• Cost-effective: The solution presented herein addresses the current dynamic range limitation of Radio-over-Fiber (RoF) systems while also providing wide-scale commercialization, in addition to the energy consumption and antenna weight advantages.

• Increased bandwidth: Because the proposed solution use fronthaul optical analog interfaces, the baseband bandwidth that each DU 200 can handle may be several GHz, which is in contrast to digital interfaces and digital transceivers where processing is limited to only hundreds of MHz.

• Suitable for scenarios having wideband signal transmission, small form factor, and/or low-power consumption requirements because the DUs 200 are not required to have downlink RF up-conversion or downlink/uplink digital signal processing, and because the electronics and photonics can be highly integrated.

Further, the solution presented herein provides the advantages of the 3GPP functional split option 8, which allows to separate the physical layer (PHY) and the RF analog front-end (AFE), as discussed in 3GPP, TR 38.801 v14.0.0 (“Radio Access Architecture and Interfaces”).

Further, the use of an analog link between the CU 100 and the DU 200 also causes the Digital Front-End (DFE) to move to the CU 100, which provides additional benefits in terms of fronthaul bandwidth. All the benefit from split option 8 can be also applied here, where analog interfaces are used:

• To shared resources facilitating maintenance and enabling network function virtualization (NFV) and software-defined networking (SDN)

• To isolate the RF components from updates to the PHY, which may improve RF/PHY scalability

• Allows reuse of the RF components to serve PHY layers of different radio access technologies, e.g., single-carrier, multi-carrier waveforms

• Allows pooling of PHY resources, which may enable a more cost-efficient dimensioning of the PHY layer

The apparatuses described herein may perform the methods herein, and any other processing, by implementing any functional means, modules, units, or circuitry. In one embodiment, for example, the apparatuses comprise respective circuits or circuitry configured to perform the steps shown in the method figures. The circuits or circuitry in this regard may comprise circuits dedicated to performing certain functional processing and/or one or more microprocessors in conjunction with memory. For example, some of the circuitry may include one or more microprocessor or microcontrollers, as well as other digital hardware, which may include digital signal processors (DSPs), special-purpose digital logic, and the like. The processing circuitry may be configured to execute program code stored in memory, which may include one or several types of memory such as read-only memory (ROM), random-access memory, cache memory, flash memory devices, optical storage devices, etc. Program code stored in memory may include program instructions for executing one or more telecommunications and/or data communications protocols as well as instructions for carrying out one or more of the techniques described herein, in several embodiments. In embodiments that employ memory, the memory stores program code that, when executed by the one or more processors, carries out the techniques described herein. Thus, various apparatus elements disclosed herein, may implement any functional means, modules, units, or circuitry, and may be embodied in hardware and/or in software (including firmware, resident software, microcode, etc.) executed on a controller or processor, including an application specific integrated circuit (ASIC).

The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

Claims

1 . A method (300), implemented by a distributed unit (200), of compensating for dynamic range limitations of a distributed wireless system comprising a central unit (100) coupled to the distributed unit (200) via at least one optical fiber (50-56), the method (300) comprising: determining (310) a first value indicative of an amplification level for a variable gain amplifier (222) from a relationship between an output of the variable gain amplifier (222) and a second value derived based on a dynamic range of the distributed unit; controlling (320) the variable gain amplifier (222) using the amplification level indicated by the determined first value to amplify an uplink signal to generate an amplified uplink signal; converting (330) the amplified uplink signal to an analog optical uplink signal

) on a first wavelength (^_L) dedicated to uplink signals; encoding (340) the determined first value indicative of the amplification level onto an analog optical state signal (S ) on a second wavelength (2 _GC) dedicated to state signals; and optically coupling (350) the analog optical uplink signal

) and the analog optical state signal (^ ) into an optical fiber (52, 53, 54, 56) for transmission to the central unit (100).

2. The method (300) of claim 1 wherein the second value is derived based on the dynamic range of the distributed unit and at least one of a saturation level of an electrical-to-optical converter (228, 236) in the distributed unit (200) and one or more characteristics of the optical fiber (52, 53, 54, 56).

3. The method (300) of any one of claims 1-2 wherein the determining (310) the first value indicative of the amplification level for the variable gain amplifier (222) comprises selecting the first value from a plurality of different first values according to the relationship between the output of the variable gain amplifier (222) and the second value, each of the plurality of different first values corresponding to a different one of a plurality of amplification levels.

4. The method (300) of claim 3 wherein the number of the plurality of different first values represents the number of amplification levels for the range between a system dynamic range requirement and the dynamic range of the distributed unit (200).

5. The method (300) of claim 3 wherein the encoding (340) the determined first value indicative of the amplification level comprises encoding the determined first value to a corresponding one of a plurality of different M-ary Pulse-Amplitude Modulation (PAM) levels, each PAM level representing a different one of the plurality of amplification levels.

6. The method (300) of claim 4 wherein the encoding (340) the determined first value indicative of the amplification level comprises encoding the determined first value to a corresponding one of a plurality of different return-to-zero pulses, each of the plurality of different return-to-zero pulses representing a different one of the plurality of amplification levels.

7. The method (300) of any one of claims 1-6 wherein the encoding (340) the determined first value indicative of the amplification level comprises driving a first laser (226) with the determined first value to encode the determined first value onto the analog optical state signal (S^ ) on the second wavelength (2 _GC).

8. The method (300) of claim 7 wherein the converting (330) the amplified uplink signal comprises driving a second laser (228) with the amplified uplink signal to generate the analog optical uplink signal (S^ ) on the first wavelength (^_L).

9. The method (300) of claim 7 wherein the converting (330) the amplified uplink signal comprises: extracting the first wavelength

from a downlink signal; and modulating the amplified uplink signal onto the first wavelength (^_L) using an intensity modulator (236) to generate the analog optical uplink signal

).

10. The method (300) of any one of claims 1 -9 wherein the analog optical state signal (S^ ) does not overlap the analog optical uplink signal (5^ ) in the frequency domain when coupled into the optical fiber (52, 53, 54, 56).

11. The method (300) of any one of claims 1-10 wherein: the variable gain amplifier (222) comprises a fixed amplifier (222 ) coupled to a variable attenuator (222_B); and the controlling (320) the variable gain amplifier comprises controlling the variable attenuator (222B) using the amplification level indicated by the determined first value.

12. The method (300) of any one of claims 1-11 wherein the determining (310) the first value indicative of the amplification level for the variable gain amplifier (222) comprises: continuously comparing the output of the variable gain amplifier (222) to the second value; and continuously determining the first value indicative of the amplification level from a result of the comparison.

13. The method (300) of any one of claims 1-12 wherein the optical fiber (52, 53, 54, 56) comprises a unidirectional fiber (52) configured to convey the coupled analog optical uplink and state signals from the distributed unit (200) to the central unit (100).

14. The method (300) of any one of claims 1-12 wherein the optical fiber (52, 53, 54, 56) comprises a bidirectional fiber (53, 55, 56) configured to convey the coupled analog optical uplink and state signals in a first direction from the distributed unit (200) to the central unit (100), and to convey analog optical downlink signals in a second direction from the central unit (100) to the distributed unit (200).

15. A distributed unit (200) for use in a distributed wireless system (10) comprising a central unit (100) coupled to each of one or more distributed units (200) via at least one corresponding optical fiber (50), the distributed unit (200) comprising: a variable gain amplifier (222); a gain control circuit (224) configured to determine a first value indicative of an amplification level for the variable gain amplifier (222) from a relationship between an output of the variable gain amplifier (222) and a second value derived based on a dynamic range of the distributed unit; wherein the amplification level indicated by the determined first value controls the variable gain amplifier (222) to amplify an uplink signal to generate an amplified uplink signal; a first electrical-to-optical converter (228, 236) configured to convert the amplified uplink signal to an analog optical uplink signal (S^ ) on a first wavelength (^_L) dedicated to uplink signals; a second electrical-to-optical converter (226) configured to encode the determined first value indicative of the amplification level onto an analog optical state signal (^ ) on a second wavelength (2 _GC) dedicated to state signals; and an optical coupler (230) configured to optically couple the analog optical uplink signal

16. The distributed unit (200) of claim 15 wherein the second value is derived based on the dynamic range of the distributed unit and at least one of a saturation level of an electrical-to- optical converter (228, 236) in the distributed unit (200) and one or more characteristics of the optical fiber (52, 53, 54, 56).

17. The distributed unit (200) of any one of claims 15-16 wherein the gain control circuit (224) determines the first value indicative of the amplification level for the variable gain amplifier (222) by selecting the v first alue from a plurality of different first values according to the relationship between the output of the variable gain amplifier (222) and the second value, each of the plurality of different first values corresponding to a different one of a plurality of amplification levels.

18. The distributed unit (200) of claim 17 wherein the number of the plurality of different first values represents the number of amplification levels for a range between a system dynamic range requirement and the dynamic range of the distributed unit (200).

19. The distributed unit (200) of claim 17 wherein the second electrical-to-optical converter (226) is configured to encode the determined first value indicative of the amplification level by encoding the determined first value to a corresponding one of a plurality of different M-ary Pulse-Amplitude Modulation (PAM) levels, each PAM level representing a different one of the plurality of amplification levels.

20. The distributed unit (200) of claim 17 wherein the second electrical-to-optical converter (226) is configured to encode the determined first value indicative of the amplification level by encoding the determined first value to a corresponding one of a plurality of different return-to- zero pulses, each of the plurality of different return-to-zero pulses representing a different one of the plurality of amplification levels.

21 . The distributed unit (200) of any one of claims 15-20 wherein the second electrical-to- optical converter (226) comprises a first directly modulated laser (226), and wherein the determined first value indicative of the amplification level drives the first directly modulated laser (226) to generate the analog optical state signal

) on the second wavelength (2 _GC).

22. The distributed unit (200) of claim 21 wherein the first electrical-to-optical converter (228, 236) comprises a second directly modulated laser (228), and wherein the amplified uplink signal drives the second directly modulated laser (228) to generate the analog optical uplink signal (S^ ) on the first wavelength

21

23. The distributed unit (200) of claim 21 wherein the first electrical-to-optical converter (228, 236) comprises an intensity modulator (236) configured to: receive the first wavelength (^_L) extracted from a downlink signal path of the distributed unit (200); and modulate the amplified uplink signal onto the first wavelength ( ^_{/ Y} ) to generate the analog optical uplink signal (S^ ) on the first wavelength (^_L).

24. The distributed unit (200) of any one of claims 15-23 wherein the analog optical state signal ) does not overlap the analog optical uplink signal

) in the frequency domain when coupled into the optical fiber (52, 53, 54, 56).

25. The distributed unit (200) of any one of claims 15-24 wherein the variable gain amplifier (222) comprises: a fixed amplifier (222_A) configured to amplify the uplink signal by a fixed gain to generate a first amplified signal; and a variable attenuator (222_B) controlled by the amplification level indicated by the determined first value to attenuate the first amplified signal to generate the amplified uplink signal.

26. The distributed unit (200) of any one of claims 15-25 wherein the gain control circuit (224) determines the first value indicative of the amplification level for the variable gain amplifier (222) by: continuously comparing the output of the variable gain amplifier (222) to the second value; and continuously determining the first value indicative of the amplification level from a result of the comparison.

27. The distributed unit (200) of any one of claims 15-26 wherein the optical fiber (52, 53, 54, 56) comprises a unidirectional fiber (52) configured to convey the coupled analog optical uplink and state signals from the distributed unit (200) to the central unit (100).

28. The distributed unit (200) of any one of claims 15-26 wherein the optical fiber (52, 53, 54, 56) comprises a bidirectional fiber (53, 55, 56) configured to convey the coupled analog optical uplink and state signals in a first direction from the distributed unit (200) to the central unit (100), and to convey analog optical downlink signals in a second direction from the central unit (100) to the distributed unit (200).

22

29. A method (400), implemented by a central unit (100), of compensating for dynamic range limitations of a distributed wireless system (10) comprising the central unit (100) coupled to at least one distributed unit (200) via at least one optical fiber (50-56), the method (400) comprising, for each of the at least one distributed units (200): receiving (410), from the corresponding distributed unit (200) via a corresponding optical fiber (52, 53, 54, 56), an analog optical signal comprising an optical uplink signal (5^ ) on a first wavelength (^_L) dedicated to uplink signals and an optical state signal (^ ) on a second wavelength (2 _GC) dedicated to state signals, the optical state signal

representing a first value indicative of an amplification level applied to a variable gain amplifier (222) of the corresponding distributed unit (200); decoupling (420) the optical uplink signal

) from the optical state signal (^ ); converting (430) the optical uplink signal (A'_z ) to a digital uplink signal; converting (440) the optical state signal (S ) to a digital state signal; and compensating (450) a gain of the digital uplink signal using the digital state signal to compensate for dynamic range limitations of the corresponding distributed unit (200).

30. The method (400) of claim 29 wherein: the digital state signal comprises an M-ary Pulse-Amplitude Modulation (PAM) signal comprising one or more of a plurality of different PAM levels; and each PAM level representing a different amplification level for the variable gain amplifier (222) of the corresponding distributed unit (200).

31 . The method (400) of claim 29 wherein: the digital state signal comprises a pulse signal comprising one or more of a plurality of different return-to-zero pulses; and each return-to-zero pulse representing a different amplification level for the variable gain amplifier (222) of the corresponding distributed unit (200).

32. The method (400) of any one of claims 29-31 wherein the compensating (450) the gain of the digital uplink signal produces a digitally compensated uplink signal, the method further comprising demodulating the digitally compensated uplink signal.

33. The method (400) of any one of claims 29-32 wherein the converting (430) the optical uplink signal

) comprises:

23 detecting the optical uplink signal ) using a first photodetector (132) to generate an electrical uplink signal; mixing the electrical uplink signal with a local oscillator frequency to downconvert the electrical uplink signal to a baseband uplink signal; and converting the baseband uplink signal to the digital uplink signal using a first analog-to- digital converter (134).

34. The method (400) of any one of claims 29-33 wherein the converting (440) the optical state signal (^ ) comprises: detecting the optical state signal (^ ) using a second photodetector (126) to generate an electrical state signal; and converting the electrical state signal to the digital state signal using a second analog-to- digital converter (128).

35. The method (400) of any one of claims 29-34 further comprising filtering the digital uplink signal to reduce an amplitude of one or more interfering signals, wherein the compensating the gain of the digital uplink signal comprises compensating the gain of the filtered digital uplink signal using the digital state signal to compensate for the dynamic range limitations of the corresponding distributed unit (200).

36. The method (400) of any one of claims 29-35 wherein the optical fiber (52, 53, 54, 56) comprises a unidirectional fiber (52) configured to convey the coupled analog optical uplink and state signals from the distributed unit (200) to the central unit (100).

37. The method (400) of any one of claims 29-35 wherein the optical fiber (52, 53, 54, 56) comprises a bidirectional fiber (53, 55, 56) configured to convey the coupled analog optical uplink and state signals in a first direction from the distributed unit (200) to the central unit (100), and to convey analog optical downlink signals in a second direction from the central unit (100) to the distributed unit (200).

38. A central unit (100) for use in a distributed wireless system comprising the central unit (100) coupled to one or more distributed units (200) via at least one optical fiber (50-56), the central unit (100) comprising for each corresponding distributed unit (200): a demultiplexer (122) configured to: receive, from the corresponding distributed unit (200) via a corresponding optical fiber (52, 53, 54, 56), an analog optical signal comprising an optical uplink signal (5 ) on a first wavelength (^_L) dedicated to uplink signals and an optical state

24 signal (S ) on a second wavelength (2 _GC) dedicated to state signals, the optical state signal ) representing a first value indicative of an amplification level applied to a variable gain amplifier (222) of the corresponding distributed unit (200); and decouple the optical uplink signal (5^ ) from the optical state signal (S^ ); a first conversion circuit (130) configured to convert the optical uplink signal

) to a digital uplink signal; a second conversion circuit (124) configured to convert the optical state signal (^ ) to a digital state signal; and a digital compensation processing circuit (140) configured to compensate a gain of the digital uplink signal using the digital state signal to compensate for dynamic range limitations of the corresponding distributed unit (200).

39. The central unit (100) of claim 38 wherein: the digital state signal comprises an M-ary Pulse-Amplitude Modulation (PAM) signal comprising one or more of a plurality of PAM levels; and each PAM level representing a different amplification level for the variable gain amplifier (222) of the corresponding distributed unit (200).

40. The central unit (100) of claim 38 wherein: the digital state signal comprises a pulse signal comprising one or more of a plurality of return-to-zero pulses; and each of the plurality of different return-to-zero pulses representing a different amplification level for the variable gain amplifier (222) of the corresponding distributed unit (200).

41 . The central unit (100) of any one of claims 38-40 wherein the digital compensation processing circuit (140) produces a digitally compensated uplink signal, the central unit (100) further comprising a demodulator (142) configured to demodulate the digitally compensated uplink signal.

42. The central unit (100) of any one of claims 38-41 wherein the first conversion circuit (130) comprises: a first photodetector (132) configured to convert the optical uplink signal (5^ ) to an electrical uplink signal; a mixer (134) configured to mix the electrical uplink signal with a local oscillator frequency to downconvert the analog uplink signal to a baseband uplink signal; and

25 a first analog-to-digital converter (136) configured to convert the baseband uplink signal to the digital uplink signal.

43. The central unit (100) of any one of claims 38-42 wherein the second conversion circuit (124) comprises: a second photodetector (126) configured to convert the optical state signal

) to an electrical state signal; and a second analog-to-digital converter (128) configured to convert the electrical state signal to the digital state signal.

44. The central unit (100) of any one of claims 38-43 further comprising a digital filter (148) configured to filter the digital uplink signal to reduce an amplitude of one or more interfering signals, wherein the digital compensation processing circuit (140) compensates the gain of the digital uplink signal by compensating the gain of the filtered digital uplink signal using the digital state signal to compensate for the dynamic range limitations of the distributed unit (200).

45. The central unit (100) of any one of claims 38-44 wherein the optical fiber (52, 53, 54, 56) comprises a unidirectional fiber (52) configured to convey the coupled analog optical uplink and state signals from the distributed unit (200) to the central unit (100).

46. The central unit (100) of any one of claims 38-44 wherein the optical fiber (52, 53, 54, 56) comprises a bidirectional fiber (53, 55, 56) configured to convey the coupled analog optical uplink and state signals in a first direction from the distributed unit (200) to the central unit (100), and to convey analog optical downlink signals in a second direction from the central unit (100) to the distributed unit (200).

26