JP2004509554A - Method and system for transmitting a signal via a non-linear transmission unit - Google Patents

Abstract

The present invention relates to a system and method for transmitting a signal via a non-linear transmission unit, the system aiming at reducing the distortion of the transmitted signal caused by the intermodulation products. To achieve intermodulation product reduction in the output signal, the system includes a power splitter 110 for splitting the signal into a main signal and an auxiliary signal. The system further includes a power fluctuation detector 120 for detecting power fluctuations in the auxiliary signal and for generating a control signal representing the power fluctuation. Further, the system is configured to maintain the power of the main signal input to the nonlinear transmission line 200a constant only when the power of the input signal exceeds a predetermined reference power value P _ref. A variable attenuator for attenuating the power of the main signal in response to a control signal; After transmission, the signal is restored by amplifying the main signal by first variable gain amplifier 310 in response to the control signal.

Description

【００３０】
ここで、
Ｌ２は、第２の光リンクにおける損失を表し、
ａ１は、可変減衰器に関する比例係数であり、
ａ２は、可変増幅器に関する比例係数である。
【００３１】
図２は、図１に示されている本発明によるシステムの伝送ユニットの第２の実施形態を示す。図２によれば、両方の信号、すなわち、主信号と補助信号とがそれぞれに、ＷＤＭ方式を使用して同一の光ファイバ２６０を経由して伝送される。
【００３２】
第２の実施形態２００′でこれを実現するために、伝送ユニットは、主信号を第１の光学信号に変換するためのレーザダイオード２５０と、制御信号を第２の光学信号に変換するための発光ダイオード２５０′からなる光ファイバリンクとして具体化されている。前記第１および第２の光学信号は両方とも、前記２つの光学信号を多重化することによって多重信号を生成するためのマルチプレクサ２１０に伝送される。多重信号も光学信号であり、前記マルチプレクサから出力されて光ファイバ２６０を経由してデマルチプレクサ２２０に伝送される。デマルチプレクサ２２０は、前記第１および第２の光学信号を前記伝送された多重信号から分離させて、光学信号を主信号と制御信号にそれぞれ変換するための２つの別個のフォトダイオード２７０、２７０′に対してその分離させた光学信号を出力する働きをする。
【００３３】
制御信号が、第１の増幅器３１０に伝送される前に第２の増幅器２００ｃによって増幅されることが好ましい。前記第２の増幅器２００ｃの動作に関しては、図１の説明を参照されたい。
【００３４】
上述のように、本発明によるシステムでは、第１および第２の実施形態による前記非線形伝送ユニットを経由して伝送される主信号は、入力信号のパワーが予め決められた基準パワー値Ｐ_ｒｅｆを超える時に、一定のパワーを有する。
【００３５】
前記非線形伝送ユニット２００ａまたは２００′の一方を経由した伝送における主信号のパワーの定常性が、前記非線形伝送ユニット２００ａまたは２００′から主信号が出力される時にその主信号における３次相互変調積ＩＭ３の減少を生じさせる。したがって、ＩＭ３積は、前記主信号からの回復の後に、すなわち、復元ユニット３００の前記制御された第１の増幅器３１０によって出力された後に、入力信号においても減少させられる。ＩＭ３積の前記減少によって、非線形伝送ユニット２００ａまたは２００′の第１または第２の実施形態によって引き起こされる主信号の歪みが減少させられ、このようにして、線形化、すなわち、言い換えると、システムのより高いダイナミックレンジが実現される。したがって、本発明によるシステムを、線形化回路と呼ぶことも可能である。
【００３６】
次に、本発明の前記要点を詳細に具現する。図８および図９を参照することによって、上述したように、相互変調積はあらゆる非線形装置において生じる可能性がある。図１または図２によるシステムでは、可変減衰器は、相互変調積を全く発生させない受動要素と見なされなければならない。しかし、さらに別の具体例では、伝送ユニット２００ａと制御された第１の増幅器３１０は両方とも、その出力信号において相互変調積をそれぞれに発生させる非線形装置と見なされなければならない。
【００３７】
したがって、図１または図２によるシステム全体を考察する時には、制御された第１の増幅器によって出力される主信号を、第１の伝送ユニット２００ａによって生じさせられた相互変調積と、制御された第１の増幅器３１０自体によって生じさせられた相互変調積との重ね合せと見なすことができる相互変調積成分からなるということが想定される。しかし、本発明では、前記非線形伝送ユニット２００ａによって生じさせられた相互変調積が減少させられ、したがって、制御された第１の増幅器の出力信号における相互変調積も減少させられる。
【００３８】
次では、この着想を図３から図７を参照して説明する。
【００３９】
図３は、特定の入力パワーＰ１の場合の、典型的な非線形装置の出力信号における３次相互変調積のパワーに対する基本信号成分のパワーの比率を示す。
【００４０】
例えば、こうした非線形装置の出力信号は、基本信号成分に加えて、３次相互変調積ＩＭ３を含む。前記装置に入力される信号がＰ１＝−４０　ｄＢｍの入力パワーを有すると想定すると、出力信号中の基本信号成分は−２５　ｄＢｍのパワーＰ＿ｆｕｎｄを有し、出力信号中のＩＭ３は約−１１０　ｄＢｍのパワーＰ＿ＩＭ３を有する。
【００４１】
しかし、図３から理解できるように、基本信号成分Ｐ＿ｆｕｎｄに関する曲線と、前記システムの出力信号中の３次相互変調積Ｐ＿ＩＭ３に関する曲線とが、「入力パワー」対「出力パワー」図において互いに異なる勾配を有する。さらに明確に述べると、基本信号成分は１の勾配を有し、一方、３次相互変調積は前記図において３の勾配を有する。
【００４２】
したがって、各々の非線形装置に関して、個々のＩＭ３阻止点（ｉｎｔｅｒｃｅｐｔｉｏｎ　ｐｏｉｎｔ）が、基本信号成分Ｐ＿ｆｕｎｄの曲線と３次相互変調積Ｐ＿ＩＭ３の曲線との交差点によって定義されることが可能である。各々の交差点は、さらに、図３による「入力パワー」対「出力パワー」図におけるその座標によって、すなわち、その出力−入力３次阻止点によって定義される。
【００４３】
図１と図２との参照において、非線形伝送ユニット２００ａの伝送特性と、制御された第１の非線形増幅器３１０の伝送特性とを、図３の個別の線図によってそれぞれに表すことが可能である。
【００４４】
次では、非線形伝送ユニット２００ａの基本信号成分と第１の制御された増幅器３１０の基本信号成分ではなく、３次相互変調積ＩＭ３だけに言及するが、これは基本信号が制御信号にしたがって可変利得増幅器ＡＭＰ１　３１０によって復元されるからである。
【００４５】
非線形伝送ユニット２００ａの３次相互変調積Ｐ＿ＩＭ３の（出力）パワー、さらに明確に述べると、光ファイバリンクｆｏｌの３次相互変調積Ｐ＿ＩＭ３の（出力）パワーを、次のように計算することが可能であり、
Ｐ＿ＩＭ３_ｆｏｌ＝３×Ｐ_ｏｕｔ−２×ＯＩＰ３_ｆｏｌ　　　　（２）
ここで、
Ｐ_ｏｕｔは、前記非線形伝送ユニット２００ａの出力信号のパワーを表し、
ＯＩＰ３_ｆｏｌは、前記非線形伝送ユニット２００ａの３次阻止点の出力パワー成分を表す。
【００４６】
式（２）は、ＯＩＰ３_ｆｏｌが一般的に利得に依存しないので、３次相互変調積のパワーに関する容易な計算を可能にする。
【００４７】
パワーＰ＿ＩＭ３_ｆｏｌは概略的に（すなわち、基準化なしに）図４に示されている。
【００４８】
図４では、Ｐ＿ＩＭ３_ｆｏｌの曲線の２つの領域を区別することが可能である。
【００４９】
１．　図１を参照することによって説明される通りの基準パワーＰ_ｒｅｆ、例えばＰ_ｒｅｆ＝−４５　ｄＢｍよりも低い入力パワーＰ_ｉｎの場合に、Ｐ＿ＩＭ３_ｆｏｌを表す曲線は３の勾配を有する通常の挙動を示す。
【００５０】
２．　Ｐ_ｒｅｆ＝−４５　ｄＢｍを超える入力パワーＰ_ｉｎの場合に、Ｐ＿ＩＭ３_ｆｏｌ曲線は水平であり、このことはＰ＿ＩＭ３がＰ_ｉｎとは無関係であることを意味する。
【００５１】
Ｐ_ｉｎ＞Ｐ_ｒｅｆの場合のＩＭ３のパワーの定常性は、前記光ファイバリンクの出力信号におけるＩＭ３の減少を意味する。これは、図１と図２を参照して説明したように、本発明による光ファイバリンクに入力される主信号のパワーの定常性を原因とする。
【００５２】
上述のＰ＿ＩＭ３_ｆｏｌとは反対に、制御された第１の増幅器３１０のＰ＿ＩＭ３_ａｍｐ１を次のように計算することが可能であり、
Ｐ＿ＩＭ３_ＡＭＰ１＝３×Ｐ_{ｏｕｔ＿ＡＭＰ１}−２×ＯＩＰ３_ＡＭＰ１　　　　（３）
ここで、
Ｐ＿ＩＭ３_ＡＭＰ１は、第１の制御された増幅器３１０の出力信号における３次相互変調積のパワーを表し、
Ｐ_{ｏｕｔ＿ＡＭＰ１}は、第１の増幅器３１０の出力信号における基本信号成分の出力パワーを表し、
ＯＩＰ３_ＡＭＰ１は、第１の増幅器３１０の３次阻止点の出力パワー成分を表す。
【００５３】
式（３）から、パラメータＯＩＰ３_ＡＭＰ１が高い時には、非線形増幅器（例えば、第１の増幅器３１０）の出力信号におけるＩＭ３のパワーが小さいということが明らかである。
【００５４】
振幅の適正な復元のために、可変光ファイバリンク利得Ｇ_{ＴＯＴＡＬ}が一定でなければならない。これは次の通りに計算され、
Ｇ_{ＴＯＴＡＬ}＝−Ｌ_ａｔｔ＋Ｇ_ｌｉｎｋ＋Ｇ_ＡＭＰ１　　　　（４）
ここで、
Ｌ_ａｔｔは、減衰器における損失（ｄＢ単位）であり、
Ｇ_ｌｉｎｋは、第１のリンク利得（ｄＢ単位）であり、
Ｇ_ＡＭＰ１は、ＡＭＰ１パワー利得（ｄＢ単位）である。
【００５５】
Ｌ_ａｔｔは、図１に示すＯＰＡＭ　１２６からのフィードバックによって、入力パワーＰ_ｉｎに依存し、次のように記述でき、
Ｌ_ａｔｔ＝Ｌ_０＋（Ｐ_ｉｎ−Ｐ_ｒｅｆ）＋ａｌ　　　　（５）
ここで、
Ｌ_０は、初期減衰（ｄＢ単位）であり、
ａｌは、可変減衰器に関する比例係数である。
【００５６】
Ｇ_ｌｉｎｋは飽和まではＰ_ｉｎに依存せず、定数と見なすことが可能である。
【００５７】
Ｇ_ＡＭＰ１は入力パワーＰ_ｉｎに依存し、
Ｇ_ＡＭＰ１＝Ｇ_０＋（Ｐ_ｉｎ−Ｐ_ｒｅｆ）＋ａ２＋Ｇ_{ｌｉｎｋ２}＋Ｇ_ＡＭＰ２　　　　（６）
であり、ここで、
Ｇ_０は、ＡＭＰ１に関する初期パワー利得であり、
ａ２は、可変増幅器に関する比例変数であり、
Ｇ_{ｌｉｎｋ２}とＧ_ＡＭＰ２はそれぞれに第２のリンクと第２の増幅器の利得である。
【００５８】
振幅復元Ｇ_ＡＭＰ１はＬ_ａｔｔに等しくなければならず、または、Ｐ_ｉｎに依存しない一定の差を持たなければならない。
【００５９】
図５は、例えば第１の増幅器３１０自体のような非線形増幅器の出力信号におけるＩＭ３の挙動を示す。式（３）によって３次相互変調積Ｐ＿ＩＭ３_ＡＭＰ１のパワーが３の勾配を有するということが明らかである。
【００６０】
上述のように、図１における制御された第１の増幅器３１０によって出力される信号出力の３次相互変調積Ｐ＿ＩＭ３_{ＴＯＴＡＬ}のパワーを、非線形伝送ユニット２００ａの各々すなわち光ファイバリンクによって生じさせられた３次相互変調積Ｐ＿ＩＭ３_ｆｏｌと、第１の増幅器３１０自体によって生じさせられた３次相互変調積Ｐ＿ＩＭ３_ＡＭＰ１との重ね合せと見なすことが可能である。したがって、Ｐ＿ＩＭ３_{ＴＯＴＡＬ}を次のように計算することが可能である。
【００６１】
Ｐ＿ＩＭ３_{ＴＯＴＡＬ}＝Ｐ＿ＩＭ３_ｆｏｌ＋Ｐ＿ＩＭ３_ＡＭＰ１　　　　（７）
図６は、図１又は図２におけるシステムに対する前記重ね合せの結果を示す。さらに明確に述べると、図６は、入力信号のパワーに応答した、図１または図２によるシステムに関する第１の増幅器３１０の出力信号における３次相互変調積Ｐ＿ＩＭ３_{ＴＯＴＡＬ}のパワーを示す。
【００６２】
基準パワーＰ_ｒｅｆ＝−４５　ｄＢｍを超える入力パワーの場合に、Ｐ＿ＩＭ３_{ＴＯＴＡＬ}曲線の勾配が、基準パワーＰ_ｒｅｆよりも小さい入力パワーの場合の勾配に比較して小さいということが明らかである。この勾配の減少は、Ｐ_ｒｅｆ＝−４５　ｄＢｍを超える入力パワーの場合の、第１の増幅器３１０の出力信号における３次相互変調積合計の減少を意味する。前記減少は、図４を参照して上述した通りに、非線形伝送ユニット２００ａに入力される時の主信号のパワーの定常性を原因とする。
【００６３】
Ｐ＿ＩＭ３_{ＴＯＴＡＬ}と関係なく第１の増幅器３１０の出力信号である基本信号成分Ｐ＿ｆｕｎｄにさらに示す図６によるシミュレーションを、次のパラメータを使用して行った。
【００６４】
光ファイバリンク利得Ｇ_{ＴＯＴＡＬ}＝−１５　ｄＢ、
光ファイバリンクの３次阻止点の出力パワーＯＩＰ３_ｆｏｌ＝２０　ｄＢｍ、および、
第１の増幅器の３次阻止点の出力パワーＯＩＰ３_ＡＭＰ１＝１００　ｄＢｍ（理想的な第１の増幅器）。
【００６５】
図６に示されているシミュレーション結果から理解できるように、第１の増幅器３１０の出力パワーにおける基本信号パワーの３次相互変調積合計に対する比率は、入力パワーＰ_ｉｎ＞−４５　ｄＢｍの場合に一定である（基本信号成分の曲線とＩＭ３積の曲線は互いにほぼ平行である）。
【００６６】
−４５　ｄＢｍよりも大きいパワーを有する入力信号に応答したシステムの出力信号におけるＩＭ３の減少のせいで、前記ＩＭ３による前記出力信号の歪みが（入力パワー範囲全体にわたる３の連続的な勾配に比較して）減少している。したがって、本発明によるシステムは線形化回路のように動作し、高ダイナミックレンジを有する信号を伝送することが可能である。
【００６７】
最後に、図７は、次のパラメータが設定されている図１または図２によるシステムの別のシミュレーションの結果を示す。
【００６８】
光ファイバリンク利得Ｇ_{ＴＯＴＡＬ}＝−１５　ｄＢ、
ＯＩＰ３_ｆｏｌ＝２０　ｄＢｍ、および、
ＯＩＰ３_ＡＭＰ１＝３５　ｄＢｍ。
【００６９】
図６とは違って、図７によるシミュレーションは、理想的ではない増幅器３１０に関して行われた。この場合には、システム全体の３次阻止点、特に非線形伝送ユニット２００ａと第１の増幅器３１０との直列接続の３次阻止点を、図７に見ることができる。基準パワーＰ_ｒｅｆ＝−４５　ｄＢを超える入力パワーの場合に、システムの最終出力信号のＩＭ３、すなわち、増幅器３１０の出力信号のＩＭ３が約２０ｄＢ減少している。したがって、システム全体すなわち図１または図２による線形化回路の３次阻止点の出力パワーＯＩＰ３_{ＴＯＴＡＬ}が、約１０ｄＢ増加している。
【００７０】
したがって、図７は、本発明による現実のシステムでは、３次相互変調積の減少と、したがって伝送後の出力信号における歪みの減少とが実現されることも可能であるということを示す。
【００７１】
図９は、図１によるシステムの変型を示す。図９のシステムに示されている全ての特徴は、追加の形でマルチプレクサ１２７が含まれていることを除いて、図１の特徴に一致する。マルチプレクサ１２７はその第１の入力信号として基準電圧Ｖ_ｒｅｆを有し、および、その第２の入力信号として、ベースバンド（ＢＢ）プロセッサから生成される信号を有する。信号Ｓ_ＢＢは、従来のやり方でベースバンドプロセッサから生成される。
【００７２】
基準パワーＰ_ｒｅｆは、信号対雑音比（ＳＮＲ_ｓｐｅｃ）に関する要件にしたがって設定されることが好ましい。
【００７３】
前記システムが基地局のダウンリンクのために使用される場合には、最小のＰ_ｒｅｆが次のように計算されることが可能であり、
Ｐ_ｒｅｆ＝ＳＮＲ_ｓｐｅｃ＋ＮＦ_ｌｉｎｋ＋Ｎ_{ｆｌｏｏｒ}＋１０＊ｌｏｇ（ＢＷ）
ここで、
ＮＦ_ｌｉｎｋは、光リンクの雑音値（ｄＢ単位）であり、
Ｎｆ_ｌｏｏｒは、伝送器のノイズフロア（ｎｏｉｓｅ　ｆｌｏｏｒ）（ｄＢｍ／Ｈｚ）であり、および、
ＢＷは、伝送信号の周波数帯域幅（Ｈｚ単位）である。
【００７４】
前記システムがアップリンク伝送に使用される場合には、Ｐ_ｒｅｆの設定はより一層複雑である。この場合には、同一の周波数帯域内に、必要とされる信号よりも強い不要信号が存在している可能性がある。したがって、到来信号合計のＳＮＲではなくＰ_ｒｅｆは必要とされる信号のＳＮＲにしたがって設定されなければならない。これは、図９ではマルチプレクサ１２７によって実現されているベースバンド（ＢＢ）プロセッサからの応答を必要とする。
【図面の簡単な説明】
【図１】
本発明の第１の実施形態によるシステムを示したブロック図である。
【図２】
図１に示されているシステムの伝送ユニットの第２の実施形態を示したブロック図である。
【図３】
非線形装置の出力信号における基本信号成分と３次相互変調積とのパワー比率を示す図である。
【図４】
本発明による光ファイバリンクにおける３次相互変調積のパワーの減少を示す図である。
【図５】
入力パワーに応答した典型的な非線形装置の出力信号における３次相互変調生成物のパワーを示す「入力パワー」対「出力パワー」図である。
【図６】
第１の組のシミュレーションパラメタの場合の、本発明によるシステムの出力信号における３次相互変調積の減少を示す「入力パワー」対「出力パワー」図である。
【図７】
第２の組のシミュレーションパラメータの場合の、本発明によるシステムにおいて実現される３次相互変調積の減少を示す「入力パワー」対「出力パワー」図である。
【図８】
当業で公知の非線形伝送装置の実施形態としての光ファイバリンクを示したブロック図である。
【図９】
図１による変型のブロック図である。[0001]
[Field of the Invention]
The present invention relates to a method and system for transmitting a signal via a non-linear transmission unit, in particular a fiber optic link, including a preparation unit and a restoration unit.
[Background of the Invention]
This system is known for example in satellite or telecommunications applications. These applications generally require a high dynamic range for the transmission unit, preferably in combination with a bandwidth of a few MHz, which typically means that the power of the input signal has a dynamic range of, for example, 100 dB. Because it may change within.
[0002]
The conventional approach to meeting this requirement is to use fiber optic links as shown in FIG. Such fiber optic links have the advantages of low loss, high bandwidth, and light weight. As shown in FIG. 8, the fiber optic link comprises a laser diode 850 for receiving the signal and converting the signal to a modulated optical signal. The optical fiber link further comprises a fiber 860 for transmitting the optical signal to a photodiode 870, which restores the signal input to the laser diode 850 from the modulated optical signal after transmission. I do. The high dynamic range required for such a transmission unit requires a linear transmission characteristic of the transmission unit. However, fiber optic links are non-linear transmission units, especially due to the non-linear behavior of laser diodes. The output signal of the nonlinear transmission unit consists of unwanted distortion caused by intermodulation in the nonlinear device.
[0003]
Intermodulation refers to the generation of frequency components in the output signal of a nonlinear device. Said frequency components correspond to the sum and difference frequencies of the (required) signal from the required channel and the (unwanted) signal from adjacent channels arriving at the system input.
[0004]
Further, the intermodulation product means a specific one of the frequency components.
[0005]
More specifically, the second-order intermodulation product is characterized by a frequency of "f1-f2" or "f2-f1" and a slope of 2 in the "input power" vs. "output power" diagram of the nonlinear device. . Furthermore, the third-order intermodulation product is characterized by a frequency of "2f1-f2" or "2f2-f1", where the curve has a slope of 3 in the "input power" vs. "output power" diagram.
[0006]
One approach to reducing the unwanted effects of the intermodulation products and achieving high dynamic range in fiber optic links is to use very high quality laser diodes and / or external modulators. Good. However, both of these methods are very expensive.
[0007]
U.S. Pat. Nos. 5,321,849 and 5,457,811 and an article from the IEEE publication, "A dynamic range enhancement technology for fiber optic microsystem radiosystems," Feng. , Lemson, P .; Reed, J .; H. Jacobs, I .; Vehicular Technology Conference, 1995 IEEE 45 ^th , Volume 2, 1995 pages 774-778 each disclose a system for increasing the dynamic range of a transmission link. A common feature of all of these prior art systems is that a control signal is generated from the transmission signal and that the control signal conditions the transmission signal before it enters the nonlinear transmission unit. It is.
[Object of the invention]
Starting from the prior art, it is an object of the present invention to improve the dynamic range of a system and method for transmitting a signal via a non-linear transmission unit.
[Summary of the Invention]
The object is solved by the subject matter of independent claims 1 and 9 respectively.
[0008]
According to claim 1, the object is to provide a power splitter for splitting a signal into a main signal and an auxiliary signal, a power fluctuation detector for detecting a power fluctuation of the auxiliary signal and generating a control signal representing the power fluctuation, A variable attenuator that attenuates the power of the main signal in response to the control signal so that the power of the main signal input to the non-linear transmission unit (200a) is kept constant; An amplifier that responds to the transmitted control signal and amplifies the main signal after transmission through the non-linear transmission unit.
[0009]
Since the power of the main signal output by the variable attenuator and input to the nonlinear transmission unit is constant, the ratio between the basic signal and the intermodulation product output by the transmission unit is also constant. Thus, the proposed system sufficiently enables signals with very high dynamic range to be transmitted via the non-linear transmission unit by reducing the second and third order intermodulation products.
[0010]
Therefore, the linearity of the system, and in particular the linearity of the transmission unit, is improved, and the distortion of the signal output by the system is reduced. Instead of high cost transmission units with improved linearity, it is possible to use low cost transmission units.
[0011]
In a first embodiment, the system includes a comparator for comparing the power of the auxiliary signal with a predetermined reference power value and for generating a control signal representing the result of the comparison. Advantageously, the comparator enables detection of the power fluctuation.
[0012]
The system may be configured to transmit the control signal after the control signal is transmitted via the main transmission device or via the other transmission unit, but before the control signal is input to the first amplifier. Advantageously comprises a second amplifier for amplifying. The second amplifier compensates for losses in the other transmission unit such that the control signal provided to the controlled first amplifier is the same as the input to the variable attenuator. In this way, an exact restoration of the attenuation by the controlled first amplifier is ensured.
[0013]
In another embodiment, the system advantageously comprises a further transmission unit for transmitting the control signal to the first amplifier. A high degree of linearity is not required for the other transmission units, so that low-cost components, in particular low-cost digital laser diodes or inexpensive light-emitting diodes LED, can be used. This system is capable of providing input / output power fluctuations for most cellular protocols, such as wideband code division multiple access (W-CDMA) or global system for mobile communications (GSM) transmission. It is based on the idea that it occurs on a later time scale. Therefore, a phase adjusting device is unnecessary.
[0014]
Alternatively, the control signal is not transmitted via the other transmission line, but is transmitted via the same transmission line as that used for the main signal by using the wavelength division multiplexing (WDM) method. Transmitted. To perform this scheme, the nonlinear transmission unit of the system includes a laser diode that converts a main signal into a first optical signal, a light emitting diode that converts a control signal into a second optical signal, and a first optical signal. A multiplexer that generates a multiplexed signal by multiplexing the second optical signal and the second optical signal; and an optical fiber that transmits the multiplexed signal from the multiplexer to a demultiplexer. The demultiplexer passes through the optical fiber. After the transmission, recover the first optical signal and the second optical signal from the transmitted multiplexed signal, and to two separate photodiodes for recovering the main signal and the control signal respectively. And serves to output these optical signals.
[0015]
The object of the invention is further solved by a method according to claim 8. The advantages of the method are consistent with the advantages of the system described above.
[0016]
The control signal is advantageously embodied such that the power of the main signal is kept constant only if the power of the input signal exceeds the predetermined reference power value.
[0017]
The object of the invention is further solved by a preparation unit for use in a system as described above comprising a power splitter, a power fluctuation detector and a variable attenuator. The preparatory unit is adapted to control the power of the input signal to a predetermined reference power value P _ref , Advantageously ensures that the main signal is generated from the input signal such that the power of the main signal is kept constant after some set power. The main signal is transmitted via the non-linear transmission line.
[0018]
The object of the invention is further solved by a restoration unit consisting of an amplifier, used in a system as described above. Said restoration unit advantageously enables to properly restore the signal from the transmitted main signal.
[Description of preferred embodiments of the invention]
In the following, the invention will be described in more detail with reference to FIGS. 1 to 7.
[0019]
FIG. 1 shows a first embodiment of the system according to the invention. The system includes a preparation unit 100, a transmission unit 200, and a restoration unit 300.
[0020]
The preparation unit 100 includes a power splitter 110 for dividing a signal, hereinafter referred to as an input signal, into a main signal and an auxiliary signal, and the ratio of the power of the main signal to the power of the auxiliary signal is, for example, 1:10. The auxiliary signal is sent to a power fluctuation detector 120 for detecting a power fluctuation of the auxiliary signal and for generating a control signal representing the power fluctuation. To achieve this, the power fluctuation detector 120 comprises a power detector 124, which extracts a power signal from the auxiliary signal by filtering and likewise includes it in the power fluctuation detector 120. And outputs the power signal to the comparator 126. The comparator 126 compares the power of the power signal with a reference voltage V _ref Predetermined reference power value represented by _Pref Compare with In this manner, the comparator 126 detects the power fluctuation of the auxiliary signal and generates a control signal representing the power fluctuation.
[0021]
The control signal is transmitted to the variable attenuator 130 to attenuate the power of the main signal according to the control signal.
[0022]
By feeding the control signal into the variable attenuator 130, the preparation unit 100 ensures that the power of the main signal remains constant when the power P1 of the auxiliary signal exceeds the reference power value.
[0023]
As shown in FIG. 1, the main signal and the control signal are restored from the preparation unit 100 via the transmission unit 200 of the first embodiment, that is, via the individual transmission units 200a and 200b, respectively. Transmitted to unit 300. Both transmission units 200a, 200b are, for example, fiber optic links as known in the art and as described above with reference to FIG. In particular, the transmission unit 200a for transmitting the main signal is assumed to be a nonlinear transmission unit due to the nonlinear characteristics of the laser diode 250.
[0024]
Laser diode 250 may be any type of laser diode that does not have application specific linearity requirements. Therefore, it may be an inexpensive laser such as a Fabry-Perot-Laser generally used for digital applications (digital lasers), or a DFB laser generally used for analog applications. An expensive high linearity laser may be used. However, it must be emphasized once more that the laser diode 250 in the transmission unit 200a for transmitting the main signal is not assumed to have sufficient linearity for the application.
[0025]
In addition, the linearity requirements for the second transmission unit 200b for transmitting control signals from the preparation unit 100 to the restoration unit 300 are also very low, so that the laser diode in the fiber optic link 200b is provided by a cheap Fabry-Perot laser It may be replaced by a simple light emitting diode LED.
[0026]
After transmission via the non-linear transmission unit 200a, the power of the input signal is P _ref , The main signal having a constant power is received and amplified by the amplifier 310 in the restoration unit 300. The amplification of the amplifier 310, and thus of the received main signal, is controlled by a control signal transmitted to the restoration unit via the second transmission unit 200b.
[0027]
A second amplifier 200c for amplifying the control signal after the control signal has been transmitted and before the control signal is input to the controlled first amplifier 310, It preferably comprises Preferably, the gain of the second amplifier 200c is adapted to compensate for losses in the second transmission unit 200b. In this case, the control signal supplied to the variable attenuator 130 and the control signal supplied to the controlled first amplifier 310 are the same as each other. In this way, the system according to the invention and, more specifically, the controlled first amplifier 310 restores the main signal at the output of the first amplifier 310 to its initial value (ie the input signal). It has become. In other words, the controlled first amplifier 310 is adapted to compensate for the attenuation provided by the variable attenuator 130.
[0028]
Generally, the gain G of the second amplifier 200c ₂ Can be calculated as
[0029]
(Equation 1)

[0030]
here,
L2 represents the loss in the second optical link;
a1 is a proportional coefficient relating to the variable attenuator,
a2 is a proportional coefficient for the variable amplifier.
[0031]
FIG. 2 shows a second embodiment of the transmission unit of the system according to the invention shown in FIG. According to FIG. 2, both signals, the main signal and the auxiliary signal, are each transmitted via the same optical fiber 260 using the WDM method.
[0032]
To achieve this in the second embodiment 200 ', the transmission unit comprises a laser diode 250 for converting the main signal into a first optical signal and a laser diode 250 for converting the control signal into a second optical signal. It is embodied as an optical fiber link comprising light emitting diodes 250 '. Both the first and second optical signals are transmitted to a multiplexer 210 for multiplexing the two optical signals to generate a multiplex signal. The multiplex signal is also an optical signal, which is output from the multiplexer and transmitted to the demultiplexer 220 via the optical fiber 260. Demultiplexer 220 separates the first and second optical signals from the transmitted multiplexed signal and converts the optical signals into a main signal and a control signal by two separate photodiodes 270, 270 ', respectively. And outputs the separated optical signal.
[0033]
Preferably, the control signal is amplified by the second amplifier 200c before being transmitted to the first amplifier 310. For the operation of the second amplifier 200c, refer to the description of FIG.
[0034]
As described above, in the system according to the present invention, the main signal transmitted via the non-linear transmission unit according to the first and second embodiments is such that the power of the input signal is a predetermined reference power value P _ref Has a constant power when
[0035]
The continuity of the power of the main signal in transmission via one of the nonlinear transmission units 200a or 200 'depends on the third-order intermodulation product IM3 in the main signal when the main signal is output from the nonlinear transmission unit 200a or 200'. Causes a decrease in Thus, the IM3 product is also reduced in the input signal after recovery from the main signal, ie, after being output by the controlled first amplifier 310 of the recovery unit 300. The reduction of the IM3 product reduces the distortion of the main signal caused by the first or second embodiment of the non-linear transmission unit 200a or 200 ', thus linearizing, ie, in other words, of the system A higher dynamic range is realized. Therefore, the system according to the invention can also be called a linearization circuit.
[0036]
Next, the gist of the present invention will be described in detail. With reference to FIGS. 8 and 9, as described above, intermodulation products can occur in any non-linear device. In the system according to FIG. 1 or FIG. 2, the variable attenuator must be regarded as a passive element which does not generate any intermodulation products. However, in yet another embodiment, both the transmission unit 200a and the controlled first amplifier 310 must be considered as non-linear devices that respectively generate intermodulation products in their output signals.
[0037]
Thus, when considering the entire system according to FIG. 1 or FIG. 2, the main signal output by the controlled first amplifier is combined with the intermodulation product produced by the first transmission unit 200a and the controlled second product. It is assumed that it consists of an intermodulation product component that can be considered as a superposition with the intermodulation product produced by one of the amplifiers 310 itself. However, in the present invention, the intermodulation products caused by the nonlinear transmission unit 200a are reduced, and therefore the intermodulation products in the output signal of the controlled first amplifier are also reduced.
[0038]
Next, this idea will be described with reference to FIGS.
[0039]
FIG. 3 shows the ratio of the power of the fundamental signal component to the power of the third-order intermodulation product in the output signal of a typical nonlinear device for a specific input power P1.
[0040]
For example, the output signal of such a nonlinear device includes a third-order intermodulation product IM3 in addition to the fundamental signal component. Assuming that the signal input to the device has an input power of P1 = −40 dBm, the fundamental signal component in the output signal has a power P_fund of −25 dBm, and IM3 in the output signal is approximately −110 dBm. Power P_IM3.
[0041]
However, as can be seen from FIG. 3, the curves for the fundamental signal component P_fund and for the third-order intermodulation product P_IM3 in the output signal of the system have different slopes in the "input power" vs. "output power" diagram. Having. More specifically, the fundamental signal component has a slope of one, while the third-order intermodulation product has a slope of three in the figure.
[0042]
Thus, for each nonlinear device, an individual IM3 interruption point can be defined by the intersection of the curve of the fundamental signal component P_fund and the curve of the third-order intermodulation product P_IM3. Each intersection is further defined by its coordinates in the “input power” vs. “output power” diagram according to FIG. 3, ie by its output-input tertiary stop.
[0043]
Referring to FIGS. 1 and 2, the transmission characteristics of the nonlinear transmission unit 200a and the transmission characteristics of the controlled first nonlinear amplifier 310 can be respectively represented by the individual diagrams of FIG. .
[0044]
In the following, not the fundamental signal component of the non-linear transmission unit 200a and the fundamental signal component of the first controlled amplifier 310, but only the third-order intermodulation product IM3, which means that the fundamental signal has a variable gain according to the control signal This is because the signal is restored by the amplifier AMP1 310.
[0045]
The (output) power of the third-order intermodulation product P_IM3 of the non-linear transmission unit 200a, and more specifically, the (output) power of the third-order intermodulation product P_IM3 of the optical fiber link fol, can be calculated as follows. And
P_IM3 _fol = 3 x P _out -2 x OIP3 _fol (2)
here,
P _out Represents the power of the output signal of the nonlinear transmission unit 200a,
OIP3 _fol Represents the output power component of the third-order stop point of the nonlinear transmission unit 200a.
[0046]
Equation (2) is expressed as OIP3 _fol Is generally independent of gain, which allows for easy calculation of the power of the third-order intermodulation product.
[0047]
Power P_IM3 _fol Is shown schematically (ie, without scaling) in FIG.
[0048]
In FIG. 4, P_IM3 _fol It is possible to distinguish two regions of the curve
[0049]
1. Reference power P as described with reference to FIG. _ref , For example, P _ref = Input power P lower than -45 dBm _in , P_IM3 _fol Shows the normal behavior with a slope of 3.
[0050]
2. P _ref = Input power P exceeding -45 dBm _in , P_IM3 _fol The curve is horizontal, which means that P_IM3 _in Has nothing to do with.
[0051]
P _in > P _ref The continuity of the power of the IM3 in the case of means the reduction of the IM3 in the output signal of the optical fiber link. This is due to the stationarity of the power of the main signal input to the optical fiber link according to the present invention, as described with reference to FIGS.
[0052]
P_IM3 described above _fol In contrast, P_IM3 of controlled first amplifier 310 _amp1 Can be calculated as follows:
P_IM3 _AMP1 = 3 x P _{out_AMP1} -2 x OIP3 _AMP1 (3)
here,
P_IM3 _AMP1 Represents the power of the third order intermodulation product in the output signal of the first controlled amplifier 310,
P _{out_AMP1} Represents the output power of the fundamental signal component in the output signal of the first amplifier 310,
OIP3 _AMP1 Represents the output power component of the first amplifier 310 at the third-order stop point.
[0053]
From equation (3), the parameter OIP3 _AMP1 It is clear that when is high, the power of IM3 in the output signal of the non-linear amplifier (eg, first amplifier 310) is small.
[0054]
To properly restore the amplitude, the variable fiber link gain G _TOTAL Must be constant. This is calculated as:
G _TOTAL = -L _att + G _link + G _AMP1 (4)
here,
L _att Is the loss (in dB) at the attenuator;
G _link Is the first link gain (in dB),
G _AMP1 Is the AMP1 power gain (in dB).
[0055]
L _att Is the input power P due to the feedback from OPAM 126 shown in FIG. _in And can be written as
L _att = L ₀ + (P _in −P _ref ) + Al (5)
here,
L ₀ Is the initial decay (in dB),
al is a proportionality factor for the variable attenuator.
[0056]
G _link Is P until saturation _in And can be regarded as a constant.
[0057]
G _AMP1 Is the input power P _in Depends on
G _AMP1 = G ₀ + (P _in −P _ref ) + A2 + G _link2 + G _AMP2 (6)
Where
G ₀ Is the initial power gain for AMP1;
a2 is a proportional variable for the variable amplifier,
G _link2 And G _AMP2 Is the gain of the second link and the second amplifier, respectively.
[0058]
Amplitude restoration G _AMP1 Is L _att Must be equal to or P _in Must have a certain difference that does not depend on
[0059]
FIG. 5 shows the behavior of IM3 in the output signal of a non-linear amplifier, for example, the first amplifier 310 itself. According to equation (3), the third-order intermodulation product P_IM3 _AMP1 It is clear that this power has a slope of 3.
[0060]
As described above, the third order intermodulation product P_IM3 of the signal output output by the controlled first amplifier 310 in FIG. _TOTAL Of the third order intermodulation product P_IM3 generated by each of the nonlinear transmission units 200a, ie, the fiber optic links. _fol And the third-order intermodulation product P_IM3 generated by the first amplifier 310 itself. _AMP1 Can be considered as a superposition of Therefore, P_IM3 _TOTAL Can be calculated as follows:
[0061]
P_IM3 _TOTAL = P_IM3 _fol + P_IM3 _AMP1 (7)
FIG. 6 shows the result of the superposition for the system in FIG. 1 or FIG. More specifically, FIG. 6 shows a third-order intermodulation product P_IM3 in the output signal of the first amplifier 310 for the system according to FIG. 1 or 2 in response to the power of the input signal. _TOTAL Shows the power of
[0062]
Reference power P _ref = _45 dBm for input powers above P_IM3 _TOTAL The slope of the curve is the reference power P _ref It is clear that the slope is small compared to the slope for smaller input powers. This decrease in slope is due to P _ref == − 45 dBm means a reduction in the sum of the third-order intermodulation products in the output signal of the first amplifier 310 for input powers greater than −45 dBm. As described above with reference to FIG. 4, the decrease is caused by the stationarity of the power of the main signal when input to the nonlinear transmission unit 200a.
[0063]
P_IM3 _TOTAL Regardless of this, the simulation according to FIG. 6 further illustrated for the basic signal component P_fund, which is the output signal of the first amplifier 310, was performed using the following parameters.
[0064]
Optical fiber link gain G _TOTAL = −15 dB,
Output power OIP3 at tertiary stop point of optical fiber link _fol = 20 dBm, and
Output power OIP3 of third-order stop point of first amplifier _AMP1 = 100 dBm (ideal first amplifier).
[0065]
As can be understood from the simulation results shown in FIG. 6, the ratio of the fundamental signal power to the sum of the third-order intermodulation products at the output power of the first amplifier 310 is equal to the input power P. _in > -45 dBm (the curve of the fundamental signal component and the curve of the IM3 product are substantially parallel to each other).
[0066]
Due to the decrease in IM3 in the output signal of the system in response to the input signal having a power greater than -45 dBm, the distortion of the output signal by the IM3 (compared to 3 continuous slopes over the entire input power range) T) is decreasing. Thus, the system according to the invention operates like a linearization circuit and is capable of transmitting signals with a high dynamic range.
[0067]
Finally, FIG. 7 shows the result of another simulation of the system according to FIG. 1 or 2 with the following parameters set:
[0068]
Optical fiber link gain G _TOTAL = −15 dB,
OIP3 _fol = 20 dBm, and
OIP3 _AMP1 = 35 dBm.
[0069]
Unlike FIG. 6, the simulation according to FIG. 7 was performed for a non-ideal amplifier 310. In this case, the third-order stop point of the entire system, in particular, the third-order stop point of the series connection of the nonlinear transmission unit 200a and the first amplifier 310 can be seen in FIG. Reference power P _ref For input powers above = -45 dB, the IM3 of the final output signal of the system, i.e. the IM3 of the output signal of the amplifier 310, has been reduced by about 20 dB. Therefore, the output power OIP3 of the entire system, ie the third-order stop point of the linearization circuit according to FIG. _TOTAL Is increased by about 10 dB.
[0070]
Thus, FIG. 7 shows that in a real system according to the invention, it is also possible to achieve a reduction of the third-order intermodulation products and thus of the distortion in the output signal after transmission.
[0071]
FIG. 9 shows a variant of the system according to FIG. All features shown in the system of FIG. 9 correspond to those of FIG. 1 except that multiplexer 127 is included in an additional manner. Multiplexer 127 has as its first input signal reference voltage V _ref And has as its second input signal a signal generated from a baseband (BB) processor. Signal S _BB Are generated from the baseband processor in a conventional manner.
[0072]
Reference power P _ref Is the signal-to-noise ratio (SNR) _spec ) Is preferably set according to the requirements for
[0073]
If the system is used for base station downlink, the minimum P _ref Can be calculated as:
P _ref = SNR _spec + NF _link + N _floor + 10 * log (BW)
here,
NF _link Is the noise value (in dB) of the optical link;
Nf _lower Is the noise floor of the transmitter (dBm / Hz), and
BW is the frequency bandwidth (in Hz) of the transmission signal.
[0074]
If the system is used for uplink transmission, P _ref Is more complicated. In this case, there is a possibility that an unnecessary signal stronger than a required signal exists in the same frequency band. Therefore, instead of the SNR of the total incoming signal, _ref Must be set according to the required signal SNR. This requires a response from a baseband (BB) processor implemented in FIG. 9 by multiplexer 127.
[Brief description of the drawings]
FIG.
FIG. 1 is a block diagram showing a system according to a first embodiment of the present invention.
FIG. 2
FIG. 2 is a block diagram showing a second embodiment of the transmission unit of the system shown in FIG.
FIG. 3
FIG. 7 is a diagram illustrating a power ratio between a fundamental signal component and a third-order intermodulation product in an output signal of a nonlinear device.
FIG. 4
FIG. 4 is a diagram illustrating a reduction in the power of the third-order intermodulation product in the optical fiber link according to the present invention.
FIG. 5
FIG. 4 is an “input power” versus “output power” diagram illustrating the power of a third-order intermodulation product in the output signal of a typical nonlinear device in response to input power.
FIG. 6
FIG. 4 is an “input power” vs. “output power” diagram showing the reduction of the third order intermodulation product in the output signal of the system according to the invention for a first set of simulation parameters.
FIG. 7
FIG. 3 is an “input power” vs. “output power” diagram illustrating the reduction of the third-order intermodulation product realized in the system according to the invention for a second set of simulation parameters.
FIG. 8
1 is a block diagram illustrating an optical fiber link as an embodiment of a nonlinear transmission device known in the art.
FIG. 9
FIG. 2 is a block diagram of a modification according to FIG. 1.

Claims (12)

In a system for transmitting a signal via a non-linear transmission unit (200a, 200 '), especially an optical fiber link,
A power splitter (110) for dividing the signal into a main signal and an auxiliary signal;
A power fluctuation detector (120) for detecting a power fluctuation in the auxiliary signal and generating a control signal representing the power fluctuation;
A variable attenuator (130) for attenuating the power of the main signal in response to the control signal so that the power of the main signal input to the nonlinear transmission units (200a, 200 ') is kept constant; ,
An amplifier for amplifying the main signal after transmission via the non-linear transmission unit in response to the transmitted control signal to recover the signal again;
And a system comprising: The power fluctuation detector (120) is for comparing the power of the auxiliary signal with a predetermined power value (P _ref ) and for generating the control signal representing the result of the comparison. The system of claim 1, comprising (126). The control signal according to claim 2, wherein the power of the main signal is kept constant only when the power of the input signal exceeds the predetermined reference power value. system. The power fluctuation detector (120) further comprises a power detector (124) for filtering the auxiliary signal before the auxiliary signal is input to the comparator (126). Item 3. The system according to Item 2. The amplifier of claim 1, further comprising a second amplifier (200c) for amplifying the control signal after transmission of the control signal and before the control signal is input to the amplifier (310). system. The system of claim 1, wherein the system further comprises another transmission unit (200b) for transmitting the control signal to the amplifier (310). The nonlinear transmission unit (200 ') comprises:
A laser diode (250) for converting the main signal into a first optical signal;
A light emitting device (250 ') for converting the control signal into a second optical signal;
A multiplexer (210) for generating a multiplexed signal by multiplexing the first optical signal and the second optical signal;
After transmitting the multiplexed signal from the multiplexer (210) via an optical fiber (260), a first optical signal and a second optical signal are restored from the transmitted multiplexed signal to obtain two signals. Optical fiber (260) for transmission to a demultiplexer (220) for outputting the first and second optical signals respectively to separate photodiodes (270, 270 ')
The system of claim 1, comprising: The method according to claim 2, wherein the predetermined reference power value (P _ref ) is output from a multiplexer (127) that multiplexes a reference voltage (V _ref ) with a response signal from a wideband processor. system. In a method for transmitting a signal via a non-linear transmission unit (200a), in particular a fiber optic link,
Splitting the signal into a main signal and an auxiliary signal;
Detecting power fluctuations in the auxiliary signal and generating a control signal representing the power fluctuations;
Attenuating the power of the main signal in response to the control signal such that the power of the main signal input to the non-linear transmission unit (200a) is kept constant;
Transmitting the main signal via the non-linear transmission unit (200a);
Recovering the signal from the transmitted main signal by amplifying the transmitted main signal in response to the transmitted control signal. The control signal is characterized in that the power of the main signal entering the transmission unit (200a) is kept constant only when the power of the input signal exceeds a predetermined reference power value. 9. The method of claim 8, wherein the method comprises: The preparation unit (100) as part of a system according to claim 1, comprising the power splitter (110), the power fluctuation detector (120), and the variable attenuator (130). The restoration unit (300) as part of a system according to claim 1, comprising a variable gain amplifier (310).

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