JP3798314B2 - Transmission system - Google Patents

Description

【００３５】
ただし、２ｎは変調多値数である。具体的には、ＱＡＭ（Quadrature Amplitude Modulation）の変調多値数として２ｎ＝４、１６、６４、２５６である。Ｎｃはサブキャリア（副搬送波）数（∝加入者端末数）である。ＢＷは無線伝送路区間の１サブキャリア当たりの信号帯域幅（単位はＨｚ）である。
【００３６】
一方、光伝送路における伝送路容量Ｃoptを算出するための評価関数は、式（２）である。
【００３７】
【数２】

【００３８】
ただし、ＢＷmaxは、ＩＭ３またはＬＤ相対雑音強度密度により劣化を受けた光ファイバ伝送路区間での１サブキャリア当たりの信号帯域幅の上限値である。Ｉphは、フォトカレントＲＦ信号の電流値である。Ｎａは、光ファイバ伝送路区間雑音電力であり、以下の式（３）より求まる。
【００３９】
【数３】

【００４０】
式（３）中のＲＩＮはＬＤ相対雑音強度密度、ｅは電荷量、<Ｉth²>は入力等化雑音電流密度（受信回路の熱雑音）である。
また、式（２）中のＤは、ＩＭ３の電力スペクトル値であり、ＬＤ入出力非線形特性曲線の３次係数（ＩＭ３の係数）ａ₃とＮｃとの関数として、式（４ａ）より求まる。
【００４１】
【数４】

【００４２】
式（４ａ）中のｒは、Ｎｃが偶数のときは、ｒ＝Ｎｃ／２であり、Ｎｃが奇数のときは、ｒ＝（Ｎｃ／２）＋１である。また、Ｄ₂（Ｎｃ，ｒ）は式（４ｂ）、Ｄ₃（Ｎｃ，ｒ）は式（４ｃ）で求まる（Ｎｃをｋ、ｒをｉとおく）。さらに、式（２）中のγはＱＡＭ変調において規定誤り率を達成するための所要ＣＮＲ（Carrier Noise Ratio）である。
【００４３】
ここで、光信号送信部２６内部のＥ／Ｏ変換部（ＬＤに該当）における光変調度ｍ（変調光振幅をＬＤ平均出力光パワーで割り算した値と同値）が、以下の式（５）の最適光変調度ｍoptに調整されていることが、光ファイバ伝送路容量Ｃopt算出の必要条件となる。
【００４４】
【数５】

【００４５】
そして、光ファイバ伝送路と無線伝送路を含めたトータルの伝送路容量である全体伝送路容量Ｃｔを算出するための評価関数は、式（６）である。すなわち、無線伝送路容量Ｃｒと光ファイバ伝送路容量Ｃoptのいずれか小さい方が、全体伝送路容量Ｃｔとなる。ｍｉｎ（Ａ，Ｂ）はＡ、Ｂのうちの小さい方を意味する。
【００４６】
【数６】

【００４７】
上記に示したような式を用いて、相互変調歪検出部２３ａ及び評価関数演算部２３ｂは、ＩＭ３の電力スペクトル値Ｄ、全体伝送路容量Ｃｔ、最適光変調度ｍoptを算出する。なお、計算に必要な各種パラメータ値は、ベースバンド信号処理部２４から渡される。
【００４８】
次に無線伝送路容量Ｃｒ、光ファイバ伝送路容量Ｃopt、全体伝送路容量Ｃｔの関係について説明する。図３は伝送路容量と変調多値数の関係を示す図である。縦軸は伝送路容量（ｂ／ｓ）、横軸はＱＡＭのディジタル変調多値数２ｎを示す。
【００４９】
式（１）から、無線伝送路容量Ｃｒは、無線信号帯域ＢＷとサブキャリア数Ｎｃと変調多値数２ｎとに単純比例して、増加することがわかる（図３の曲線（ａ））。
【００５０】
一方、式（２）から、光伝送路容量Ｃoptでは、無線伝送路の場合とは逆に、Ｎｃ及び変調多値数２ｎに対して逆比例して、劣化することがわかる（図３の曲線（ｂ））。これは、変調多値数増加による伝送路容量増加の比率に対して、光伝送路区間で発生するＩＭ３の電力（＝Ｄ）に起因する伝送路容量の劣化比率が上回るためである。また、光ファイバ伝送路容量Ｃoptは、光信号送信部２６のＥ／Ｏ変換部の光変調度ｍにも依存し、最適光変調度ｍoptのときに最大値をとる。
【００５１】
ＲＯＦシステムでは、無線及び光ファイバ両伝送路に、ＲＦ変調信号の同一の情報が通ることから、無線伝送路区間及び光ファイバ伝送路区間のトータルの全体伝送路容量Ｃｔは、ＣｒとＣoptとの最小値で制限されることになる（図３の曲線（ｃ））。
【００５２】
ここで例えば、システム設計時に変調方式を２５６ＱＡＭを採用しようとしても、全体伝送路容量を最大とする最適の変調多値数は、図に示すトレードオフ関係により、２５６ではなく６４であることがわかる。したがって、従来の固定の変調方式によるシステム設計では、当初の所望の伝送路容量を実現することはできず、高品質な伝送が維持できなかった。
【００５３】
本発明では、ＩＭ３量の動的変化に合わせ、光ファイバ伝送路区間及び無線伝送路区間の全体伝送路容量Ｃｔが最大となる最適なディジタル変調方式を選択することにより、高い伝送品質を有するＲＯＦシステムを実現するものである。
【００５４】
次に伝送装置２０の動作について詳しく説明する（第１の実施の形態とする）。図４は第１の実施の形態の伝送装置の動作を説明するための図である。以下のステップＳ１〜Ｓ５はアップストリーム側の処理、ステップＳ６〜ステップＳ８はダウンストリーム側の処理である。なお、伝送装置２０−１の構成は、図１と同様なので構成の説明は省略する。
【００５５】
〔Ｓ１〕基地局３から制御局２へ向けて送出された光信号Ｐoutは、光ファイバ伝送路を通じて光損失を生じ、光信号Ｐａとして制御局２へ到達する。光ファイバロスをＦloss、光信号Ｐoutの光パワーをＰｗとすると、光信号Ｐａの光パワーＰｗａは、Ｐｗａ＝Ｐｗ／Ｆlossである。
【００５６】
〔Ｓ２〕光信号受信部２１は、光信号Ｐａを受信して、Ｏ／Ｅ変換し、フォトカレントＲＦ信号を生成する。フォトカレントＲＦ信号の電流値Ｉphは、Ｉph＝η_PD・Ｐｗａ＝η_PD・Ｐｗ／Ｆlossである。なお、η_PDは、光信号受信部２１の受光感度であり、一般的にはＯ／Ｅ変換の割合を示す。単位はＡ／Ｗである。
【００５７】
〔Ｓ３〕相互変調歪検出部２３ａは、フォトカレントＲＦ信号に対して、周波数スペクトル演算を施して、各サブキャリアの周波数のスペクトル値を算出する。そして、加入者に割り当てている全帯域（Ｎｃ×ＢＷ）の中央の周波数ｆｍ（＝(ｆ１+ｆ_Nc)／２）の周波数成分を抽出する。
【００５８】
ここの周波数位置では、上述したように、加入者を割り当てていないため、サブキャリアは存在しない。したがって、ｆｍに残留している成分の電力スペクトル値を、ＩＭ３の電力スペクトル値として式（４ａ）により求める。
【００５９】
〔Ｓ４〕評価関数演算部２３ｂは、ベースバンド信号処理部２４から渡されるパラメータ値を用いて、式（１）、式（２）、式（５）から、ＱＡＭの変調多値数２ｎ＝４、１６、６４、２５６の各場合に対応する全体伝送路容量Ｃｔを計算する。また、式（５）より、最適光変調度ｍoptを計算する。
【００６０】
そして、評価関数演算部２３ｂは、変調多値数２ｎに対する全体伝送路容量Ｃｔの各値と、最適光変調度ｍoptの値とを、一時的に保持する。そして、変調多値数２ｎに対する全体伝送路容量Ｃｔの各値は、最適変調多値数決定部２５ａへ送信し、最適光変調度ｍoptは光信号送信部２６へ送信する。
【００６１】
〔Ｓ５〕ＲＦ復調部２２は、フォトカレントＲＦ信号を多値ＱＡＭ復調して、無線端末４からのベースバンド信号ｑ１ａ(t)、ｑ２ａ(t)、…を抽出し、ベースバンド信号処理部２４へ送信する。
【００６２】
〔Ｓ６〕最適変調多値数決定部２５ａは、変調多値数２ｎと全体伝送路容量Ｃｔの対応テーブルを作成し、全体伝送路容量Ｃｔの中の最大の全体伝送路容量Ｃｔ_maxに対応する変調多値数（最適変調多値数）を選び出す。
【００６３】
〔Ｓ７〕ＲＦ変調部２５ｂは、ベースバンド信号処理部２４からの各加入者端末に割り当てているベースバンドの送信データｑ１（ｔ）、ｑ２（ｔ）、…ｑＮｃ（ｔ）に、サブキャリア周波数ｆ₁、ｆ₂、ｆ₃、…、ｆ_Ncを割り振り、さらにこれらに最適変調多値数によるディジタル多値ＱＡＭ変調を施してＲＦ変調信号ｒ１（ｔ）、ｒ２（ｔ）、ｒ３（ｔ）、…、ｒＮｃ（ｔ）を生成し出力する。
【００６４】
〔Ｓ８〕光信号送信部２６は、ＲＦ変調信号の電流和ν(t)=Σ[ｒｉ(t)・ｃｏｓ２πｆ_i]に直流バイアス電流Ｉｂを加え、光変調度の調整を行う。この際、最適光変調度ｍoptにより、ｍopt＝|ν|／(Ｉｂ−Ｉth)となるように（ＩthはＬＤのしきい値電流値）、Ｉｂを調整後、光信号送信部２６内部のＥ／Ｏ変換部の駆動電流としてＩｂ＋ν(t)を出力する。
【００６５】
そして、光信号送信部２６内部のＥ／Ｏ変換部は、電流Ｉｂ＋ν(t)で駆動され、最適光変調度ｍoptに維持されたＲＦ光変調信号が出力する。この光信号は、光ファイバ伝送路→基地局３→無線伝送路→無線端末４の順に送信されることになる。
【００６６】
ここで、ＬＤ入出力特性曲線にもとづき、式（４ａ）にでてきたａ₃について説明する。図５はＬＤ入出力特性曲線を示す図である。縦軸はＬＤの出力光パワー、横軸はＬＤの駆動電流Ｉである。
【００６７】
出力光パワーＰは、ＲＦ変調光信号のパワーを指しており、電流Ｉ₀を中心とする変調駆動電流信号により、ＬＤが動作して、ＲＦ変調光信号が生成する。
ＬＤの出力光パワーは、ＬＤしきい値電流Ｉ_th以上での動作領域において，理想的には線形出力であるが、ＬＤのＩ-Ｌ特性の電界分布不均一性や、注入キャリアと光子間の相互作用に起因する非線形現象により、実際には非線形な出力となる。このとき出力光パワーの変調周波成分を次数毎に、べき級数展開で近似すると式（７）となる。
【００６８】
【数７】
Ｐ＝Ｐ₀（１＋ａ₁・ν＋ａ₂・ν²＋ａ₃・ν³＋…） …（７）
この式で２次以上（ａ₂・ν²の項以上）はすべて非線形成分となる。また、各係数ａ₁、ａ₂、ａ₃、…は、各変調成分の強度（振幅）を表している（正確には各々Ｐ₀倍した値が実際の強度である）。式（４ａ）のａ₃は、この式（７）中のａ₃に該当するものである。
【００６９】
次に第２の実施の形態の伝送装置について説明する。図６は第２の実施の形態の伝送装置の構成を示す図である。伝送装置２０−２では、相互変調歪検出部２３ａが、フォトカレントＲＦ信号からＩＭ３を検出するのではなく、光信号送信部２６で生成されたＲＦ変調光信号を再度、電気に変換した電気信号Ｅ１を受信して、この電気信号Ｅ１からＩＭ３を検出するものである（光信号送信部２６内部にあるＬＤ等の非線形性回路により生じるＩＭ３を検出している）。なお、その他の構成及び動作については第１の実施の形態と同様なので説明は省略する。
【００７０】
次に第３の実施の形態の伝送装置について説明する。図７は第３の実施の形態の伝送装置の構成を示す図である。伝送装置２０−３では、相互変調歪検出部２３ａが、フォトカレントＲＦ信号からＩＭ３を検出するのではなく、ＲＦ変調部２５ｂで生成されたＲＦ変調信号（図では符号をＥ２とした）を受信して、この信号Ｅ２からＩＭ３を検出するものである（ＲＦ変調部２５ｂ内部にある非線形性回路により生じるＩＭ３を検出している）。なお、その他の構成及び動作については第１の実施の形態と同様なので説明は省略する。
【００７１】
次に第４の実施の形態の伝送装置について説明する。図８は第４の実施の形態の伝送装置の構成を示す図である。伝送装置２０−４は、第１の実施の形態に対するあらたな構成要素として、無線変調多値数抽出部２３ｃがＲＦ受信制御部２３に設けられる。
【００７２】
無線変調多値数抽出部２３ｃは、フォトカレントＲＦ信号から、無線伝送路区間のみの変調多値数である無線変調多値数を算出して抽出するもので、抽出された無線変調多値数は、最適変調多値数決定部２５ａへ送信される。
【００７３】
一方、最適変調多値数決定部２５ａでは、変調多値数と、各変調多値数により算出された全体伝送路容量Ｃｔとの対応テーブルを有しており、この対応テーブル上で現在の無線変調多値数の値に対応する全体伝送路容量をＣｔｒ、あらかじめ設定してある規定劣化量をΔとした場合に、Ｃｔ_max−Ｃｔｒ＞Δの場合は、最大値の全体伝送路容量Ｃｔ_maxに対応する変調多値数をテーブルから選択して、これを最適変調多値数とし、Ｃｔ_max−Ｃｔｒ≦Δの場合は、現在の全体伝送路容量Ｃｔｒに対応する変調多値数をテーブルから選択して、これを最適変調多値数とする。
【００７４】
このような制御を行う理由は、Ｃｔ_maxとＣｔｒの差分が小さい場合にも、変調方式を頻繁に変化させると、制御局２〜無線端末４間で、変調方式が確立するまでの時間がかかり、回線通信品質を維持できなくなるからである。品質維持のためには、制御に要するオーバヘッド時間を短縮することが必要である。したがって、第４の実施の形態では、伝送路容量に対する劣化量が規定値を超えた場合に、変調方式を切り替えることにして、オーバヘッド時間を短縮するものである。
【００７５】
次に無線端末４について説明する。図９は無線端末４の構成を示す図である。無線端末４は、無線受信部４１、無線送信部４２、端末側ベースバンド信号処理部４３から構成される。無線受信部４１は、端末側ＲＦ復調部４１ａ、最適変調多値数検出部４１ｂを含み、無線送信部４２は端末側ＲＦ変調部４２ａを含む。
【００７６】
〔Ｓ１１〕制御局２から送信されたＲＦ変調光信号は、基地局３で無線信号となり、無線端末４のアンテナ(図示せず)を通じて受信される。無線端末４の端末側ＲＦ復調部４１ａは、受信信号を復調してベースバンド信号を生成し、これを端末側ベースバンド信号処理部４３へ送信する。
【００７７】
〔Ｓ１２〕最適変調多値数検出部４１ｂは、制御局２内の伝送装置２０で設定された最適変調多値数を検出する。
〔Ｓ１３〕端末側ベースバンド信号処理部４３は、ベースバンド信号を処理する。
【００７８】
〔Ｓ１４〕端末側ＲＦ変調部４２ａは、端末側ベースバンド信号処理部４３で処理されたベースバンド信号を、最適変調多値数検出部４１ｂで検出された最適変調多値数で変調し、ＲＦ変調信号（端末側ＲＦ変調信号）を生成する。そして、端末側ＲＦ変調信号は、アンテナを通じて基地局３へ無線で送出される。また、この無線信号は、基地局３で光信号に変換されて制御局２へ送信される。
【００７９】
次に第１の実施の形態の伝送装置２０−１に関する具体的な構成例及び動作について説明する。図１０は第１の実施の形態の伝送装置２０−１の構成例を示す図である。
【００８０】
ＰＩＮフォトダイオード２１ａは、光信号受信部２１に該当し、ＲＦ直交復調回路２２ａは、ＲＦ復調部２２に該当する。相互変調歪検出部２３ａは、ＦＦＴ（Fast Fourier Transform）演算処理回路２３ａ−１、ＩＭ３成分計算処理回路２３ａ−２を含む。評価関数演算部２３ｂは、伝送路容量計算処理回路２３ｂ−１、最適光変調度計算処理回路２３ｂ−２、パラメータ格納メモリ２３ｂ−３、レジスタ２３ｂ−１ａ、２３ｂ−２ａを含む。
【００８１】
また、最適変調多値数決定部２５ａは、対応テーブル格納メモリ２５ａ−１、セレクタ２５ａ−２を含み、多値ＱＡＭ変調回路２５ｂ−１は、ＲＦ変調部２５ｂに該当する。さらに、光信号送信部２６は、バイアス電流調整回路２６ａ、バイアス印加部２６ｂ、Ｅ／Ｏ変換を行うＬＤ２６ｃを含む。
【００８２】
ここで、ＰＩＮフォトダイオード２１ａは、受信光信号のＯ／Ｅ変換を行う。ＦＦＴ演算処理回路２３ａ−１は、受信電気信号を高速フーリエ変換演算し、周波数スペクトル成分を算出する。ＩＭ３成分計算処理回路２３ａ−２は、全帯域中央の周波数ｆｍに関するＩＭ３の電力スペクトル値Ｄを算出する。
【００８３】
伝送路容量計算処理回路２３ｂ−１は，電力スペクトル値Ｄをもとに、ＱＡＭ変調多値数に対する全体伝送路容量Ｃｔを算出する。最適光変調度計算処理回路２３ｂ−２は、電力スペクトル値Ｄをもとに、最適光変調度ｍoptを算出する。パラメータ格納メモリ２３ｂ−３は、ベースバンド信号処理部２４から渡された計算に必要な各種パラメータを格納し、伝送路容量計算処理回路２３ｂ−１及び最適光変調度計算処理回路２３ｂ−２に出力する。
【００８４】
レジスタ２３ｂ−１ａは、伝送路容量計算処理回路２３ｂ−１の計算結果を保持し、レジスタ２３ｂ−２ａは、最適光変調度計算処理回路２３ｂ−２の計算結果を保持する。
【００８５】
ＲＦ直交復調回路２２ａは、２ｎ値のＱＡＭ変調されている受信電気信号を直交復調し、ベースバンド信号を生成し、ベースバンド信号処理部２４へ送信する。
【００８６】
対応テーブル格納メモリ２５ａ−１は、レジスタ２３ｂ−１ａで保持された全体伝送路容量Ｃｔを受け取り、変調多値数と全体伝送路容量Ｃｔとの対応テーブルを記憶する。
【００８７】
セレクタ２５ａ−２は、対応テーブルの中で、最大となる全体伝送路容量Ｃｔ_maxを選び出し、それに相当する最適変調多値数を多値ＱＡＭ変調回路２５ｂ−１へ出力する。多値ＱＡＭ変調回路２５ｂ−１は、ベースバンド信号処理部２４から送信された送信データを、最適変調多値数で変調してＲＦ変調信号を生成する。
【００８８】
バイアス電流調整回路２６ａは、最適光変調度ｍoptにもとづいて、このｍoptを維持できるようにバイアス電流値を調整する。バイアス印加部２６ｂは、ＲＦ変調信号にバイアス電流を印加してＬＤ駆動信号を生成する。ＬＤ２６ｃは、ＬＤ駆動信号にもとづきＲＦ光変調信号を生成し送信する。
【００８９】
次に第２の実施の形態の伝送装置２０−２に関する具体的な構成例及び動作について説明する。図１１は第２の実施の形態の伝送装置２０−２の構成例を示す図である。
【００９０】
伝送装置２０−２は、図１０で上述した伝送装置２０−１の光信号送信部２６内部に、あらたにモニタ用フォトダイオード（モニタ用ＰＤ）２６ｄが設置される。モニタ用ＰＤ２６ｄの出力信号Ｅ１は、ＦＦＴ演算処理回路２３ａ−１へ入力して、その後、ＩＭ３の検出が行われる。その他の構成及び動作は、第１の実施の形態と同様なので説明は省略する。
【００９１】
次に第３の実施の形態の伝送装置２０−３に関する具体的な構成例及び動作について説明する。図１２は第３の実施の形態の伝送装置２０−３の構成例を示す図である。
【００９２】
伝送装置２０−３は、図１０で上述した伝送装置２０−１に対し、多値ＱＡＭ変調回路２５ｂ−１の出力信号Ｅ２を、ＦＦＴ演算処理回路２３ａ−１へ入力する。その後、ＩＭ３の検出が行われる。その他の構成及び動作は、第１の実施の形態と同様なので説明は省略する。
【００９３】
次に第４の実施の形態の伝送装置２０−４に関する具体的な構成例及び動作について説明する。図１３、図１４は第４の実施の形態の伝送装置２０−４の構成例を示す図である。
【００９４】
伝送装置２０−４は、図１０で上述した伝送装置２０−１に対し、ＲＦ受信制御部２３内部に、あらたに無線変調多値数抽出部２３ｃを設け、さらに最適変調多値数決定部２５ａ内部に、あらたに劣化量判断部２５ａ−４、しきい値格納メモリ２５ａ−３が設けられる。その他の構成要素は伝送装置２０−１と同様である。
【００９５】
ここで、ＰＩＮフォトダイオード２１ａは、受信光信号のＯ／Ｅ変換を行う。ＦＦＴ演算処理回路２３ａ−１は、受信光信号を高速フーリエ変換演算し、周波数スペクトル成分を算出する。ＩＭ３成分計算処理回路２３ａ−２は、全帯域中央の周波数ｆｍに関するＩＭ３の電力スペクトル値Ｄを算出する。
【００９６】
無線変調多値数抽出部２３ｃは、無線伝送路区間の伝搬特性等から、無線伝送路区間のみを考慮した場合の変調多値数を選び出す（無線変調多値数抽出部２３ｃの技術としては、例えば、特開平７−２５０１１６号公報記載の技術が適用可能である）。
【００９７】
伝送路容量計算処理回路２３ｂ−１は，ＩＭ３の電力スペクトル値Ｄをもとに、ＱＡＭ変調多値数に対する全体伝送路容量Ｃｔを算出する。最適光変調度計算処理回路２３ｂ−２は、ＩＭ３の電力スペクトル値Ｄをもとに、最適光変調度ｍoptを算出する。
【００９８】
パラメータ格納メモリ２３ｂ−３は、ベースバンド信号処理部２４から渡された計算に必要な各種パラメータを格納し、伝送路容量計算処理回路２３ｂ−１及び最適光変調度計算処理回路２３ｂ−２へ出力する。レジスタ２３ｂ−１ａは、伝送路容量計算処理回路２３ｂ−１の計算結果を保持し、レジスタ２３ｂ−２ａは、最適光変調度計算処理回路２３ｂ−２の計算結果を保持する。
【００９９】
ＲＦ直交復調回路２２ａは、２ｎ値のＱＡＭ変調された受信電気信号を直交復調し、ベースバンド信号を生成し、ベースバンド信号処理部２４へ送信する。
対応テーブル格納メモリ２５ａ−１は、レジスタ２３ｂ−１ａで保持された全体伝送路容量Ｃｔを受け取り、変調多値数と全体伝送路容量Ｃｔとの対応テーブルを記憶する。
【０１００】
しきい値格納メモリ２５ａ−３は、伝送路容量劣化の規定劣化量Δを格納する。劣化量判断部２５ａ−４は、対応テーブルを用いて、全体伝送路容量の差分（Ｃｔ_max−Ｃｔｒ）と、規定劣化量Δを比較し判定する。セレクタ２５ａ−２は、比較判定結果を受けて、最適変調多値数を決定して、多値ＱＡＭ変調回路２５ｂ−１へ出力する。
【０１０１】
多値ＱＡＭ変調回路２５ｂ−１は、ベースバンド信号処理部２４から送信された送信データを、最適変調多値数で変調してＲＦ変調信号を生成する。
バイアス電流調整回路２６ａは、最適光変調度ｍoptにもとづいて、このｍoptを維持できるようにバイアス電流値を調整する。バイアス印加部２６ｂは、ＲＦ変調信号にバイアス電流を印加してＬＤ駆動信号を生成する。ＬＤ２６ｃは、ＬＤ駆動信号にもとづきＲＦ光変調信号を生成し送信する。
【０１０２】
以上説明したように，本発明によれば、ＲＯＦのシステム内のＩＭ３発生量変化に柔軟に対応して、最適な変調多値数を決定し、制御局２から無線端末４間での全体伝送路容量を最大化して、高品質な通信を行うことが可能になる。
【０１０３】
なお、上記に示した実施の形態に対して、各実施の形態を任意に組み合わせて伝送装置を構成することができる。例えば、モニタ用ＰＤ２６ｄからの電気信号Ｅ１からＩＭ３を検出する第２の実施の形態と、全体伝送路容量の差分値を規定劣化量と比較して最適変調多値数を決定する第４の実施の形態と、を組み合わせて伝送装置を構成してもよい。
【０１０４】
（付記１） 無線による情報を光ファイバで伝送する伝送システムにおいて、光ファイバ伝送路を通じて送信された光信号を受信し、光／電気変換して、フォトカレントＲＦ信号を生成する光信号受信部と、前記フォトカレントＲＦ信号を復調してベースバンド帯域に周波数変換してベースバンド信号を生成するＲＦ復調部と、前記フォトカレントＲＦ信号から周波数毎のスペクトルを算出し、全帯域中央に位置する周波数のスペクトルから、相互変調歪の歪成分情報を検出する相互変調歪検出部と、前記歪成分情報にもとづき評価関数を演算し、最適光変調度と、変調多値数毎に、光ファイバ伝送路と無線伝送路を含めた全体伝送路容量とを算出する評価関数演算部と、から構成されるＲＦ受信制御部と、前記ベースバンド信号を処理し、送信データを生成し、評価関数演算時に必要なパラメータ値を出力するベースバンド信号処理部と、前記全体伝送路容量の最大値をとる最適変調多値数を決定する最適変調多値数決定部と、前記最適変調多値数を用いて、前記送信データにＲＦ変調を行ってＲＦ変調信号を生成するＲＦ変調部と、から構成されるＲＦ送信制御部と、前記最適光変調度を用いて、前記ＲＦ変調信号の電気／光変換を制御して、ＲＦ変調光信号を生成し送信する光信号送信部と、から構成される伝送装置と、
無線信号を受信する無線受信部と、前記伝送装置で決定された変調多値数でベースバンド信号を変調して端末側ＲＦ変調信号を生成し、無線送信する無線送信部と、から構成される無線端末と、
前記無線端末と無線伝送路で接続し、前記伝送装置と光ファイバ伝送路で接続して、前記無線端末と前記伝送装置間の中継伝送を行う中継装置と、
を有することを特徴とする伝送システム。
【０１０５】
（付記２） 前記相互変調歪検出部は、相互変調歪の前記歪成分情報として、加入者に割り当てた全帯域に対し、全帯域中央の位置の周波数に残留する、３トーンで形成される３次相互変調歪の歪成分情報を検出することを特徴とする付記１記載の伝送システム。
【０１０６】
（付記３） 前記相互変調歪検出部は、前記フォトカレントＲＦ信号の代わりに、前記ＲＦ変調光信号を電気に変換した電気信号または前記ＲＦ変調信号から、相互変調歪の前記歪成分情報を検出することを特徴とする付記１記載の伝送システム。
【０１０７】
（付記４） 前記フォトカレントＲＦ信号から、無線伝送路区間のみの変調多値数である無線変調多値数を算出して抽出する無線変調多値数抽出部をさらに有し、前記最適変調多値数決定部は、算出した全体伝送路容量と変調多値数とのテーブルを作成し、最大値となる全体伝送路容量をＣｔ_max、前記テーブル上で前記無線変調多値数の値に対応する現在の全体伝送路容量をＣｔｒ、規定劣化量をΔとした場合に、Ｃｔ_max−Ｃｔｒ＞Δの場合は、全体伝送路容量Ｃｔ_maxに対応する変調多値数を最適変調多値数とし、Ｃｔ_max−Ｃｔｒ≦Δの場合は、現在の全体伝送路容量Ｃｔｒに対応する変調多値数を最適変調多値数とすることを特徴とする付記１記載の伝送システム。
【０１０８】
（付記５） 無線による情報を光ファイバで伝送する伝送装置において、
光ファイバ伝送路を通じて送信された光信号を受信し、光／電気変換して、フォトカレントＲＦ信号を生成する光信号受信部と、
前記フォトカレントＲＦ信号を復調してベースバンド帯域に周波数変換してベースバンド信号を生成するＲＦ復調部と、
前記フォトカレントＲＦ信号から周波数毎のスペクトルを算出し、全帯域中央に位置する周波数のスペクトルから、相互変調歪の歪成分情報を検出する相互変調歪検出部と、前記歪成分情報にもとづき評価関数を演算し、最適光変調度と、変調多値数毎に、光ファイバ伝送路と無線伝送路を含めた全体伝送路容量とを算出する評価関数演算部と、から構成されるＲＦ受信制御部と、
前記ベースバンド信号を処理し、送信データを生成し、評価関数演算時に必要なパラメータ値を出力するベースバンド信号処理部と、
前記全体伝送路容量の最大値をとる最適変調多値数を決定する最適変調多値数決定部と、前記最適変調多値数を用いて、前記送信データにＲＦ変調を行ってＲＦ変調信号を生成するＲＦ変調部と、から構成されるＲＦ送信制御部と、
前記最適光変調度を用いて、前記ＲＦ変調信号の電気／光変換を制御して、ＲＦ変調光信号を生成し送信する光信号送信部と、
を有することを特徴とする伝送装置。
【０１０９】
（付記６） 前記相互変調歪検出部は、相互変調歪の前記歪成分情報として、加入者に割り当てた全帯域に対し、全帯域中央の位置の周波数に残留する、３トーンで形成される３次相互変調歪の歪成分情報を検出することを特徴とする付記５記載の伝送装置。
【０１１０】
（付記７） 前記相互変調歪検出部は、前記フォトカレントＲＦ信号の代わりに、前記ＲＦ変調光信号を電気に変換した電気信号または前記ＲＦ変調信号から、相互変調歪の前記歪成分情報を検出することを特徴とする付記５記載の伝送装置。
【０１１１】
（付記８） 前記フォトカレントＲＦ信号から、無線伝送路区間のみの変調多値数である無線変調多値数を算出して抽出する無線変調多値数抽出部をさらに有し、前記最適変調多値数決定部は、算出した全体伝送路容量と変調多値数とのテーブルを作成し、最大値となる全体伝送路容量をＣｔ_max、前記テーブル上で前記無線変調多値数の値に対応する現在の全体伝送路容量をＣｔｒ、規定劣化量をΔとした場合に、Ｃｔ_max−Ｃｔｒ＞Δの場合は、全体伝送路容量Ｃｔ_maxに対応する変調多値数を最適変調多値数とし、Ｃｔ_max−Ｃｔｒ≦Δの場合は、現在の全体伝送路容量Ｃｔｒに対応する変調多値数を最適変調多値数とすることを特徴とする付記５記載の伝送装置。
【０１１２】
（付記９） 無線通信を行う無線端末において、
無線信号を受信する無線受信部と、
局側に設置された伝送装置で決定された変調多値数でベースバンド信号を変調して端末側ＲＦ変調信号を生成し、無線送信する無線送信部と、
を有することを特徴とする無線端末。
【０１１３】
（付記１０） 信号受信制御を行うＲＦ受信装置において、
光ファイバ伝送路を通じて送信された光信号を受信し、光／電気変換して、フォトカレントＲＦ信号を生成する光信号受信部と、
前記フォトカレントＲＦ信号から周波数毎のスペクトルを算出し、全帯域中央に位置する周波数のスペクトルから、相互変調歪の歪成分情報を検出する相互変調歪検出部と、
前記歪成分情報にもとづき評価関数を演算し、最適光変調度と、変調多値数毎に、光ファイバ伝送路と無線伝送路を含めた全体伝送路容量とを算出する評価関数演算部と、
を有することを特徴とするＲＦ受信装置。
【０１１４】
（付記１１） 信号送信制御を行うＲＦ送信装置において、
評価関数演算時に求めた、光ファイバ伝送路と無線伝送路を含めた全体伝送路容量に対する最大値をとる最適変調多値数を決定する最適変調多値数決定部と、前記最適変調多値数を用いて、送信データにＲＦ変調を行ってＲＦ変調信号を生成するＲＦ変調部と、
評価関数演算時に求めた最適光変調度を用いて、前記ＲＦ変調信号の電気／光変換を制御して、ＲＦ変調光信号を生成し送信する光信号送信部と、
を有することを特徴とするＲＦ送信装置。
【０１１５】
【発明の効果】
以上説明したように、本発明の伝送システムは、伝送装置では、相互変調歪の歪成分情報から、最適光変調度と、光ファイバ伝送路と無線伝送路を含めた全体伝送路容量とを算出して、全体伝送路容量の最大値をとる最適変調多値数を決定する。そして、最適変調多値数を用いてＲＦ変調信号を生成し、最適光変調度を用いて、ＲＦ変調信号の電気／光変換の制御をして、ＲＦ変調光信号を生成し送信する。また、無線端末では、伝送装置で決定された変調多値数でベースバンド信号を変調して端末側ＲＦ変調信号を生成する構成とした。これにより、相互変調歪の動的変化に合わせて、最適な変調多値数が決定され、制御局から無線端末間での全体伝送路容量を最大化できるので、高品質な通信を行うことが可能になる。
【図面の簡単な説明】
【図１】本発明の伝送システムの原理図である。
【図２】サブキャリアに対するＩＭ３の発生イメージを示す図である。
【図３】伝送路容量と変調多値数の関係を示す図である。
【図４】第１の実施の形態の伝送装置の動作を説明するための図である。
【図５】ＬＤ入出力特性曲線を示す図である。
【図６】第２の実施の形態の伝送装置の構成を示す図である。
【図７】第３の実施の形態の伝送装置の構成を示す図である。
【図８】第４の実施の形態の伝送装置の構成を示す図である。
【図９】無線端末の構成を示す図である。
【図１０】第１の実施の形態の伝送装置の構成例を示す図である。
【図１１】第２の実施の形態の伝送装置の構成例を示す図である。
【図１２】第３の実施の形態の伝送装置の構成例を示す図である。
【図１３】第４の実施の形態の伝送装置の構成例を示す図である。
【図１４】第４の実施の形態の伝送装置の構成例を示す図である。
【図１５】ＲＯＦシステムの全体構成を示す図である。
【符号の説明】
１ 伝送システム
２ 制御局
３ 基地局
４ 無線端末
５ ネットワーク
２０ 伝送装置
２１ 光信号受信部
２２ ＲＦ復調部
２３ ＲＦ受信制御部
２３ａ 相互変調歪検出部
２３ｂ 評価関数演算部
２４ ベースバンド信号処理部
２５ ＲＦ送信制御部
２５ａ 最適変調多値数決定部
２５ｂ ＲＦ変調部
２６ 光信号送信部
３０ 中継装置
４１ 無線受信部
４２ 無線送信部
Ｃｔ 全体伝送路容量
ｍopt 最適光変調度[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a transmission system, and more particularly to a transmission system that transmits information by radio using an optical fiber.
[0002]
[Prior art]
In recent years, demand for wireless communication such as mobile communication and fixed wireless access (FWA) has spread to general consumers, and the number of wireless communication subscribers is rapidly increasing. Under such circumstances, a system called ROF (Radio on Fiber) is currently attracting attention as a technique for transmitting radio signal information using an optical fiber.
[0003]
FIG. 15 is a diagram showing the overall configuration of the ROF system. The ROF system (optical fiber radio communication system) 100 includes a control station (CS) 200, a base station (BS) 300, and a wireless terminal (WT) 400.
[0004]
The control station 200 and the base station 300 are connected by optical fibers (for upstream and downstream), and the base station 300 and the wireless terminal 400 are connected wirelessly through respective antennas. The control station 200 is connected to a network 5 including the Internet.
[0005]
The ROF system uses a wide bandwidth of optical fiber to superimpose information on RF (Radio Frequency) signals specific to various services on an optical carrier and transmits it through an optical fiber. It is also possible to transmit the optical signal in the same optical fiber.
[0006]
By using such an ROF system 100 that combines optical fiber communication and wireless communication, it is possible to integrate wireless services of different frequencies and realize a multi-service wireless transmission system that transmits using a single optical fiber. become.
[0007]
On the other hand, in the conventional wireless transmission, a method in which the modulation multilevel number is fixed to a constant value is mainly used regardless of the transmission path capacity. However, in recent years, the modulation multilevel number has been set to be variable. A method for controlling the transmission line capacity has been proposed.
[0008]
For example, in "adaptive modulation system" (Shozo Komaki, "Study on variable capacitance microwave system", IEICE Transactions B-II, J 73-B-II, No. 10, Oct. 1990) A radio modulation scheme is selected according to the situation of the transmission path section characteristics to increase the transmission path capacity.
[0009]
[Problems to be solved by the invention]
However, the prior art as described above is an optimization considering only the wireless transmission path section between the base station and the wireless terminal, and the existence section of the signal is not only between the base station and the wireless terminal but also via the optical fiber. It cannot be applied directly to the ROF system 100 that is extended to the section of the control station.
[0010]
On the other hand, there is intermodulation that should be considered in terms of noise and interference in the transmission system. This is a phenomenon in which when a plurality of signals are input to a non-linear circuit, a signal having a third frequency different from the frequency of the original signal is generated. The distortion component generated by the intermodulation deteriorates the transmission quality.
[0011]
Since the ROF system 100 uses a nonlinear circuit such as an LD (laser diode) when converting an RF signal into light, third-order intermodulation distortion (IM3: 3rd order Inter Modulation) occurs.
[0012]
This third-order intermodulation distortion depends on the number of subcarriers (number of subcarriers) proportional to the number of subscribers accommodated, and when the number of subscribers changes, the amount of third-order intermodulation distortion also changes in the same way. Capacity also changes dynamically. For this reason, there is a problem that high-quality transmission becomes difficult particularly in mobile radio transmission in which traffic changes are drastic.
[0013]
Therefore, for the ROF system 100, the modulation multi-level number is variably set to maximize the transmission path capacity (total transmission path capacity of the optical fiber transmission path section and the wireless transmission path section) to improve the quality. Need to perform optimization control flexibly corresponding to the change of the third-order intermodulation distortion.
[0014]
The present invention has been made in view of such points, and in accordance with the dynamic change of the third-order intermodulation distortion, the optimum modulation multi-value number is determined, the transmission line capacity is maximized, and the transmission quality is improved. It is an object of the present invention to provide an improved transmission system.
[0015]
[Means for Solving the Problems]
In the present invention, in order to solve the above-described problem, in a transmission system 1 for transmitting wireless information through an optical fiber as shown in FIG. 1, an optical signal transmitted through an optical fiber transmission line is received, and optical / electrical conversion is performed. An optical signal receiver 21 that generates a photocurrent RF signal, an RF demodulator 22 that demodulates the photocurrent RF signal and converts the frequency into a baseband band to generate a baseband signal, and a photocurrent RF signal. A spectrum for each frequency is calculated, an intermodulation distortion detector 23a that detects distortion component information of the intermodulation distortion from a spectrum of frequencies located at the center of the entire band, an evaluation function is calculated based on the distortion component information, and the optimum light An evaluation function calculator 23b for calculating the modulation degree mopt and the total transmission line capacity Ct including the optical fiber transmission line and the wireless transmission line for each modulation multi-value number; The configured RF reception control unit 23, the baseband signal processing unit 24 that processes the baseband signal, generates transmission data, and outputs the parameter value necessary for the evaluation function calculation, and the maximum value of the total transmission line capacity Ct An optimum modulation multi-level number determining unit 25a that determines the optimal modulation multi-level number that takes the following: an RF modulation unit 25b that performs RF modulation on transmission data using the optimal modulation multi-level number and generates an RF modulation signal An RF transmission control unit 25 configured and an optical signal transmission unit 26 that generates and transmits an RF modulated optical signal by controlling electrical / optical conversion of the RF modulated signal using the optimum optical modulation degree mopt. Transmission device 20, a wireless reception unit 41 that receives a wireless signal, a baseband signal that is modulated by a modulation multi-value determined by the transmission device 20 to generate a terminal-side RF modulated signal, and wireless transmission Transmitter 42 Are connected to the wireless terminal 4 via a wireless transmission path, connected to the transmission apparatus 20 via an optical fiber transmission path, and perform relay transmission between the wireless terminal 4 and the transmission apparatus 20. And a transmission system 1 characterized by comprising:
[0016]
Here, the optical signal receiving unit 21 receives a signal transmitted through the optical fiber transmission line, performs optical / electrical conversion, and generates a photocurrent RF signal. The RF demodulator 22 demodulates the photocurrent RF signal, converts the frequency into a baseband band, and generates a baseband signal. The intermodulation distortion detector 23a calculates a spectrum for each frequency from the photocurrent RF signal, and detects distortion component information of the intermodulation distortion from the spectrum of the frequency located at the center of the entire band. The evaluation function calculation unit 23b calculates an evaluation function based on the distortion component information, and calculates the optimum optical modulation degree mopt and the total transmission line capacity Ct including the optical fiber transmission line and the wireless transmission line for each modulation multilevel number. calculate. The baseband signal processing unit 24 processes the baseband signal, generates transmission data, and outputs parameter values necessary for calculating the evaluation function. The optimum modulation multilevel number determination unit 25a determines the optimum modulation multilevel number that takes the maximum value of the total transmission path capacity Ct. The RF modulation unit 25b performs RF modulation on the transmission data using the optimum modulation multi-value number to generate an RF modulation signal. The optical signal transmission unit 26 generates and transmits an RF modulated optical signal by controlling the electrical / optical conversion of the RF modulated signal using the optimum optical modulation degree mopt. The wireless receiving unit 41 receives a wireless signal. The wireless transmission unit 42 modulates the baseband signal with the modulation multi-value number determined by the transmission apparatus 20 to generate a terminal-side RF modulated signal, and wirelessly transmits it. The relay device 30 is connected to the wireless terminal 4 via a wireless transmission path, and is connected to the transmission device 20 via an optical fiber transmission path to perform relay transmission between the wireless terminal 4 and the transmission device 20.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a principle diagram of a transmission system according to the present invention. The transmission system 1 is an ROF system that transmits information transmitted by radio using an optical fiber, and includes a control station 2 including a transmission apparatus 20, a base station 3 including a relay apparatus 30, and a radio terminal 4.
[0018]
The control station 2 and the base station 3 are connected by upstream and downstream optical fibers, and the base station 3 and the wireless terminal 4 are connected by radio. Further, a network 5 including the Internet is connected to the control station 2.
[0019]
The transmission device 20 includes an optical signal receiver 21, an RF demodulator 22, an RF reception controller 23, a baseband signal processor 24, an RF transmission controller 25, and an optical signal transmitter 26.
[0020]
The optical signal receiving unit 21 receives an upstream optical signal transmitted through the optical fiber transmission line, and performs optical / electrical (hereinafter referred to as O / E) conversion to generate a photocurrent RF signal.
[0021]
The RF demodulator 22 demodulates the photocurrent RF signal, converts the frequency into a baseband band, and generates a baseband signal.
The RF reception controller 23 includes an intermodulation distortion detector 23a and an evaluation function calculator 23b. The intermodulation distortion detector 23a calculates a spectrum for each frequency from the photocurrent RF signal, and detects distortion component information of the intermodulation distortion from the spectrum of the frequency located at the center of the entire band.
[0022]
In the system of the present invention, no subscriber is assigned to the frequency located at the center of the entire band. In the present invention, since the third-order intermodulation distortion is targeted as the intermodulation distortion, the intermodulation distortion is hereinafter referred to as IM3.
[0023]
The evaluation function calculation unit 23b calculates an evaluation function based on the distortion component information of IM3, and the total transmission path capacity including the optical fiber transmission path and the wireless transmission path for each of the optimum optical modulation degree mopt and the modulation multi-value number. Ct is calculated. Details of the evaluation function and the calculation method will be described later.
[0024]
The baseband signal processing unit 24 processes the baseband signal, generates transmission data, and outputs parameter values necessary for calculating the evaluation function.
The RF transmission control unit 25 includes an optimum modulation multilevel number determination unit 25a and an RF modulation unit 25b. The optimum modulation multilevel number determining unit 25a determines an optimal modulation multilevel number (the optimum value of the QAM digital modulation multilevel number) that takes the maximum value of the overall transmission line capacity Ct. That is, among the plurality of total transmission line capacities Ct calculated for each modulation multilevel number, the total transmission line capacity (Ct _max The modulation multi-level number corresponding to (3) is the optimum modulation multi-level number.
[0025]
The RF modulation unit 25b performs RF modulation on the transmission data using the optimum modulation multi-value number to generate an RF modulation signal.
The optical signal transmission unit 26 controls the electrical / optical (hereinafter referred to as E / O) conversion of the RF modulation signal using the optimum optical modulation degree mopt, generates an RF modulated optical signal, and downstream optical fiber Send through.
[0026]
For the wireless terminal 4, the wireless reception unit 41 receives a wireless signal. The wireless transmission unit 42 modulates the baseband signal with the modulation multi-level number (optimum modulation multi-level number) determined by the transmission apparatus 20, generates a terminal-side RF modulation signal, and wirelessly transmits it.
[0027]
The relay device 30 is connected to the wireless terminal 4 through a wireless transmission line, and is connected to the transmission device 20 through an optical fiber transmission line. Then, relay transmission between the wireless terminal 4 and the transmission device 20 is performed. Next, IM3 will be described. Now, frequency f ₁ And f ₂ The frequency f of the intermodulation wave generated when the above signal is input to the nonlinear circuit _i Is | m · f ₁ ± n · f ₂ (M and n are positive integers), and m + n is called the order of the intermodulation wave. IM3 is an intermodulation wave of m + n = 3.
[0028]
FIG. 2 is a diagram showing an image of IM3 generation for a subcarrier. The vertical axis is power, and the horizontal axis is frequency. Here, consider the case where Nc = 3, where Nc is the number of subcarriers. Furthermore, the frequency of each subcarrier is f ₁ , F ₂ , F _Three And the bandwidth per subcarrier is BW.
[0029]
The frequency of the intermodulation wave I1 is 2f with respect to the intermodulation waves I1 to I3 of IM3. ₂ -F _Three The frequency of the intermodulation wave I2 is f ₁ -F ₂ + F _Three The frequency of the intermodulation wave I3 is 2f ₂ -F ₁ (Both m + n = 3).
[0030]
Further, IM3 generated from a two-wave signal is called a two-tone IM3, and IM3 generated from a three-wave signal is called a three-tone IM3. Intermodulation waves I1 and I3 are two-tone IM3, and intermodulation wave I2 is three-tone IM3.
[0031]
As can be seen from the figure, the 3-tone IM3 occurs in the center of the entire band, and the power spectrum value is larger (closer to twice) than the 2-tone IM3. Further, as the number of subcarriers (the number of subscribers) increases, the power spectrum value of the 3-tone IM3 in the center of the entire band increases. For this reason, when extracting IM3 information, as a worst-case evaluation, three-tone IM3 generated in the center of all bands allocated to subscribers may be considered.
[0032]
In the system of the present invention, the center frequency fm (= (f ₁ + F _Nc Since no subscribers are assigned to) / 2), there is no subcarrier at this frequency position. Therefore, the power spectrum value of the component remaining at the frequency fm can be regarded as the power spectrum value of IM3. The power spectrum value of IM3 corresponds to D calculated by the equation (4a) described later, and corresponds to distortion component information.
[0033]
Next, a calculation method of IM3 and a calculation method of the transmission path capacity and the optimum optical modulation degree mopt in the evaluation function will be described. The evaluation function for calculating the transmission path capacity Cr in the wireless transmission path is Expression (1).
[0034]
[Expression 1]

[0035]
However, 2n is a modulation multilevel number. Specifically, 2n = 4, 16, 64, 256 as the modulation multilevel number of QAM (Quadrature Amplitude Modulation). Nc is the number of subcarriers (subcarriers) (the number of subscriber terminals). BW is a signal bandwidth (unit: Hz) per subcarrier in the wireless transmission path section.
[0036]
On the other hand, the evaluation function for calculating the transmission line capacity Copt in the optical transmission line is Equation (2).
[0037]
[Expression 2]

[0038]
However, BWmax is the upper limit value of the signal bandwidth per subcarrier in the optical fiber transmission line section that has been degraded by IM3 or LD relative noise intensity density. Iph is the current value of the photocurrent RF signal. Na is the optical fiber transmission line section noise power, and is obtained from the following equation (3).
[0039]
[Equation 3]

[0040]
RIN in equation (3) is the LD relative noise intensity density, e is the charge amount, <Ith ² > Is the input equalization noise current density (thermal noise of the receiving circuit).
Further, D in the formula (2) is the power spectrum value of IM3, and the third-order coefficient (coefficient of IM3) a of the LD input / output nonlinear characteristic curve a _Three And Nc can be obtained from the equation (4a).
[0041]
[Expression 4]

[0042]
R in the formula (4a) is r = Nc / 2 when Nc is an even number, and r = (Nc / 2) +1 when Nc is an odd number. D ₂ (Nc, r) is the equation (4b), D _Three (Nc, r) is obtained by the equation (4c) (Nc is set to k and r is set to i). Further, γ in Equation (2) is a required CNR (Carrier Noise Ratio) for achieving a specified error rate in QAM modulation.
[0043]
Here, the optical modulation degree m (equivalent to the value obtained by dividing the modulated light amplitude by the LD average output light power) in the E / O converter (corresponding to LD) in the optical signal transmitter 26 is expressed by the following equation (5). Adjustment of the optimum optical modulation degree mopt is a necessary condition for calculating the optical fiber transmission line capacity Copt.
[0044]
[Equation 5]

[0045]
The evaluation function for calculating the total transmission line capacity Ct, which is the total transmission line capacity including the optical fiber transmission line and the wireless transmission line, is Expression (6). That is, the smaller one of the wireless transmission line capacity Cr and the optical fiber transmission line capacity Copt is the total transmission line capacity Ct. min (A, B) means the smaller one of A and B.
[0046]
[Formula 6]

[0047]
Using the equations as described above, the intermodulation distortion detector 23a and the evaluation function calculator 23b calculate the power spectrum value D of IM3, the total transmission line capacity Ct, and the optimum light modulation degree mopt. Various parameter values necessary for the calculation are passed from the baseband signal processing unit 24.
[0048]
Next, the relationship among the wireless transmission line capacity Cr, the optical fiber transmission line capacity Copt, and the overall transmission line capacity Ct will be described. FIG. 3 is a diagram showing the relationship between the transmission path capacity and the modulation multi-level number. The vertical axis represents the transmission path capacity (b / s), and the horizontal axis represents the QAM digital modulation multilevel number 2n.
[0049]
From the equation (1), it can be seen that the radio transmission line capacity Cr increases in proportion to the radio signal band BW, the number of subcarriers Nc, and the modulation multilevel number 2n (curve (a) in FIG. 3).
[0050]
On the other hand, it can be seen from equation (2) that the optical transmission line capacity Copt deteriorates in inverse proportion to Nc and the modulation multilevel number 2n, contrary to the case of the wireless transmission line (the curve in FIG. 3). (B)). This is because the deterioration rate of the transmission line capacity due to the power (= D) of IM3 generated in the optical transmission line section exceeds the ratio of the transmission line capacity increase due to the increase in the modulation multi-level number. The optical fiber transmission line capacity Copt also depends on the optical modulation degree m of the E / O conversion unit of the optical signal transmission unit 26, and takes a maximum value when the optimum optical modulation degree mopt is reached.
[0051]
In the ROF system, since the same information of the RF modulation signal passes through both the wireless and optical fiber transmission lines, the total total transmission line capacity Ct of the wireless transmission line section and the optical fiber transmission line section is obtained by combining Cr and Copt. It is limited by the minimum value (curve (c) in FIG. 3).
[0052]
Here, for example, even if 256QAM is adopted as the modulation method at the time of system design, the optimum modulation multi-level number that maximizes the overall transmission path capacity is 64 instead of 256 due to the trade-off relationship shown in the figure. . Therefore, in the conventional system design based on the fixed modulation method, the initial desired transmission line capacity cannot be realized, and high quality transmission cannot be maintained.
[0053]
In the present invention, an ROF having a high transmission quality is selected by selecting an optimal digital modulation method that maximizes the total transmission line capacity Ct of the optical fiber transmission line section and the wireless transmission path section in accordance with the dynamic change of the IM3 amount. The system is realized.
[0054]
Next, the operation of the transmission apparatus 20 will be described in detail (referred to as the first embodiment). FIG. 4 is a diagram for explaining the operation of the transmission apparatus according to the first embodiment. The following steps S1 to S5 are upstream processing, and steps S6 to S8 are downstream processing. The configuration of the transmission apparatus 20-1 is the same as that in FIG.
[0055]
[S1] The optical signal Pout transmitted from the base station 3 to the control station 2 causes an optical loss through the optical fiber transmission line, and reaches the control station 2 as an optical signal Pa. When the optical fiber loss is Floss and the optical power of the optical signal Pout is Pw, the optical power Pwa of the optical signal Pa is Pwa = Pw / Floss.
[0056]
[S2] The optical signal receiver 21 receives the optical signal Pa, performs O / E conversion, and generates a photocurrent RF signal. The current value Iph of the photocurrent RF signal is Iph = η _PD ・ Pwa = η _PD -Pw / Floss. Η _PD Is the light receiving sensitivity of the optical signal receiving unit 21 and generally indicates the ratio of O / E conversion. The unit is A / W.
[0057]
[S3] The intermodulation distortion detector 23a performs frequency spectrum calculation on the photocurrent RF signal to calculate the spectrum value of the frequency of each subcarrier. Then, the center frequency fm (= (f1 + f) of the entire band (Nc × BW) allocated to the subscriber. _Nc ) / 2) frequency components are extracted.
[0058]
At this frequency position, as described above, since no subscriber is assigned, there is no subcarrier. Therefore, the power spectrum value of the component remaining in fm is obtained by the formula (4a) as the power spectrum value of IM3.
[0059]
[S4] The evaluation function calculation unit 23b uses the parameter values passed from the baseband signal processing unit 24 to calculate the QAM modulation multi-value number 2n = 4 from the equations (1), (2), and (5). , 16, 64, and 256, the total transmission line capacity Ct corresponding to each case is calculated. Further, the optimum light modulation degree mopt is calculated from the equation (5).
[0060]
Then, the evaluation function calculator 23b temporarily holds each value of the overall transmission line capacity Ct for the modulation multi-value number 2n and the value of the optimum light modulation degree mopt. Then, each value of the total transmission path capacity Ct with respect to the modulation multilevel number 2n is transmitted to the optimum modulation multilevel number determination unit 25a, and the optimum optical modulation degree mopt is transmitted to the optical signal transmission unit 26.
[0061]
[S5] The RF demodulator 22 performs multilevel QAM demodulation on the photocurrent RF signal to extract baseband signals q1a (t), q2a (t),... From the wireless terminal 4, and a baseband signal processor 24. Send to.
[0062]
[S6] The optimum modulation multi-level number determining unit 25a creates a correspondence table between the modulation multi-level number 2n and the total transmission line capacity Ct, and the maximum total transmission line capacity Ct in the total transmission line capacity Ct. _max The modulation multilevel number (optimum modulation multilevel number) corresponding to is selected.
[0063]
[S7] The RF modulation unit 25b converts the baseband transmission data q1 (t), q2 (t),... QNc (t) assigned to each subscriber terminal from the baseband signal processing unit 24 to the subcarrier frequency. f ₁ , F ₂ , F _Three ... f _Nc Are further subjected to digital multilevel QAM modulation with an optimal modulation multilevel number to generate and output RF modulated signals r1 (t), r2 (t), r3 (t),..., RNc (t).
[0064]
[S8] The optical signal transmitter 26 calculates the current sum ν (t) = Σ [ri (t) · cos2πf of the RF modulation signal. _i ] Is added with a DC bias current Ib to adjust the degree of light modulation. At this time, after adjusting Ib so that mopt = | ν | / (Ib−Ith) by the optimum optical modulation degree mopt (Ith is the threshold current value of the LD), E in the optical signal transmitter 26 is adjusted. Ib + ν (t) is output as the drive current of the / O converter.
[0065]
The E / O converter in the optical signal transmitter 26 is driven by the current Ib + ν (t) and outputs an RF light modulation signal maintained at the optimum light modulation degree mopt. This optical signal is transmitted in the order of optical fiber transmission line → base station 3 → wireless transmission line → wireless terminal 4.
[0066]
Here, based on the LD input / output characteristic curve, a _Three Will be described. FIG. 5 is a diagram showing an LD input / output characteristic curve. The vertical axis represents the output light power of the LD, and the horizontal axis represents the LD drive current I.
[0067]
The output optical power P indicates the power of the RF modulated optical signal, and the current I ₀ The LD operates by the modulation drive current signal centered at, and an RF modulated optical signal is generated.
The output optical power of the LD is the LD threshold current I _th In the above operation region, the output is ideally linear, but in reality it is non-linear due to the non-uniformity of the electric field distribution of the IL characteristic of the LD and the non-linear phenomenon caused by the interaction between the injected carrier and the photon. Output. At this time, when the modulation frequency component of the output light power is approximated by power series expansion for each order, Expression (7) is obtained.
[0068]
[Expression 7]
P = P ₀ (1 + a ₁ ・ Ν + a ₂ ・ Ν ² + A _Three ・ Ν ^Three + ...) (7)
2nd order or higher in this formula (a ₂ ・ Ν ² All of the above items are nonlinear components. Each coefficient a ₁ , A ₂ , A _Three ,... Represent the intensity (amplitude) of each modulation component (precisely, each P ₀ The doubled value is the actual intensity). A in formula (4a) _Three In the equation (7) _Three It corresponds to.
[0069]
Next, a transmission apparatus according to the second embodiment will be described. FIG. 6 is a diagram illustrating a configuration of the transmission apparatus according to the second embodiment. In the transmission apparatus 20-2, the intermodulation distortion detector 23a does not detect IM3 from the photocurrent RF signal, but again converts the RF modulated optical signal generated by the optical signal transmitter 26 into electricity again. E1 is received and IM3 is detected from this electric signal E1 (IM3 generated by a nonlinear circuit such as an LD in the optical signal transmitter 26 is detected). Since other configurations and operations are the same as those in the first embodiment, description thereof will be omitted.
[0070]
Next, a transmission apparatus according to a third embodiment will be described. FIG. 7 is a diagram illustrating the configuration of the transmission apparatus according to the third embodiment. In the transmission device 20-3, the intermodulation distortion detection unit 23a does not detect IM3 from the photocurrent RF signal, but receives the RF modulation signal generated by the RF modulation unit 25b (indicated by E2 in the figure). Thus, IM3 is detected from this signal E2 (IM3 generated by a nonlinear circuit in the RF modulation unit 25b is detected). Since other configurations and operations are the same as those in the first embodiment, description thereof will be omitted.
[0071]
Next, a transmission apparatus according to a fourth embodiment will be described. FIG. 8 is a diagram illustrating a configuration of a transmission apparatus according to the fourth embodiment. In the transmission device 20-4, a radio modulation multi-level number extraction unit 23c is provided in the RF reception control unit 23 as a new component for the first embodiment.
[0072]
The radio modulation multilevel number extraction unit 23c calculates and extracts the radio modulation multilevel number that is the modulation multilevel number only in the radio transmission path section from the photocurrent RF signal. Is transmitted to the optimum modulation multilevel number determination unit 25a.
[0073]
On the other hand, the optimum modulation multi-level number determining unit 25a has a correspondence table between the modulation multi-level number and the total transmission path capacity Ct calculated by each modulation multi-level number. When the total transmission line capacity corresponding to the value of the modulation multi-value number is Ctr and the predetermined deterioration amount set in advance is Δ, Ct _max When -Ctr> Δ, the maximum total transmission line capacity Ct _max Is selected from the table as an optimum modulation multilevel number, and Ct _max When −Ctr ≦ Δ, the modulation multilevel number corresponding to the current entire transmission line capacity Ctr is selected from the table, and this is set as the optimum modulation multilevel number.
[0074]
The reason for such control is Ct _max This is because, even when the difference between Ctr and Ctr is small, if the modulation method is changed frequently, it takes time until the modulation method is established between the control station 2 and the wireless terminal 4, and the line communication quality cannot be maintained. . In order to maintain quality, it is necessary to reduce the overhead time required for control. Therefore, in the fourth embodiment, the overhead time is shortened by switching the modulation method when the amount of degradation with respect to the transmission path capacity exceeds a specified value.
[0075]
Next, the wireless terminal 4 will be described. FIG. 9 is a diagram illustrating the configuration of the wireless terminal 4. The wireless terminal 4 includes a wireless reception unit 41, a wireless transmission unit 42, and a terminal-side baseband signal processing unit 43. The radio reception unit 41 includes a terminal-side RF demodulation unit 41a and an optimum modulation multilevel number detection unit 41b, and the radio transmission unit 42 includes a terminal-side RF modulation unit 42a.
[0076]
[S11] The RF modulated optical signal transmitted from the control station 2 becomes a radio signal at the base station 3 and is received through an antenna (not shown) of the radio terminal 4. The terminal-side RF demodulation unit 41 a of the wireless terminal 4 demodulates the received signal to generate a baseband signal, and transmits this to the terminal-side baseband signal processing unit 43.
[0077]
[S12] The optimum modulation multilevel number detection unit 41b detects the optimum modulation multilevel number set by the transmission apparatus 20 in the control station 2.
[S13] The terminal-side baseband signal processing unit 43 processes the baseband signal.
[0078]
[S14] The terminal-side RF modulation unit 42a modulates the baseband signal processed by the terminal-side baseband signal processing unit 43 with the optimal modulation multilevel number detected by the optimal modulation multilevel number detection unit 41b, and RF A modulation signal (terminal-side RF modulation signal) is generated. The terminal-side RF modulation signal is transmitted wirelessly to the base station 3 through the antenna. The radio signal is converted into an optical signal by the base station 3 and transmitted to the control station 2.
[0079]
Next, a specific configuration example and operation related to the transmission apparatus 20-1 according to the first embodiment will be described. FIG. 10 is a diagram illustrating a configuration example of the transmission apparatus 20-1 according to the first embodiment.
[0080]
The PIN photodiode 21 a corresponds to the optical signal receiver 21, and the RF quadrature demodulator circuit 22 a corresponds to the RF demodulator 22. The intermodulation distortion detector 23a includes an FFT (Fast Fourier Transform) arithmetic processing circuit 23a-1 and an IM3 component calculation processing circuit 23a-2. The evaluation function calculation unit 23b includes a transmission line capacity calculation processing circuit 23b-1, an optimum light modulation degree calculation processing circuit 23b-2, a parameter storage memory 23b-3, and registers 23b-1a and 23b-2a.
[0081]
The optimum modulation multilevel number determination unit 25a includes a correspondence table storage memory 25a-1 and a selector 25a-2, and the multilevel QAM modulation circuit 25b-1 corresponds to the RF modulation unit 25b. Further, the optical signal transmission unit 26 includes a bias current adjustment circuit 26a, a bias application unit 26b, and an LD 26c that performs E / O conversion.
[0082]
Here, the PIN photodiode 21a performs O / E conversion of the received optical signal. The FFT operation processing circuit 23a-1 performs a fast Fourier transform operation on the received electrical signal to calculate a frequency spectrum component. The IM3 component calculation processing circuit 23a-2 calculates the power spectrum value D of IM3 related to the frequency fm at the center of the entire band.
[0083]
The transmission path capacity calculation processing circuit 23b-1 calculates the total transmission path capacity Ct for the QAM modulation multilevel number based on the power spectrum value D. The optimum light modulation degree calculation processing circuit 23b-2 calculates the optimum light modulation degree mopt based on the power spectrum value D. The parameter storage memory 23b-3 stores various parameters necessary for the calculation passed from the baseband signal processing unit 24, and outputs them to the transmission path capacity calculation processing circuit 23b-1 and the optimum optical modulation degree calculation processing circuit 23b-2. To do.
[0084]
The register 23b-1a holds the calculation result of the transmission line capacity calculation processing circuit 23b-1, and the register 23b-2a holds the calculation result of the optimum light modulation degree calculation processing circuit 23b-2.
[0085]
The RF quadrature demodulation circuit 22 a performs quadrature demodulation on the 2n-value QAM modulated received electrical signal, generates a baseband signal, and transmits the baseband signal to the baseband signal processing unit 24.
[0086]
The correspondence table storage memory 25a-1 receives the overall transmission line capacity Ct held in the register 23b-1a, and stores a correspondence table between the modulation multi-value number and the overall transmission line capacity Ct.
[0087]
The selector 25a-2 selects the maximum total transmission line capacity Ct in the correspondence table. _max And the optimum modulation multi-level number corresponding to it is output to the multi-level QAM modulation circuit 25b-1. The multi-level QAM modulation circuit 25b-1 modulates the transmission data transmitted from the baseband signal processing unit 24 with the optimal modulation multi-level number to generate an RF modulation signal.
[0088]
The bias current adjustment circuit 26a adjusts the bias current value so as to maintain this mopt based on the optimum optical modulation degree mopt. The bias application unit 26b applies a bias current to the RF modulation signal to generate an LD drive signal. The LD 26c generates and transmits an RF light modulation signal based on the LD drive signal.
[0089]
Next, a specific configuration example and operation related to the transmission apparatus 20-2 according to the second embodiment will be described. FIG. 11 is a diagram illustrating a configuration example of the transmission apparatus 20-2 according to the second embodiment.
[0090]
In the transmission device 20-2, a monitoring photodiode (monitoring PD) 26d is newly installed in the optical signal transmission unit 26 of the transmission device 20-1 described above with reference to FIG. The output signal E1 of the monitoring PD 26d is input to the FFT arithmetic processing circuit 23a-1, and then IM3 is detected. Since other configurations and operations are the same as those in the first embodiment, description thereof will be omitted.
[0091]
Next, a specific configuration example and operation regarding the transmission apparatus 20-3 according to the third embodiment will be described. FIG. 12 is a diagram illustrating a configuration example of the transmission apparatus 20-3 according to the third embodiment.
[0092]
The transmission apparatus 20-3 inputs the output signal E2 of the multilevel QAM modulation circuit 25b-1 to the FFT arithmetic processing circuit 23a-1 with respect to the transmission apparatus 20-1 described above with reference to FIG. Thereafter, IM3 is detected. Since other configurations and operations are the same as those in the first embodiment, description thereof will be omitted.
[0093]
Next, a specific configuration example and operation regarding the transmission apparatus 20-4 according to the fourth embodiment will be described. 13 and 14 are diagrams illustrating a configuration example of the transmission apparatus 20-4 according to the fourth embodiment.
[0094]
In the transmission apparatus 20-4, a radio modulation multilevel number extraction unit 23c is newly provided in the RF reception control unit 23 with respect to the transmission apparatus 20-1 described above with reference to FIG. 10, and an optimum modulation multilevel number determination unit 25a is further provided. A deterioration amount determination unit 25a-4 and a threshold value storage memory 25a-3 are newly provided inside. Other components are the same as those of the transmission apparatus 20-1.
[0095]
Here, the PIN photodiode 21a performs O / E conversion of the received optical signal. The FFT operation processing circuit 23a-1 performs a fast Fourier transform operation on the received optical signal and calculates a frequency spectrum component. The IM3 component calculation processing circuit 23a-2 calculates the power spectrum value D of IM3 related to the frequency fm at the center of the entire band.
[0096]
The radio modulation multi-level number extraction unit 23c selects a modulation multi-level number in consideration of only the radio transmission path section from the propagation characteristics of the radio transmission path section (as a technique of the radio modulation multi-level number extraction unit 23c, For example, the technique described in JP-A-7-250116 can be applied).
[0097]
The transmission path capacity calculation processing circuit 23b-1 calculates the total transmission path capacity Ct for the QAM modulation multilevel number based on the power spectrum value D of IM3. The optimum light modulation degree calculation processing circuit 23b-2 calculates the optimum light modulation degree mopt based on the power spectrum value D of IM3.
[0098]
The parameter storage memory 23b-3 stores various parameters necessary for the calculation passed from the baseband signal processing unit 24, and outputs them to the transmission path capacity calculation processing circuit 23b-1 and the optimum optical modulation degree calculation processing circuit 23b-2. To do. The register 23b-1a holds the calculation result of the transmission line capacity calculation processing circuit 23b-1, and the register 23b-2a holds the calculation result of the optimum light modulation degree calculation processing circuit 23b-2.
[0099]
The RF quadrature demodulation circuit 22 a performs quadrature demodulation on the 2n-value QAM modulated received electrical signal, generates a baseband signal, and transmits the baseband signal to the baseband signal processing unit 24.
The correspondence table storage memory 25a-1 receives the overall transmission line capacity Ct held in the register 23b-1a, and stores a correspondence table between the modulation multi-value number and the overall transmission line capacity Ct.
[0100]
The threshold value storage memory 25a-3 stores a specified deterioration amount Δ of transmission path capacity deterioration. The degradation amount determination unit 25a-4 uses the correspondence table to determine the difference (Ct _max -Ctr) and the specified deterioration amount Δ are compared. The selector 25a-2 receives the comparison determination result, determines the optimum modulation multilevel number, and outputs it to the multilevel QAM modulation circuit 25b-1.
[0101]
The multi-level QAM modulation circuit 25b-1 modulates the transmission data transmitted from the baseband signal processing unit 24 with the optimal modulation multi-level number to generate an RF modulation signal.
The bias current adjustment circuit 26a adjusts the bias current value so as to maintain this mopt based on the optimum optical modulation degree mopt. The bias application unit 26b applies a bias current to the RF modulation signal to generate an LD drive signal. The LD 26c generates and transmits an RF light modulation signal based on the LD drive signal.
[0102]
As described above, according to the present invention, the optimum modulation multi-level number is determined flexibly in response to changes in the amount of IM3 generated in the ROF system, and the entire transmission between the control station 2 and the wireless terminal 4 is performed. It is possible to maximize the path capacity and perform high-quality communication.
[0103]
Note that the transmission apparatus can be configured by arbitrarily combining the embodiments described above. For example, the second embodiment for detecting IM3 from the electrical signal E1 from the monitor PD 26d, and the fourth embodiment for determining the optimum modulation multilevel number by comparing the difference value of the total transmission path capacity with the specified deterioration amount. The transmission apparatus may be configured by combining the above.
[0104]
(Additional remark 1) In the transmission system which transmits the information by radio | wireless with an optical fiber, the optical signal receiver which receives the optical signal transmitted through the optical fiber transmission line, carries out optical / electrical conversion, and produces | generates a photocurrent RF signal; An RF demodulator that demodulates the photocurrent RF signal and converts the frequency into a baseband band to generate a baseband signal; a frequency for each frequency is calculated from the photocurrent RF signal; An intermodulation distortion detector that detects distortion component information of the intermodulation distortion, and an evaluation function based on the distortion component information. And an RF reception control unit configured to calculate an overall transmission path capacity including a wireless transmission path, and an RF reception control unit configured to process and transmit the baseband signal. A baseband signal processing unit that generates transmission data and outputs a parameter value necessary for calculating the evaluation function, and an optimal modulation multilevel number determination unit that determines an optimal modulation multilevel number that takes the maximum value of the overall transmission path capacity; An RF modulation control unit configured to perform RF modulation on the transmission data and generate an RF modulation signal using the optimum modulation multi-value number, and using the optimum light modulation degree, An optical signal transmitter that controls electrical / optical conversion of the RF modulated signal to generate and transmit an RF modulated optical signal;
A radio reception unit that receives a radio signal, and a radio transmission unit that modulates a baseband signal with a modulation multi-value determined by the transmission device to generate a terminal-side RF modulation signal and wirelessly transmits the signal. A wireless terminal,
A relay device that is connected to the wireless terminal via a wireless transmission line, is connected to the transmission device via an optical fiber transmission line, and performs relay transmission between the wireless terminal and the transmission device;
A transmission system comprising:
[0105]
(Supplementary Note 2) The intermodulation distortion detector is formed of three tones that remain in the frequency at the center of the entire band as the distortion component information of the intermodulation distortion with respect to the entire band allocated to the subscriber. The transmission system according to appendix 1, wherein distortion component information of the next intermodulation distortion is detected.
[0106]
(Supplementary Note 3) The intermodulation distortion detector detects the distortion component information of the intermodulation distortion from an electric signal obtained by converting the RF modulated optical signal into electricity or the RF modulation signal instead of the photocurrent RF signal. The transmission system according to supplementary note 1, wherein:
[0107]
(Supplementary Note 4) A radio modulation multilevel number extraction unit that calculates and extracts a radio modulation multilevel number that is a modulation multilevel number only in a radio transmission path section from the photocurrent RF signal, The value number determination unit creates a table of the calculated total transmission path capacity and the modulation multi-level number, and determines the maximum total transmission path capacity as Ct. _max , Ct is the current overall transmission line capacity corresponding to the value of the wireless modulation multi-level number on the table, and Ct _max If -Ctr> Δ, the total transmission line capacity Ct _max The modulation multi-level number corresponding to is the optimum modulation multi-level number, and Ct _max In the case of −Ctr ≦ Δ, the transmission system according to appendix 1, wherein the modulation multilevel number corresponding to the current overall transmission line capacity Ctr is set as the optimum modulation multilevel number.
[0108]
(Supplementary Note 5) In a transmission apparatus that transmits information by radio using an optical fiber,
An optical signal receiving unit that receives an optical signal transmitted through an optical fiber transmission line and performs optical / electrical conversion to generate a photocurrent RF signal;
An RF demodulator that demodulates the photocurrent RF signal and converts the frequency into a baseband to generate a baseband signal;
An intermodulation distortion detector that calculates a spectrum for each frequency from the photocurrent RF signal and detects distortion component information of the intermodulation distortion from a spectrum of frequencies located at the center of the entire band; and an evaluation function based on the distortion component information And an evaluation function calculation unit that calculates an optimal optical modulation degree and an overall transmission line capacity including an optical fiber transmission line and a wireless transmission line for each modulation multi-level number. When,
A baseband signal processing unit that processes the baseband signal, generates transmission data, and outputs a parameter value necessary for evaluation function calculation;
An optimal modulation multilevel number determining unit that determines an optimal modulation multilevel number that takes the maximum value of the overall transmission path capacity, and using the optimal modulation multilevel number, RF modulation is performed on the transmission data to generate an RF modulation signal. An RF modulation control unit configured to generate an RF transmission control unit;
An optical signal transmission unit that generates and transmits an RF modulated optical signal by controlling electrical / optical conversion of the RF modulated signal using the optimum optical modulation degree;
A transmission apparatus comprising:
[0109]
(Supplementary Note 6) The intermodulation distortion detection unit is formed of three tones that remain in the frequency at the center of the entire band as the distortion component information of the intermodulation distortion with respect to the entire band allocated to the subscriber. 6. The transmission apparatus according to appendix 5, wherein distortion component information of the next intermodulation distortion is detected.
[0110]
(Supplementary note 7) The intermodulation distortion detector detects the distortion component information of the intermodulation distortion from an electric signal obtained by converting the RF modulated optical signal into electricity or the RF modulation signal instead of the photocurrent RF signal. The transmission apparatus according to appendix 5, wherein:
[0111]
(Supplementary Note 8) A radio modulation multilevel number extraction unit that calculates and extracts a radio modulation multilevel number that is a modulation multilevel number only in a radio transmission path section from the photocurrent RF signal, The value number determination unit creates a table of the calculated total transmission path capacity and the modulation multi-level number, and determines the maximum total transmission path capacity as Ct. _max , Ct is the current overall transmission line capacity corresponding to the value of the wireless modulation multi-level number on the table, and Ct _max If -Ctr> Δ, the total transmission line capacity Ct _max The modulation multi-level number corresponding to is the optimum modulation multi-level number, and Ct _max The transmission apparatus according to appendix 5, wherein the modulation multilevel number corresponding to the current overall transmission line capacity Ctr is set as the optimum modulation multilevel number when -Ctr ≦ Δ.
[0112]
(Supplementary Note 9) In a wireless terminal that performs wireless communication,
A wireless receiver for receiving wireless signals;
A radio transmitting unit that modulates a baseband signal with a modulation multi-value determined by a transmission apparatus installed on the station side to generate a terminal-side RF modulated signal and wirelessly transmits
A wireless terminal characterized by comprising:
[0113]
(Supplementary Note 10) In an RF receiver that performs signal reception control,
An optical signal receiving unit that receives an optical signal transmitted through an optical fiber transmission line and performs optical / electrical conversion to generate a photocurrent RF signal;
An intermodulation distortion detector that calculates a spectrum for each frequency from the photocurrent RF signal and detects distortion component information of the intermodulation distortion from the spectrum of the frequency located at the center of the entire band;
An evaluation function calculation unit that calculates an evaluation function based on the distortion component information, calculates an optimum optical modulation degree, and an overall transmission line capacity including an optical fiber transmission line and a wireless transmission line for each modulation multi-value number;
An RF receiver characterized by comprising:
[0114]
(Supplementary Note 11) In an RF transmission apparatus that performs signal transmission control,
Optimum modulation multilevel number determining unit for determining the optimum modulation multilevel number that takes the maximum value for the total transmission path capacity including the optical fiber transmission line and the wireless transmission path obtained during the evaluation function calculation, and the optimal modulation multilevel number An RF modulation unit that performs RF modulation on transmission data to generate an RF modulation signal;
An optical signal transmitter that generates and transmits an RF-modulated optical signal by controlling electrical / optical conversion of the RF-modulated signal using the optimum optical modulation degree obtained at the time of calculating the evaluation function;
An RF transmission device comprising:
[0115]
【The invention's effect】
As described above, in the transmission system of the present invention, the transmission apparatus calculates the optimum optical modulation degree and the total transmission line capacity including the optical fiber transmission line and the wireless transmission line from the distortion component information of the intermodulation distortion. Then, the optimum modulation multi-level number that takes the maximum value of the overall transmission path capacity is determined. Then, an RF modulation signal is generated using the optimal modulation multi-level number, and the electrical / optical conversion of the RF modulation signal is controlled using the optimal optical modulation degree, and an RF modulation optical signal is generated and transmitted. Further, the wireless terminal is configured to generate the terminal-side RF modulated signal by modulating the baseband signal with the modulation multi-value number determined by the transmission apparatus. As a result, the optimum modulation multi-level number is determined in accordance with the dynamic change of the intermodulation distortion, and the entire transmission line capacity between the control station and the wireless terminal can be maximized, so that high-quality communication can be performed. It becomes possible.
[Brief description of the drawings]
FIG. 1 is a principle diagram of a transmission system according to the present invention.
FIG. 2 is a diagram showing an image of occurrence of IM3 for a subcarrier.
FIG. 3 is a diagram illustrating a relationship between transmission line capacity and the number of modulation multi-values.
FIG. 4 is a diagram for explaining the operation of the transmission apparatus according to the first embodiment.
FIG. 5 is a diagram showing an LD input / output characteristic curve.
FIG. 6 is a diagram illustrating a configuration of a transmission apparatus according to a second embodiment.
FIG. 7 is a diagram illustrating a configuration of a transmission apparatus according to a third embodiment.
FIG. 8 is a diagram illustrating a configuration of a transmission apparatus according to a fourth embodiment.
FIG. 9 is a diagram illustrating a configuration of a wireless terminal.
FIG. 10 is a diagram illustrating a configuration example of a transmission apparatus according to the first embodiment;
FIG. 11 is a diagram illustrating a configuration example of a transmission apparatus according to the second embodiment;
FIG. 12 is a diagram illustrating a configuration example of a transmission apparatus according to a third embodiment.
FIG. 13 is a diagram illustrating a configuration example of a transmission apparatus according to a fourth embodiment.
FIG. 14 is a diagram illustrating a configuration example of a transmission apparatus according to a fourth embodiment;
FIG. 15 is a diagram showing an overall configuration of an ROF system.
[Explanation of symbols]
1 Transmission system
2 control stations
3 base stations
4 wireless terminals
5 network
20 Transmission equipment
21 Optical signal receiver
22 RF demodulator
23 RF reception controller
23a Intermodulation distortion detector
23b Evaluation function calculator
24 Baseband signal processor
25 RF transmission controller
25a Optimum modulation multilevel number determination unit
25b RF modulator
26 Optical signal transmitter
30 Relay device
41 Wireless receiver
42 Wireless transmitter
Ct Total transmission line capacity
mopt Optimum light modulation

Claims (5)

In transmission systems that transmit wireless information over optical fibers,
An optical signal transmitted through an optical fiber transmission line is received, optical / electrically converted to generate a photocurrent RF signal, and the photocurrent RF signal is demodulated and frequency converted to a baseband. An RF demodulator that generates a baseband signal, calculates a spectrum for each frequency from the photocurrent RF signal, and detects distortion component information of the intermodulation distortion from the spectrum of the frequency located at the center of the entire band. An evaluation function that calculates an evaluation function based on the detection unit and the distortion component information, and calculates an optimum optical modulation degree and an overall transmission line capacity including an optical fiber transmission line and a wireless transmission line for each modulation level. An RF reception control unit comprising a calculation unit, a base that processes the baseband signal, generates transmission data, and outputs a parameter value necessary for calculating an evaluation function A signal processing unit, an optimum modulation multi-level number determining unit that determines an optimum modulation multi-level number that takes the maximum value of the overall transmission line capacity, and RF modulation is performed on the transmission data using the optimal modulation multi-level number. An RF modulation control unit configured to generate an RF modulation signal, and an RF transmission control unit configured to control the electrical / optical conversion of the RF modulation signal using the optimum optical modulation degree, thereby generating an RF modulation optical signal An optical signal transmitter that generates and transmits a transmission device;
A radio reception unit that receives a radio signal, and a radio transmission unit that modulates a baseband signal with a modulation multi-value determined by the transmission device to generate a terminal-side RF modulation signal and wirelessly transmits the signal. A wireless terminal,
A relay device that is connected to the wireless terminal via a wireless transmission line, is connected to the transmission device via an optical fiber transmission line, and performs relay transmission between the wireless terminal and the transmission device;
A transmission system comprising: The intermodulation distortion detection unit is a third-order intermodulation distortion formed of three tones that remains in the frequency at the center of the entire band for the entire band allocated to the subscriber as the distortion component information of the intermodulation distortion. The transmission system according to claim 1, wherein the distortion component information is detected. The intermodulation distortion detection unit detects the distortion component information of the intermodulation distortion from an electric signal obtained by converting the RF modulation optical signal into electricity or the RF modulation signal instead of the photocurrent RF signal. The transmission system according to claim 1. A radio modulation multilevel number extraction unit that calculates and extracts a radio modulation multilevel number that is a modulation multilevel number only in a radio transmission path section from the photocurrent RF signal; and the optimum modulation multilevel number determination unit Creates a table of the calculated total transmission path capacity and the modulation multi-level number, Ct _max is the maximum total transmission path capacity, and the current total corresponding to the value of the radio modulation multi-level number on the table Ctr transmission channel capacity, when the prescribed amount of degradation was delta, in the case of Ct _max -Ctr> Δ, the modulation level corresponding to the entire transmission channel capacity Ct _max the optimum modulation level, Ct _max - 2. The transmission system according to claim 1, wherein when Ctr ≦ Δ, the modulation multilevel number corresponding to the current overall transmission line capacity Ctr is set as the optimum modulation multilevel number. In a transmission device that transmits information by radio using an optical fiber, an optical signal receiving unit that receives an optical signal transmitted through an optical fiber transmission line and performs optical / electrical conversion to generate a photocurrent RF signal;
An RF demodulator that demodulates the photocurrent RF signal and converts the frequency into a baseband to generate a baseband signal;
An intermodulation distortion detector that calculates a spectrum for each frequency from the photocurrent RF signal and detects distortion component information of the intermodulation distortion from a spectrum of frequencies located at the center of the entire band; and an evaluation function based on the distortion component information And an evaluation function calculation unit that calculates an optimal optical modulation degree and an overall transmission line capacity including an optical fiber transmission line and a wireless transmission line for each modulation multi-level number. When,
A baseband signal processing unit that processes the baseband signal, generates transmission data, and outputs a parameter value necessary for evaluation function calculation;
An optimal modulation multilevel number determining unit that determines an optimal modulation multilevel number that takes the maximum value of the overall transmission path capacity, and using the optimal modulation multilevel number, RF modulation is performed on the transmission data to generate an RF modulation signal. An RF modulation control unit configured to generate an RF transmission control unit;
An optical signal transmission unit that generates and transmits an RF modulated optical signal by controlling electrical / optical conversion of the RF modulated signal using the optimum optical modulation degree;
A transmission apparatus comprising:

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