JP2004117246A

JP2004117246A - Antenna assembly

Info

Publication number: JP2004117246A
Application number: JP2002282692A
Authority: JP
Inventors: Noriyuki Inaba; 稲葉　敬之
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 2002-09-27
Filing date: 2002-09-27
Publication date: 2004-04-15
Anticipated expiration: 2022-09-27
Also published as: JP3861785B2

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem that when a radar is tracking a target at a low altitude, a countermeasure for phasing caused by a multi-path phenomenon is insufficient. <P>SOLUTION: This antenna system is provided with: an array antenna for transmitting/receiving a radar pulse; a phase shifter which controls an amplitude and a phase of a signal transmitted/received by the array antenna; a synthesizer which synthesizes voltage of an output signal of the synthesizer; a spatial maximum specific synthesizing weight estimating device which estimates the weight for synthesizing the output signal of frequency counter at a maximum S/N (Signal to Noise ratio); and a spatial maximum specific synthesizer which synthesizes the output signal of the frequency converter in accordance with the weight estimated by the spatial maximum specific synthesizing weight estimating device. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
この発明は、追尾レーダや航空管制レーダにおいて低高度目標を追尾するとき，直接波と海面反射（あるいは地面反射）によるマルチパス現象により受信信号が消滅し目標追尾が困難となる現象を回避するレーダフェージング対策技術に関するのものである。
【０００２】
【従来の技術】
低高度目標を追尾するときのマルチパス環境を説明する図を図１１に示す．このような状況では，直接波と海面反射波は同一ビーム内の僅少な角度差となる．一方，ドップラ周波数差や時間遅延差はほぼ無視できる（観測できない）大きさとなる．すなわち，直接波と海面反射波は位相のみシフトした完全相関信号に近い状況となっている．このようなマルチパス環境では目標の位置（距離・高度）によっては，マルチパス現象により電力が消滅するフェージングが発生する．このようなマルチパス環境では，送受信ビーム方向を目標より上方にむけることでフェージングを緩和する方法が考えられる．しかし，直接波と海面反射波方法のビーム利得差の信号が得られるのみであり，フェージング対策としては十分でない．
【０００３】
アレーアンテナを用いたフェージング対策として空間ダイバーシティ法が知られている．空間ダイバーシティ法には，アレーアンテナで一番受信状態が良いアンテナを選択するだけの選択方式，および等利得合成，最大比合成法（ＭＲＣ：　Ｍａｘｉｍａｌ　ｒａｔｉｏ　ｃｏｍｂｉｎｉｎｇ）がある．最大比合成法は，アレーアンテナ各素子で得られた信号の位相をそろえ，かつＳ／Ｎで重み付けがなされる最も高性能な空間ダイバーシティ法である．最大比合成法を用いたフェージング対策は，［１］唐沢好男，井上隆，神谷幸宏，田野哲，“ソフトウエアアンテナ［２］，”信学技報，ＲＣＳ９８−１５２，ｐｐ７−１２，　Ｎｏｖ．１９９８．［２］神谷幸宏，唐沢好男，“ソフトウエアアンテナ［３］，”信学技報，ＡＰ−９８−１３９，　ｐｐ６５−７２，　Ｊａｎ．　１９９８．［３］　唐沢好男，“ＩＴＳミリ波車車間通信における路面反射フェージングとスペースダイバシティに関する基礎的検討，”信学論Ｂ，　ｖｏｌ．Ｊ８３−Ｂ，　Ｎｏ．４，　ｐｐ．５１８−５２４，　Ａｐｒｉｌ，　２０００．　にて報告されている．図１２は，アレーアンテナで受信した信号に最大比合成法を適用するこれら従来法のアンテナ装置の構成を説明する図である．図１２において，１はアレーアンテナ，２は移相器，３は合成器，４は周波数変換装置，５はＡ／Ｄ変換機，６はビーム形成装置，７は送信機である．
【０００４】
アレーアンテナ１で受信された信号は，移相器２で送受信ビーム方向を目標より上方にむけるビームステアリングのための位相が与えられ，合成器３で電力合成されビーム形成される．合成器によるビーム形成された信号は周波数変換装置で，ＲＦ信号からベースバンド信号に周波数変換される．ベースバンド信号はＡ／Ｄ変換機５でデジタル化されたデジタル信号が得られる．複数の周波数を利用する周波数ダイバーシティ法である．図１３は送信周波数を変えることでフェージングが発生する距離が変わること説明する図である．上図は，ある送信周波数ｆ０に対しｆ０＋１０％の周波数，下図はｆ０−１０％の周波数を用いたときの相対受信電力の距離依存性である．このように周波数を変えることで，ある位置にいる目標に対し受信電力が得られる周波数が存在することが分かる．複数の周波数を用いる周波数の制御方法は，図１４のタイミングチャートで示すよに，送信パルス毎にｆ１，ｆ２，・・と周波数を変える周波数ホッピング法（ｆｒｅｑｕｅｎｃｙ　ｈｏｐｐｉｎｇ）と，送受信系が十分な帯域をもつ場合は，一つのパルス内でｆ１＋ｆ２＋・・ｆｋという複数の周波数を多重化して送信する周波数多重法がある．周波数ホッピングは，予め定められた周波数の順で順次送信するものであるが，その類似方式として周波数アジリティ法があり，受信電力が得られる周波数となるまでランダムに周波数を変更する方式である．　周波数多重は送受信系に広い帯域が必要となるため，複数の周波数を用いる従来法としては，周波数ホッピングが用いられている．
【０００５】
図１５は第二の従来法の一例として，周波数ホッピングを用いたアンテナ装置の構成を説明する図であり，１〜７は図１２と同じものであり，８は電力算出装置，９は電力加算器である．電力算出装置８では，各パルス毎に，各周波数ｆ１，ｆ２，・・ｆｋで送信した信号を，個別に電力を算出する．電力加算器９では，各周波数各周波数ｆ１，ｆ２，・・ｆｋで送信した信号電力をそのまま全て加算する．周波数によっては，フェージングが発生しない，パルスが含まれるため，全て加算した信号はフェージングが緩和されることが期待される．
【０００６】
【発明が解決しようとする課題】
上記の第一の従来法におけるアンテナ装置は，ビーム指向性により直接波と海面反射波の到来角度差から受信電力を得る方法である．しかし，目標高度が低く距離が遠いと直接波と海面反射波の角度差は僅少となり，ビーム指向性の差ではフェージング対策として不十分である．
【０００７】
一方，複数の周波数を用いる，第二の従来法では，ある周波数で得られた信号の電力を求め，電力が得られる周波数を選択する（周波数アジリティ法），あるいは予め定められた周波数で送信したパルスの受信信号電力を全て加算する方法（周波数ホッピング）である．周波数を変更すればフェージングが発生する距離が変化することは前記したが，直接波と海面反射波の路長差と海面反射時の位相シフトの関係によっては，複数の周波数を用いてもいずれの周波数においても電力が得られないという状況も発生しうる．このように，複数の周波数を利用し，各周波数で電力を求めた後に加算する従来法２でもフェージング対策として十分でないという問題がある．
【０００８】
この発明は、上記のような課題を解決するためになされたものであり，フェージング対策を行うものであり，この中で，単一の周波数のみを用いる解決法と，複数の周波数を用いる解決法とを示す．
【０００９】
【課題を解決するための手段】
第１の発明は，レーダパルスを送受信するアレーアンテナと，前記アレーアンテナにより送受信された信号の振幅・位相を制御する移相器と，前記位相器の出力信号を電圧合成する合成器と，前記合成器の出力信号を周波数変換する周波数変換装置と，前記周波数変換装置の出力信号を最大Ｓ／Ｎ（Ｓｉｇｎａｌ　ｔｏ　Ｎｏｉｓｅ　ｒａｔｉｏ）で合成するためのウエイトを推定する空間最大比合成ウエイト推定装置と，前記空間最大比合成ウエイト推定装置により推定されたウエイトに従い前記周波数変換装置の出力信号を合成する空間最大比合成装置とを備えるアンテナ装置である．
【００１０】
【発明の実施の形態】
実施の形態１．
図１はこの第一発明の実施の形態を示すもので、図において、１〜７は従来法と同じである．１０は空間最大比合成ウエイト推定装置，１１は空間最大比合成装置である．
【００１１】
アレーアンテナ１で送信した周波数ｆ１の送信パルスは，目標から反射し同じくアレーアンテナ１に入射し受信信号となる．アレー素子数をＮ，サブアレー数をＭ（図１では一例としてＭ＝４としている．），サブアレー内の素子数をＮｓとする．アレーアンテナに入力する受信信号のうち目標が含まれる受信信号（サンプル数をＴとする．）を行列形式でＸｒ∈Ｃ^Ｎ ^× ^Ｔとする．ここで，Ｃ^Ｎ ^× ^Ｔは，複素数を要素としたＮ行Ｔ列の行列であることを表す．受信信号Ｘｒは，位相器２に入力される．移相器２では，送信方向と同じ方向に受信ビーム指向する各サブアレー別のビームステアリング操作を行う．ここで，ステアリングベクトルをＢ（θ）　∈Ｃ^Ｎ ^× ^Ｔと書く．合成器３では，サブアレーの出力信号として，“数１”が得られる．
【００１２】
【数１】

【００１３】
ここで，アレーアンテナ１がリニアアレーの場合を例とすると，Ｂ（θ）　∈Ｃ^Ｎ ^× ^Ｔは，“数２”となる
【００１４】
【数２】

【００１５】
ここで，行列の右肩のＴは転置行列を表し，θは電波の入射角，ｄはアレーアンテナの各アンテナの間隔，λは送信波の波長を示す．また，合成器３の出力は，“数３”となる．
【００１６】
【数３】

【００１７】
ここで，ｓｕｂｍａｔｒｉｘ（Ｘ，ａ：ｂ，ｃ：ｄ）は，マトリックスの〜行，〜ｄ列のサブマトリックスを切り出す操作を表す．第一の発明では，“数３”で書かれる合成器出力信号は，ベースバンドへ周波数変換されＡ／Ｄ変換された後，空間最大比合成装置１０に入力される．空間最大比合成装置では，合成器３出力であるサブアレー間の信号に最大比合成が適用される．空間最大比合成について説明する．各サブアレーの出力Ｙｍを一つの行列に纏めて，空間最大比合成への入力データ行列Ｚ　∈Ｃ^Ｎ ^× ^Ｔを，“数４”と記述する．
【００１８】
【数４】

【００１９】
このデータ行列を用いて空間最大比合成ウエイト推定装置１１では，空間最大比合成ウエイトを求めるために，ｚに関する相関行列＿Ｒを，“数５”で求める．
【００２０】
【数５】

【００２１】
さらに，相関行列Ｒの最大固有値に対応する固有ベクトルＷ１を求めることで，空間最大比合成のウエイトベクトルが得られる．こうして得られた空間最大比合成ウエイトは，空間最大比合成装置１０に入力され，空間最大比合成装置ではこのウエイトベクトルを用い図２のタイミング図に示すように，目標の有無に関わらず送信パルスと送信パルス間の全ての距離サンプルｒに亙り“数６”の処理を行う．
【００２２】
【数６】

【００２３】
次に，電力Ｐ（ｒ）は，“数７”となる．
【００２４】
【数７】

【００２５】
サブアレー構成による空間最大比合成装置の動作である．空間最大比合成によりフェージング対策が図られたＳ／Ｎの良い信号が得られる．一般的には，この空間最大比合成装置の出力は，距離方向に　閾値処理が適用され目標検出が検出される．また，Ｓ／Ｎが向上したことにより目標検出サンプルの区間の中で，より正確な距離の測定が可能となる．また，サブアレー構成とすることで，数５の相関行列の固有ベクトル計算などの次元が削減され，計算量が著しく低減されている．この実施の形態１では，図２に示すように各送信パルスに対し以上を繰り返す．
【００２６】
単一の周波数を用いれば、アレーアンテナをサブアレー構成とする方法を採用しているため，マルチパス環境で空間最大比合成を採用しフェージングによる受信信号の消滅を回避する上で，送受信系ハードウエアの削減，空間最大比合成ウエイト計算負荷低減を可能とする効果がある．図３は，単一の周波数を用いる受信空間最大比合成，および送受信空間最大比合成の効果を示す図である．図３の（ａ）は，目標高度を一定として横軸は目標までの距離であり，縦軸は受信相対電力である．実線は，フルＤＢＦ（Ｄｅｇｉｔａｌ　Ｂｅａｍ　ｆｏｒｍｉｎｎｇ：サブアレー構成でなく，各放射素子での受信信号をベースバンドまで周波数変換し最大比合成したもの．）による受信空間最大比合成法で得られる受信電力である．点線は，同じく送受信空間最大比合成法，破線は参考としてのモノパルス和信号，一点鎖線は同じく参考としてのマルチパスのない自由空間でのモノパルス和信号である．図３は，目標高度が低く，遠方にある場合であり，直接波と海面反射波の角度がほとんど０であるため空間最大比合成は，自由空間でもノモパルス和信号とう同等となるレベルまでの電力回復は得られていない．しかし，角度差が大きかったり（すなわち目標高度がもう少し高い場合），アレーアンテナの開口径が大きければ，図中の近距離で見られるように空間最大比合成法でも電力回復が期待できる．図３の（ｂ）（ｃ）は，サブアレー構成による空間最大比合成の効果を説明する図である．
図３（ｂ）はフルＤＢＦに比べた２サブアレー構成での利得低下を，（ｃ）は同様にフルＤＢＦに比べた４サブアレー構成での利得低下の例を示す．いずれも，フルＤＢＦに比べ利得の低下は少なく，第一の発明によるサブアレー構成での空間最大比合成でも十分満足する効果が得られることが可能である．
【００２７】
実施の形態２．
実施の形態２は，構成は図１と同じであるが，図４のタイミング図に示すように受信信号から得られた空間最大比合成ウエイトを，つぎの送信波の送信ウエイトとして用いることを特徴としており，送信と受信双方で空間最大比合成を実現するものである．
【００２８】
実施の形態３．
実施の形態３は第二の発明（周波数ホッピング空間・周波数最大比合成）の実施例である．図５はこの実施の形態を示すもので、図において、１〜７は従来法と同じである．１２は空間・周波数最大比合成ウエイト推定装置，１３は空間・周波数最大比合成装置である．第二の発明では，第一の発明と同様にサブアレー構成によりパルス毎に周波数ホッピングを行い送信したときの受信パルスデータを保持しておいて，空間・周波数最大比合成法を実現する方法である．
【００２９】
図６は，この実施の形態３の，送受信および処理タイミングを説明する図である．図６に示すように，送信パルスの周波数をｆ１，ｆ２・・・ｆｘとパルス毎で変更し送信する．この間，各送信パルスの目標反射からのアレーアンテナに入力する受信信号は，第一の発明と同様に，位相器２で送信方向と同じ方向に受信ビーム指向する各サブアレー別のビームステアリング操作される．周波数番号をとすると，サブアレー番号での合成器３出力は，式（１）と同様にＹｍ，ｋ∈Ｃ^１ ^× ^Ｔが得られる．空間・周波数最大比合成装置１２では，このＹｍ，ｋ∈Ｃ^１ ^× ^Ｔを，全ての周波数での送受信が完了するまで保持しておく．次に，これらを列方向にならべて行列Ｖ∈Ｃ^ＭＫ ^× ^Ｔ作成する．
【００３０】
数８”で定義した次元の列ベクトルに対し，空間最大比合成と同じ数（５）〜（７）の処理を行う．こうして，各周波数，および各サブアレーで得られた受信信号の最大比合成が容易に実現可能となる
【００３１】
【数８】

【００３２】
実施の形態４．
実施の形態４は第二の発明（周波数ホッピング空間・周波数最大比合成）の別の実施例である．図７は，この実施の形態４の，送受信および処理タイミングを説明する図である．実施の形態４は，実施の形態２と３を組み合せたものである．実施の形態３による周波数ホッピングした送信パルス列を送信する．そのパルス列から空間・周波数最大比合成ウエイト推定装置で推定したウエイトを受信の空間・周波数最大比合成に持ちいるのみにならず，次の送信パルスは空間・周波数最大比合成ウエイトの要素のうち振幅の大きな要素に対応する周波数と，その要素を送信ウエイトとして用いるものである．続く送受信は，実施の形態２を繰り返しつつ，電力をモニタすることで，空間最大比合成だけでは，フェージングによる受信電力の回復が困難となり周波数を変更すべきかを判断する．周波数を変更すべきと判断された場合は，次の送信パルスとして実施の形態４の初めと同様に周波数ホッピングした送信パルス列を送信する実施の形態３を実施するものである．以下，同様の制御を繰り返す．このように制御することで，実施の形態２にくらべ周波数ホッピングを併用しているためフェージング改善効果が大きく，かつ周波数ホッピングした送信を繰り返す実施の形態３に比べ，実施の形態２の空間周波数最大比合成で十分な場合は，それを，繰り返すことになり，全体としての電力利用効率を向上させることが可能である．
【００３３】
実施の形態５．
実施の形態５は，第三の発明（周波数多重空間・周波数最大比合成）の実施例である．図８この実施の形態を示すもので、図において、１〜７は図５の実施の形態３と同じである．１２は空間・周波数最大比合成ウエイト推定装置，１３は空間・周波数最大比合成装置である．１４は周波数多重して送信されたパルスの受信信号を各周波数に弁別する周波数弁別装置である．第三の発明は，空間・周波数最大比合成ウエイト推定装置，空間・周波数最大比合成装置の機能としては，第二の発明と同様である．図９を用いて実施の形態５（すなわち第三の発明）を説明する．図９は送受信および処理タイミングを説明する図である．実施の形態５では，実施の形態３で各送信パルスを周波数ホッピングした周波数を同時に一つの送信パルスで周波数多重化して送信するものである．送受信系が広い帯域をもち，複数の周波数を同時に送信可能な場合に適用可能となる．周波数弁別により各周波数に弁別された各サブアレーの信号を，空間・周波数最大比合成ウエイト推定装置１２にて，第二の発明と同じ数（８）を作成する．第二の発明と同様にして得られた空間・最大比合成ウエイトを用いて，空間・周波数最大比合成装置１３にて受信信号を合成する．こうして，各周波数，および各サブアレーで得られた受信信号の最大比合成が１パルスにて容易に実現可能となる．更に，この空間・周波数最大比合成ウエイトを，送信機７，位相器２に入力し，次の送信時の周波数多重パルスのウエイトとして用いることで送信の空間・周波数最大比合成が可能となる．これを繰り返すことで，送受信の空間・周波数最大比合成が実現可能となる．
【００３４】
図１０は，複数の周波数を利用する第二の発明の効果を説明する図である．図１０（ａ）は，図３（ａ）と同様の図であり，実線は周波数ホッピングを用いたときのフルＤＢＦによる空間・周波数最大比合成による受信電力である．周波数は，中心周波数をｆ０（＝６ＧＨｚ）として，ｆ０−１０％，ｆ０，ｆ０＋１０％の３種を用いた例である．点線は参考としてのモノパルス和信号，一点鎖線は同じく参考としてのマルチパスのない自由空間でのモノパルス和信号である．この図から，空間・周波数最大比合成により全ての距離範囲において自由空間でのモノパルス和信号以上の受信電力が得られることが確認される．同様に，図１０（ｂ）は，周波数ホッピング空間・周波数最大比合成，または周波数多重空間・周波数最大比合成であるサブアレー構成による空間・周波数最大比合成の効果を説明する図である．図１０（ｂ）は，フルＤＢＦに比べた２および４サブアレーでの利得の低下と，従来の第二の方法（周波数ホッピングしたパスルの各電力を加算する方法．ノンコヒーレントＦ／Ｈと呼ぶ．）での利得の低下を説明する図である．２，および４サブアレーでもフルＤＢＦに比べて利得の低下は，２ｄＢ以下に止まっているが，第二の従来法（ノンコヒーレントＦ／Ｈ）では，近距離で１０ｄＢ近いフェージングが発生している．この理由は，図１０（ｃ）　は，３種の各周波数での受信電力を示す．図１０（ｃ）から明らかなように，使用した３種の周波数のいずれにおいても電力のヌルとなる距離が一致してしまったため，第二の従来法ではフェージングによる電力の消滅を回避しきれないことが分かる．このように，空間・周波数最大比合成によりこのような悪条件下でも自由空間でのモノパルス和信号以上の電力が得られるという効果が得られる．
【００３５】
【発明の効果】
上記によりフェージング対策を行うことができる．
【図面の簡単な説明】
【図１】実施の形態１および２を示すアンテナ装置のブロック図である．
【図２】実施の形態１の動作タイミングを説明する図である．
【図３】実施の形態１および２の効果を説明する図である
【図４】実施の形態２の動作タイミングを説明する図である．
【図５】実施の形態３および４を示すアンテナ装置のブロック図である．
【図６】実施の形態３の動作タイミングを説明する図である．
【図７】実施の形態４の動作タイミングを説明する図である．
【図８】実施の形態５を示すアンテナ装置のブロック図である．
【図９】実施の形態５の動作タイミングを説明する図である．
【図１０】実施の形態４または５の効果を説明する図である．
【図１１】マルチパス環境の電波伝播を説明する図である．
【図１２】従来の第一の技術によるアンテナ装置のブロック図である．
【図１３】マルチパス環境での受信電力の周波数依存性を説明する図である．
【図１４】周波数ホッピング送信と周波数多重送信のタイミングを説明する図である．
【図１５】従来の第二の技術によるアンテナ装置のブロック図である．
【符号の説明】
１　アレーアンテナ，２　移相器，３　合成器，４　周波数変換装置，５Ａ／Ｄ変換機，６　ビーム形成器，７　送信機，８　電力算出器，９　電力加算器，１０　空間最大比合成ウエイト推定装置，１１　空間最大比合成装置，１２　空間・周波数最大比合成ウエイト推定装置，１３　空間・周波数最大比合成装置，１４　目標[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention provides a radar for avoiding a phenomenon in which when a low-altitude target is tracked in a tracking radar or an air traffic control radar, a received signal disappears due to a multipath phenomenon caused by a direct wave and sea surface reflection (or ground reflection), making target tracking difficult. It is related to fading countermeasure technology.
[0002]
[Prior art]
Figure 11 shows a diagram explaining the multipath environment when tracking a low altitude target. In such a situation, the direct wave and the sea surface reflected wave have a small angle difference in the same beam. On the other hand, the Doppler frequency difference and the time delay difference are almost negligible (not observable). In other words, the direct wave and the sea surface reflected wave are in a situation close to a perfect correlation signal shifted only in phase. In such a multipath environment, depending on the target position (distance / altitude), fading occurs in which the power disappears due to the multipath phenomenon. In such a multipath environment, there is a method to reduce fading by directing the transmitting and receiving beam directions above the target. However, only the signal of the beam gain difference between the direct wave and the sea surface reflected wave method is obtained, which is not enough as a countermeasure for fading.
[0003]
A spatial diversity method is known as a fading countermeasure using an array antenna. The spatial diversity method includes a selection method for simply selecting an antenna having the best reception state among array antennas, an equal gain combining method, and a maximum ratio combining method (MRC). The maximum ratio combining method is the most efficient spatial diversity method in which the phases of the signals obtained by each element of the array antenna are aligned and weighted by S / N. Countermeasures against fading using the maximum ratio combining method are described in [1] Yoshio Karasawa, Takashi Inoue, Yukihiro Kamiya, Satoshi Tano, "Software Antenna [2]," IEICE Technical Report, RCS 98-152, pp7-12, Nov . 1998. [2] Yukihiro Kamiya, Yoshio Karasawa, "Software Antenna [3]," IEICE Technical Report, AP-98-139, pp65-72, Jan. 1998. [3] Yoshio Karasawa, "Basic Study on Road Reflection Fading and Space Diversity in ITS Millimeter-Wave Inter-Vehicle Communication," IEICE B, vol. J83-B, No. 4, pp. 518-524, April, 2000. Reported in. FIG. 12 is a diagram illustrating the configuration of these conventional antenna devices that apply the maximum ratio combining method to the signal received by the array antenna. In FIG. 12, 1 is an array antenna, 2 is a phase shifter, 3 is a combiner, 4 is a frequency converter, 5 is an A / D converter, 6 is a beam forming device, and 7 is a transmitter.
[0004]
The signal received by the array antenna 1 is provided with a phase for beam steering in which a transmission / reception beam direction is directed above a target by a phase shifter 2, and the power is combined by a combiner 3 to form a beam. The signal formed by the synthesizer is frequency-converted from the RF signal to the baseband signal by the frequency converter. As the baseband signal, a digital signal digitized by the A / D converter 5 is obtained. This is a frequency diversity method that uses multiple frequencies. FIG. 13 is a diagram illustrating that changing the transmission frequency changes the distance at which fading occurs. The upper diagram shows the distance dependence of the relative received power when a frequency of f0 + 10% is used for a certain transmission frequency f0, and the lower diagram shows the frequency dependence of f0-10%. By changing the frequency in this way, it can be seen that there is a frequency at which the received power can be obtained for the target at a certain position. As shown in the timing chart of FIG. 14, a frequency control method using a plurality of frequencies includes a frequency hopping method (frequency hopping) in which the frequency is changed to f1, f2,. When there is a frequency multiplexing method, there is a frequency multiplexing method in which a plurality of frequencies of f1 + f2 + .. fk are multiplexed and transmitted in one pulse. Frequency hopping is a method of transmitting data sequentially in the order of a predetermined frequency. A similar method is the frequency agility method, in which the frequency is changed randomly until the frequency at which the received power can be obtained. Since frequency multiplexing requires a wide band for the transmission and reception systems, frequency hopping is used as a conventional method using multiple frequencies.
[0005]
FIG. 15 is a diagram for explaining the configuration of an antenna device using frequency hopping as an example of a second conventional method, wherein 1 to 7 are the same as those in FIG. 12, 8 is a power calculation device, and 9 is a power addition device. Container. The power calculation device 8 individually calculates the power of the signal transmitted at each frequency f1, f2,... Fk for each pulse. The power adder 9 adds all the signal powers transmitted at the respective frequencies f1, f2,... Fk as they are. Depending on the frequency, fading does not occur and pulses are included, so it is expected that the fading of all added signals will be reduced.
[0006]
[Problems to be solved by the invention]
The antenna device in the first conventional method is a method to obtain the received power from the arrival angle difference between the direct wave and the sea surface reflected wave by the beam directivity. However, if the target altitude is low and the distance is long, the angle difference between the direct wave and the sea surface reflected wave is small, and the difference in beam directivity is not enough as a countermeasure against fading.
[0007]
On the other hand, in the second conventional method that uses a plurality of frequencies, the power of a signal obtained at a certain frequency is obtained, and the frequency at which the power is obtained is selected (frequency agility method), or transmitted at a predetermined frequency. This is a method (frequency hopping) in which all received signal powers of pulses are added. As described above, if the frequency is changed, the distance at which fading occurs will change. However, depending on the relationship between the path length difference between the direct wave and the sea surface reflected wave and the phase shift at the time of sea surface reflection, any of a plurality of frequencies may be used. In some cases, power cannot be obtained even at frequencies. As described above, the conventional method 2 in which a plurality of frequencies are used and power is obtained at each frequency and then added is not sufficient as a countermeasure against fading.
[0008]
SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and is a countermeasure for fading. Among them, a solution using only a single frequency and a solution using a plurality of frequencies. Is shown.
[0009]
[Means for Solving the Problems]
A first invention provides an array antenna for transmitting and receiving radar pulses, a phase shifter for controlling the amplitude and phase of a signal transmitted and received by the array antenna, a combiner for voltage-synthesizing an output signal of the phase shifter, A frequency converter for frequency-converting the output signal of the synthesizer, a spatial maximum ratio combined weight estimator for estimating a weight for synthesizing the output signal of the frequency converter with a maximum signal-to-noise ratio (S / N), An antenna device comprising: a spatial maximum ratio combining device that combines output signals of the frequency conversion device according to the weight estimated by the spatial maximum ratio combining weight estimating device.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiment 1 FIG.
FIG. 1 shows an embodiment of the first invention, in which 1 to 7 are the same as in the conventional method. Numeral 10 is a spatial maximum ratio combining weight estimating device, and 11 is a spatial maximum ratio combining device.
[0011]
The transmission pulse of the frequency f1 transmitted by the array antenna 1 is reflected from the target, is also incident on the array antenna 1, and becomes a received signal. The number of array elements is N, the number of subarrays is M (M = 4 as an example in FIG. 1), and the number of elements in the subarray is Ns. Among the received signals input to the array antenna, a received signal containing a target (T is the number of samples) is defined as Xr∈C ^N ^× ^T in a matrix format. Here, C ^N ^× ^T indicates that it is a matrix of N rows and T columns having a complex number as an element. The received signal Xr is input to the phase shifter 2. The phase shifter 2 performs a beam steering operation for each sub-array for directing the reception beam in the same direction as the transmission direction. Here, the steering vector is written as B (θ) ∈C ^N ^× ^T. In the synthesizer 3, "Equation 1" is obtained as an output signal of the sub-array.
[0012]
(Equation 1)

[0013]
Here, if the case where the array antenna 1 is a linear array is taken as an example, B (θ) C ^N ^× ^T becomes “Equation 2”.
(Equation 2)

[0015]
Here, T on the right shoulder of the matrix represents the transposed matrix, θ is the incident angle of the radio wave, d is the distance between the array antennas, and λ is the wavelength of the transmitted wave. The output of the synthesizer 3 is "Equation 3".
[0016]
[Equation 3]

[0017]
Here, submatrix (X, a: b, c: d) represents an operation of cutting out a sub-matrix of ~ row and ~ d column of the matrix. In the first invention, the synthesizer output signal written by "Equation 3" is frequency-converted to baseband, A / D-converted, and then input to the spatial maximum ratio synthesizer 10. In the spatial maximum ratio combining device, the maximum ratio combining is applied to the signal between the sub-arrays which is the output of the combiner 3. The spatial maximum ratio composition is described. The output Ym of each sub-array is combined into one matrix, and the input data matrix Z ∈ C ^N ^× ^T for spatial maximum ratio combining is described as “Equation 4”.
[0018]
(Equation 4)

[0019]
Using this data matrix, the spatial maximum ratio composite weight estimating device 11 obtains the correlation matrix _R for z by "Equation 5" in order to obtain the spatial maximum ratio composite weight.
[0020]
(Equation 5)

[0021]
Further, a weight vector for spatial maximum ratio combining is obtained by obtaining an eigenvector W1 corresponding to the maximum eigenvalue of the correlation matrix R. The spatial maximum ratio combining weight thus obtained is input to the spatial maximum ratio combining device 10, and the spatial maximum ratio combining device uses this weight vector to transmit the transmission pulse irrespective of the presence or absence of the target as shown in the timing diagram of FIG. The processing of "Equation 6" is performed over all distance samples r between and the transmission pulse.
[0022]
(Equation 6)

[0023]
Next, the power P (r) becomes “Equation 7”.
[0024]
(Equation 7)

[0025]
This is the operation of the spatial maximum ratio synthesizer using a sub-array configuration. A signal with good S / N ratio with fading countermeasures achieved by spatial maximum ratio combining is obtained. Generally, threshold detection is applied to the output of this spatial maximum ratio synthesizer in the distance direction, and target detection is detected. In addition, the improved S / N allows more accurate distance measurement in the section of the target detection sample. In addition, by using a sub-array configuration, dimensions such as eigenvector calculation of the correlation matrix of Equation 5 are reduced, and the amount of calculation is significantly reduced. In the first embodiment, the above is repeated for each transmission pulse as shown in FIG.
[0026]
If a single frequency is used, the method of using a sub-array configuration of the array antenna is adopted. Therefore, in order to avoid the disappearance of the received signal due to fading by adopting spatial maximum ratio combining in a multipath environment, the transmission / reception hardware This has the effect of reducing the load and reducing the computational load on the spatial maximum ratio composite weight. Fig. 3 is a diagram showing the effects of maximum ratio combining of the receiving space and maximum ratio combining of the transmitting and receiving spaces using a single frequency. In FIG. 3A, the horizontal axis is the distance to the target while the target altitude is constant, and the vertical axis is the received relative power. The solid line is the reception power obtained by the reception space maximum ratio combining method using a full DBF (Digital Beam forming: not a sub-array configuration, but a signal obtained by frequency-converting a received signal from each radiating element to baseband and performing maximum ratio combining). . The dotted line is the transmission / reception space maximum ratio combining method, the broken line is the reference monopulse sum signal, and the dashed line is the reference monopulse sum signal in free space without multipath. Fig. 3 shows the case where the target altitude is low and the distance is far, and the angle between the direct wave and the sea surface reflected wave is almost 0. No recovery has been obtained. However, if the angle difference is large (that is, the target altitude is slightly higher) or if the aperture diameter of the array antenna is large, power recovery can be expected even with the spatial maximum ratio combining method as seen at a short distance in the figure. FIGS. 3B and 3C are diagrams illustrating the effect of spatial maximum ratio combining by the sub-array configuration.
FIG. 3B shows an example of the gain reduction in the 2-subarray configuration compared to the full DBF, and FIG. 3C shows an example of the gain reduction in the 4-subarray configuration similarly to the full DBF. In each case, the reduction of the gain is smaller than that of the full DBF, and it is possible to obtain a sufficiently satisfactory effect even in the spatial maximum ratio combining in the sub-array configuration according to the first invention.
[0027]
Embodiment 2 FIG.
Embodiment 2 has the same configuration as that of FIG. 1, but is characterized in that the spatial maximum ratio combining weight obtained from the received signal is used as the transmission weight of the next transmission wave as shown in the timing chart of FIG. It realizes spatial maximum ratio combining in both transmission and reception.
[0028]
Embodiment 3 FIG.
Embodiment 3 is an embodiment of the second invention (frequency hopping space / frequency maximum ratio combining). FIG. 5 shows this embodiment, in which 1 to 7 are the same as in the conventional method. Reference numeral 12 denotes a space / frequency maximum ratio combining weight estimating device, and reference numeral 13 denotes a spatial / frequency maximum ratio combining device. In the second invention, as in the first invention, a method for realizing the spatial-frequency maximum ratio combining method by retaining received pulse data when frequency hopping is performed for each pulse by a sub-array configuration and transmitted. .
[0029]
FIG. 6 is a diagram for explaining transmission / reception and processing timing according to the third embodiment. As shown in FIG. 6, the frequency of the transmission pulse is changed to f1, f2,. During this time, the received signal input to the array antenna from the target reflection of each transmission pulse is subjected to beam steering operation for each sub-array for directing the reception beam in the same direction as the transmission direction by the phase shifter 2, as in the first invention. . Assuming a frequency number, the output of the synthesizer 3 at the sub-array number is Ym, k ， C ¹ ^× ^T , as in the case of the equation (1). The space / frequency maximum ratio synthesizing device 12 holds this Ym, k∈C ¹ ^× ^T until transmission and reception at all frequencies are completed. Next, we create the matrix V∈C ^MK ^× ^T side by side these in the column direction.
[0030]
The same number of processes (5) to (7) as in the spatial maximum ratio combining are performed on the column vector of the dimension defined by equation 8 ". Thus, the maximum ratio combining of the received signals obtained at each frequency and each sub-array is performed. Can be easily realized [0031]
(Equation 8)

[0032]
Embodiment 4 FIG.
Embodiment 4 is another example of the second invention (frequency hopping space / frequency maximum ratio combining). FIG. 7 is a diagram for explaining transmission / reception and processing timing according to the fourth embodiment. The fourth embodiment is a combination of the second and third embodiments. The transmission pulse train subjected to frequency hopping according to Embodiment 3 is transmitted. Not only does the weight estimated by the space / frequency maximal ratio combining weight estimator from the pulse train be used for receiving spatial / frequency maximal ratio combining, but the next transmitted pulse also has the amplitude The frequency corresponding to the large element of and the element is used as the transmission weight. In the subsequent transmission and reception, while monitoring the power while repeating Embodiment 2, it is difficult to recover the received power due to fading only with the spatial maximum ratio combination, and it is determined whether to change the frequency. If it is determined that the frequency should be changed, the third embodiment in which the transmission pulse train subjected to frequency hopping is transmitted as the next transmission pulse as in the beginning of the fourth embodiment is performed. Hereinafter, the same control is repeated. By controlling in this way, the frequency hopping is used together as compared with the second embodiment, so the fading improvement effect is large, and the spatial frequency maximum in the second embodiment is larger than that in the third embodiment in which frequency-hopped transmission is repeated. If ratio combining is sufficient, it will be repeated, and it is possible to improve the power use efficiency as a whole.
[0033]
Embodiment 5 FIG.
Embodiment 5 is an embodiment of the third invention (frequency multiplexing space / frequency maximum ratio combining). FIG. 8 shows this embodiment. In the drawing, reference numerals 1 to 7 are the same as those of the third embodiment shown in FIG. Reference numeral 12 denotes a space / frequency maximum ratio combining weight estimator, and reference numeral 13 denotes a spatial / frequency maximum ratio combining device. A frequency discriminator 14 discriminates a received signal of a pulse transmitted by frequency multiplexing into each frequency. The third invention has the same functions as those of the second invention in terms of the functions of the space / frequency maximum ratio combining weight estimation device and the space / frequency maximum ratio combining device. Embodiment 5 (ie, the third invention) will be described with reference to FIG. FIG. 9 is a diagram illustrating transmission / reception and processing timing. In the fifth embodiment, the frequency obtained by frequency-hopping each transmission pulse in the third embodiment is simultaneously frequency-multiplexed with one transmission pulse and transmitted. It can be applied when the transmission and reception system has a wide band and can transmit multiple frequencies simultaneously. The same number (8) as that of the second invention is created by the spatial / frequency maximum ratio combined weight estimating device 12 from the signals of the respective sub-arrays discriminated into the respective frequencies by the frequency discrimination. Using the space / maximum ratio combining weight obtained in the same manner as in the second invention, the received signal is synthesized by the space / frequency maximum ratio combining device 13. Thus, the maximum ratio combining of the received signals obtained at each frequency and each sub-array can be easily realized with one pulse. Furthermore, the maximum spatial / frequency ratio combining weight is input to the transmitter 7 and the phase shifter 2 and used as the weight of the frequency multiplexed pulse at the next transmission, so that the maximum spatial / frequency ratio combining for transmission becomes possible. By repeating this, it is possible to realize the spatial / frequency maximum ratio combining of transmission and reception.
[0034]
FIG. 10 is a diagram illustrating the effect of the second invention using a plurality of frequencies. FIG. 10 (a) is a diagram similar to FIG. 3 (a), and the solid line is the received power by spatial-frequency maximum ratio combining by full DBF when frequency hopping is used. The frequency is an example using three types of f0-10%, f0, and f0 + 10% with the center frequency f0 (= 6 GHz). The dotted line is the monopulse sum signal for reference, and the dashed line is the reference monopulse sum signal in free space without multipath. From this figure, it is confirmed that the received power equal to or higher than the monopulse sum signal in free space can be obtained in the entire range of distances by spatial-frequency maximum ratio combining. Similarly, FIG. 10B is a diagram for explaining the effect of space / frequency maximum ratio combining using a sub-array configuration that is frequency hopping space / frequency maximum ratio combining or frequency multiplexing space / frequency maximum ratio combining. FIG. 10 (b) shows a reduction in gain in the 2 and 4 sub-arrays compared to the full DBF, and a second conventional method (a method of adding each power of a frequency-hopped pulse; referred to as non-coherent F / H). It is a figure explaining the fall of the gain in (). In the 2 and 4 sub-arrays, the decrease in the gain is less than 2 dB compared to the full DBF, but in the second conventional method (non-coherent F / H), fading close to 10 dB occurs at a short distance. The reason for this is that Fig. 10 (c) shows the received power at each of the three frequencies. As is clear from FIG. 10 (c), since the null distance of the power coincides with any of the three frequencies used, the disappearance of the power due to fading cannot be avoided in the second conventional method. You can see that. In this way, the effect that the power more than the monopulse sum signal in free space can be obtained even in such a bad condition by the space-frequency maximum ratio combination is obtained.
[0035]
【The invention's effect】
As a result, fading countermeasures can be taken.
[Brief description of the drawings]
FIG. 1 is a block diagram of an antenna device according to first and second embodiments.
FIG. 2 is a diagram for explaining the operation timing of the first embodiment.
FIG. 3 is a diagram illustrating effects of the first and second embodiments. FIG. 4 is a diagram illustrating operation timings of the second embodiment.
FIG. 5 is a block diagram of an antenna device according to the third and fourth embodiments.
FIG. 6 is a diagram for explaining the operation timing of the third embodiment.
FIG. 7 is a diagram for explaining the operation timing of the fourth embodiment.
FIG. 8 is a block diagram of an antenna device according to a fifth embodiment.
FIG. 9 is a diagram for explaining the operation timing of the fifth embodiment.
FIG. 10 is a diagram illustrating the effect of the fourth or fifth embodiment.
FIG. 11 is a diagram illustrating radio wave propagation in a multipath environment.
FIG. 12 is a block diagram of an antenna device according to a first conventional technique.
FIG. 13 is a diagram illustrating the frequency dependence of received power in a multipath environment.
FIG. 14 is a diagram illustrating timings of frequency hopping transmission and frequency multiplex transmission.
FIG. 15 is a block diagram of an antenna device according to a second conventional technique.
[Explanation of symbols]
Reference Signs List 1 array antenna, 2 phase shifter, 3 synthesizer, 4 frequency converter, 5 A / D converter, 6 beamformer, 7 transmitter, 8 power calculator, 9 power adder, 10 spatial maximum ratio combined weight estimation Device, 11 spatial maximum ratio combining device, 12 spatial / frequency maximum ratio combining weight estimating device, 13 spatial / frequency maximum ratio combining device, 14 target

Claims

An array antenna for transmitting and receiving radar pulses,
A phase shifter for controlling the amplitude and phase of a signal transmitted and received by the array antenna, a synthesizer for voltage-synthesizing an output signal of the phase shifter,
A frequency converter for frequency-converting the output signal of the synthesizer;
A spatial maximum ratio combining weight estimating apparatus for estimating a weight for combining output signals of the frequency converting apparatus with a maximum signal-to-noise ratio (S / N);
A spatial maximum ratio synthesizing device that synthesizes an output signal of the frequency conversion device according to the weight estimated by the spatial maximum ratio combining weight estimating device;
An antenna device comprising:

A transmitter for transmitting a radar pulse based on the weight estimated by the spatial maximum ratio combined weight estimating device;
The antenna device according to claim 1, further comprising:

An array antenna for transmitting and receiving radar pulses,
A phase shifter for controlling the amplitude and phase of a signal transmitted and received by the array antenna, a synthesizer for voltage-synthesizing an output signal of the phase shifter,
A frequency converter for frequency-converting the output signal of the synthesizer;
A space / frequency maximum ratio combining weight estimating apparatus for estimating a weight for combining a frequency-hopped output signal of the frequency converting apparatus with a maximum signal-to-noise ratio (S / N);
A space / frequency maximum ratio combining device that combines output signals of the frequency conversion device according to the weight estimated by the space / frequency maximum ratio combining weight estimating device;
An antenna device comprising:

An array antenna for transmitting and receiving radar pulses,
A phase shifter for controlling the amplitude and phase of a signal transmitted and received by the array antenna, a synthesizer for voltage-synthesizing an output signal of the phase shifter,
A frequency discriminator for discriminating a frequency multiplexed signal of the output signal of the synthesizer into respective frequencies;
A frequency converter for frequency-converting an output signal of the frequency discriminator;
A space / frequency maximum ratio combining weight estimating apparatus for estimating a weight for combining an output signal of the frequency converting apparatus with a maximum signal to noise ratio (S / N);
A space / frequency maximum ratio combining device that combines output signals of the frequency conversion device according to the weight estimated by the space / frequency maximum ratio combining weight estimating device;
An antenna device comprising:

5. The antenna device according to claim 3, further comprising: a transmitter that transmits a radar pulse frequency-hopped based on the weight estimated by the space / frequency maximum ratio combined weight estimation device.