JP2004173153A - Transmitter and transversal filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transmitter for transmitting an OFDM (orthogonal frequency division multiplexing) signal whose out-of-band radiation level is reduced under the condition that satisfies a Nyquist first reference and prevents the occurrence of an inter-code interference. <P>SOLUTION: In this transmitter, a serial/parallel converter 1 converts a transmission symbol string of one sequence into a parallel transmission symbol string of L (the number of subcarriers) sequences. An IFT 2 converts the parallel transmission symbol string of L sequences into an OFDM signal. I and Q channel signals of conversion results are subjected to spectral shaping by root rolloff filters 3I and 3Q, and then, inputted to multipliers 4I and 4Q to be orthogonally modulated. In a receiver side, I and Q channel signals of orthogonal demodulation signals outputted from multipliers 9I and 9Q are subjected to spectral shapings by root rolloff filters 11I and 11Q and then inputted to an FT part 12. <P>COPYRIGHT: (C)2004,JPO

Description

上述したスペクトル整形フィルタ出力ｖ_１（ｔ）の自己相関関数Ｒ（ｔ）は、次式の通りである。
【数２】

定数Ｋは送信フィルタのインパルスレスポンス長であり、有限相関長のディジタルフィルタ（トランスバーサルフィルタ）のタップ数に相当している。
電力スペクトルＰ（ｗ）は、自己相関関数のフーリエ変換であるから、次式で与えられる。
【数３】

なお、ディジタルフィルタを用いることを想定しているので、式（ｂ），（ｃ）における積分は離散的な数値積分で、Ｔ_ｓ ／ Ｍの時間間隔に４点取る（４倍オーバサンプ）とした。すなわち、サンプリング間隔を、Ｔ_ｓ ／４Ｍ、最高周波数を２Ｍ／Ｔ_ｓとした。
【００３６】
図６は、ロールオフ率＝０．２のルート・ロールオフフィルタを送信側と受信側に用いた場合の出力電力スペクトルを示すグラフである。フィルタのタップ数は１０１，２０１または４０１とした。
図１１に示した従来のＯＦＤＭ信号の出力電力スペクトルと比較すると、低い帯域外輻射レベルが達成される。信号帯域幅は、図１１に示したような、従来の時間領域ウインドウを乗算する場合と比べて、帯域外輻射レベル−４０ ｄＢで６０％程度になることがわかる。
【００３７】
上述したルート・ロールオフフィルタをディジタルフィルタで実現する場合、有限のインパルスレスポンス長で作らざるを得ない。その結果、インパルスレスポンスを途中で打ち切ることになり、打ち切りが不連続を発生させることになるため、図６に示されるように、出力電力スペクトルは長い裾を引き、帯域外輻射レベルが増加している。
ロールオフ率を小さくすれば、遷移領域で急速に電力スペクトル密度が低下することになるので、周波数利用効率を高められるので好ましい。しかし、ロールオフ率を小さくするほど、裾の引きが長くなり帯域外輻射レベルが大きくなる。この原因は、小さいロールオフ率ほど、インパルス応答の時間相関長が長くなってしまうので、打ち切り誤差の影響が大となるためである。
【００３８】
そこで、以下に説明する本発明の第２の実施形態においては、上述した打ち切りの影響を低減するために、送信機側のルート・ロールオフフィルタについて、インパルスレスポンスの最適化を行う。
最適化した結果得られる送受信フィルタの総合伝送特性は、２乗余弦ロールオフ特性とは異なるものとなり、インパルスレスポンスの打ち切りが存在する条件下において、帯域外輻射レベルの低減を実現するものである。
最適化の具体的条件の一例として、（１）送信側と受信側とを合わせた総合伝送特性が、ナイキスト第１基準（符号間干渉なし）を満たすこと、（２）受信機のフィルタは理想的な２乗余弦のルート・ロールオフフィルタとした。
【００３９】
図２は、本発明の第２実施形態を説明するためのブロック構成図である。
図中、図９，図１と同様な部分には同じ符号を付して説明を省略する。
２１Ｉ，２１Ｑはインパルスレスポンスを最適化したスペクトル整形フィルタである。受信側のルート・ロールオフフィルタ１１Ｉ，１１Ｑは、図１と同様のものである。
【００４０】
図３は、インパルスレスポンスを最適化したときのロールオフフィルタの説明図である。図３（ａ）は最適化の第１具体例、図３（ｂ）は最適化の第２具体例の説明図である。
図中、横軸は正規化周波数（正側のみを示す）、縦軸は出力レベルである。
ナイキスト第１基準によれば、理想低域通過フィルタの遮断周波数（ナイキスト周波数、正規化周波数＝０．５）の出力レベル０．５（−６ｄＢ）の点に対して、肩特性が奇関数であれば、符号間干渉なしの条件を満たす。
【００４１】
図３（ａ）において、３１はロールオフ率α＝０．２としたときの２乗余弦ロールオフ特性であり、ナイキスト第１基準を満たすフィルタ特性である。矩形特性（理想低域通過フィルタ），３角形特性，台形特性といったものも、ナイキスト第１基準を満たすが、傾きに非連続点があるため、インパルスレスポンス長が増加するため用いられない。
この実施の形態においては、ここで、この２乗余弦ロールオフ特性３１を基本伝達関数とし、さらに、遷移領域の中心周波数に対し奇関数である１または複数の補正伝達関数を加算して帯域外輻射レベルを低減したものである。ナイキスト第１基準は満たしているので、符号間干渉は生じない。
図３（ａ）の例では、遷移領域の外側境界３２（正規化周波数０．６）を、所要の帯域と帯域外との境界（周波数ｆ_１）としている。必ずしも、このようにする必要はなく、所要の帯域と帯域外との境界とする周波数ｆ_１は、任意に設定して最適化計算をすることができる。
図３（ａ）に示したような、遷移領域の外側境帯域外輻射レベルが僅かな値であるため、リニアスケールでは、補正後の伝達関数と２乗余弦ロールオフ特性３１との差は識別しにくいが、図７を参照して後述するログスケールの電力スペクトルでは識別できる。
送信側のスペクトル整形フィルタ２１Ｉ，２１Ｑは、総合伝送特性の一部を分配された特性となる。そのため、その特性は、総合伝送特性の伝達関数を受信フィルタの伝達関数で除算した伝達関数で表される。補正伝達関数としては、前記遷移領域における、前記１または複数の補正伝達関数は、奇数次の余弦関数となるものであり、総合伝送特性の伝達関数は、前記遷移領域の両端の各周波数において連続している。
【００４２】
この実施の形態における送信フィルタと受信フィルタとの総合伝送特性Ｆ（ｗ）は、次式の通りである。ｗは角周波数であり、正負の値をとる。ｐは次数、α_ｐは次数ｐにおける係数であって、α_ｐＦ_ｐ（ｗ）は各次数ｐ（１次以上Ｐ次以下）における周波数特性である。
Ｆ_０（ｗ）＋Ｆ_１（ｗ）のみの場合は、従来の２乗余弦ロールオフ特性にとなる。
【数４】

ここで、各次数ｐ（１次以上Ｐ次以下）における係数α_ｐの総和は１である。すなわち、
【数５】

【００４３】
式（２）は、周波数特性が、遷移領域の両側で連続となるための条件である。
式（１）（２）を変形して次式（３）を得る。
【数６】

ここで、
【数７】

【数８】

上述した式（４）（５）は、式（３）中のＦ_０（ｗ），Ｆ_１（ｗ）を示している。αはロールオフ率、Ｍはサブキャリア数（総数）、Ｔ_ｓは、送信側におけるシリアル・パラレル変換後の並列シンボル列のシンボル間隔（ＯＦＤＭ信号のシンボル区間長）である。
式（１）でＰ＝１（すなわち、ｐ＝０，１のときの和）の場合は、図３の３１に示す、従来の２乗余弦ロールオフフィルタとなる。
Ｆ_ｐ（ｗ） （ｐ≧２）は、インパルスレスポンスを最適化（帯域外輻射レベルを低減）するためにＦ_ｐ（ｗ） （ｐ＝１）を拡張した関数である。
【００４４】
すなわち、従来の２乗余弦ロールオフフィルタは、通過周波数帯域と遮断周波数帯域の間の肩特性（遷移特性）として、余弦関数ｃｏｓ（ｘ）を使用している。この実施の形態における送受信フィルタの総合伝送特性では、肩特性として、奇数次（基本周期の奇数分の１の周期）の余弦（ｃｏｓｉｎｅ）関数群、すなわち、ｃｏｓ（ｘ） ，ｃｏｓ（３ｘ） ，ｃｏｓ（５ｘ） ，… ，ｃｏｓ［（２Ｐ−１）ｘ］ の組み合わせを使用している。
図４は、ロールオフ率α＝０．２としたときのＦ_ｐ（ｗ）を示すグラフである。あわせて、Ｆ_０（ｗ）も図示している。正規化周波数が負の領域は図示していないが正負対称である。
図４から明らかなように、上述したｃｏｓｉｎｅ関数群は、いずれも遷移領域の中心（正規化周波数０．５）に対して奇関数である。従って、符号間干渉なしの条件を満たし、拡張２乗余弦ロールオフフィルタとでもいうべきものが実現される。
【００４５】
この実施の形態における送信信号、すなわち、スペクトル整形フィルタが出力する送信信号の帯域外輻射レベルを求め、この帯域外輻射レベルが最小となるように次数ｐにおける係数α_ｐを設定した場合の、出力電力スペクトルを求める。
この実施の形態では、受信側のフィルタ（図２の１１Ｉ，１１Ｑ）を理想ルート・ロールオフ（ｒｏｏｔ ｒｏｌｌ−ｏｆｆ）特性（２乗余弦のロールオフ特性）と仮定した。従って、上述した式（１）の総合伝送特性を、受信側の特性（理想ルート・ロールオフ特性）で割り算すれば、送信側のフィルタ特性、すなわち、図２のスペクトル整形フィルタ２１Ｉ，２１Ｑの特性が求まる。
【００４６】
送信側のフィルタ特性をＧ（ｗ）とすると、次式の通りである。
【数９】

ここで、式（６）中のＧ_１（ｗ），Ｇ_ｐ（ｗ）は、次式の通りである。なお、式（７）のＧ_１（ｗ）は理想ルート・ロールオフ特性である。
【数１０】

【００４７】
上述した式（６）のＧ（ｗ）を送信フィルタの特性とした場合、その出力電力スペクトルＰ（ｗ）は次式となる。数式の導出過程は省略する。
【数１１】

ここで、ｃ（ｗ），Ｈｐ（ｗ）は、次式の通りである。
【数１２】

式（１０），（１１）において、ｗ_ｍはｍ番目のサブキャリアの角周波数、Ｔ_ｓは送信側における直並列変換後の並列シンボル列のシンボル間隔（ＯＦＤＭ信号のシンボル区間長）、定数Ｋは送信フィルタのインパルスレスポンス長であり、有限相関長のディジタルフィルタ（トランスバーサルフィルタ）のタップ数に相当している。
【００４８】
帯域外輻射レベルＰｔは、式（９）の出力電力スペクトルを、帯域外周波数において積分して得られるため、次式のように与えられる。
【数１３】

ここでｗ_１は既に説明したように、最適化の際の帯域内と帯域外との境界であると定義した角周波数、ｗ_２はサンプリング定理から決まる最高角周波数である。この境界を周波数ｆ_１で定義すればｆ_１＝ｗ_１／２π、最高角周波数ｗ_２を周波数で定義すればｆ_２＝ｗ_２／２πで与えられる。ディジタルフィルタを前提にすれば、最高周波数はサンプリング周波数の１／２、４倍オーバサンプリングであれば、サンプリング周波数は正規化周波数で４であるから、最高周波数は２となる。
上述した式（１２）は、各α_Ｐ （ｐ≧２）の２次関数であり（α_ｐとα_ｑとを、ｐ＝２，…Ｐ、ｑ＝２，…Ｐに関して乗算している）、かつ、２次係数が全て正であるため、その極値が帯域外輻射レベルの最小値を与える。各α_Ｐ （ｐ≧２）について式（１２）を微分した各式において、極値を与える各α_Ｐ （ｐ≧２）の値を求め、これらを（２）式に代入してα_１を得、さらに、このα_１とともに、（６）式に代入してフーリエ変換すれば、帯域外輻射レベルを最小にするインパルスレスポンスが求まる。
【００４９】
図３（ｂ）に示すように、境界３４を周波数ｆ_１＝０．５５とした場合のロールオフ特性３５としては、ロールオフ率α＝０．１（遷移領域０．４５≦ｆ≦０．５５）とした場合とほぼ同様の急峻な特性が得られる。
すなわち、境界３４の周波数ｆ_１よりも外の帯域の輻射レベルが最小になるように係数を設定したために、境界３４の周波数ｆ_１＝０．５５から外側の送信フィルタ伝送特性の振幅は著しく低減される。これは、後述する図７（ｂ）に示す最適化後の出力電力スペクトルと図１２の振幅制御なしの出力電力スペクトルを比較すれば明らかである。結果として周波数ｆ_１から外側の総合伝送特性の振幅は著しく低減される。総合伝送特性はナイキスト周波数（正規化周波数０．５）の出力０．５（−６ｄＢ）の点の前後で奇関数となる特性を維持するので、周波数ｆ＝０．４５においては、ほぼ１．０（０ｄＢ）となる。その結果、ロールオフ率α＝０．１（遷移領域０．４５≦ｆ≦０．５５）とした場合と同様の急峻な遷移特性が得られる。しかも、この境界３４の周波数ｆ_１からの帯域外輻射レベルが最小であるので、単なるロールオフ率α＝０．１をディジタルフィルタで実現した場合と比べて、帯域外輻射電力レベルが小さくなる。
【００５０】
キャリア数Ｍ＝６４、４倍オーバサンプルでディジタルフィルタのタップ数＝２０１、ロールオフ率α＝０．２、境界の周波数ｆ_１＝０．５５、Ｐ＝５（余弦関数の次数としては９次まで）としたときのα_ｐ（１≦ｐ≦５）は次の通りである。
α_１＝１−（α_２＋α_３＋α_４＋α_５）
α_２＝０．３０８４００４０６
α_３＝−０．１００２７９１６７
α_４＝０．０２５００５５１４
α_５＝−０．００３２６８２１６９１
【００５１】
図５は、帯域外輻射レベルを最小にしたときの送信側のスペクトル整形フィルタ２１Ｉ，２１Ｑのインパルスレスホンスを示すグラフである。図中、横軸はサンプルタイミング、縦軸は出力値（リニアスケール）である。スペクトル整形フィルタ２１Ｉ，２１Ｑは、例えば、直列化された遅延要素の各タップの出力にそれぞれタップ係数を乗算し、各乗算結果を加算して出力するトランスバーサルフィルタ（ＦＩＲフィルタ：有限長インパルス応答フィルタ）で実現される。この場合、上述したインパルスレスポンスの値は横軸をタップ番号としたときのタップ係数を与える。既に説明したように、２０１タップで、４倍オーバサンプリングを行った場合を示す。
なお、スペクトル整形フィルタ２１Ｉ，２１Ｑの出力はＤ／Ａ変換されるが、Ｄ／Ａ変換器の出力は、インパルスではなく矩形となるため、いわゆるアパーチャ効果により、実質的な伝送路の総合伝達特性の高域が下がることになるが、オーバサンプル率が高ければ事実上無視できる。無視できないときには、例えば、このスペクトル整形フィルタ２１Ｉ，２１Ｑなどにおいて特性を補償すればよい。
【００５２】
図７は、帯域外輻射レベルが最小になるように最適化したときの、出力電力スペクトルを示す図である。図６と同じ設定条件での特性である。
図７（ａ）は、所要帯域と帯域外との境界と定義する周波数ｆ_１＝０．６、図７（ｂ）はｆ_１＝０．５５とした場合を示している。
最適化によりサイドローブのない準矩形の出力電力スペクトルが得られた。電力密度が−１００ｄＢ以下の範囲では裾を引いているが、図示の範囲外となっている。比較例として、第１の実施形態である２乗余弦ロールオフフィルタの場合を示している。
図７（ｂ）において、帯域外輻射レベルが−４０ｄＢとなる信号帯域は、ナイキスト周波数の１．０６倍、帯域外輻射レベルが−６０ｄＢとなる信号帯域は、ナイキスト周波数の１．０８倍にすぎないので、周波数利用効率が著しく改善される。図１１に図示したような、従来の時間領域ウインドウを乗算した場合よりも、遙かに低い帯域外輻射レベルが達成される。
【００５３】
図８は、第２の実施形態における出力電力スペクトルのキャリア数依存性を示すグラフである。キャリア数Ｍは、１６，３２および６４の３通りとした。キャリア数の増加に伴い帯域外輻射レベルが減少することがわかる。キャリア数６４では著しく低減される。キャリア数があまりにも少ない場合には、フィルタのタップ数を増やす必要がある。
【００５４】
上述した説明では、２乗余弦ロールオフ特性の関数（基本伝達関数）に加える関数（補正伝達関数）として３次以上の奇数次ｃｏｓ関数群（ｃｏｓ（３ｘ），ｃｏｓ（５ｘ）…）とした。原理上、ナイキスト周波数に対して奇関数となる関数群であれば、ナイキストの第１条件を満たすフィルタとなり、かつ、帯域外輻射レベルを低減するように関数群の係数を最適化することができる。
従って、上述した具体例以外の基本伝達関数、補正伝達関数を採用してもよい。例えば、補正伝達関数として、正弦（ｓｉｎ）関数群を用いることができる。また、ｓｉｎ関数群と上述したｃｏｓ関数群とを組み合わせたものでも構わない。
また、ロールオフ特性の関数に加える関数として実数部のみからなる関数を用いたが、虚数部を有するものでもよい。虚数部が、ナイキスト周波数に対して偶関数となる場合にもナイキスト第１基準を満たすことが知られている。
【００５５】
上述した説明では、サブキャリア数（Ｌ）は、通常、ＩＦＴ２のポイント数（Ｍ）に等しいとしたが、他の通信システムとの間の相互干渉を避ける等のために、意図的に任意の数のサブキャリアを使用しない場合がある。このような場合でもＩＦＴ２のポイント数（Ｍ）は変更しないため、ＬとＭの値は必ずしも一致しない。
【００５６】
上述した説明では、無線ＬＡＮシステムに用いるＯＦＤＭ通信システムを前提として説明したので、送信複素シンボル列としては、ディジタル変調規則に従ってマッピングされた複素信号となる。しかし、送信複素シンボル列としては、ディジタル変調規則に従ってマッピングされた複素信号が時間軸方向に拡散符号で符号拡散されたものであってもよい。あるいは、直並列変換されたサブキャリアの複素振幅レベルが、周波数軸方向（サブキャリアの配列方向）に拡散符号が割り当てられて符号拡散されて、ＩＦＴに出力されるものであってもよい。このように、ＩＦＴに入力される前の送信複素シンボルが何であっても構わない。
また、通信システムの用途は、無線ＬＡＮに限られるものではない。携帯電話などの移動無線通信ネットワークにおける移動局基地局間通信システムにおける上りおよび下り伝送路の少なくとも一方の通信システムに適用してもよい。
【００５７】
上述した説明では、本発明のフィルタをＯＦＤＭ通信システムの送信フィルタに適用したが、他の用途に使用して、ナイキスト第１基準を満足するとともに、帯域外出力信号電力レベルを低減することも可能である。例えば、ディジタル変調を用いずに、２値あるいは多値の実数部のみの信号を用いる有線あるいは光通信ベースバンド通信システムに適用することができる。
また、ナイキスト第１基準を満たす特性の一端を担う基本となる伝達関数を、ナイキスト第１基準を満たす１または複数の伝達関数群を使用して修正する手法を、帯域外輻射レベル低減以外の目的のために採用することもできる。
【００５８】
【発明の効果】
上述した説明から明らかなように、本発明の送信装置は、帯域外輻射電力レベルを低減させることが難しかった、ＩＦＴにより作成されたＯＦＤＭ信号を送信する通信システムにおいて、ナイキスト第１基準を満足して符号間干渉を生じることなく、所要帯域外の輻射電力レベルが低減するという効果がある。
また、本発明のフィルタ装置は、上述した通信システム等の送信フィルタとして使用して、ナイキスト第１基準を満足して符号間干渉を生じることなく、所要帯域外の輻射電力レベルが低減するという効果がある。
【図面の簡単な説明】
【図１】本発明の第１実施形態の説明図である。図１（ａ）はブロック構成図、図１（ｂ）は動作説明図である。
【図２】本発明の第２実施形態を説明するためのブロック構成図である。
【図３】インパルスレスポンスを最適化したロールオフフィルタの説明図である。
【図４】ロールオフ率α＝０．２としたときのＦ_ｐ（ｗ）を示すグラフである。
【図５】帯域外輻射レベルを最小にしたときのスペクトル整形フィルタ２１Ｉ，２１Ｑのインパルスレスホンスを示すグラフである。
【図６】ロールオフ率０．２のルート・ロールオフフィルタを送信側と受信側に用いた場合の出力電力スペクトルを示すグラフである。
【図７】帯域外輻射レベルが最小になるように最適化したときの、出力電力スペクトルを示す図である。
【図８】第２の実施形態における出力電力スペクトルのキャリア数依存性を示すグラフである。
【図９】従来のＯＦＤＭ通信システムのブロック構成図である。
【図１０】従来のＯＦＤＭ通信システムにおける、送信機側における振幅制御を説明するための窓関数の説明図である。
【図１１】従来のＯＦＤＭ通信システムにおける、出力信号電力スペクトルを示すグラフである。
【図１２】従来のシングルキャリア通信システムのブロック構成図およびロールオフ特性を示すグラフである。
【符号の説明】
１…直並列変換器、２…ＩＦＴ（逆フーリエ変換）部、３Ｉ，３Ｑ…送信機側のルート・ロールオフフィルタ、４Ｉ，４Ｑ…乗算器、５…キャリア信号発振器、６…加算器、７…伝送路、８…分岐部、９Ｉ，９Ｑ…乗算器、１０…キャリア信号発振器、１１Ｉ，１１Ｑ…受信機側のルート・ロールオフフィルタ、１２…ＦＴ（フーリエ変換）部、１３…並直列変換器、２１Ｉ，２１Ｑ…インパルスレスポンスを最適化したスペクトル整形フィルタ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a transmitting apparatus used in an OFDM (Orthogonal Frequency Division Multiplexing) communication system, and a transversal filter that reduces an out-of-band output signal power level without generating intersymbol interference. .
[0002]
[Prior art]
2. Description of the Related Art OFDM communication technology using a plurality of orthogonal subcarriers is used in various wireless communication systems such as a wireless LAN (IEEE802.11a standard, see Non-Patent Document 1) and a digital audio / video communication system.
FIG. 9 is a block diagram of a conventional OFDM communication system.
In the figure, 1 is a serial-parallel converter, and 2 is an IFT (Inverse Fourier Transform) unit. The transmission data is input to the serial / parallel converter 1 as a transmission symbol (complex number) sequence after digital modulation (BPSK, QPSK, 16QAM, etc.), and is converted into L parallel transmission symbol (complex number) sequences. Output to IFT2.
Here, L is the number of subcarriers of the OFDM signal. That is, the serial-to-parallel converter 1 specifies the complex amplitude of the signal that modulates the subcarriers that are orthogonal to each other by the parallel transmission symbol (complex number) sequence. The IFT 2 generates a waveform corresponding to each complex amplitude of the subcarrier and outputs an OFDM signal for each of I channel (real part) and Q channel (imaginary part). The IFT 2 is usually realized by digital signal processing using an IDFT (Inverse Discrete Fourier Transform: discrete inverse Fourier transform) such as IFFT (Inverse Fast Fourier Transform).
[0003]
41 is an amplitude controller, 4I and 4Q are multipliers, 5 is a carrier (carrier) signal oscillator, 6 is an adder, and 7 is a transmission line.
The amplitude control unit 41 smoothly attenuates the amplitude of each of the I-channel and Q-channel OFDM signals output from the IFT 2 at each symbol boundary. Specifically, it will be described later with reference to FIG.
The time axis waveform of the I channel whose amplitude is controlled is quadrature modulated. That is, the multiplier 4I multiplies the in-phase output of the carrier (carrier) signal oscillator 5 and outputs the result to the adder 6. On the other hand, the Q-channel time axis waveform whose amplitude has been controlled is multiplied by the π / 2 phase shift output of the carrier signal oscillator 5 in the multiplier 4Q and output to the adder 6. The adder 6 outputs the sum of the outputs of the multipliers 4I and 4Q to the transmission path 7 as an OFDM signal.
[0004]
When an IDFT is used for the IFT 2 and the amplitude control unit 41 is also realized by digital processing, the output of the amplitude control unit 41 is converted into an analog signal by a D / A converter (not shown) and then output to the multipliers 4I and 4Q. Supplied.
Further, in the actual device configuration, the added signal is directly output as a signal in a high frequency band (RF) to the transmission line 7 or temporarily becomes a signal in an intermediate frequency band (IF). The frequency is converted to a high frequency band (RF) and output to the transmission path 7 (wireless transmission path).
Normally, a guard time is inserted at the beginning of one symbol section of the OFDM signal, but is not shown. The above is the configuration on the transmitter side.
[0005]
On the receiver side, the processing on the transmitter side is reversed. Reference numeral 8 denotes a branch unit, 9I and 9Q denote multipliers, and 10 denotes a carrier signal oscillator. On the receiver side, the branching unit 8 supplies the received OFDM signal to an I-channel multiplier 9I and a Q-channel multiplier 9Q. In each of the multipliers 9I and 9Q, the received OFDM signal is converted into an in-phase output and a π / 2 phase-shifted output from a carrier signal oscillator 10 which oscillates at the same frequency in phase synchronization with the carrier signal oscillator 5 on the transmitter side. The multiplication results in quadrature demodulation. The orthogonally demodulated I-channel and Q-channel OFDM signals are output to an FT (Fourier Transform) unit 12.
[0006]
The FT unit 12 extracts a complex amplitude of a signal that modulates a plurality of subcarriers forming the OFDM signal and outputs the complex amplitude to the parallel-serial converter 13. The parallel-serial converter 13 outputs a serialized received symbol (complex number) sequence. The received symbol (complex number) sequence is digitally demodulated based on a modulation method corresponding to digital modulation on the transmitter side, and received data is obtained.
The FT unit 12 is generally realized by digital signal processing using a discrete Fourier transform (DFT) such as a fast Fourier transform (FFT). When the DFT is used, the outputs of the multipliers 9I and 9Q are supplied to the FT section 12 after being converted into digital signals by an A / D converter (not shown).
[0007]
The frequency spectrum of the time axis waveform output from the IFT unit 2 approaches an ideal rectangular frequency spectrum if the number of subcarriers is very large. However, when the number of subcarriers is not so large, the frequency spectrum is spread. When a wireless transmission path is used as the transmission path 7, the unnecessary radiation level out of the band allocated to the communication system is restricted by the standard. Therefore, subcarriers cannot be set up to the limit of the allocated band, and there is a problem that the frequency utilization efficiency is reduced.
It is conceivable to combine the modulated subcarriers after limiting the band in advance. However, when IFT2 is used, modulation is performed in IFT2, so this method cannot be applied.
[0008]
Therefore, conventionally, in order to suppress the out-of-band component of the OFDM signal, the insertion of the amplitude control unit 41 shown in FIG. 10 and 11 are known.
FIG. 10 is an explanatory diagram of a window function for explaining amplitude control (Windowing) on the transmitter side.
In the figure, reference numerals 51, 52, and 53 denote windows for multiplying the OFDM signals of three adjacent symbol periods, respectively.
Each of the illustrated windows 51, 52, and 53 has a time width that slightly exceeds one symbol section of the OFDM signal, and is a window function in a time domain that rises smoothly and falls smoothly. The waveform of the OFDM signal may change discontinuously at the boundary of each symbol section due to a change in the value of a transmitted symbol. In such a case, an out-of-band radiation component is generated. Therefore, by multiplying the time axis waveform output from IFT2 by the window function, the OFDM transmission signal is smoothly attenuated at the boundary of each symbol section, and as a result, the out-of-band radiation component is reduced.
[0009]
FIG. 11 is a graph showing an output signal power spectrum in a conventional OFDM communication system. A case where the signal amplitude at both ends of one OFDM symbol section is spectrally shaped by a time-domain window function (roll-off rate of 0.025) of a raised cosine to limit the amplitude at the boundary of one symbol section; A case where such amplitude limitation is not performed will be described. The horizontal axis is the frequency normalized so that the symbol rate (symbol / sec) of the original complex transmission symbol before the serial-to-parallel conversion by the serial-to-parallel converter 1 shown in FIG.
The required band required to transmit the OFDM signal is 0.5 of the above-mentioned normalized frequency. The integrated value of the power spectrum in the frequency band exceeding 0.5 is the out-of-band radiation level.
As is clear from FIG. 11, the reduction of the out-of-band radiation level is realized by the amplitude control. However, it is still not enough. In order to use OFDM signals in a wireless LAN, there is a demand to reduce the power density to at least about -60 dB. If the roll-off rate of the time domain window is increased, the out-of-band radiation level is reduced, but intersymbol interference occurs, so that orthogonality between subcarriers cannot be maintained, and the data rate must be reduced. As a result, the frequency utilization efficiency eventually decreases.
[0010]
Non-Patent Document 1 mentioned above suggests that filtering is performed in the frequency domain as a method other than the above-described windowing, and a research paper using an analog filter such as a Butterworth filter or a Chebyshev filter is suggested. However, if the design is made to greatly reduce out-of-band radiation, the characteristics of the transition region of the filter will cause the modulated signal of the subcarrier near the band boundary to be asymmetric, and will not satisfy the Nyquist criterion. The symbol sequence is not completely restored at the receiving end.
[0011]
[Non-patent document 1]
Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications High-speed Physical Layer in the 5 GHz Band <URL: HYPERLINK "http://standards.ieee.org/getieee802/download/802.11a -1999.pdf "http: // standards. ieee. org / getieee 802 / download / 802.11a-1999. pdf>
[0012]
[Problems to be solved by the invention]
SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and a transmitting apparatus that transmits an OFDM signal that satisfies the Nyquist first criterion and has a reduced out-of-band radiation level under the condition that no intersymbol interference occurs. It is another object of the present invention to provide a transversal filter that can be used as a transmission filter of the transmission device and that reduces the out-of-band output signal power level under the condition that intersymbol interference does not occur.
[0013]
[Means for Solving the Problems]
According to the first aspect of the present invention, there is provided an inverse Fourier transform unit for inputting a parallel transmission symbol sequence and outputting an OFDM signal, and inputting the OFDM signal output from the inverse Fourier transform unit for band limiting. And a transmitting means for outputting a transmission signal to be transmitted to the receiving apparatus by orthogonally modulating the output of the transmitting filter, wherein the total transmission characteristic including the receiving filter on the receiving apparatus side is provided. Is a roll-off characteristic satisfying the Nyquist first criterion, and the transmission filter is a characteristic obtained by distributing a part of the roll-off characteristic satisfying the Nyquist first criterion, and is output by the inverse Fourier transform means. It has a characteristic of reducing the out-of-band radiation level of the OFDM signal.
Therefore, an OFDM signal with a reduced out-of-band radiation level can be transmitted under the condition that the Nyquist first criterion is satisfied and no intersymbol interference occurs.
[0014]
In the invention according to claim 2, in the transmission device according to claim 1, the characteristic of the transmission filter is a characteristic obtained by equally dividing a roll-off characteristic satisfying the first Nyquist criterion with the reception filter. There is something.
[0015]
According to a third aspect of the present invention, in the transmission device according to the first or second aspect, the roll-off characteristic satisfying the first Nyquist criterion is a raised cosine roll-off characteristic.
[0016]
According to a fourth aspect of the present invention, in the transmission device according to the first aspect, the total transmission characteristic in a transition region of the roll-off characteristic satisfying the first Nyquist criterion is a roll-off characteristic satisfying the first Nyquist criterion. And a sum of a basic transfer function and one or more corrected transfer functions that are odd functions with respect to the center frequency of the transition region, and have a characteristic of reducing the out-of-band output signal power level more than the basic transfer function. Is represented by a transfer function corrected so as to have the characteristic of the reception filter is represented by a transfer function in which a part of the basic transfer function is distributed, and the characteristic of the transmission filter is a transfer function of the total transmission characteristic. Is divided by the transfer function of the reception filter.
Therefore, by using one or a plurality of corrected transfer functions, the degree of freedom for determining the characteristics of the transition region is increased rather than simply changing the roll-off rate of the basic transfer function, so that the out-of-band radiation level of the OFDM signal can be reduced. Design for obtaining reduced characteristics is facilitated.
If, for example, a periodic function that is an odd function with respect to the center frequency of the transition region and that is orthogonal to the center frequency of the transition region is used as one or a plurality of correction transfer functions, the design becomes easier.
[0017]
According to a fifth aspect of the present invention, in the transmission apparatus according to the fourth aspect, in the transition region, the weight coefficient of the basic transfer function and each weight coefficient of the one or more correction transfer functions are out of band. The radiation level is regarded as an integral value of the power density in a frequency region outside the band exceeding a predetermined boundary frequency, and is optimized so that the out-of-band radiation level is minimized.
Therefore, the characteristic of easily reducing the out-of-band radiation level of the OFDM signal can be obtained.
[0018]
In the invention according to claim 6, in the transmission apparatus according to claim 5, the predetermined boundary frequency is within a range of a transition region of a roll-off characteristic satisfying the first Nyquist criterion, and The frequency is set to a frequency that is closer to the outside of the band than the center frequency of the transition region.
Accordingly, it is possible to substantially reduce the roll-off rate of the basic transfer function and suppress the spread of the bandwidth.
[0019]
According to a seventh aspect of the present invention, in the transmission apparatus according to any one of the fourth to sixth aspects, the one or more correction transfer functions in the transition region are odd-order cosine functions. Wherein the transfer function of the overall transmission characteristic is continuous at each frequency at both ends of the transition region.
Therefore, the characteristic of easily reducing the out-of-band radiation level of the OFDM signal can be realized.
[0020]
According to an eighth aspect of the present invention, in the transmission apparatus according to any one of the fourth to seventh aspects, the basic transfer function has a raised cosine roll-off characteristic.
[0021]
In the invention according to the ninth aspect, the transversal filter realizes a characteristic in which the overall transmission characteristic is distributed among other filters so that the overall transmission characteristic is a roll-off characteristic satisfying the Nyquist first criterion. Wherein the total transmission characteristic in the transition region of the roll-off characteristic that satisfies the Nyquist first criterion is a basic transfer function that is a roll-off characteristic that satisfies the Nyquist first criterion, and an odd value with respect to a center frequency of the transition region. A sum of one or more corrected transfer functions represented by a function, represented by a transfer function corrected to have a characteristic of reducing the out-of-band output signal power level from the basic transfer function, and Is represented by a transfer function in which a part of the basic transfer function is distributed, and the characteristic of the transversal filter is the transfer function of the overall transmission characteristic. As represented function by a transfer function divided by the transfer function of the other filter, in which the tap coefficient is set.
Therefore, a signal having a reduced out-of-band output signal power level can be output under the condition that the Nyquist first criterion is satisfied and no intersymbol interference occurs. By using one or a plurality of corrected transfer functions, the degree of freedom for determining the characteristics of the transition region is increased, rather than simply changing the roll-off rate of the basic transfer function, so that the characteristic of reducing the out-of-band output signal power level can be obtained. The design for obtaining is easy.
If, for example, a periodic function that is an odd function with respect to the center frequency of the transition region and that is orthogonal to the center frequency of the transition region is used as one or a plurality of correction transfer functions, the design becomes easier.
[0022]
According to a tenth aspect of the present invention, in the transversal filter according to the ninth aspect, in the transition region, the weight coefficient of the basic transfer function and each weight coefficient of the one or more correction transfer functions are equal to the band. The external output signal power level is regarded as an integral value of the power density in a frequency region outside the band exceeding a predetermined boundary frequency, and is optimized so that the out-of-band output signal power level is minimized.
Therefore, it is possible to easily obtain the characteristic of reducing the out-of-band output signal power level.
[0023]
In the invention according to claim 11, in the transversal filter according to claim 10, the predetermined boundary frequency is within a range of a transition region of a roll-off characteristic satisfying the Nyquist first criterion, and The frequency is set to a frequency that is closer to the outside of the band than the center frequency of the transition region.
Accordingly, it is possible to substantially reduce the roll-off rate of the basic transfer function and suppress the spread of the bandwidth.
[0024]
According to a twelfth aspect of the present invention, in the transversal filter according to any one of the ninth to eleventh aspects, the one or more correction transfer functions in the transition region are an odd-order cosine function and an odd-order cosine function. Wherein the transfer function of the total transmission characteristic is continuous at each frequency at both ends of the transition region.
Therefore, the characteristic of easily reducing the out-of-band output signal level can be realized.
[0025]
According to a thirteenth aspect of the present invention, in the transversal filter according to any one of the ninth to twelfth aspects, the basic transfer function has a raised cosine roll-off characteristic.
[0026]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is an explanatory diagram of the first embodiment of the present invention. FIG. 1A is a block diagram, and FIG. 1B is an operation explanatory diagram.
In FIG. 1A, the same parts as in FIG. 9 of the conventional example are denoted by the same reference numerals. Reference numerals 3I and 3Q denote root roll-off filters on the transmitter side, and 11I and 11Q denote root roll-off filters on the receiver side.
On the transmitter side, the serial-to-parallel converter 1 converts one sequence of transmission symbol sequences into L (number of subcarriers) sequences of parallel transmission symbol sequences. The obtained L series of parallel transmission symbol sequences are converted into OFDM signals by IFT2. The I and Q channel signals resulting from the conversion are spectrally shaped (band-limited in the frequency domain) by root roll-off filters 3I and 3Q, and then input to multipliers 4I and 4Q to be quadrature modulated. , IF or RF signals.
[0027]
On the receiver side, the I and Q channel signals of the quadrature demodulated signals output from the multipliers 9I and 9Q are input to the FT unit 12 after being spectrally shaped by the root roll-off filters 11I and 11Q.
In the figure, an enclosed portion indicated by reference numeral 14 indicates a substantial transmission path from when the OFDM signal output from the IFT 2 on the transmitter side is received via the transmission path 7 and is input to the FT 12. In this transmission path, the total transmission characteristic (multiplication of the transfer function) obtained by combining the characteristics of both the root roll-off filters 3I and 3Q on the transmitter side and the root roll-off filters 11I and 11Q on the receiver side is rolled off. And
At this time, the root roll-off filters 3I and 3Q on the transmitter side have a characteristic obtained by distributing a part of the roll-off characteristic satisfying the first Nyquist criterion, and the out-of-band radiation level of the OFDM signal output from the IFT2. Has the property of reducing
[0028]
A transfer function obtained by adding a transfer function that is an odd function to the cutoff frequency of the ideal low-pass filter to the transfer function of the ideal low-pass filter satisfies the Nyquist first criterion.
Among the satisfying transfer functions, a transfer function in which the transfer function is continuous and the slope of the transfer function with respect to the frequency is continuous and changes smoothly is referred to as a roll-off characteristic in this specification. In particular, a transition region having a raised cosine is called a raised cosine roll-off characteristic.
Since the total transmission characteristic satisfies the Nyquist first criterion, there is no inter-symbol interference, and the transmission symbol is completely reproduced. Under this condition, the out-of-band radiation level can be reduced as compared with the conventional case.
[0029]
It may seem surprising that the communication system described above can completely recover the transmitted symbol sequence. Therefore, a detailed description will be given in comparison with a conventional communication system using a single carrier (single carrier).
FIG. 12 is a block diagram of a conventional single carrier communication system and a graph showing roll-off characteristics as a comparative example.
FIG. 12A is a block diagram of a conventional single carrier communication system using a root roll-off filter. FIG. 12B is a graph showing a transfer function of the roll-off characteristic.
In FIG. 12A, the same parts as those in FIG. 9 are denoted by the same reference numerals. 61I and 61Q are route roll-off filters on the transmitter side, and 62I and 62Q are route roll-off filters on the receiver side.
The boxed portion indicated by the reference numeral 63 indicates a substantial transmission path until a series of complex transmission symbol strings is received via the transmission path 7 and restored as a complex reception symbol string. In this substantial transmission line, the roll-off characteristic is realized in this substantial transmission line by matching the characteristics of the route roll-off filters on both the transmitter side and the receiver side (multiplication of the transfer function). .
[0030]
FIG. 12B is a graph showing a transfer characteristic of a raised cosine roll-off characteristic. The horizontal axis is the normalized frequency, and the vertical axis is the output (linear scale). Although the characteristics on the negative side of the normalized frequency are not shown, they are left-right symmetric.
The frequency is normalized such that the symbol rate (reciprocal of the symbol interval) of the transmission symbol sequence is 1. At this time, the frequency at which the output level decreases to 1/2 (6 dB) is 0.5.
The roll-off characteristics are shown for three cases where the roll-off rate is α = 1.0, 0.2, and 0.1. When the roll-off rate is α, a region (transition region) of a shoulder characteristic in which a transition from (1−α) / 2 to (1 + α) / 2 in a normalized frequency transitions in a raised cosine manner is obtained.
The distribution of the transfer characteristic of the roll-off characteristic to the transmitter and the receiver may be arbitrarily determined for the sole purpose of eliminating intersymbol interference. Usually, the characteristics of the transmitter and the receiver are the same. A raised cosine root roll-off filter is used.
In the transmission path configuration of the route-off characteristic shown in FIG. 12A, if the received signal can be sampled at the correct timing (if the sampling points are correct), the transmission symbol sequence can be completely restored at the receiver without intersymbol interference. Known as the Nyquist primary criterion.
[0031]
On the other hand, in the block diagram showing one embodiment of the present invention shown in FIG. 1A, a portion 14 surrounded by a dotted line as a substantial transmission path is the same as the conventional single carrier communication system shown in FIG. In the block diagram shown, it corresponds to a portion 63 surrounded by a dotted line as a substantial transmission path.
Therefore, the converted data (OFDM signal) after IFT2 on the transmitter side is regarded as a transmission symbol sequence. The transmission symbol sequence is band-limited to satisfy the first Nyquist criterion by the root roll-off filters 3I, 3Q, 11I, and 11Q, and symbol synchronization is established between the transmitter and the receiver. If sampling is performed at the correct timing, the converted data (OFDM signal) after IFT2 matches the input signal (OFDM signal) to FT12 on the receiver side. Furthermore, the FT12 on the receiver side outputs the same parallel complex symbol as the parallel complex symbol before IFT2 on the transmitter side. Therefore, the complex transmission symbol sequence can be completely restored on the receiving side.
[0032]
The entire transformed data (OFDM signal) after IFT2 is regarded as one transmission symbol sequence, and the transmission symbol sequence is band-limited to satisfy the first Nyquist criterion. Since one symbol section of the OFDM signal includes M sampling points, the apparent transmission speed is M times. M is called the number of points of IFT2.
Therefore, if the band is limited by a roll-off filter having an M-fold bandwidth and sampling is performed at the receiver at M-fold sampling points (M sampling points in one symbol section of the OFDM signal), the sampling point is Based on the output, an original series of complex transmission symbols can be restored. Here, sampling at the M times sampling point is performed by the FT unit 12. The FT unit 12 extracts the complex amplitude of the modulation signal for modulating the orthogonal subcarriers by sampling M times.
Although depending on the signal configuration of OFDM, the number of points of IFT2 is usually made equal to the number of subcarriers (L) of the OFDM signal. Therefore, hereinafter, for the sake of simplicity, M is also referred to as the number of subcarriers, assuming that the number of subcarriers (L) is equal to the number of points (M) of IFT2.
[0033]
FIG. 1B shows an example where M = 4. One symbol of one carrier in the OFDM signal is shown, and a case is shown in which one cycle of the sine wave 15 is output in one OFDM symbol section. Assuming that the section length of the OFDM symbol is Ts, the sampling interval is Ts / M = Ts / 4. Assuming that the half of the reciprocal of the sampling interval is the Nyquist frequency (normalized frequency = 0.5) and a transmission / reception filter that satisfies the first criterion of Nyquist is adopted, as shown in FIG. It is apparent that the complex transmission symbol sequence can be completely restored at the receiving side without intersymbol interference as shown at 17a to 17d if the sample is sampled at the correct sampling point since the impulse response 16a to 16d is obtained. .
In FIG. 1 (b), the number of carriers is set to 1. However, even when the number of carriers is set to be plural, the complex transmission symbol sequence is also completely Can be restored. Since the sampling interval of the OFDM signal is Ts / M, the sampling frequency is M / Ts and the Nyquist frequency is M / 2Ts.
[0034]
Within a -6 dB pass band having a roll-off characteristic (from -0.5 to +0.5 in normalized frequency), M subcarriers included in the OFDM signal are arranged. Among them, the modulated signal of the subcarrier closer to the end of the band is more susceptible to the band limitation, but has the roll-off characteristic and satisfies the first Nyquist criterion. There is no intersymbol interference for the carrier modulation signal.
Note that the transmission symbol sequence can be completely restored when there is no noise, interference, distortion of the propagation path, and fixed deterioration of the wireless device.For example, when there is fixed deterioration such as a sample timing error or an interference wave, The restoration of the transmission symbol sequence may be incomplete.
[0035]
The transmission power spectrum according to this embodiment is calculated according to the following procedure.
First, a conversion data string that has been subjected to parallel conversion is defined. Symbol a _mn Indicates the n-th converted data sequence of the m-th subcarrier. f (t) is the impulse response of the root roll-off filter. w _m Is the angular frequency of the m-th subcarrier, T _s Is the symbol interval (symbol section length of the OFDM signal) of the parallel symbol string after serial-to-parallel conversion on the transmitting side, the output of the spectrum shaping filter (root roll-off filter) is as follows.
(Equation 1)

The above-described spectrum shaping filter output v ₁ The autocorrelation function R (t) of (t) is as follows.
(Equation 2)

The constant K is the impulse response length of the transmission filter, and corresponds to the number of taps of a digital filter (transversal filter) having a finite correlation length.
Since the power spectrum P (w) is a Fourier transform of the autocorrelation function, it is given by the following equation.
[Equation 3]

Since it is assumed that a digital filter is used, the integrals in equations (b) and (c) are discrete numerical integrals, _s Four points were taken at a time interval of / M (4 times oversump). That is, the sampling interval is set to T _s / 4M, the highest frequency is 2M / T _s And
[0036]
FIG. 6 is a graph showing an output power spectrum when a root roll-off filter having a roll-off rate of 0.2 is used on the transmission side and the reception side. The number of taps of the filter was 101, 201 or 401.
As compared with the output power spectrum of the conventional OFDM signal shown in FIG. 11, a low out-of-band radiation level is achieved. It can be seen that the signal bandwidth is about 60% at the out-of-band radiation level of -40 dB, as compared with the case where the conventional time domain window is multiplied as shown in FIG.
[0037]
When the above-described root roll-off filter is realized by a digital filter, it must be formed with a finite impulse response length. As a result, the impulse response is interrupted in the middle, and the discontinuation causes discontinuity. Therefore, as shown in FIG. 6, the output power spectrum has a long tail, and the out-of-band radiation level increases. I have.
It is preferable to reduce the roll-off rate because the power spectrum density rapidly decreases in the transition region, and the frequency use efficiency can be increased. However, the lower the roll-off rate, the longer the tailing becomes, and the higher the out-of-band radiation level. The reason for this is that the smaller the roll-off rate, the longer the time correlation length of the impulse response, and the greater the effect of the truncation error.
[0038]
Therefore, in the second embodiment of the present invention described below, the impulse response is optimized for the root roll-off filter on the transmitter side in order to reduce the influence of the above-mentioned truncation.
The overall transmission characteristic of the transmission / reception filter obtained as a result of the optimization is different from the raised cosine roll-off characteristic, and realizes the reduction of the out-of-band radiation level under the condition where the impulse response is discontinued.
As an example of specific conditions for optimization, (1) the total transmission characteristics of the transmitting side and the receiving side satisfy the first Nyquist criterion (no intersymbol interference), and (2) the filter of the receiver is ideal. Root-roll-off filter with a typical raised cosine.
[0039]
FIG. 2 is a block diagram illustrating a second embodiment of the present invention.
In the figure, the same parts as those in FIGS. 9 and 1 are denoted by the same reference numerals, and description thereof will be omitted.
21I and 21Q are spectrum shaping filters that optimize the impulse response. The route roll-off filters 11I and 11Q on the receiving side are the same as those in FIG.
[0040]
FIG. 3 is an explanatory diagram of the roll-off filter when the impulse response is optimized. FIG. 3A is a diagram illustrating a first specific example of optimization, and FIG. 3B is a diagram illustrating a second specific example of optimization.
In the figure, the horizontal axis is the normalized frequency (only the positive side is shown), and the vertical axis is the output level.
According to the Nyquist first standard, the shoulder characteristic is an odd function with respect to the point of the output level 0.5 (−6 dB) of the cutoff frequency (Nyquist frequency, normalized frequency = 0.5) of the ideal low-pass filter. If so, the condition for no intersymbol interference is satisfied.
[0041]
In FIG. 3A, reference numeral 31 denotes a raised cosine roll-off characteristic when the roll-off rate α = 0.2, which is a filter characteristic satisfying the Nyquist first criterion. A rectangular characteristic (ideal low-pass filter), a triangular characteristic, and a trapezoidal characteristic also satisfy the Nyquist first criterion, but are not used because the slope has discontinuous points and the impulse response length increases.
In this embodiment, the raised cosine roll-off characteristic 31 is used as a basic transfer function, and one or more corrected transfer functions, which are odd functions with respect to the center frequency of the transition region, are added to the out-of-band area. The radiation level is reduced. Since the first Nyquist criterion is satisfied, no intersymbol interference occurs.
In the example of FIG. 3A, the outer boundary 32 (normalized frequency 0.6) of the transition region is defined as the boundary between the required band and the out-of-band (frequency f ₁ ). It is not always necessary to do so, and the frequency f as the boundary between the required band and the out-of-band ₁ Can be arbitrarily set to perform the optimization calculation.
As shown in FIG. 3A, since the radiation level outside the boundary region of the transition region is a small value, the difference between the transfer function after correction and the raised cosine roll-off characteristic 31 is identified on the linear scale. Although it is difficult to do so, it can be identified in a log-scale power spectrum described later with reference to FIG.
The transmission-side spectrum shaping filters 21I and 21Q have characteristics obtained by distributing a part of the total transmission characteristics. Therefore, the characteristic is represented by a transfer function obtained by dividing the transfer function of the total transmission characteristic by the transfer function of the reception filter. As the corrected transfer function, the one or more corrected transfer functions in the transition region are odd-order cosine functions, and the transfer function of the overall transmission characteristic is continuous at each frequency at both ends of the transition region. are doing.
[0042]
The total transmission characteristic F (w) of the transmission filter and the reception filter in this embodiment is as follows. w is an angular frequency and has a positive or negative value. p is the order, α _p Is the coefficient at the order p, α _p F _p (W) is a frequency characteristic in each order p (1st order or more and Pth order or less).
F ₀ (W) + F ₁ In the case of only (w), the conventional raised cosine roll-off characteristic is obtained.
(Equation 4)

Here, the coefficient α in each order p (1st order or more and Pth order or less) _p Are 1. That is,
(Equation 5)

[0043]
Equation (2) is a condition for the frequency characteristic to be continuous on both sides of the transition region.
Equations (1) and (2) are modified to obtain the following equation (3).
(Equation 6)

here,
(Equation 7)

(Equation 8)

Equations (4) and (5) described above are equivalent to F in equation (3). ₀ (W), F ₁ (W) is shown. α is the roll-off rate, M is the number of subcarriers (total), T _s Is the symbol interval (symbol section length of the OFDM signal) of the parallel symbol sequence after serial-parallel conversion on the transmission side.
In the case of P = 1 (that is, the sum when p = 0, 1) in the equation (1), a conventional raised cosine roll-off filter shown at 31 in FIG. 3 is obtained.
F _p (W) (p ≧ 2) is used to optimize the impulse response (reduce the out-of-band radiation level) _p (W) This is a function obtained by expanding (p = 1).
[0044]
That is, the conventional raised cosine roll-off filter uses the cosine function cos (x) as a shoulder characteristic (transition characteristic) between the pass frequency band and the cut-off frequency band. In the overall transmission characteristic of the transmission / reception filter in this embodiment, as the shoulder characteristic, a cosine function group of an odd-order (a period of an odd number of the fundamental period), that is, cos (x), cos (3x), A combination of cos (5x),..., cos [(2P-1) x] is used.
FIG. 4 is a graph of F when the roll-off rate α = 0.2. _p It is a graph which shows (w). In addition, F ₀ (W) is also illustrated. Although the area where the normalized frequency is negative is not shown, the area is symmetric with positive and negative.
As is clear from FIG. 4, the above-described cosine function group is an odd function with respect to the center of the transition region (normalized frequency 0.5). Therefore, what is referred to as an extended raised cosine roll-off filter that satisfies the condition of no intersymbol interference is realized.
[0045]
The out-of-band radiation level of the transmission signal in this embodiment, that is, the transmission signal output from the spectrum shaping filter is obtained, and the coefficient α in the order p is set so that the out-of-band radiation level is minimized. _p The output power spectrum in the case where is set is obtained.
In this embodiment, the filters on the receiving side (11I and 11Q in FIG. 2) are assumed to have ideal root roll-off characteristics (roll-off characteristics of raised cosine). Therefore, if the total transmission characteristic of the above equation (1) is divided by the characteristic on the receiving side (ideal route / roll-off characteristic), the filter characteristic on the transmitting side, that is, the characteristic of the spectrum shaping filters 21I and 21Q in FIG. Is found.
[0046]
Assuming that the filter characteristic on the transmission side is G (w), the following equation is obtained.
(Equation 9)

Here, G in equation (6) ₁ (W), G _p (W) is as follows. Note that G in equation (7) ₁ (W) is an ideal route roll-off characteristic.
(Equation 10)

[0047]
When G (w) in the above equation (6) is used as the characteristic of the transmission filter, the output power spectrum P (w) is as follows. The process of deriving the formula is omitted.
[Equation 11]

Here, c (w) and Hp (w) are as follows.
(Equation 12)

In equations (10) and (11), w _m Is the angular frequency of the m-th subcarrier, T _s Is the symbol interval (symbol section length of the OFDM signal) of the parallel symbol sequence after serial-to-parallel conversion at the transmission side, the constant K is the impulse response length of the transmission filter, and the number of taps of a digital filter (transversal filter) having a finite correlation length Is equivalent to
[0048]
The out-of-band radiation level Pt is obtained by integrating the output power spectrum of Expression (9) at the out-of-band frequency, and is given by the following expression.
(Equation 13)

Where w ₁ Is the angular frequency defined as the boundary between the in-band and out-of-band at the time of optimization, w ₂ Is the highest angular frequency determined from the sampling theorem. The frequency f ₁ Defined by f ₁ = W ₁ / 2π, highest angular frequency w ₂ If f is defined by frequency, f ₂ = W ₂ / 2π. Assuming a digital filter, the maximum frequency is 1/2 of the sampling frequency, and if oversampling is four times the sampling frequency, the sampling frequency is 4 as the normalized frequency, and the maximum frequency is 2.
The above equation (12) is given by _P (P ≧ 2) is a quadratic function (α _p And α _q , And p = 2,... P, q = 2,... P), and since the quadratic coefficients are all positive, the extreme value gives the minimum value of the out-of-band radiation level. Each α _P In each equation obtained by differentiating equation (12) with respect to (p ≧ 2), each α that gives an extreme value _P (P ≧ 2) are obtained, and these values are substituted into equation (2) to obtain α ₁ And furthermore, this α ₁ At the same time, by substituting into the equation (6) and performing Fourier transform, an impulse response that minimizes the out-of-band radiation level is obtained.
[0049]
As shown in FIG. 3B, the boundary 34 is ₁ As the roll-off characteristic 35 when = 0.55, almost the same steep characteristic as when the roll-off rate α = 0.1 (transition region 0.45 ≦ f ≦ 0.55) is obtained.
That is, the frequency f of the boundary 34 ₁ Since the coefficient is set so that the radiation level in the band outside the frequency band becomes minimum, the frequency f ₁ From 0.55, the amplitude of the outer transmission filter transmission characteristic is significantly reduced. This is apparent from a comparison between the output power spectrum after optimization shown in FIG. 7B described later and the output power spectrum without amplitude control shown in FIG. As a result, the frequency f ₁ The amplitude of the overall transmission characteristic outside of is significantly reduced. Since the overall transmission characteristic maintains the characteristic that becomes an odd function before and after the point of the output 0.5 (−6 dB) of the Nyquist frequency (normalized frequency 0.5), at the frequency f = 0.45, approximately 1. 0 (0 dB). As a result, a steep transition characteristic similar to the case where the roll-off rate α = 0.1 (transition region 0.45 ≦ f ≦ 0.55) is obtained. Moreover, the frequency f of the boundary 34 ₁ , The out-of-band radiation power level is smaller than when a simple roll-off rate α = 0.1 is realized by a digital filter.
[0050]
Number of carriers M = 64, number of taps of digital filter with 4 times oversampling = 201, roll-off rate α = 0.2, boundary frequency f ₁ = 0.55, P = 5 (the order of the cosine function is up to the order 9). _p (1 ≦ p ≦ 5) is as follows.
α ₁ = 1− (α ₂ + Α ₃ + Α ₄ + Α ₅ )
α ₂ = 0.308400406
α ₃ = -0.100279167
α ₄ = 0.025005514
α ₅ = -0.003262681691
[0051]
FIG. 5 is a graph showing the impulse response of the spectrum shaping filters 21I and 21Q on the transmission side when the out-of-band radiation level is minimized. In the figure, the horizontal axis is the sample timing, and the vertical axis is the output value (linear scale). The spectrum shaping filters 21I and 21Q, for example, multiply the output of each tap of the serialized delay element by a tap coefficient, add each multiplication result and output the result, and output the result (FIR filter: finite length impulse response filter). ). In this case, the value of the above impulse response gives a tap coefficient when the horizontal axis is a tap number. As described above, a case in which quadruple oversampling is performed with 201 taps is shown.
Although the outputs of the spectrum shaping filters 21I and 21Q are D / A converted, the output of the D / A converter is not an impulse but a rectangle. Is lowered, but can be effectively ignored if the oversampling rate is high. If not negligible, for example, the characteristics may be compensated for in the spectrum shaping filters 21I and 21Q.
[0052]
FIG. 7 is a diagram illustrating an output power spectrum when optimization is performed to minimize the out-of-band radiation level. This is a characteristic under the same setting conditions as in FIG.
FIG. 7A shows a frequency f defined as a boundary between a required band and an out-of-band. ₁ = 0.6, and FIG. ₁ = 0.55.
A quasi-rectangular output power spectrum without sidelobes was obtained by optimization. In the range where the power density is -100 dB or less, the tail is drawn, but outside the range shown in the figure. As a comparative example, a case of a raised cosine roll-off filter according to the first embodiment is shown.
In FIG. 7B, the signal band having an out-of-band radiation level of −40 dB is 1.06 times the Nyquist frequency, and the signal band having an out-of-band radiation level of −60 dB is only 1.08 times the Nyquist frequency. As a result, the frequency utilization efficiency is significantly improved. A much lower out-of-band radiation level is achieved than when multiplied by a conventional time domain window, as shown in FIG.
[0053]
FIG. 8 is a graph illustrating the dependence of the output power spectrum on the number of carriers in the second embodiment. The number of carriers M was set to 16, 32 and 64. It can be seen that the out-of-band radiation level decreases as the number of carriers increases. When the number of carriers is 64, the number is significantly reduced. If the number of carriers is too small, it is necessary to increase the number of taps of the filter.
[0054]
In the above description, as a function (correction transfer function) to be added to the function of the raised cosine roll-off characteristic (basic transfer function), an odd-order or higher-order cos function group (cos (3x), cos (5x)...) Is used. . In principle, if the function group becomes an odd function with respect to the Nyquist frequency, the function group becomes a filter satisfying the first condition of Nyquist, and the coefficients of the function group can be optimized so as to reduce the out-of-band radiation level. .
Therefore, a basic transfer function and a corrected transfer function other than the specific examples described above may be employed. For example, a group of sine functions can be used as the correction transfer function. Further, a combination of the sin function group and the above-described cos function group may be used.
Further, although a function having only a real part is used as a function to be added to the function of the roll-off characteristic, a function having an imaginary part may be used. It is known that the Nyquist first criterion is satisfied even when the imaginary part is an even function with respect to the Nyquist frequency.
[0055]
In the above description, the number of subcarriers (L) is usually equal to the number of points (M) of IFT2. However, in order to avoid mutual interference with other communication systems, any number of subcarriers are intentionally used. May not use the number of subcarriers. Even in such a case, since the number of points (M) of IFT2 is not changed, the values of L and M do not always match.
[0056]
In the above description, the description has been made on the assumption that the OFDM communication system is used for the wireless LAN system. Therefore, the transmission complex symbol sequence is a complex signal mapped according to a digital modulation rule. However, the transmission complex symbol sequence may be a complex signal mapped according to the digital modulation rule and code-spread with a spreading code in the time axis direction. Alternatively, the complex amplitude level of the subcarrier that has been subjected to serial / parallel conversion may be assigned a spreading code in the frequency axis direction (subcarrier arrangement direction), code-spread, and output to the IFT. Thus, the transmission complex symbol before being input to the IFT may be any.
Further, the application of the communication system is not limited to the wireless LAN. The present invention may be applied to at least one of an uplink and a downlink transmission line in a communication system between mobile stations and base stations in a mobile radio communication network such as a mobile phone.
[0057]
In the above description, the filter of the present invention is applied to the transmission filter of the OFDM communication system. However, the filter can be used in other applications to satisfy the Nyquist first criterion and to reduce the out-of-band output signal power level. It is. For example, the present invention can be applied to a wired or optical communication baseband communication system that uses only binary or multivalued signals of the real part without using digital modulation.
Also, a technique for correcting a basic transfer function that plays a part of the characteristic satisfying the Nyquist first criterion by using one or a plurality of transfer functions that satisfy the Nyquist first criterion is used for purposes other than the reduction of the out-of-band radiation level. Can also be adopted for.
[0058]
【The invention's effect】
As is apparent from the above description, the transmitting apparatus of the present invention satisfies the Nyquist first criterion in a communication system that transmits an OFDM signal created by IFT, in which it was difficult to reduce the out-of-band radiation power level. Therefore, there is an effect that the radiation power level outside the required band is reduced without causing intersymbol interference.
Further, the filter device of the present invention is used as a transmission filter of the above-described communication system or the like, and satisfies the first Nyquist criterion, thereby reducing the radiation power level outside the required band without causing intersymbol interference. There is.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a first embodiment of the present invention. FIG. 1A is a block diagram, and FIG. 1B is an operation explanatory diagram.
FIG. 2 is a block diagram for explaining a second embodiment of the present invention.
FIG. 3 is an explanatory diagram of a roll-off filter having an optimized impulse response.
FIG. 4 shows F when roll-off rate α = 0.2 _p It is a graph which shows (w).
FIG. 5 is a graph showing the impulse response of the spectrum shaping filters 21I and 21Q when the out-of-band radiation level is minimized.
FIG. 6 is a graph showing an output power spectrum when a root roll-off filter having a roll-off rate of 0.2 is used on the transmission side and the reception side.
FIG. 7 is a diagram illustrating an output power spectrum when optimization is performed so that an out-of-band radiation level is minimized.
FIG. 8 is a graph showing the dependence of the output power spectrum on the number of carriers in the second embodiment.
FIG. 9 is a block diagram of a conventional OFDM communication system.
FIG. 10 is an explanatory diagram of a window function for explaining amplitude control on a transmitter side in a conventional OFDM communication system.
FIG. 11 is a graph showing an output signal power spectrum in a conventional OFDM communication system.
FIG. 12 is a block diagram of a conventional single carrier communication system and a graph showing roll-off characteristics.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Serial-parallel converter, 2 ... IFT (inverse Fourier transform) part, 3I, 3Q ... Root roll-off filter at the transmitter side, 4I, 4Q ... Multiplier, 5 ... Carrier signal oscillator, 6 ... Adder, 7 ... Transmission line, 8 ... Branching section, 9I, 9Q ... Multiplier, 10 ... Carrier signal oscillator, 11I, 11Q ... Route / roll-off filter on receiver side, 12 ... FT (Fourier transform) section, 13 ... Parallel-serial conversion , 21I, 21Q ... Spectrum shaping filter with optimized impulse response

Claims (13)

Inverse Fourier transform means for inputting a parallel transmission symbol sequence and outputting an OFDM signal;
A transmission filter that receives the OFDM signal output from the inverse Fourier transform unit and performs band limitation;
A transmission device having a transmission unit that outputs a transmission signal to be transmitted to a reception device by orthogonally modulating an output of the transmission filter,
The overall transmission characteristic including the reception filter on the receiving device side is a roll-off characteristic satisfying the Nyquist first criterion,
The transmission filter is a characteristic obtained by distributing a part of a roll-off characteristic satisfying the Nyquist first criterion, and has a characteristic of reducing an out-of-band radiation level of the OFDM signal output by the inverse Fourier transform unit. ,
A transmitting device characterized by the above-mentioned. The transmission apparatus according to claim 1, wherein the characteristic of the transmission filter is a characteristic obtained by equally dividing a roll-off characteristic satisfying the Nyquist first criterion between the transmission filter and the reception filter. The transmission device according to claim 1, wherein the roll-off characteristic satisfying the first Nyquist criterion is a raised cosine roll-off characteristic. The overall transmission characteristic in the transition region of the roll-off characteristic that satisfies the Nyquist first criterion is an odd function with respect to the basic transfer function that becomes the roll-off characteristic that satisfies the Nyquist first criterion and the center frequency of the transition region. Or a sum of a plurality of corrected transfer functions, represented by a transfer function corrected to have a characteristic of reducing the out-of-band output signal power level than the basic transfer function,
The characteristic of the receiving filter is represented by a transfer function in which a part of the basic transfer function is distributed,
The characteristic of the transmission filter is represented by a transfer function obtained by dividing the transfer function of the total transmission characteristic by the transfer function of the reception filter.
The transmitting device according to claim 1, wherein: In the transition region, the weighting factor of the basic transfer function and each weighting factor of the one or more correction transfer functions is a power density in a frequency region on the out-of-band side exceeding a predetermined boundary frequency. Is regarded as an integral value, and is optimized so that the out-of-band radiation level is minimized.
The transmission device according to claim 4, wherein: The predetermined boundary frequency is set to a frequency that is within the range of the transition region of the roll-off characteristic that satisfies the Nyquist first criterion and that is closer to the outside of the band than the center frequency of the transition region. ,
The transmitting device according to claim 5, wherein: In the transition region, the one or more correction transfer functions are odd-order cosine functions,
The transfer function of the overall transmission characteristic is continuous at each frequency at both ends of the transition region,
The transmitting device according to claim 4, wherein the transmitting device comprises: The transmission device according to any one of claims 4 to 7, wherein the basic transfer function has a raised cosine roll-off characteristic. A transversal filter that realizes characteristics distributed among other filters so that the total transmission characteristics are roll-off characteristics satisfying the Nyquist first criterion,
The overall transmission characteristic in the transition region of the roll-off characteristic that satisfies the Nyquist first criterion is expressed by an odd function with respect to a basic transfer function that is a roll-off characteristic that satisfies the Nyquist first criterion and a center frequency of the transition region. And a transfer function corrected to have a characteristic of reducing the out-of-band output signal power level from the basic transfer function, and
The characteristic of the other filter is represented by a transfer function in which a part of the basic transfer function is distributed,
Tap coefficients are set so that the characteristic of the transversal filter is represented by a transfer function obtained by dividing the transfer function of the total transmission characteristic by the transfer function of the other filter.
A transversal filter, characterized in that: In the transition region, the weighting factor of the basic transfer function and each weighting factor of the one or more correction transfer functions are such that the out-of-band output signal power level exceeds the predetermined boundary frequency in a frequency region outside the band. It is regarded as an integrated value of the power density and optimized so that the out-of-band output signal power level is minimized.
The transversal filter according to claim 9, wherein: The predetermined boundary frequency is set to a frequency that is within the range of the transition region of the roll-off characteristic that satisfies the Nyquist first criterion and that is closer to the outside of the band than the center frequency of the transition region. ,
The transversal filter according to claim 10, wherein: In the transition region, the one or more correction transfer functions are odd-order cosine functions,
The transfer function of the overall transmission characteristic is continuous at each frequency at both ends of the transition region,
The transversal filter according to any one of claims 9 to 11, wherein: The transversal filter according to any one of claims 9 to 12, wherein the basic transfer function has a raised cosine roll-off characteristic.

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