JP4171842B2 - Transmitter and transversal filter - Google Patents

Description

上述したスペクトル整形フィルタ出力ｖ₁(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f₁）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f₁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。
F₀(w)＋F₁(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F₀(w),F₁(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、F₀(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G₁(w),G_p(w)は、次式の通りである。なお、式(7)のG₁(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w₁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、w₂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f₁で定義すればf₁＝w₁/2π、最高角周波数w₂を周波数で定義すればf₂＝w₂/2π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α₁を得、さらに、このα₁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f₁＝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f₁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f₁＝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f₁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f₁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f₁＝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。
α₁＝1-(α₂+α₃+α₄+α₅)
α₂＝0.308400406
α₃＝-0.100279167
α₄＝0.025005514
α₅＝-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ｆ₁ ＝0.6 、図７（ｂ）はｆ₁ ＝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、参考実施形態である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、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】 本発明の参考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】 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]
BACKGROUND OF THE INVENTION
The present invention relates to a transmission device 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 causing intersymbol interference. .
[0002]
[Prior art]
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, an OFDM communication technique using a plurality of orthogonal subcarriers is used.
FIG. 9 is a block diagram of a conventional OFDM communication system.
In the figure, 1 is a series-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 being digitally modulated (BPSK, QPSK, 16QAM, etc.), converted to 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 designates 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 I channel (real part) and Q channel (imaginary part). The IFT 2 is usually realized by digital signal processing using IDFT (Inverse Discrete Fourier Transform) such as IFFT (Inverse Fast Fourier Transform).
[0003]
Reference numeral 41 denotes an amplitude control unit, 4I and 4Q denote multipliers, 5 denotes a carrier (carrier wave) signal oscillator, 6 denotes an adder, and 7 denotes a transmission path.
The amplitude control unit 41 smoothly attenuates the amplitude of each I-channel and Q-channel OFDM signal output from the IFT 2 at each symbol boundary. Specifically, this will be described later with reference to FIG.
The amplitude-controlled I channel time axis waveform is orthogonally modulated. That is, the multiplier 4I multiplies the in-phase output of the carrier (carrier wave) signal oscillator 5 and outputs the result to the adder 6. On the other hand, the amplitude-controlled Q channel time axis waveform 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 line 7 as an OFDM signal.
[0004]
When 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 sent to the multipliers 4I and 4Q. Supplied.
In addition, in an actual apparatus configuration, the addition signal is directly output as a high frequency band (RF) signal to the transmission path 7 or once becomes an intermediate frequency band (IF) signal. The frequency is converted into a high frequency band (RF) and output to the transmission line 7 (wireless transmission line).
Normally, a guard time is inserted at the beginning of one symbol period of the OFDM signal, but the illustration is omitted. The above is the configuration on the transmitter side.
[0005]
On the receiver side, the processing on the transmitter side is reversed. 8 is a branching unit, 9I and 9Q are multipliers, and 10 is 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 has the same phase output and π / 2 phase shift output from the carrier signal oscillator 10 that oscillates at the same frequency in phase synchronization with the carrier signal oscillator 5 on the transmitter side. By multiplying, quadrature demodulation is performed. 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 the complex amplitude of the signal that modulates each of the plurality of subcarriers constituting the OFDM signal and outputs it to the parallel-serial converter 13. The parallel-serial converter 13 outputs a serialized reception symbol (complex number) sequence. The received symbol (complex number) sequence is digitally demodulated based on a modulation scheme corresponding to digital modulation on the transmitter side to obtain received data.
The FT unit 12 is usually realized by digital signal processing using DFT (Discrete Fourier Transform) such as FFT (Fast Fourier Transform). When DFT is used, the outputs of the multipliers 9I and 9Q are converted into digital signals by an A / D converter (not shown) and then supplied to the FT unit 12.
[0007]
The frequency spectrum of the time axis waveform output from the IFT unit 2 approaches a rectangular ideal frequency spectrum if the number of subcarriers is very large. However, when the number of subcarriers is not so large, the frequency spectrum is broadened. When a wireless transmission path is used as the transmission path 7, the level of unnecessary radiation outside the band allocated to the communication system is limited by the standard. Therefore, subcarriers cannot be set up to the allocated bandwidth limit, and there is a problem that the frequency utilization efficiency is lowered.
It is conceivable to combine the modulated subcarriers after band-limiting them in advance. However, when IFT2 is used, this method cannot be applied because modulation is performed within IFT2.
[0008]
Therefore, conventionally, for example, p. 10 and 11 of Non-Patent Document 1 is known to insert the amplitude control unit 41 shown in FIG.
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 adjacent three symbol sections, respectively.
Each of the windows 51, 52, and 53 shown in the figure has a time width that slightly exceeds one symbol period of the OFDM signal, and is a time-domain window function that rises smoothly and falls smoothly. The waveform of the OFDM signal may change discontinuously at the boundary of each symbol interval 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 by IFT2 and the window function, the OFDM transmission signal is smoothly attenuated at the boundary of each symbol period, 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. When the signal amplitude at both ends of one OFDM symbol interval is spectrum-shaped with a raised cosine time domain window function (roll-off rate 0.025) and amplitude is limited at the boundary of one symbol interval, like this A case where no amplitude limitation is performed will be described. The horizontal axis is a frequency normalized so that the symbol rate (symbol / sec) of the original complex transmission symbol is 1 before being serial-parallel converted by the serial-parallel converter 1 shown in FIG.
The required bandwidth necessary for transmitting the OFDM signal is 0.5 of the normalized frequency described above. The integral value of the power spectrum in the frequency band exceeding 0.5 is the out-of-band radiation level.
As is apparent from FIG. 11, the out-of-band radiation level is reduced by amplitude control. But still not enough. In order to use an OFDM signal in a wireless LAN, there is a demand for reducing the power density to at least about -60 dB. Increasing the roll-off rate of the time domain window reduces the out-of-band radiation level, but intersymbol interference occurs, so that the orthogonality between subcarriers cannot be maintained, and the data rate must be lowered. As a result, the frequency utilization efficiency eventually decreases.
[0010]
In the non-patent document 1 mentioned above, a research paper that actually uses an analog filter such as a Butterworth filter or a Chebyshev filter is suggested as a method other than the above-described windowing, in which it is suggested to apply a filter in the frequency domain. However, if it is designed to greatly reduce out-of-band radiation, the modulation signal of the subcarrier near the band boundary becomes asymmetric due to the characteristics of the transition region of the filter and does not satisfy the Nyquist criterion, causing intersymbol interference and complex transmission. The symbol sequence is not completely restored on the receiving side.
[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/getieee802/download/802.11a-1999.pdf>
[0012]
[Problems to be solved by the invention]
The present invention has been made in order to solve the above-described problems, and transmits a OFDM signal that satisfies the Nyquist first standard and has an out-of-band radiation level reduced under the condition that no intersymbol interference occurs. An object of the present invention is to provide a transversal filter that can be used as a transmission filter of the transmission apparatus and that reduces the out-of-band output signal power level under the condition that no intersymbol interference occurs.
[0013]
[Means for Solving the Problems]
In the invention according to claim 1, the present invention provides: fundamentally, Inverse Fourier transform means for inputting a parallel transmission symbol string and outputting an OFDM signal, a transmission filter for inputting the OFDM signal output from the inverse Fourier transform means for band limiting, and orthogonally modulating the output of the transmission filter A transmission device having a transmission means for outputting a transmission signal to be transmitted to the reception device, the total transmission characteristic including the reception filter on the reception device side is a roll-off characteristic that satisfies the Nyquist first standard, The transmission filter has a characteristic in which a part of a roll-off characteristic satisfying the Nyquist first criterion is distributed and has a characteristic of reducing an out-of-band radiation level of the OFDM signal output by the inverse Fourier transform unit. Is.
Therefore, an OFDM signal with a reduced out-of-band radiation level can be transmitted under the condition that the Nyquist first standard is satisfied and no intersymbol interference is generated.
[0016]
Furthermore, in the invention according to claim 1, The total transmission characteristic in the transition region of the roll-off characteristic satisfying the Nyquist first criterion is an odd function 1 with respect to a basic transfer function serving as a roll-off characteristic satisfying the Nyquist first criterion and a center frequency of the transition region. Or a sum of a plurality of corrected transfer functions, represented by a transfer function that is corrected to have a characteristic that reduces an out-of-band output signal power level than the basic transfer function, and the characteristics of the reception filter are A part of the basic transfer function is expressed by a distributed transfer function, and the characteristic of the transmission filter is expressed by a transfer function obtained by dividing the transfer function of the total transmission characteristic by the transfer function of the reception filter.
Therefore, by using one or more correction transfer functions, the degree of freedom in determining the characteristics of the transition region is increased rather than simply changing the roll-off rate of the basic transfer function. The design for obtaining the characteristics to be reduced is facilitated.
For example, if one or a plurality of correction transfer functions is a periodic function that is an odd function and orthogonal to the center frequency of the transition region, the design is further facilitated.
[0017]
Claim 2 In the invention described in Claim 1 In the transmission device according to claim 1, in the transition region, the weighting coefficient of the basic transfer function and each weighting coefficient of the one or more correction transfer functions are set so that the out-of-band radiation level exceeds an outside band exceeding a predetermined boundary frequency. It is regarded as an integral value of the power density in the frequency region on the side, and is optimized so that the out-of-band radiation level is minimized.
Therefore, it is possible to easily obtain the characteristic of reducing the out-of-band radiation level of the OFDM signal.
[0018]
Claim 3 In the invention described in Claim 2 The predetermined boundary frequency is a frequency that is within a transition region of a roll-off characteristic that satisfies the Nyquist first criterion and that is outside the band than the center frequency of the transition region. Is set.
Therefore, it is possible to substantially reduce the roll-off rate of the basic transfer function and suppress the bandwidth spread.
[0019]
Claim 4 In the invention described in Claims 1 to 3 In the transmission device according to any one of the above, the one or more correction transfer functions in the transition region are odd-order cosine functions, and the transfer function of the total transmission characteristic is the transition It is continuous at each frequency at both ends of the region.
Therefore, it is possible to easily realize the characteristic of reducing the out-of-band radiation level of the OFDM signal.
[0020]
Claim 5 In the invention described in Claims 1 to 4 In the transmission device according to any one of the above, the basic transfer function has a raised cosine roll-off characteristic.
[0021]
Claim 6 The transversal filter that realizes a characteristic in which the total transmission characteristic is distributed to other filters so that the total transmission characteristic is a roll-off characteristic that satisfies the Nyquist first criterion. The total transmission characteristic in the transition region of the roll-off characteristic satisfying the Nyquist first criterion is expressed as an odd function with respect to the basic transfer function serving as the roll-off characteristic satisfying the Nyquist first criterion and the center frequency of the transition region. One or a plurality of corrected transfer functions, expressed by a transfer function corrected to have a characteristic of reducing an out-of-band output signal power level than the basic transfer function, The characteristic 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 total transmission characteristic. Serial as represented by a transfer function divided by the transfer function of the other filter, in which the tap coefficient is set.
Therefore, a signal with a reduced out-of-band output signal power level can be output under the condition that the Nyquist first standard is satisfied and no intersymbol interference is generated. By using one or more correction transfer functions, the degree of freedom in 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 characteristics of reducing the out-of-band output signal power level The design to obtain is easy.
For example, if one or a plurality of correction transfer functions is a periodic function that is an odd function and orthogonal to the center frequency of the transition region, the design is further facilitated.
[0022]
Claim 7 In the invention described in Claim 6 In the transversal filter according to claim 1, the weighting coefficient of the basic transfer function and each weighting coefficient of the one or more correction transfer functions in the transition region exceed the out-of-band output signal power level by a predetermined boundary frequency. It is regarded as an integral value of the power density in the frequency region on the out-of-band side, 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]
Claim 8 In the invention described in Claim 7 In the transversal filter described in (1), the predetermined boundary frequency is within a transition region of a roll-off characteristic that satisfies the Nyquist first criterion, and is outside the band from the center frequency of the transition region. The frequency is set.
Therefore, it is possible to substantially reduce the roll-off rate of the basic transfer function and suppress the bandwidth spread.
[0024]
Claim 9 In the invention described in Claims 6 to 8 In the transversal filter according to any one of the above, the one or more correction transfer functions in the transition region are odd-order cosine functions, and the transfer function of the total transmission characteristic is the It is continuous at each frequency at both ends of the transition region.
Therefore, it is possible to easily realize the characteristic of reducing the out-of-band output signal level.
[0025]
Claim 10 In the invention described in Claims 6 to 9 In the transversal filter according to any one of the above, the basic transfer function has a raised cosine roll-off characteristic.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates the present invention. Reference embodiment It is explanatory drawing of. FIG. 1A is a block diagram, and FIG. 1B is an operation explanatory diagram.
In FIG. 1 (a), the same parts as those in FIG. 3I and 3Q are root roll-off filters on the transmitter side, and 11I and 11Q are root roll-off filters on the receiver side.
On the transmitter side, the serial-parallel converter 1 converts one series of transmission symbol sequences into L (number of subcarriers) series of parallel transmission symbol sequences. The obtained L series of parallel transmission symbol sequences are converted into OFDM signals by IFT 2. The resulting I and Q channel signals are subjected to spectrum shaping (band-limited in the frequency domain) by the route roll-off filters 3I and 3Q, and then input to the multipliers 4I and 4Q to be orthogonally modulated. Get IF or RF signal.
[0027]
On the receiver side, the I and Q channel signals of the quadrature demodulated signals output from the multipliers 9I and 9Q are spectrum-shaped by the route roll-off filters 11I and 11Q, and then input to the FT unit 12.
In the figure, the encircled portion indicated by reference numeral 14 indicates a substantial transmission path from when the OFDM signal output from the transmitter IFT 2 is received via the transmission path 7 and input to the FT 12. In this transmission line, the total transmission characteristics (multiplication of the transfer function) combining the characteristics of the route / roll-off filters 3I and 3Q on the transmitter side and the route / roll-off filters 11I and 11Q on the receiver side are roll-off characteristics. And
At this time, the root-side roll-off filters 3I and 3Q on the transmitter side are characteristics in which a part of the roll-off characteristic satisfying the Nyquist first standard is distributed, and the out-of-band radiation level of the OFDM signal output by the IFT 2 It has the characteristic which reduces.
[0028]
A transfer function in which a transfer function that is an odd function with respect to the cutoff frequency of the ideal low-pass filter is added to the transfer function of the ideal low-pass filter satisfies the Nyquist first criterion.
Among the satisfied transfer functions, a transfer function that is continuous and that has a characteristic in which the slope of the transfer function with respect to 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 overall transmission characteristics satisfy the Nyquist first criterion, there is no intersymbol interference and the transmitted symbols are completely reproduced. Under this condition, the out-of-band radiation level can be reduced as compared with the prior art.
[0029]
It may seem surprising that the communication system described above can completely recover the transmitted symbol sequence. Therefore, it will be described in detail in comparison with a conventional communication system using a 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 route roll-off filter. FIG. 12B is a graph showing a transfer function of roll-off characteristics.
In FIG. 12A, the same parts as those in FIG. 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 encircled portion indicated by reference numeral 63 indicates a substantial transmission path until one series of complex transmission symbol sequences is received via the transmission path 7 and restored as a complex reception symbol sequence. In this substantial transmission line, the roll-off characteristics are realized in this substantial transmission line by combining the characteristics of both the root and roll-off filters on the transmitter side and the receiver side (multiplication of the transfer function). .
[0030]
FIG. 12B is a graph showing the transfer characteristic of the raised cosine roll-off characteristic. The horizontal axis is the normalized frequency, and the vertical axis is the output (linear scale). The characteristics on the negative side of the normalized frequency are omitted, but are symmetrical.
The frequency normalization is performed so that the symbol rate of the transmission symbol sequence (the reciprocal of the symbol interval) is 1. At this time, the frequency at which the output level drops to 1/2 (6 dB) is 0.5.
The roll-off characteristics in three cases where the roll-off rate is α = 1.0, 0.2, 0.1 are shown. When the roll-off factor is α, the normalized frequency (1-α) / 2 to (1 + α) / 2 has a shoulder characteristic region (transition region) that transitions in a raised cosine shape. Become.
Distribution of the transfer characteristics to the transmitter and receiver with roll-off characteristics is arbitrary as long as the single purpose of eliminating intersymbol interference is eliminated. Usually, the characteristics on the transmitter side and the receiver side are the same. A raised cosine root roll-off filter is used.
In the transmission line configuration having the route-off characteristic shown in FIG. 12A, if the received signal can be sampled at the correct timing (if the sampling point is correct), the transmission symbol sequence can be completely restored without intersymbol interference in the receiver. Known as the Nyquist first standard.
[0031]
On the other hand, the present invention shown in FIG. Reference embodiment 14 is a substantial transmission line surrounded by a dotted line 14 in the block diagram showing the conventional single carrier communication system shown in FIG. 12, and a substantial transmission line 63 is surrounded by a dotted line. Corresponds to the part.
Therefore, the converted data (OFDM signal) after IFT 2 on the transmitter side is regarded as a transmission symbol string. This transmission symbol sequence is subjected to band limitation that satisfies the Nyquist's first criteria by the route 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 IFT 2 matches the input signal (OFDM signal) to the FT 12 on the receiver side. Further, the FT 12 on the receiver side outputs the same parallel complex symbol as the parallel complex symbol before IFT 2 on the transmitter side. Therefore, the complex transmission symbol sequence can be completely restored on the receiving side.
[0032]
The entire converted data (OFDM signal) after IFT2 is regarded as one transmission symbol string, and the transmission symbol string is subjected to band limitation that satisfies the Nyquist first criterion. Since M sampling points are included in one symbol section of the OFDM signal, the apparent transmission rate is M times. M is called the number of IFT2 points.
Therefore, if the band is limited by a roll-off filter with M times the bandwidth and sampling is performed at the M times sampling points (M sampling points in one symbol period of the OFDM signal) on the receiver side, Based on the output, the original sequence of complex transmission symbols can be recovered. Here, sampling at the sampling point of M times is performed by the FT unit 12. The FT unit 12 extracts the complex amplitude of the modulation signal that modulates each orthogonal subcarrier by M times sampling.
Although depending on the OFDM signal configuration, the number of points of IFT2 is usually equal to the number of subcarriers (L) of the OFDM signal. Therefore, in the following, for simplicity of explanation, 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 when M = 4. One symbol of one carrier in the OFDM signal is shown, and the case where a sine wave 15 of one cycle is output in one OFDM symbol section is shown. If the interval length of the OFDM symbol is Ts, the sampling interval is Ts / M = Ts / 4. If a transmission / reception filter that satisfies Nyquist's first criterion is adopted with 1/2 of the reciprocal of the sampling interval as the Nyquist frequency (normalized frequency = 0.5), as shown in the figure, zero crossing occurs at the sampling interval Ts / M. It is clear that the complex transmission symbol sequence can be completely restored on the receiving side without intersymbol interference as in 17a to 17d if it is sampled at a correct sampling point because it has a random impulse response 16a to 16d.
Although the number of carriers is 1 in FIG. 1B, even when the number of carriers is plural, the modulation signal corresponding to each carrier is simply linearly added at the input of the transmission filter. 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]
M subcarriers included in the OFDM signal are arranged in a −6 dB passband having a roll-off characteristic (from −0.5 to +0.5 in normalized frequency). Among these, the modulation signal of the subcarrier closer to the end of the band is more susceptible to the band limitation, but the roll-off characteristic satisfies the Nyquist's first standard. Intersymbol interference does not occur even for a carrier modulation signal.
Note that the transmission symbol sequence can be completely restored when there is no noise, interference, propagation path distortion, or fixed deterioration of the radio equipment.For example, when there is fixed deterioration such as a sample timing error or an interference wave There is a case where the restoration of the transmission symbol sequence becomes incomplete.
[0035]
this Reference embodiment The transmission power spectrum is calculated by the following procedure.
First, a conversion data string converted in parallel is defined. Symbol a _mn Indicates the nth conversion data string of the mth subcarrier. f (t) is the impulse response of the root roll-off filter. w _m Is the angular frequency of the mth subcarrier, T _s Where is the symbol interval (symbol length of the OFDM signal symbol) of the parallel symbol sequence after serial-parallel conversion on the transmission side, the output of the spectrum shaping filter (root / roll-off filter) is
[Expression 1]

Spectral shaping filter output v described above ₁ The autocorrelation function R (t) of (t) is as follows:
[Expression 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 the 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 integration in equations (b) and (c) is a discrete numerical integration, and T _S / Four points were taken at a time interval of M (4 times oversampling). That is, the sampling interval is T _S / 4M, maximum frequency is 2M / T _S It was.
[0036]
FIG. 6 is a graph showing an output power spectrum when a route roll-off filter with a roll-off rate = 0.2 is used on the transmission side and the reception side. The number of filter taps was 101, 201, or 401.
Compared to 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 an out-of-band radiation level of −40 dB, as compared to the case of multiplying the conventional time domain window as shown in FIG.
[0037]
When the above-described root roll-off filter is realized by a digital filter, it must be made with a finite impulse response length. As a result, the impulse response is interrupted and the discontinuity causes discontinuity. Therefore, as shown in FIG. 6, the output power spectrum has a long tail and the out-of-band radiation level increases. Yes.
If the roll-off rate is reduced, the power spectral density rapidly decreases in the transition region, which is preferable because the frequency utilization efficiency can be increased. However, the lower the roll-off rate, the longer the tail 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 influence of the truncation error.
[0038]
Therefore, the present invention described below Embodiment In order to reduce the influence of the above-mentioned truncation, the impulse response is optimized for the route / roll-off filter on the transmitter side.
The total 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 reduction of the out-of-band radiation level under the condition that the impulse response is censored.
As an example of the specific conditions for optimization, (1) the total transmission characteristics of the transmitting side and the receiving side satisfy the Nyquist first criterion (no intersymbol interference), and (2) the receiver filter is ideal A root-off filter with a square cosine.
[0039]
FIG. 2 illustrates the present invention. Embodiment It is a block block diagram for demonstrating.
In the figure, parts similar to those in FIGS. 9 and 1 are denoted by the same reference numerals, and description thereof is omitted.
21I and 21Q are spectrum shaping filters that optimize the impulse response. The receiving side roll-off filters 11I and 11Q 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 an explanatory diagram of a first specific example of optimization, and FIG. 3B is an explanatory diagram of a second specific example of optimization.
In the figure, the horizontal axis represents the normalized frequency (only the positive side is shown), and the vertical axis represents the output level.
According to the Nyquist first standard, if the shoulder characteristic is an odd function with respect to the point of the output level 0.5 (-6 dB) of the cutoff frequency of the ideal low-pass filter (Nyquist frequency, normalized frequency = 0.5), the sign Satisfy the condition of no interference.
[0041]
In FIG. 3A, 31 is a raised cosine roll-off characteristic when the roll-off rate α = 0.2, and is a filter characteristic that satisfies the Nyquist first criterion. A rectangular characteristic (ideal low-pass filter), a triangular characteristic, and a trapezoid characteristic also satisfy the Nyquist first criterion, but are not used because the impulse response length increases because there is a discontinuous point in the slope.
In this embodiment, the square cosine roll-off characteristic 31 is used as a basic transfer function, and one or more correction transfer functions that are odd functions are added to the center frequency of the transition region to add out-of-band. The radiation level is reduced. Since the Nyquist first 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 this, but the frequency f at the boundary between the required band and the out-of-band ₁ Can be arbitrarily set for optimization calculation.
As shown in FIG. 3A, since the radiation level outside the outer boundary band of the transition region is a slight value, the difference between the corrected transfer function 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 by a log-scale power spectrum described later with reference to FIG.
The spectrum shaping filters 21I and 21Q on the transmission side have a characteristic in which a part of the total transmission characteristic is distributed. 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 correction transfer function, the one or more correction transfer functions in the transition region are odd-order cosine functions, and the transfer function of the total transmission characteristic is continuous at each frequency at both ends of the transition region. is 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 takes a positive or negative value. p is the order, α _p Is the coefficient in order p, α _p F _p (w) is a frequency characteristic in each order p (from 1st order to Pth order).
F ₀ (w) + F ₁ In the case of (w) only, the conventional raised cosine roll-off characteristic is obtained.
[Expression 4]

Here, the coefficient α in each order p (1st to Pth) _p The sum of is 1. That is,
[Equation 5]

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

here,
[Expression 7]

[Equation 8]

Equations (4) and (5) 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 interval length of the OFDM signal) of the parallel symbol string after serial / parallel conversion on the transmission side.
When P = 1 in the equation (1) (that is, the sum when p = 0, 1), a conventional raised cosine roll-off filter 31 shown in FIG. 3 is obtained.
F _p (w) (p ≧ 2) is F to optimize the impulse response (reduce out-of-band radiation level). _p (w) This is an extended function of (p = 1).
[0044]
That is, the conventional square 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 a shoulder characteristic, cosine function groups of odd order (periods of an odd number of the basic period), that is, cos (x), cos (3x), The combination of cos (5x), ..., cos [(2P-1) x] is used.
FIG. 4 shows 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 region where the normalized frequency is negative is not shown, it is symmetrical.
As is clear from FIG. 4, all the cosine function groups described above are odd functions with respect to the center of the transition region (normalized frequency 0.5). Therefore, the condition that there is no intersymbol interference is satisfied, and what is called an extended square cosine roll-off filter 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 this out-of-band radiation level is minimized. _p When the is set, the output power spectrum 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 (square cosine roll-off characteristics). Therefore, if the total transmission characteristic of the above equation (1) is divided by the reception side characteristic (ideal route roll-off characteristic), the transmission side filter characteristic, that is, the characteristics of the spectrum shaping filters 21I and 21Q in FIG. Is obtained.
[0046]
If the transmission side filter characteristic is G (w), the following equation is obtained.
[Equation 9]

Where 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.
[Expression 10]

[0047]
When G (w) in the above equation (6) is the characteristic of the transmission filter, the output power spectrum P (w) is as follows. The derivation process of the mathematical formula is omitted.
## EQU11 ##

Here, c (w) and Hp (w) are as follows.
[Expression 12]

In equations (10) and (11), w _m Is the angular frequency of the mth subcarrier, T _s Is the symbol interval (symbol length of the OFDM signal) after serial-parallel conversion on the transmitting side, the constant K is the impulse response length of the transmission filter, and the number of taps of the digital filter (transversal filter) with a finite correlation length It corresponds to.
[0048]
Since the out-of-band radiation level Pt is obtained by integrating the output power spectrum of Equation (9) at the out-of-band frequency, it is given by the following equation.
[Formula 13]

Where w ₁ , As already explained, the angular frequency defined as the boundary between in-band and out-of-band during optimization, w ₂ Is the highest angular frequency determined from the sampling theorem. This boundary is the frequency f ₁ F ₁ = W ₁ / 2π, maximum angular frequency w ₂ Is defined by frequency, f ₂ = W ₂ Is given by / 2π. If the digital filter is assumed, if the maximum frequency is 1/2, 4 times oversampling of the sampling frequency, the sampling frequency is 4 as the normalized frequency, so the maximum frequency is 2.
The above equation (12) _P It is a quadratic function (p ≧ 2) (α _p And α _q And p = 2,... P, q = 2,... P), and the second order coefficients are all positive, so that 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 giving an extreme value _P Find the value of (p ≧ 2) and substitute these into equation (2) ₁ In addition, this α ₁ At the same time, an impulse response that minimizes the out-of-band radiation level can be obtained by substituting into equation (6) and performing Fourier transform.
[0049]
As shown in FIG. 3B, the boundary 34 has a frequency f. ₁ As the roll-off characteristic 35 when 0.55, 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 as to minimize the radiation level in the outer band, the frequency f of the boundary 34 ₁ The amplitude of the transmission filter transmission characteristic outside from 0.55 is significantly reduced. This is apparent when comparing the optimized output power spectrum shown in FIG. 7B described later and the output power spectrum without amplitude control in FIG. The resulting frequency f ₁ The amplitude of the overall transmission characteristic from the outside is significantly reduced. The overall transmission characteristic maintains an odd function before and after the output 0.5 (−6 dB) point of the Nyquist frequency (normalized frequency 0.5), and thus is approximately 1.0 (0 dB) at the frequency f = 0.45. As a result, the same steep transition characteristics as when the roll-off rate α = 0.1 (transition region 0.45 ≦ f ≦ 0.55) are obtained. Moreover, the frequency f of this boundary 34 ₁ Therefore, the out-of-band radiation power level is smaller than that when a simple roll-off rate α = 0.1 is realized by a digital filter.
[0050]
Number of carriers M = 64, 4 times oversampling, number of digital filter taps = 201, roll-off rate α = 0.2, boundary frequency f ₁ = 0.55, P = 5 (the order of the cosine function is up to 9th order) α _p (1 ≦ p ≦ 5) is as follows.
α ₁ = 1- (α ₂ + α _Three + α _Four + α _Five )
α ₂ = 0.308400406
α _Three = -0.100279167
α _Four = 0.025005514
α _Five = -0.00326821691
[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 represents the sample timing, and the vertical axis represents the output value (linear scale). The spectrum shaping filters 21I and 21Q are, for example, transversal filters (FIR filters: finite-length impulse response filters) that output the tap coefficients of the serialized delay elements multiplied by tap coefficients and add the multiplication results, respectively. ). In this case, the impulse response value described above gives the tap coefficient when the horizontal axis is the tap number. As described above, a case where oversampling is performed four times 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. However, if the oversampling rate is high, it can be ignored. When it cannot be ignored, for example, the characteristics may be compensated in the spectrum shaping filters 21I and 21Q.
[0052]
FIG. 7 is a diagram showing an output power spectrum when the out-of-band radiation level is optimized to be minimized. This is a characteristic under the same setting conditions as in FIG.
FIG. 7A shows the frequency f defined as the boundary between the required band and the out-of-band. ₁ = 0.6, FIG. ₁ = 0.55 is shown.
The quasi-rectangular output power spectrum without side lobes was obtained by optimization. In the range where the power density is -100 dB or less, the tail is drawn, but it is outside the range shown in the figure. As a comparative example, Reference embodiment The case of a raised cosine roll-off filter is shown.
In Fig. 7 (b), the signal band with an out-of-band radiation level of -40dB is 1.06 times the Nyquist frequency, and the signal band with an out-of-band radiation level of -60dB is only 1.08 times the Nyquist frequency. The utilization efficiency is remarkably improved. A much lower out-of-band radiation level is achieved than when multiplied by a conventional time domain window as illustrated in FIG.
[0053]
FIG. Embodiment It is a graph which shows the carrier number dependence of the output power spectrum in. The number of carriers M was three, 16, 32 and 64. It can be seen that the out-of-band radiation level decreases as the number of carriers increases. The number of carriers is significantly reduced at 64. When the number of carriers is too small, it is necessary to increase the number of filter taps.
[0054]
In the above description, an odd-order cos function group (cos (3x), cos (5x)...) Of the third or higher order is added as a function (correction transfer function) to be added to the function of the raised cosine roll-off characteristic (basic transfer function). . In principle, if the function group is an odd function with respect to the Nyquist frequency, it becomes a filter that satisfies the first Nyquist condition, and the coefficient 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 sine function group can be used as the correction transfer function. Also, a combination of the sin function group and the above-described cos function group may be used.
Further, although a function including only a real part is used as a function to be added to the function of the roll-off characteristic, the 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, but in order to avoid mutual interference with other communication systems, etc. The number of subcarriers may not be used. Even in such a case, since the number of points (M) of IFT2 is not changed, the values of L and M do not necessarily match.
[0056]
In the above description, the OFDM communication system used in the wireless LAN system has been described. As a transmission complex symbol sequence, a complex signal mapped according to a digital modulation rule is used. However, the transmission complex symbol sequence may be a complex signal mapped in accordance with a digital modulation rule and code-spread with a spreading code in the time axis direction. Alternatively, the complex amplitude level of the subcarriers subjected to serial / parallel conversion may be code-spread with a spreading code assigned in the frequency axis direction (subcarrier arrangement direction) and output to the IFT. In this way, any transmission complex symbol before being input to the IFT may be used.
The use of the communication system is not limited to wireless LAN. The present invention may be applied to at least one of the uplink and downlink transmission paths in the communication system between mobile 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, it can be used for other applications to satisfy the Nyquist first standard and 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 a binary or multilevel real part signal without using digital modulation.
In addition to reducing the out-of-band radiation level, a method for correcting a basic transfer function that plays a part in characteristics satisfying the Nyquist first criterion by using one or a plurality of transfer function groups satisfying the Nyquist first criterion. Can also be adopted for.
[0058]
【The invention's effect】
As is apparent from the above description, the transmission apparatus of the present invention satisfies the Nyquist first standard in a communication system for transmitting an OFDM signal created by IFT, which is difficult to reduce the out-of-band radiation power level. Thus, 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 in the above-described communication system or the like, and the radiation power level outside the required band is reduced without satisfying the Nyquist first standard and causing intersymbol interference. There is.
[Brief description of the drawings]
FIG. 1 of the present invention Reference embodiment It is explanatory drawing of. FIG. 1A is a block diagram, and FIG. 1B is an operation explanatory diagram.
FIG. 2 of the present invention Embodiment It is a block block diagram for demonstrating.
FIG. 3 is an explanatory diagram of a roll-off filter with an optimized impulse response.
Fig. 4 F when roll-off rate α = 0.2 _p It is a graph which shows (w).
FIG. 5 is a graph showing 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 route 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 showing an output power spectrum when the out-of-band radiation level is optimized to be minimum.
[Fig. 8] Embodiment It is a graph which shows the carrier number dependence of the output power spectrum in.
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 the 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 ... Series-parallel converter, 2 ... IFT (inverse Fourier transform) part, 3I, 3Q ... Root roll-off filter on the transmitter side, 4I, 4Q ... Multiplier, 5 ... Carrier signal oscillator, 6 ... Adder, 7 ... Transmission path, 8 ... Branch unit, 9I, 9Q ... Multiplier, 10 ... Carrier signal oscillator, 11I, 11Q ... Route roll-off filter on receiver side, 12 ... FT (Fourier transform) part, 13 ... Parallel serial conversion , 21I, 21Q ... Spectral shaping filter with optimized impulse response

Claims (10)

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 means and limits the bandwidth;
A transmission device having a transmission means for outputting a transmission signal to be transmitted to a reception device by orthogonally modulating the output of the transmission filter,
The overall transmission characteristic including the reception filter on the receiver side is a roll-off characteristic that satisfies the Nyquist first standard,
The transmission filter is a characteristic distributed part of the roll-off characteristic that satisfies the Nyquist first criterion, and have the characteristic of reducing out-of-band radiation level of the OFDM signal, wherein the inverse Fourier transform means outputs And
The total transmission characteristic in the transition region of the roll-off characteristic satisfying the Nyquist first criterion is an odd function 1 with respect to a basic transfer function serving as a roll-off characteristic satisfying the Nyquist first criterion and a 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-band output signal power level than the basic transfer function,
The characteristic of the reception 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.
A transmission apparatus characterized by the above . 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 the power density in the frequency region outside the band exceeding the predetermined boundary frequency. And is optimized to minimize the out-of-band radiation level.
The transmission apparatus according to claim 1 , wherein: The predetermined boundary frequency is set to a frequency that is within a transition region of a roll-off characteristic that satisfies the Nyquist first criterion and that is outside the band of the center frequency of the transition region. ,
The transmission apparatus according to claim 2 , wherein: The one or more correction transfer functions in the transition region are odd-order cosine functions,
The transfer function of the total transmission characteristic is continuous at each frequency at both ends of the transition region,
The transmission apparatus according to any one of claims 1 to 3, wherein The transmission device according to claim 1 , wherein the basic transfer function has a raised cosine roll-off characteristic. A transversal filter that realizes a characteristic in which the total transmission characteristic is distributed to other filters so that the total transmission characteristic is a roll-off characteristic that satisfies the Nyquist first criterion,
The total transmission characteristic in the transition region of the roll-off characteristic satisfying the Nyquist first criterion is expressed as an odd function with respect to a basic transfer function serving as a roll-off characteristic satisfying the Nyquist first criterion and a center frequency of the transition region. One or a plurality of corrected transfer functions, and expressed 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 other filter is represented by a transfer function in which a part of the basic transfer function is distributed,
The tap coefficient is 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 by 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 used to calculate the out-of-band output signal power level in the frequency region outside the band exceeding a predetermined boundary frequency. It is regarded as an integral value of power density, and is optimized to minimize the out-of-band output signal power level.
The transversal filter according to claim 6 . The predetermined boundary frequency is set to a frequency that is within a transition region of a roll-off characteristic that satisfies the Nyquist first criterion and that is outside the band of the center frequency of the transition region. ,
The transversal filter according to claim 7 . The one or more correction transfer functions in the transition region are odd-order cosine functions,
The transfer function of the total transmission characteristic is continuous at each frequency at both ends of the transition region,
The transversal filter according to any one of claims 6 to 8 , wherein the transversal filter is provided. The transversal filter according to any one of claims 6 to 9 , wherein the basic transfer function has a raised cosine roll-off characteristic.

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