DE3322954A1 - 7-level phase shift keying/amplitude shift keying (PSK/ASK) system - Google Patents

7-level phase shift keying/amplitude shift keying (PSK/ASK) system

Info

Publication number
DE3322954A1
DE3322954A1 DE19833322954 DE3322954A DE3322954A1 DE 3322954 A1 DE3322954 A1 DE 3322954A1 DE 19833322954 DE19833322954 DE 19833322954 DE 3322954 A DE3322954 A DE 3322954A DE 3322954 A1 DE3322954 A1 DE 3322954A1
Authority
DE
Germany
Prior art keywords
ask
psk
amplitude
shift keying
coding
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
DE19833322954
Other languages
German (de)
Inventor
Hermann Dipl.-Ing. 6802 Ladenburg Weidenfeller
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Robert Bosch GmbH
Original Assignee
ANT Nachrichtentechnik GmbH
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from DE19823243373 external-priority patent/DE3243373A1/en
Application filed by ANT Nachrichtentechnik GmbH filed Critical ANT Nachrichtentechnik GmbH
Priority to DE19833322954 priority Critical patent/DE3322954A1/en
Publication of DE3322954A1 publication Critical patent/DE3322954A1/en
Granted legal-status Critical Current

Links

Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/36—Modulator circuits; Transmitter circuits
    • H04L27/362—Modulation using more than one carrier, e.g. with quadrature carriers, separately amplitude modulated
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/345—Modifications of the signal space to allow the transmission of additional information
    • H04L27/3455—Modifications of the signal space to allow the transmission of additional information in order to facilitate carrier recovery at the receiver end, e.g. by transmitting a pilot or by using additional signal points to allow the detection of rotations
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/38—Demodulator circuits; Receiver circuits
    • H04L27/3818—Demodulator circuits; Receiver circuits using coherent demodulation, i.e. using one or more nominally phase synchronous carriers
    • H04L27/3827—Demodulator circuits; Receiver circuits using coherent demodulation, i.e. using one or more nominally phase synchronous carriers in which the carrier is recovered using only the demodulated baseband signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/0014—Carrier regulation
    • H04L2027/0018—Arrangements at the transmitter end
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/0014—Carrier regulation
    • H04L2027/0024—Carrier regulation at the receiver end
    • H04L2027/0026—Correction of carrier offset
    • H04L2027/0028—Correction of carrier offset at passband only
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/0014—Carrier regulation
    • H04L2027/0024—Carrier regulation at the receiver end
    • H04L2027/0026—Correction of carrier offset
    • H04L2027/0036—Correction of carrier offset using a recovered symbol clock
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/0014—Carrier regulation
    • H04L2027/0044—Control loops for carrier regulation
    • H04L2027/0053—Closed loops
    • H04L2027/0057—Closed loops quadrature phase

Landscapes

  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Digital Transmission Methods That Use Modulated Carrier Waves (AREA)

Abstract

In a 7-level PSK/ASK (phase shift keying/amplitude shift keying) system according to the main patent P 3243373, 3 parallel binary baseband signals with 3 sine-wave carriers of the carrier frequency representing an integral multiple of the bit clock or line clock, which sine-wave carriers are in each case phaseshifted through the angles @1 to @3, are amplitude-modulated at the transmitting end in 3 ring-type modulators and are combined in a hybrid coupler with 3 inputs following transmission-end coding and receiving-end decoding of the one ambiguous signal condition to form a 7-level PSK/ASK transmission signal. For transmission-end coding of the one ambiguous signal condition, 6 digital information steps are in each case combined with one code step to form a block. For receiving-end decoding, each of the 6 information steps of a block is modulo-2 added to the assigned code step. <IMAGE>

Description

Deutsche Bundespost 251?German Federal Post 251?

Die Erfindung betrifft ein 7-stufiges Phasenumtastungs/ Ainplitudenumtastungs-System (PSK/ASK-System), nach Patent P 32 43 373, bei dem sendeseitig 3 parallele binäre Basisbandsignale mit 3 um jeweils die Winkel <ΡΛ bis f5 phasenverschober.enSinusträgern der ein ganzzahliges Vielfaches dec rdttaktes oder Schrittaktes darstellenden Trägerfrequenz in 3 Ringmodulatoren amplitudenmoduliert werden und in einem Hybridkoppler mit 3 Eingängen nach sendeseitiger Codierung und empfangsseitiger Decodierung des einen nicht eindeutigen Signalzustandes zu einem 7-stufigen PSK/ASK-Sendesignal verknüpft werden. Wie in der Hauptanmeldung beschrieben, treten allgemein bei nach dem Hauptpatent konzipierten 1 = 2n-k-stufigen PSK/ASK-Systemen k Mehrdeutigkeiten der Zuordnung der Signalzustände auf (n = Anzahl derThe invention relates to a 7-stage phase shift keying / Ainplitude shift keying system (PSK / ASK system), according to patent P 32 43 373, in which 3 parallel binary baseband signals with 3 phase-shifting sinus carriers of an integer by angles <ΡΛ to f5 on the transmission side Multiple dec rdttaktes or step clocks representing carrier frequency are amplitude-modulated in 3 ring modulators and linked in a hybrid coupler with 3 inputs after coding on the transmission side and decoding of the one ambiguous signal state on the receiving side to form a 7-stage PSK / ASK transmission signal. As described in the main application, in 1 = 2 n -k-stage PSK / ASK systems designed according to the main patent, k ambiguities in the assignment of the signal states occur (n = number of

3 bit/Übertragungsschritt, k = ganzzahlig). Bei 1 = 2-1 = 7-stufigen PSK/ASK-Systemen ist demzufolge ein Signal-3 bit / transmission step, k = integer). If 1 = 2-1 = 7-stage PSK / ASK systems is therefore a signal

i zustand mehrdeutig. 'i state ambiguous. '

Zur Vermeidung dieser Mehrdeutigkeit wird im Hauptpatent eine einfache sendeseitige Codierung und empfangsseitige Decodierung dieses Signalzustandes vorgeschlagen.To avoid this ambiguity, the main patent uses a simple coding on the transmit side and coding on the receive side Decoding of this signal state is proposed.

Bei diesem Codierverfahren erhöht sich die Übertragungs-Schrittgeschwindigkeit Vsü und damit die BandbreiteWith this coding method, the transmission step speed increases Vsü and thus the bandwidth

durch die eingefügte Code-Redundanz auf ;jby the inserted code redundancy; j

Vsü = I Vs jVsü = I Vs j

Vs = Schrittgeschwindigkeit vor derVs = walking speed in front of the

Codierung ;Coding;

Die Zusatzerfindung bringt nun eine besonders einfache -The additional invention now brings a particularly simple -

Lösung dieser zusätzlichen Codierung bzw. Decodierung, ·Solution of this additional coding or decoding,

wobei sich hier die Übertragungsschrittgeschwindigkeit,where the transmission step speed is here,

infolge der eingefügten Redundanz, nur auf I due to the inserted redundancy, only on I.

Vsü = ^ Vs :Vsü = ^ Vs:

Deutsche Bundespost 251?German Federal Post 251?

erhöht,elevated,

d.h. auch die eingefügte Redundanz und die Bandbreite sind "bei diesem Verfahren geringer.that is, the redundancy and bandwidth introduced are "less" with this method.

Gemäß der Erfindung werden zur sendeseitigen Codierung des einen nicht eindeutigen Signalzustandes bei einem 7 PSK/ASK-System jeweils 6 digitale Inforaationsschritte mit einem Codeschritt zu einem Block vereinigt. Zur empfangsseitigen Decodierung wird jeder der 6 Informationsschritte eines Blocks mit dem zugeordneten Codeschritt modulo-2 addiert.According to the invention, the coding of the one ambiguous signal state at the transmission end In a 7 PSK / ASK system, 6 digital information steps are combined with one code step to form a block. Each of the 6 information steps of a block with the assigned code step modulo-2 added.

Im folgenden wird das Block-Codierverfahren nach der Erfindung im einzelnen erläutert. Die Figur zeigt den Phasenstern des 7-PSK/ASK-Systems und das zugehörige Aufbauträgersystem, sowie die Zuordnung der 8 Signalzustände S1 - Sg zu den einzelnen Kombinationsmöglichkeiter c^r Aufbauträger g.. bis g·?. Im gewählten Beispie] entspri -h t, ein jeweils positives Vorzeichen des Aufbauträgers einer logischen Eins und ein jeweils negatives Vorzeichen eines logischen Null. Eine nach diesen logischen Zuständen aufgeschlüsselte Tabelle ist in Tabelle 1 dargestellt. The block coding method according to the invention is explained in detail below. The figure shows the phase star of the 7-PSK / ASK system and the associated mounting support system, as well as the assignment of the 8 signal states S 1 - Sg to the individual possible combinations c ^ r mounting supports g .. to g · ?. In the example chosen , a positive sign of the mounting support corresponds to a logical one and a negative sign to a logical zero. A table broken down according to these logical states is shown in Table 1.

Mit einem 7-PSK/ASK-Modem müssen 8 mögliche Basisband-Übertragungsschritte, die in stochastischer Folge auftreten, übertragen werden (1 Übertragungsschritt = 3 Bit). Die in der folgenden Tabelle 1 eingerahmten Zustände sind jedoch bei dem 7-PSK/ASK-System der Figur (OLO = -+-, LOL = +-+) nicht eindeutig einzuordnen.With a 7-PSK / ASK modem, 8 possible baseband transmission steps must be which occur in a stochastic sequence are transmitted (1 transmission step = 3 bits). The states framed in the following table 1 are, however, in the 7-PSK / ASK system of the figure (OLO = - + -, LOL = + - +) cannot be clearly classified.

000
0OL000
0OL

OLLOLL

LOOLOO

LL0
LLL LL0
LLL

Deutsche Bundespost 2512German Federal Post 2512

Diese Zweideutigkeit wird nach der Erfindung durch Blockcodierung beseitigt werden.According to the invention, this ambiguity is eliminated by block coding.

Die zu übertragende stochastische Schrittfolge wird "bei der Blockcodierung in Blöcke aus 6 Schritten gruppiert, Dabei besteht die Forderung, daß nach der Codierung in keinem dieser Blöcke ein zweideutiger Zustand auftreten darf.The stochastic sequence of steps to be transmitted is "grouped into blocks of 6 steps in block coding, The requirement is that no ambiguous state occurs in any of these blocks after the coding allowed.

Zugelassen sind nachdem in der Figur dargestellten 7 PSK/ ASK-System die 7 eindeutigen Sendeschritte nach Tabelle According to the 7 PSK / ASK system shown in the figure, the 7 unambiguous transmission steps according to the table are permitted

000 0OL OLO '000 0OL OLO '

OLL LOO LLO LLL 20OLL LOO LLO LLL 20

Um die Eindeutigkeit zu erreichen, wird für jeden Block ein Codeschritt ermittelt (nicht LOL beim System nach der Figur), zu jedem Schritt des Blocks modulo-2 addiert und als 7. Schritt übertragen. 25In order to achieve uniqueness, a code step is determined for each block (not LOL in the system according to of the figure), added modulo-2 to each step of the block and transferred as the 7th step. 25th

Tabelle 3 zeigt 2 Beispiele:Table 3 shows 2 examples: codierte Sendefolgecoded transmission sequence UrsprungsfolgeOrigin sequence OLLOLL 0OL0OL 000000 OLOOLO 0OL0OL OLL mod. 2OLL mod. 2 LLOLLO LOO ©LOO © LLLLLL LOLLOL LOOLOO LLOLLO

OLO - Codeschritt -OLO - code step -

"iesp"iesp ■ort Γ51?■ location Γ5 1 ? 000000 33229543322954 LLLLLL LICLIC 0OL0OL LOOLOO OLLOLL EGd . ?Ground floor ? OLLOLL LOOLOO ©© OLOOLO LOLLOL 001001 LLOLLO

LLL - Codeschritt u.s.w.
10LLL - code step etc.
10

Aufgrund der Gruppierung in Blöcke mit nur 6 Schritten ist es immer möglich, einen geeigneten Codenchritt (nicht LOL beim System nach der Figur) zu ermitteln.Due to the grouping in blocks with only 6 steps it is always possible to find a suitable code step (not LOL with the system according to the figure).

Tritt in einem Block der verbotene Zustand nicht auf (LOL beim System nach der Figur) wird als Codeschritt 000 gesendet. If the forbidden state does not occur in a block (LOL in the system according to the figure), code step 000 is sent.

Bei diesem Codiert:· fahren werden anstelle von 6 Informationsschritten 6 ccuierte Sendeschritte und ein Codeschritt übertragen. Damit erhöht sich die Übertragungs-Schrittgeschwindigkeit aufWith this coding: · there are information steps instead of 6 6 encoded transmission steps and one code step transmitted. This increases the transmission step speed on

Vsü = ^ Vs
(Vs = Schrittgeschwindigkeit)Vsü = ^ Vs
(Vs = walking speed)

Die eingefügte Redundanz beträgtThe inserted redundancy is

ΣΡ1 ld I1 - έ PI I1 7 3 _ 6 β3 r = ^ i=J .ίοΟ = 2 -—S .100 * M,3ΣΡ1 ld I 1 - έ PI I 1 7 3 _ 6 β3 r = ^ i = J .ίοΟ = 2 -—S .100 * M, 3

Σ Pi.ld I1 I · 3 Σ Pi.ld I 1 I 3

i1 pi i1 pi

Im Decodierer erzielt man die Ursprungsschrittfolge wieder, indem man jeden Schritt eines Blocks mit seinem zugehörigen 7. Schritt (Codeschritt) modulo-2 addiert, Tabelle The original sequence of steps is restored in the decoder by adding each step of a block with its associated 7th step (code step) modulo-2, table

Deutsch'? Bundespost 251? "*German'? Federal Post 251? "*

codierte Enipfangsfolge decodierte Ursprungsfolgeencoded reception sequence decoded original sequence

0OL0OL mod. 2 mod. 2 000000 OLLOLL ©© OLOOLO LOOLOO LOLLOL LOOLOO LOLLOL LLOLLO LLI.LLI. uOuO

Der Codeschritt wird nach der Decodierung jeweils unterdrückt. The code step is suppressed after the decoding.

Claims (1)

Deutsche Bundespost""^1? ·"-..'-Deutsche Bundespost "" ^ 1 ? · "-..'- 7-stufiges Pha senumta stungs/Amplitud enucita stungs (PSK/ ASK)-System7-stage phase / amplitude enucita tion (PSK / ASK) system (Zusatz zu P 32 43 373)(Addition to P 32 43 373) (1) Patentanspruch(1) Claim 7-stufiges Phasenumtastungs/Amplitudenuctastungs (PSK/ ASK)-System nach Patent P 32 43 373, "bei dem sendeseitig 3 parallele binäre Basisbandsignale mit 3 um jeweils die Winkel <M Ms ψ 3 phasenverschobenen Sinusträgern der ein ganzzahliges Vielfaches des Bittaktes oder Schrittaktes darstellenden Trägerfrequenz in 3 Ringmodulatoren amplitudenmoduliert und in einem Hybridkoppler mit 3 Eingängen nach sendeseitiger Codierung und empfangsseitiger Decodierung des einen nicht eindeutigen Signalzustands zu einem 7-stufigen PSK/ASK-Sendesignal verknüpft werden, dadurch gekennzeichnet , daß zur sendeseitigen Codierung des" einen nicht eindeutigen ^i^nalzustandes jeweils 6 digitale Informationsschritte r.' t einem Codeschritt zu einem Block vereinigt werden -.·■::& zur empfangsseitigen Decodierung jeder der 6 Informationsschritte eines Blocks mit den zugeordneten Codeschritt modulo-2 addiert wird.7-stage phase shift keying / amplitude keying (PSK / ASK) system according to patent P 32 43 373, "with 3 parallel binary baseband signals with 3 sinusoidal carriers phase-shifted by angles <M Ms ψ 3 which represent an integral multiple of the bit cycle or step cycle Carrier frequency is amplitude-modulated in 3 ring modulators and amplitude-modulated in a hybrid coupler with 3 inputs after coding on the transmission side and decoding of the one ambiguous signal state on the receiver side to form a 7-stage PSK / ASK transmission signal, characterized in that for the coding of the "one ambiguous ^ i ^ nalstatuses each 6 digital information steps r . ' t can be combined into a block with a code step -. · ■ :: & each of the 6 information steps of a block with the assigned code step is added modulo-2 for the decoding at the receiving end.
DE19833322954 1982-11-24 1983-06-25 7-level phase shift keying/amplitude shift keying (PSK/ASK) system Granted DE3322954A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
DE19833322954 DE3322954A1 (en) 1982-11-24 1983-06-25 7-level phase shift keying/amplitude shift keying (PSK/ASK) system

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE19823243373 DE3243373A1 (en) 1982-11-24 1982-11-24 Phase shift keying/amplitude shift keying (PSK/ASK) system with any number of stages
DE19833322954 DE3322954A1 (en) 1982-11-24 1983-06-25 7-level phase shift keying/amplitude shift keying (PSK/ASK) system

Publications (1)

Publication Number Publication Date
DE3322954A1 true DE3322954A1 (en) 1985-01-03

Family

ID=25806030

Family Applications (1)

Application Number Title Priority Date Filing Date
DE19833322954 Granted DE3322954A1 (en) 1982-11-24 1983-06-25 7-level phase shift keying/amplitude shift keying (PSK/ASK) system

Country Status (1)

Country Link
DE (1) DE3322954A1 (en)

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2744109A1 (en) * 1976-10-08 1978-04-13 Data General Corp ARRANGEMENT FOR DATA BIT COMPRESSION

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2744109A1 (en) * 1976-10-08 1978-04-13 Data General Corp ARRANGEMENT FOR DATA BIT COMPRESSION

Similar Documents

Publication Publication Date Title
DE2364874B2 (en) Coding arrangement for a differential phase modulation system
DE69330591T2 (en) Differentially coded pulse position modulation with protection interval for use in communication networks
DE3111384C2 (en) Encoding / decoding system with zero disparity
DE2625038C3 (en) Converter for converting a sequence of digital binary signals into a sequence of multiphase phase-modulated carrier pulses or vice versa
DE69127544T2 (en) SYSTEM FOR CODED SQUARE AMPLITUDE MODULATION
DE2417124A1 (en) METHOD FOR DATA TRANSFER AND SYSTEM FOR USING THIS METHOD
DE69131503T2 (en) RADIO FOR DIFFERENT TYPES OF MODULATION
DE2403098A1 (en) SYSTEM FOR TRANSMITTING COLUMN-PHASE MANCHESTER-CODED TWO-VALUE DATA SIGNALS
DE3789815T2 (en) Data modulation and demodulation system for a magnetic recording system.
DE2445926A1 (en) SYSTEM FOR TRANSMISSION OF MULTI-LEVEL DATA
DE2656054A1 (en) DEVICE FOR WORD SYNCHRONIZATION IN AN OPTICAL COMMUNICATION SYSTEM
DE3318673A1 (en) CIRCUIT ARRANGEMENT FOR FRAME SYNCHRONIZATION AND ELIMINATION OF PHASE AMBIGUITY IN QAM TRANSMISSION SYSTEMS
DE3030148C2 (en)
DE3322954A1 (en) 7-level phase shift keying/amplitude shift keying (PSK/ASK) system
DE3852376T2 (en) LORAN-C navigation system.
DE3042394C2 (en)
US4821038A (en) Method of and apparatus for Loran-C message communication with reduced skywave navigation location errors and the like
DE2416121A1 (en) DEMODULATOR DEVICE FOR PHASE DIFFERENCE MODULATED PULSE SEQUENCES
DE2431519B2 (en) Circuit for the recovery of data recorded by means of pulse frequency modulation
DE2405534A1 (en) Video signal transmission system - uses difference between input signal and predicted signal which is quantised and transmitted
DE2336707B2 (en) Method for data processing when sending and / or transferring information and device for carrying out the method
DE66622T1 (en) METHOD AND DEVICE FOR DEMODULATING DIFFERENTLY CODED, PHASE-MODULATED DATA BROADCASTING.
DE3718632C1 (en) Data decoding method
DE3526052A1 (en) Digital communication system with an nB/(n+1)B line code
DE1537016C (en) Method and circuit arrangement for the transmission of binary input data by means of a modified duobinary carrier modulation

Legal Events

Date Code Title Description
AF Is addition to no.

Ref country code: DE

Ref document number: 3243373

Format of ref document f/p: P

8110 Request for examination paragraph 44
8176 Proceedings suspended because of application no:

Ref document number: 3243373

Country of ref document: DE

Format of ref document f/p: P

8178 Suspension cancelled
AF Is addition to no.

Ref country code: DE

Ref document number: 3243373

Format of ref document f/p: P

AF Is addition to no.

Ref country code: DE

Ref document number: 3243373

Format of ref document f/p: P

D2 Grant after examination
8364 No opposition during term of opposition
8320 Willingness to grant licences declared (paragraph 23)
8327 Change in the person/name/address of the patent owner

Owner name: ROBERT BOSCH GMBH, 70469 STUTTGART, DE

8340 Patent of addition ceased/non-payment of fee of main patent