CN1989722B

CN1989722B - Time varying cyclic delay diversity of OFDM

Info

Publication number: CN1989722B
Application number: CN2005800241026A
Authority: CN
Inventors: 艾曼·福兹·纳吉布; 阿夫内什·阿格拉瓦拉
Original assignee: Qualcomm Inc
Current assignee: Qualcomm Inc
Priority date: 2004-05-17
Filing date: 2005-04-29
Publication date: 2011-05-18
Anticipated expiration: 2025-04-29
Also published as: ES2351152T3; CN1989722A

Abstract

Methods and apparatuses that apply a time-varying delay to symbols to be transmitted from one or more antennas are provided.

Description

Time-varying cyclic delay diversity for OFDM systems

Cross referencing

The present application claims priority from provisional application No.60/572,137 entitled "Systems time varying Cyclic Delay Diversity of OFDM" (OFDM system time varying Cyclic Delay Diversity), filed on day 17, month 5, 2004, which is assigned to the assignee of the present application.

Technical Field

This document relates generally to wireless communications, and more particularly to signal transmission in multiple antenna systems.

Background

In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver through multiple propagation paths. The characteristics of the propagation path typically change over time due to a variety of factors such as fading and multipath. To provide diversity to suppress deleterious path effects and improve performance, multiple transmit and receive antennas may be used. Multiple-in multiple-out (MIMO) communication systems employing multiple (N)_TA plurality of transmitting antennas and a plurality of (N)_RAnd) receiving antennas for data transmission. From N_TA transmitting antenna and N_RA MIMO channel composed of multiple receiving antennas can be decomposed into N_SA separate channel of which N_S≤min{N_T，N_R}。N_SEach of the independent channels may also be referred to as a spatial subchannel (or a transmission channel) of the MIMO channel and corresponds to a dimension.

It is true, at least to some extent, that if the propagation paths between the transmit and receive antennas are linearly independent (i.e., transmissions on one path do not form as a linear combination of transmissions on the other paths), the likelihood of correctly receiving a data transmission increases as the number of antennas increases. In general, as transmit and receive antennas increase, diversity increases and performance improves.

To further improve the diversity of the channel, transmit diversity techniques may be used. Many transmit diversity techniques have been developed. One such technique is transmit delay diversity. In transmit delay diversity, a transmitter transmits the same signal using two antennas, where the signal transmitted by the second antenna is a delay of the signal transmitted by the first antenna. In this way, the second antenna forms diversity by establishing a second set of independent multipath elements that can be collected at the receiver. If the multipath generated by the first transmitter fades, the multipath generated by the second transmitter may not fade, in which case an acceptable signal-to-noise ratio (SNR) will be maintained at the receiver. This technique is easy to implement because only the composite TX0+ TX1 channel needs to be estimated at the receiver. The biggest disadvantage of transmit delay diversity is that it increases the effective delay spread of the channel and performs poorly when multipath caused by the second antenna starts to appear and interacts destructively with multipath of the first antenna, thus reducing the overall level of diversity.

To address the standard delay diversity problem, other delay diversity techniques have been developed. One such technique is known as cyclic delay diversity. The cyclic delay is n_iThe samples for each of the symbols are shifted in the order in which they are transmitted as part of the symbol. Those samples that are outside the active part of the symbol are transmitted at the beginning of the symbol. In this technique, a prefix is added (pre-pend) from the front to each sample with a fixed delay or order so that the sample is transmitted from a particular antenna as part of a symbol. However, the cyclic delay causes a longer delay, and thus it will be limited to a partial guard interval period to avoid inter-symbol interference.

The cyclic delay diversity scheme may cause frequency selectivity of the channel, and thus it may provide the benefits of diversity for flat channels. However, cyclic delay diversity does not provide any time diversity when the channel is not in its time selection. For example, if two transmit antennas are in a slow fading or static channel, the cyclic shift Δ_mIt is possible to always have two channels (e.g., H)₁(n) and H₂(n)) are destructively (or structurally) superimposed (addition).

It is therefore desirable to propose a delay diversity scheme that can minimize the possibility of destructive or structural superposition of the channels used to provide diversity.

Disclosure of Invention

In one aspect, a method for providing transmit diversity comprises: providing a first symbol to a first antenna after a first delay period; providing a second symbol to the first antenna after a second delay period different from the first delay period; the third symbol is provided to the first antenna after a third delay period different from the first delay period and the second delay period.

In another aspect, a transmitter includes: at least two antennas; a modulator; and a delay circuit that delays the symbol output from the modulator to the antenna by a delay period that varies with time.

In an additional aspect, a wireless transmitter includes: at least two antennas; a memory storing a plurality of symbols, each symbol comprising a plurality of samples, wherein the memory outputs the plurality of samples of a first symbol to one of the at least two antennas after a first delay and outputs the plurality of samples of a second symbol of the plurality of symbols to the one antenna after a second delay. The first delay and the second delay are different.

In another aspect, a transmitter includes: at least three antennas; a modulator; a first delay circuit connected between the modulator and one of the at least two antennas, which delays a symbol output from the modulator to the antenna by one delay period varying with time; and a second delay circuit connected between the modulator and the other antenna of the at least two antennas, which delays the symbol output from the modulator to the other antenna by another delay period that varies with time. The another delay period is different from the delay period.

In another aspect, a method of providing transmit diversity in a multi-channel communication system includes: applying a first phase shift to a first symbol to be transmitted on a first antenna; and applying a second phase shift, different from the first phase shift, to the first symbol to be transmitted on the second antenna.

In another aspect, a transmitter includes: at least two antennas; a modulator; and a phase shift circuit that applies a phase shift that changes with time to the symbol output by the modulator to the antenna.

Drawings

The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

fig. 1 shows a block diagram of an embodiment of a transmitter system and a receiver system in a MIMO system;

FIG. 2 shows a block diagram of an embodiment of a transmitter unit that provides time-varying delay diversity;

FIG. 3 shows a block diagram of an embodiment of a time-varying delay applied to symbols transmitted from the same antenna;

FIG. 4 illustrates a block diagram of an embodiment of time-varying delays applied to symbols transmitted from multiple antennas;

FIG. 5 shows a block diagram of another embodiment of a transmitter unit that provides time-varying delay diversity;

FIG. 6 illustrates a block diagram of an embodiment of a receiver unit that may use time-varying delay diversity;

FIG. 7 shows a block diagram of an embodiment of a delay component;

FIG. 8 shows a flow diagram of an embodiment of a method for providing time-varying diversity;

FIG. 9 shows a block diagram of another embodiment of a transmitter unit that provides time-varying delay diversity;

and

fig. 10 shows a flow diagram of another embodiment of a method for providing time-varying diversity.

Detailed Description

The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments and is not intended to represent the only embodiments in which the present invention may be practiced. The term "exemplary" as used in this specification means "serving as an example, instance, or illustration," and should not be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the concepts of the present invention.

Multi-channel communication systems include multiple-input multiple-output (MIMO) communication systems, Orthogonal Frequency Division Multiplexing (OFDM) communication systems, MIMO systems employing OFDM (i.e., MIMO-OFDM systems), and other types of transmissions. For clarity, various aspects and embodiments are described in detail for a MIMO system.

MIMO systems employing multiple (N)_TMultiple) transmitting antenna and multiple (N)_RAnd) receiving antennas for data transmission. From N_TA transmitting antenna and N_RThe MIMO channel formed by the receiving antennas can be decomposed into N_SA separate channel of which N_S≤min{N_T，N_R}。N_SEach of the separate channels may also be referred to as a spatial subchannel (or transmission channel) of the MIMO channel. The number of spatial subchannels is determined by the MIMO signalThe number of eigenmodes (eigenmodes) of a channel is determined, which in turn depends on the channel response matrixHWhereinHDescription of N_TA transmitting antenna and N_RThe response between the receiving antennas. Channel response matrixHIs composed of independent Gaussian random variables h_i，jComposition, i ═ 1, 2_R，j＝1，2，...N_TWherein h is_i，jIs the coupling (i.e., complex gain) between the jth transmit antenna and the ith receive antenna. For simplicity, it is assumed that the channel response matrixHIs of full rank (i.e. N)_S＝N_T≤N_R) And an independent data stream may be from N_TEach of the transmit antennas transmits.

Fig. 1 is a block diagram of an embodiment of a transmitter system 110 and a receiver system 150 in a MIMO system 100. At the transmitter system 110, traffic data for a number of data streams is provided from a data source 112 to a Transmit (TX) data processor 114. In one embodiment, each data stream is transmitted over a respective transmit antenna. TX data processor 114 formats, codes, and interleaves the traffic data for each data stream based on a particular coding scheme selected for that data stream to provide coded data.

The coded data for each data stream can be multiplexed with pilot data using, for example, Time Division Multiplexing (TDM) or Code Division Multiplexing (CDM). The pilot data is typically a known data type that is processed in a known manner and may be used at the receiver system to estimate the channel response. The multiplexed pilot and coded data for each data stream is then modulated (i.e., symbol mapped) based on a particular modulation scheme (e.g., BPSK, QPSK, M-PSK, or M-QAM) selected for that data stream to provide modulation symbols. The data rate, coding, and modulation for each data stream may be determined by controls provided by processor 130.

The modulation symbols for all data streams are then provided to a TX MIMO processor 120, which TX MIMO processor 120 further processes the modulation symbols (e.g., OFDM). TX MIMO processor 120 then passes N_TOne modulation symbol stream is provided to N_TAnd Transmitters (TMTR)122a through 122 t. Each transmitter 122 receives and processes a respective symbol stream to provide one or more analog signals, and further conditions (e.g., amplifies, filters, and upconverts) the analog signals to provide a modulated signal suitable for transmission over the MIMO channel.

At the receiver system 150, the transmitted modulated signal consists of N_RAntennas 152a through 152r receive and the received signal from each antenna 152 is provided to a respective receiver (RCVR) 154. Each receiver 154 conditions (e.g., filters, amplifies, and downconverts) a respective received signal, digitizes the conditioned signal to provide samples, and processes the samples to provide a corresponding "received" symbol stream.

RX MIMO/data processor 160 then receives N_RA receiver 154 receives N_RA stream of received symbols, and N based on a particular receiver processing technique_RProcessing the received symbol streams to provide N_TA "detected" symbol stream. The processing by RX MIMO/data processor 160 is described in detail below. Each detected symbol stream includes symbols that are estimates of the modulation symbols transmitted for the corresponding data stream. RX MIMO/data processor 160 then demodulates, deinterleaves, and decodes each detected symbol stream to recover the traffic data for the data stream. The processing by RX MIMO/data processor 160 is complementary to that performed by TX MIMO processor 120 and TX data processor 114 at transmitter system 110.

RX MIMO processor 160 may derive pairs N based on, for example, pilot multiplexed with traffic data_TA transmitting antenna and N_REstimation of the channel response between the receive antennas. The channel response estimate may be used for spatial or space/time processing at the receiver. RX MIMO processor 160 may further estimate the signal-to-noise-and-interference ratios (SNRs) of the detected symbol streams, and possibly other channel characteristics, and provides these quantities to a processor 170. RX MIMO/data processor 160 or processor 170 may also derive an estimate of the "operating" SNR for the system, which may indicate a communicationThe condition of the link. Processor 170 then provides Channel State Information (CSI), which may comprise various types of information regarding the communication link and/or the received data stream. For example, the CSI may comprise only the operating SNR. The CSI is then processed by a TX data processor 178, modulated by a modulator 180, conditioned by transmitters 154a through 154r, and transmitted back to transmitter system 110.

At transmitter system 110, the modulated signals from receiver system 150 are received by antennas 124, conditioned by receivers 122, demodulated by a demodulator 140, and processed by a RX data processor 142 to recover the CSI reported by the receiver system. The reported CSI is then provided to processor 130 and used to (1) determine the data rates and coding and modulation schemes to be used for the data streams and (2) generate various controls for TX data processor 114 and TX MIMO processor 120.

Processors

130 and 170 control operations at the transmitter and receiver systems, which are coupled to appropriate transmit and receive data processors.

Memories

132 and 172 provide storage for program codes and data used by

processors

130 and 170, respectively.

The model for an OFDM MIMO system can be expressed as:

y＝Hx+nequation (1)

Wherein,yis receiving a vector, i.e.

<math><mrow><munder><mi>y</mi><mo>&OverBar;</mo></munder><mo>=</mo><msup><mfenced open='[' close=']'><mtable><mtr><mtd><msub><mi>y</mi><mn>1</mn></msub></mtd><mtd><msub><mi>y</mi><mn>2</mn></msub></mtd><mtd><mo>.</mo><mo>.</mo><mo>.</mo></mtd><mtd><msub><mi>y</mi><msub><mi>N</mi><mi>R</mi></msub></msub></mtd></mtr></mtable></mfenced><mi>T</mi></msup><mo>,</mo></mrow></math>

Wherein { y_iIs the term received from the ith receive antenna, i e {1_R}；

xIs a transmit vector, i.e.

<math><mrow><munder><mi>x</mi><mo>&OverBar;</mo></munder><mo>=</mo><msup><mfenced open='[' close=']'><mtable><mtr><mtd><msub><mi>x</mi><mn>1</mn></msub></mtd><mtd><msub><mi>x</mi><mn>2</mn></msub></mtd><mtd><mo>·</mo><mo>·</mo><mo>·</mo></mtd><mtd><msub><mi>x</mi><msub><mi>N</mi><mi>T</mi></msub></msub></mtd></mtr></mtable></mfenced><mi>T</mi></msup><mo>,</mo></mrow></math>

Where { x_jIs the term transmitted from the jth transmit antenna, j ∈ {1_T}；

HIs a channel response matrix for the MIMO channel;

nis a mean vector of0And covariance matrixΛ _n＝σ² IAdditive White Gaussian Noise (AWGN), wherein0Is a vector consisting of 0 s and is,Iis an identity matrix with 1 on the diagonal and 0 at other positions, σ²Is the noise variance; and

[.]^Tis expressed as.]The transposing of (1).

From N due to scattering in the propagation environment_TN transmitted by transmitting antenna_TThe symbol streams interfere with each other at the receiver. In particular, a given symbol stream transmitted from a transmit antenna may be divided by N_RThe receive antennas receive at different amplitudes and phases. Thus, each received signal may include N_TA component of each of the transmit symbol streams. N is a radical of_RThe received signals will together include all N_TA stream of transmit symbols. However, the N_TA stream of symbols is spread over N_RAmong the received signals.

At the receiver, N may be processed using various processing techniques_RA received signal to detect N_TA stream of transmit symbols. These receiver processing techniques can be divided into two broad categories:

spatial and space-time receiver processing techniques (also called equalization techniques), and

receiver processing techniques of "successive nulling/equalization and interference cancellation" (also known as "successive interference cancellation" or "successive cancellation" receiver processing techniques).

Fig. 2 is a block diagram of a portion of a transmitter unit 200, which may be an embodiment of a transmitter portion of a transmitter system, such as transmitter system 110 in fig. 1. In one embodiment, the number of pairs to be in N may be_TN transmitted on a transmitting antenna_TEach of the data streams uses a separate data rate and coding and modulation scheme (i.e., separate coding and modulation on a per antenna basis). The particular data rate and coding and modulation schemes to be used for each transmit antenna may be determined based on control provided by processor 130 and the data rates may be determined as described above.

In one embodiment, transmitter unit 200 includes a transmit data processor 202 that receives, codes, and modulates each data stream according to a separate coding and modulation scheme to provide modulation symbols and transmit, and MIMO transmit data processor 202 and transmit data processor 204 are one embodiment of transmit data processor 114 and transmit MIMO processor 120, respectively, of fig. 1.

In one embodiment, as shown in FIG. 2, transmit data processor 202 includes a demultiplexer 210, N_TEncoders 212a to 212t and N_TA plurality of channel interleavers 214a to 214t (i.e. one set of demultiplexer, encoder and channel interleaver for each transmit antenna). Demultiplexer 210 demultiplexes the data (i.e., information bits) to correspond to N_TN of transmitting antenna_TA data stream for data transmission. N is a radical of_TThe individual data streams may be associated with different data rates, which may be determined by a rate control function provided by processor 130 or 170 (fig. 1) in one embodiment. Each data stream is provided to a respective encoder 212a to 212 t.

Each encoder 212 a-212 t receives a respective data stream and encodes the data stream based on a particular coding scheme selected for the data stream to provide coded bits. In one embodiment, encoding may be used to increase the reliability of data transmission. In one embodiment, the coding scheme may include any combination of Cyclic Redundancy Check (CRC) coding, convolutional coding, Turbo coding, block coding, and the like. The coded bits from each encoder 212a through 212t are then provided to a respective channel interleaver 214a through 214t, which channel interleaver 214a through 214t interleaves the coded bits based on a particular interleaving scheme. Interleaving provides time diversity for the coded bits, allows data to be transmitted based on the average SNR of the transmission channel used for the data stream, suppresses fading, and eliminates correlation between the coded bits used to form each modulation symbol.

The coded interleaved bits from each channel interleaver 214a through 214t are provided to a respective symbol mapping module 222a through 222t in transmit MIMO processor 204, which symbol mapping modules 222a through 222t map the bits to form modulation symbols.

The particular modulation scheme implemented by each symbol mapping module 222a through 222t is determined by the modulation control provided by processor 130. Each symbol mapping module 222a to 222t groups out a plurality of q symbols_jThe set of interleaved bits is coded to form non-binary symbols, each of which is then mapped to a particular point in a signal constellation corresponding to a selected modulation scheme (e.g., QPSK, M-PSK, M-QAM, or other modulation scheme). Each mapped signal point corresponds to an M_jVitamin (M)_j-ary) modulation symbols, where M_jCorresponds to a particular modulation scheme selected for the jth transmit antenna, and

M_{j} = 2^{q_{j}} .

the symbol mapping modules 222a through 222t then provide N_TA stream of modulation symbols.

In the particular embodiment shown in fig. 2, transmit MIMO processor 204 includes a modulator 224 and Inverse Fast Fourier Transform (IFFT) modules 226a through 226t in addition to symbol mapping modules 222a through 222 t. Modulator 224 modulates the samples to form N on the appropriate subbands and transmit antennas_TModulation symbols for each stream. In addition, modulator 224 provides N at a prescribed power level_TEach of the symbol streams. In one embodiment, modulator 224 may modulate symbols according to a frequency hopping sequence controlled by a processor, such as

processor

130 or 170. In this embodiment, for N_TThe frequency at which the streams of symbols are modulated may vary for each group or block of symbols of a transmission cycle, for each frame, or for a portion of a frame.

Each IFFT block 226a through 226t receives a respective modulation symbol stream from modulator 224. Each IFFT module 226 a-226 t groups out a plurality of groups N_FThe sets of individual modulation symbols form respective vectors of modulation symbols, and each vector of modulation symbols is converted to its time-domain representation (referred to as an OFDM symbol) using an inverse fast fourier transform. The IFFT modules 226 a-226 t may be designed to perform at any number (8, 16, 32_F) Is inverse transformed on the frequency sub-channel of (1).

The time domain representation of each modulation symbol vector generated by IFFT blocks 226a through 226t is provided to an associated cyclic prefix generator 228a through 228 t. Cyclic prefix generators 228a to 228t add a prefix having a fixed number of samples from the front to N constituting an OFDM symbol_SA fixed number of samples, typically a number of samples from the end of an OFDM symbol, to form a corresponding transmission symbol. The prefix is designed to improve performance to suppress deleterious path effects such as channel dispersion caused by frequency selective fading. Cyclic prefix generators 228a through 228t then provide the transport symbol streams to associated delay elements 230a through 230 t-1.

Each delay element 230a through 230t-1 provides a delay to each symbol output from cyclic prefix generators 228a through 228 t. In one embodiment, the delay provided by each delay element 230 a-230 t-1 changes over time. In one embodiment, this delay is such that: which changes between consecutive symbols output by the cyclic prefix generator or between consecutive symbols to be transmitted consecutively from the transmitter unit 200. In another embodiment, the delay may vary between groups of two, three, four, or more symbols, where each symbol within a group has the same delay. In additional examples, all symbols within a frame or a burst period (burst period) have the same delay, with each symbol in each frame or burst period having a different delay than each symbol in a preceding or following frame or burst period.

Furthermore, in the embodiment shown in FIG. 2, the delay provided by each delay element 230a through 230t-1 is different from the delay provided by the other delay cells. Furthermore, although fig. 2 shows cyclic prefix generator 228a not connected to a delay component, other embodiments may provide a delay component to the output of each of cyclic prefix generators 228 a-228 t.

The symbols output by delay elements 230a through 230t-1 are provided to associated transmitters 232a through 232t such that antennas 232a through 232t transmit the symbols in accordance with the delays provided by delay elements 230a through 230 t-1.

As described above, in one embodiment, the time-varying delay Δ provided by each delay element 230 a-230 t-1_mChanges occur over time. In one embodiment, the ith OFDM symbol is transmitted from antenna m as a transmit symbol according to the delay of equation 2:

(equation 2)

The entire channel obtained in this case can be represented as

(equation 3)

Wherein H_m(i, n) are the channel n-th order Discrete Fourier Transform (DFT) coefficients for the channel impulse response from the mth transmit antenna to the receive antenna.

Frequency selectivity and time selectivity can be introduced into the channel by using such time-varying delays, which can be used to improve performance. For example, by applying time-varying delays to the transmission symbols on different subcarriers and different OFDM symbols, both time-selectivity and frequency-selectivity may be provided. Furthermore, in the case of transmission to multiple users, since the channel condition of each user receiver is different from the channel conditions of the other user receivers, the time variation of the channel provided by varying the delay of the symbols can be used to provide diversity gain to each of the multiple users.

In one embodiment, the delay Δ_m(i) Each successive symbol or group of successive symbols may be delayed by N x β samples in a linear fashion over time, where β is a constant, N varies between 0, 1. In another embodiment, the delay Δ_m(i) May be relative to the adjacent channel (i.e., N)_TAntennas of the antennas) and the random delays of the preceding and/or following symbols based on the pseudo-random sequence. In additional embodiments, the delay varies with f (x), where f is a function, such as a sine, cosine, or other time varying function, x varies between 0, 1.. multidot., N-1, or some multiple thereof, and where N is a frame, a burst period, or a number of symbols in a symbol stream. In each of the embodiments described above, the delay may also be varied based on feedback information, in which case the receiver sends back a channel quality indicator describing the overall channel conditions, and varies the delay Δ_m(i) To improve overall quality.

Referring to fig. 3, an embodiment of a time-varying delay applied to symbols transmitted from the same antenna is shown. Generating a symbol S₁、S₂、S₃And S₄So as to be respectively in successive time slots T₁、T₂、T₃And T₄During which transmission takes place. Each symbol S₁、S₂、S₃And S₄Comprising nine samples N_S1、N_S2、N_S3、N_S4、N_S5、N_S6、N_S7、N_S8、N_S9And dual sampling cyclic prefix N_C1And N_C2In which N is_C1And N_C2Are respectively a sample N_S8And N_S9. It should be noted that the content of each sample may be different for each symbol. It should be noted that sample N_S1、N_S2、N_S3、N_S4、N_S5、N_S6、N_S7、N_S8、N_S9Can be according to N_S1、N_S2、N_S3、N_S4、N_S5、N_S6、N_S7、N_S8、N_S9Are combined to form the symbol S₁。

Then, a delay section (e.g., delay section 230a) transmits the symbol S to the same antenna₁、S₂、S₃And S₄A delay is provided. In the embodiment shown in FIG. 3, the symbol S₁Is one sampling period t₁. To be immediately preceded by a symbol S on the same antenna₁The next symbol S transmitted thereafter₂Is delayed by two sampling periods t₁And t₂. To be immediately preceded by a symbol S on the same antenna₂The next symbol S transmitted thereafter₃Is delayed by three sampling periods t₁、t₂And t₃. To be immediately preceded by a symbol S on the same antenna₃The next symbol S transmitted thereafter₄Is delayed by four sampling periods t₁、t₂、t₃And t₄. If additional symbols are to be transmitted on the same antenna, the next consecutive symbol will be transmitted with five sample periods t₁、t₂、t₃、t₄And t₅Is transmitted with a delay of (1). In this way, a linear time-varying delay may be applied to transmissions from antennas that may or may not be part of a MIMO system.

It should be noted that the linear variation of the delay period need not be a sequence in units of one sampling period, but may be a sequence in units of 2 or more sampling periods, for example, the first symbol S₁Can be delayed by three sample periods, the second symbol S₂Can be delayed by six sample periods, the third symbol S₃Can be delayed by nine sample periods, the fourth symbol S₄May be delayed by twelve sample periods. Furthermore, the linear variation need not change between each successive symbol, but may change for groups of symbols, e.g. symbol S₁And S₂Are delayed by one sampling period, symbol respectivelyNumber S₃And S₄Respectively delayed by two or more sample periods.

Referring to fig. 4, an embodiment of applying time-varying delays to symbols transmitted from multiple antennas is shown. To be driven from antenna A₁、A₂、A₃And A₄Up-transmitting the same symbol S₁. Symbol S₁Comprising nine samples N_S1、N_S2、N_S3、N_S4、N_S5、N_S6、N_S7、N_S8、N_S9And double sampling cyclic prefix N_C1And N_C2In which N is_C1And N_C2Are respectively a sample N_SAnd N_S9. At the first antenna A₁Top unpaired symbol S₁A delay of any sample period is applied. At the second antenna A₂Will sign S₁Delayed by one sampling period t₁. At the third antenna A₃Will sign S₁Delayed by two sampling periods t₁And t₂. At the fourth antenna A₄Will sign S₁Delayed by three sampling periods t₁、t₂And t₃. Thus, in a MIMO system, except by antenna A₁、A₂、A₃And A₄In addition to providing spatial diversity, time and frequency diversity may also be provided.

The time diversity and its variation provided for the scheme shown in fig. 4 can reduce the probability of collision of the same samples of the same symbol, thereby minimizing the probability of destructive or structural superposition of the channels.

It should be noted that the delay variation between the same symbols transmitted on the same antenna need not be linear or related to the delay on the other antennas, as long as the symbols are delayed by different amounts on each antenna if they are to be transmitted substantially simultaneously.

It should be noted that the number of stages used does not necessarily correspond to the number of antennas, and may be changed corresponding to a smaller or larger number of groups than the number of antennas.

Further, as discussed with respect to FIG. 2, the delay may be random and may be based on a function such as a sine, cosine, or other function. In some embodiments, the delay period is limited to a number of samples in one symbol, where the delay period may be repeated after a fixed or random number of symbols. Further, it should be noted that the inter-symbol delay may be a fraction of the sampling period and is not limited to a multiple of the entire sampling period. In one embodiment, the partial delay may be implemented by using a partial clock period of one or more clocks of the transmitter unit 200.

Referring to fig. 5, a block diagram of another embodiment of a transmitter unit providing time-varying delay diversity is shown. Transmitter unit 500 is substantially identical to transmitter unit 200. Further, scaling circuits (scaling circuits) 554a through 554t-1 are connected to the output terminals of one of the delay elements 530a through 530t-1, respectively. Scaling circuits 534a through 534t-1 provide a fixed scaling shift (scalar shift) to the delay provided by each of delay components 534a through 534 t-1. For example, a fixed shift is applied to each delay, such that if, for example, a constant shift of 0.5 is applied, one single sample period delay would be 0.5 sample periods, one double sample period delay would be one sample period, and one five sample period delay would be 2.5 sample periods. In one embodiment, each of scaling circuits 554a through 554t-1 provides a different shift than the other scaling circuits. In one embodiment, linear progression is provided over scaling circuits 554a through 534t-1, i.e., scaling circuit 554a provides less than 554b, 554b provides less than 554c, and so on.

It should be noted that although fig. 5 shows that cyclic prefix generator 228a is not connected to a delay component, other embodiments may provide a delay component to the output of each of cyclic prefix generators 228a through 228 t. Furthermore, although fig. 5 shows that cyclic prefix generator 228a is not connected to the scaling circuit, other embodiments may provide the scaling circuit to the output of each of cyclic prefix generators 228a through 228t, regardless of whether a delay circuit is connected to the cyclic prefix generator.

Referring to fig. 6, a block diagram of an embodiment of a receiver unit that may use time-varying delay diversity is shown. The transmitted signals are received by antennas 602a through 602r and processed by receivers 604a through 604r, respectively, to provide N_RA sample stream, N_RThe sample streams are then provided to an RX processor 606.

In demodulator 608, cyclic prefix removal components 612a through 612r and FFT modules 614a through 614r provide N_RA stream of symbols. Cyclic prefix removal components 612a through 612r remove the cyclic prefix contained in each transmission symbol to provide a corresponding recovered OFDM symbol.

FFT modules 614a through 614r then transform each recovered symbol of the symbol stream using a fast fourier transform to provide a symbol stream having a symbol duration corresponding to N for each transmitted symbol period_FN of one frequency subchannel_FAnd recovering the vector of modulated signals. FFT modules 614a through 614r pass N_RThe received symbol streams are provided to a spatial processor 620.

Spatial processor 620 to N_RThe received symbol streams are spatially or space-time processed to provide N_TA stream of detected symbols, N_TOne detected symbol stream is for N_TAn estimate of the transmitted symbol stream. Spatial processor 620 may implement a linear ZF equalizer, a Channel Correlation Matrix Inversion (CCMI) equalizer, a Minimum Mean Square Error (MMSE) equalizer, an MMSE linear equalizer (MMSE-LE), a Decision Feedback Equalizer (DFE), or other equalizer, such implementations being described and illustrated in U.S. patent application nos. 09/993,087, 09/854,235, 09/826,481, and 09/956,444, each of which is incorporated herein by reference in its entirety.

The spatial processor 620 is capable of compensating for the time-varying delay provided by the delay components and/or scaling circuits of the transmitter, which has been discussed with respect to fig. 2 and 5. In one embodiment, such compensation may be provided, for example, by a delay scheme that makes the receiver 600 known a priori to be linear, random based on a pseudo-random sequence, or a function. This knowledge may be provided, for example, by having all transmitters use the same scheme or providing information about the scheme used as part of the initialization of the communication between the transmitters and the receiver unit 600.

Multiplexer/demultiplexer 622 then multiplexes/demultiplexes the detected symbols and will correspond to N_DN of one data stream_DProviding the aggregated detected symbol stream to N_DAnd symbol demapping elements 624a through 624 r. Each symbol demapping element 624a through 624r then demodulates the detected symbols according to a demodulation scheme that is complementary to the modulation scheme used for the data stream. Then, will come from N_DN of symbol demapping elements 624a through 624r_DThe demodulated data streams are provided to an RX data processor 610.

In RX data processor 610, channel deinterleavers 632a through 632r deinterleave each demodulated data stream in a manner complementary to the processing performed on the data stream at the transmitter system, and decoders 634a through 634r decode the deinterleaved data in a manner complementary to the processing performed at the transmitter system. For example, decoders 634a through 634r may employ Turbo decoders or Viterbi decoders if Turbo or convolutional coding, respectively, is performed at the transmitter unit. The decoded data stream from each decoder 634a through 634r represents an estimate of the transmitted data stream. Decoders 634a through 634r may also provide the status of each received packet (e.g., which indicates whether correct or incorrect reception was made). Decoders 634a through 634r may also store demodulated data for packets that are not decoded correctly so that the data may be combined and decoded with data from subsequent incremental transmissions.

In the embodiment shown in fig. 6, channel estimator 640 estimates the channel response and the noise variance and provides these estimates to processor 650. The channel response and noise variance may be estimated based on the detected symbols for the pilot.

Processor 650 may be designed to perform a variety of rate selection related functions. For example, processor 650 may determine a maximum data rate available for each data stream based on the channel estimates and other parameters (e.g., modulation scheme).

Referring to fig. 7, a block diagram of an embodiment of a delay component is shown. The processor 700 is connected to the memory 704 via a bus 702. Memory 704 is used to store samples of a time-domain representation of the modulation symbols provided for transmission. The samples for each symbol are stored in memory addresses known to processor 700. Processor 700 may then instruct memory 704 to output samples for each symbol by using any desired time-varying delay for successive groups of symbols or symbols in a frame or burst period.

As described with respect to fig. 3 and 4, the delay per symbol may vary between each symbol in a group of symbols to be transmitted consecutively, as well as between symbols in different frames or burst periods. The use of memory allows any predetermined or suitable scheme to be used to provide delay for the symbols and thus time diversity that can be varied (e.g., linearly varied) based on channel conditions and the predetermined scheme.

Referring to fig. 8, a flow diagram of an embodiment of a method for providing time-varying delay diversity is shown. At block 800, samples representing one or more modulation symbols after being subjected to an inverse fast fourier transform are provided. A cyclic prefix is then prepended to each modulation symbol, block 802. The size of the prefix may vary as desired, and in one embodiment, the prefix may be 32 or more samples.

The samples including the cyclic prefix are then stored in a memory, which may be a buffer in one embodiment, at block 804. In one embodiment, the samples for each modulation symbol are stored in memory according to the order in which they are provided after a cyclic prefix is added from the front. In other embodiments, the samples for each modulation symbol may be stored in any desired order. At block 806, the first symbol to be transmitted is removed according to the first delay N. At block 808, the next symbol to be transmitted is removed according to a second delay, wherein the second delay is different from the first delay. The second delay and the additional delay of the following symbol may be an N + β delay, where β may increase or decrease linearly from N, vary randomly based on N, or as a result of some function.

Then, at block 810, a determination is made whether the symbol requiring delay has been transmitted or used. If the determination is negative, then at block 808, additional delay is provided to the output of the following symbol from the memory or buffer according to the same time variation. If the determination is yes, the process ends and additional symbols are provided at block 812 as described for blocks 800-804.

Referring to fig. 9, a block diagram of another embodiment of a transmitter unit providing time varying delay diversity is shown. Transmitter unit 900 is substantially identical to transmitter unit 200. However, instead of using delay elements 230a to 230t-1 connected to the outputs of IFFT blocks 226a to 226t, phase shift circuits 930a to 930t-1 are connected before IFFT blocks 926a to 926t to receive the outputs of modulators 924. Phase shift circuits 930a through 930 t-provide time-varying phase shifts to the samples of each symbol. For example, phase shift circuit 930a may provide a phase shift Φ to a sample of a first symbol of the modulator output₁Providing a phase shift Φ to the sample of the next or following symbol₁. The samples of the following symbols may have the same or different amounts of phase shift. This phase shift may be used as a delay in the time domain after the IFFT performed by IFFT blocks 926a through 926 t.

Each phase shift circuit 930 a-930 t may provide a different phase shift than the other phase shift circuits 930 a-930 t so that the delay of the same symbol transmitted from multiple antennas is different at each antenna. This change may or may not be a function of the phase shift applied to the other antenna.

In one embodiment, the phase shift provided by each phase shift circuit 930 a-930 t is such that the phase shift varies between successive symbols of the modulator output. In other embodiments, the phase shift may vary between groups of two, three, four, or more symbols, where each symbol within a group has the same phase shift. In additional embodiments, all symbols in a frame or a burst period have the same phase shift, and each frame or each burst period has a different phase shift for each symbol than the preceding or following frame or burst period.

It should be noted that although fig. 9 shows the phase shift circuit not connected to cyclic prefix generator 928a, other embodiments may provide a phase shift circuit to the output of each of cyclic prefix generators 928a to 928 t.

In some embodiments, the modulator and phase shift circuit may include a processor.

Referring to fig. 10, a flow diagram of an embodiment of a method for providing time-varying delay diversity is shown. At block 1000, a first phase shift Φ is applied to samples of a first symbol of a modulator output₁. Then, at block 1002, a phase shift Φ is applied to the samples of the second symbol₂Wherein phi₂And phi₁Different. Then, at block 1004, a determination is made whether there are more symbols to be phase shifted. If not, then the next symbol of the modulator output may be sampled using the sum of phi at block 106₁Or phi₂The same or different phase shift. The process is then repeated until there are no more symbols to apply a phase shift.

If there are no more symbols to be phase shifted, the symbols are IFFT at block 1008, a cyclic prefix is added from the front at block 1010, and the symbols are stored in memory, which may be a buffer in one embodiment, at block 1012. In one embodiment, the samples for each modulation symbol are stored in memory according to the order in which they were provided after the cyclic prefix was appended from the front. In other embodiments, the samples for each modulation symbol may be stored in any desired order.

In some embodiments, the phase shift may differ between successive symbols, groups of symbols, or frames by a phase equal to a constant angle (constant angle) multiplied by a varying value corresponding to the position of the symbol in the symbol stream or other sequential value. The fixed angle may be fixed or may vary with other time constants. Furthermore, the fixed angle applied on the symbol may be different for different antennas.

In other embodiments, the phase shift may be changed according to a random phase relative to other symbols. This can be achieved by using a pseudo random code to generate the phase shift.

It should be noted that although fig. 10 illustrates waiting for performing IFFT and cyclic prefix addition from the front until phase shift is applied to all symbols of one frame or burst period, each symbol may be IFFT performed and cyclic prefix added from the front individually or in groups before completing the phase shift for each symbol of one frame or burst period.

It should be noted that transmitters 200 and 500 may receive and process respective modulation symbol streams (for MIMO without OFDM) or transmission symbol streams (for MIMO with OFDM) to generate modulation signals, which are then transmitted from associated antennas. Other transmitter unit designs may also be implemented which would be within the scope of the present invention.

Coding and modulation for MIMO systems with and without OFDM is described in detail in the following u.s. patent applications:

U.S. patent application No.09/993,087, entitled "Multiple-access Multiple-Input Multiple-output (mimo) Communication System", filed 11/6/2001;

U.S. patent application No.09/854,235, entitled "Method and apparatus for Processing Data in a Multiple-Input Multiple-output (mimo) Communication System Utilizing Channel State Information", filed on day 5, 11, 2001;

U.S. patent application nos. 09/826,481 and 09/956,449, both entitled "Method and Apparatus for using Channel State Information in an automated Communication System", filed on 3/23 and 9/18 of 2001, respectively;

U.S. patent application No.09/776,075, entitled "Coding Scheme for a wireless Communication System", filed on 2/1/2001; and

U.S. patent application No.09/532,492, entitled "High Efficiency, High performance Communications System Employing Multi-Carrier modulation", was filed 3/30/2000.

All of the above applications are assigned to the assignee of the present application and are hereby incorporated by reference. Application No.09/776,075 describes a coding scheme in which different rates can be obtained by encoding data with the same basic code (e.g., a convolutional or Turbo code), and the desired rate can be obtained by adjusting the puncturing. Other coding and modulation schemes may also be used and would be within the scope of the present invention.

Those of skill would appreciate that the various illustrative logical blocks, modules, circuits, and algorithms described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and algorithms have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design requirements imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.

The various illustrative logical blocks, processors, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a Digital Signal Processor (DSP), a circuit, an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, microprocessor, or state machine. A processor may also be implemented as a combination of devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microcontrollers and a DSP core, a plurality of logic components, a plurality of circuits, or other such configuration.

The methods or algorithms described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method of providing transmit diversity in a multi-channel communication system, comprising:

providing a first symbol comprising a first set of samples to a first antenna;

providing said first symbol to a second antenna after a first delay period;

providing a second symbol comprising a second set of samples to the first antenna;

providing the second symbol to the second antenna after a second delay period different from the first delay period;

providing a third symbol comprising a third set of samples to the first antenna;

providing said third symbol to said second antenna after a third delay period different from said first delay period and said second delay period,

wherein each of said symbols includes a cyclic prefix having one or more samples, wherein said delay period varies according to a defined time-varying function, wherein said delay period varying according to a defined time-varying function has bound, where time diversity can be provided even if the channel does not have its time selection.

2. The method of claim 1, wherein the first symbol, second symbol and third symbol are consecutive symbols in the same symbol stream.

3. The method of claim 1, wherein the third symbol is transmitted immediately after the second symbol, wherein the second symbol is transmitted immediately after the first symbol.

4. The method of claim 1, wherein the first symbol, second symbol, and third symbol are non-consecutive symbols in the same symbol stream.

5. The method of claim 4, wherein the first symbol and the second symbol are separated by a plurality of symbols, and the second symbol and the third symbol are separated by the plurality of symbols.

6. The method of claim 1, wherein the first symbol, second symbol, and third symbol are respectively transmitted in different frames.

7. The method of claim 1, wherein

providing said first symbol after said first delay period includes applying a first phase shift to each sample in said first set of samples;

The step of providing the second symbol after the second delay period different from the first delay period includes applying a second phase shift different from the first phase shift to each sample in the second set of samples. phase shift; and

The step of providing the third symbol after a third delay period different from the first delay period and the second delay period includes applying to each sample in the third set of samples a third phase shift different from the second phase shift.

8. The method of claim 1, wherein the first delay period comprises a delay of X sampling periods, the second delay period comprises a delay of X+β sampling periods, and the third delay period A delay of X+2β sample periods is included, where β is a constant.

9. The method of claim 1 , wherein the first delay period comprises a delay of X sample periods and the second delay period comprises a delay of Y sample periods, wherein Y sample periods are different from X and the third delay period includes a delay of Z sample periods, where Z sample periods is any number of sample periods different from X and Y.

10. The method of claim 1, wherein the first delay period comprises a delay of f(x) sample periods, the second delay period comprises a delay of f(x+1) sample periods, and The third delay period includes a delay of f(x+1) sampling periods, where f is a function and x is a value that changes according to symbol positions.

11. The method of claim 1, further comprising modulating the first, second, and third symbols according to different carrier frequencies.

12. The method of claim 1, further comprising applying a scaling shift to the first delay period, second delay period, and third delay period.

13. A transmitter comprising:

at least two antennas;

a modulator connected to the at least two antennas; and

a delay circuit connected between the modulator and one of the at least two antennas, the delay circuit outputting from the modulator to the antenna with a delay period that varies according to a defined time-varying function The symbols are delayed,

wherein each said symbol comprises a cyclic prefix with one or more samples, wherein said delay period varying according to a defined time-varying function has bounds, wherein a time can be provided even if the channel does not have its time selection separation.

14. The transmitter of claim 13, wherein the delay circuit provides the delay period such that the delay period differs between consecutive symbols output by the modulator to the antenna.

15. The transmitter of claim 13 , wherein the delay circuit provides the delay period such that the delay period differs between a plurality of consecutive symbols by a plurality of sampling periods, the plurality of sampling periods being equal to one The constant is multiplied by a varying value corresponding to the position of the symbol.

16. The transmitter of claim 13 , wherein the delay circuit provides the delay period such that the delay period differs between consecutive symbols by a random number of sampling periods relative to other consecutive symbols .

17. The transmitter of claim 13 , wherein the delay circuit provides the delay period such that the delay period differs by f(x) sample periods between consecutive symbols, where f is a function, x is a value that changes according to the position of the symbol.

18. The transmitter of claim 13, further comprising a scaling circuit connected between the delay circuit and the antenna.

19. The transmitter of claim 13, wherein the modulator modulates symbols using multiple carrier frequencies.

20. The transmitter of claim 13, wherein the modulator and the delay circuit comprise a processor.

21. The transmitter of claim 13, wherein said at least two antennas include at least three antennas, said transmitter further comprising another delay circuit connected between said modulator and said at least three antennas Between the other of the antennas, the further delay circuit provides a further delay period that varies with time, wherein the further delay period and the delay period are different for each symbol.

22. The transmitter of claim 21 , wherein said delay period differs between consecutive symbols output by said modulator to said antenna, and said another delay period differs between successive symbols output by said modulator different between a plurality of consecutive symbols output by the detector to the other antenna, and wherein the delay period between a plurality of consecutive symbols changes by n*β samples, and the other The delay period varies by n*(β+k) samples, where n is the position of the symbol in the symbol stream and β and k are constants.

23. The transmitter of claim 13, further comprising a cyclic prefix generator circuit connected between the modulator and the delay circuit, wherein the cyclic prefix generator circuit adds to each of the symbols The cyclic prefix.

24. A wireless transmitter comprising:

at least two antennas; and

a memory connected to the antenna, the memory storing a plurality of symbols, each symbol comprising a plurality of samples, wherein the memory outputs the plurality of samples to one of the at least two antennas after a first delay period a plurality of samples of the first symbol of the symbols, and outputs a plurality of samples of the second symbol of the plurality of symbols to the one antenna after a second delay period, wherein the first delay period and the The second delay period is different,

25. The wireless transmitter of claim 24, wherein the second delay period is n*β samples greater than the first delay period, where n is the sequential position of the second symbol in the symbol stream , β is a constant.

26. The wireless transmitter of claim 24, wherein the first delay period is f(x) samples and the second delay period is f(x+1) samples, where f is a function , x is a value that changes according to the position of the symbol.

27. The wireless transmitter of claim 24, wherein the memory comprises a buffer.

28. The wireless transmitter of claim 24, further comprising a cyclic prefix generator connected to said memory, said cyclic prefix generator sending Each symbol of the plurality of symbols is appended with the cyclic prefix.

29. The wireless transmitter of claim 24 , wherein the at least two antennas include at least three antennas, wherein the memory provides a signal to another antenna in the at least three antennas after a third delay period. providing the first symbol and providing the second symbol to the other antenna after a fourth delay period, wherein the first delay period, the second delay period, the third delay period and the fourth delay period are mutually All are different.

30. The wireless transmitter of claim 29, wherein the second delay period differs from the first delay period by n*β samples, and the fourth delay period differs from the third delay period by n*(β+k) samples, where n is the position of the symbol in the symbol stream, and β and k are constants.

31. A transmitter for transmitting a plurality of symbols comprising:

multiple antennas;

a modulator coupled to the plurality of antennas that modulates a plurality of symbols transmitted on the plurality of antennas; and

means for providing a delay period for each symbol output by said modulator and varying said delay period between consecutive symbols output by said modulator,

32. The transmitter of claim 31 , wherein said means for providing a delay period comprises means for providing a delay period varying by n*β samples between consecutive symbols, where n is the sequential position of the symbol in the symbol stream, and β is a constant.

33. The transmitter of claim 31, wherein the means for providing a delay period comprises means for providing a delay period that varies by a random number of sampling periods between consecutive symbols.

34. The transmitter of claim 31 , wherein said means for providing a delay period comprises means for providing a delay period varying by f(x) samples between a plurality of consecutive symbols, wherein f is the function and x is the value that changes according to the position of the symbol.

35. The transmitter of claim 31, wherein the modulator modulates the plurality of symbols using a plurality of carrier frequencies.

36. The transmitter of claim 31 , further comprising means for applying a scaled shift to the delay period provided to each of the plurality of symbols.

37. The transmitter of claim 31 , further comprising means for adding the cyclic prefix consisting of N samples to each symbol of the plurality of symbols.

38. A transmitter comprising:

at least two antennas;

a modulator connected to said at least two antennas;

a first delay circuit connected between the modulator and one of the at least two antennas, the first delay circuit outputting from the modulator to the antenna with a time-varying delay period pair The symbols are delayed;

A second delay circuit, which is connected between the modulator and the other antenna of the at least two antennas, the second delay circuit utilizes another delay period that varies according to a defined time-varying function from delaying symbols output by said modulator to said another antenna,

wherein said another delay period is different from said delay period, wherein each said symbol comprises a cyclic prefix having one or more samples, wherein said delay period varying according to a defined time-varying function has bound, where time diversity can be provided even if the channel does not have its time selection.

39. The transmitter of claim 38, wherein the first delay circuit provides the delay period such that the delay period differs between consecutive symbols output by the modulator to the antenna , and the second delay circuit provides the further delay period such that the further delay period differs between consecutive symbols output by the modulator to the further antenna.

40. The transmitter of claim 38, wherein the first delay circuit provides the delay period such that the delay period differs between consecutive symbols by a plurality of sampling periods, the plurality of sampling periods equal to a constant multiplied by a varying value corresponding to the symbol position; and said second delay circuit provides said another delay period such that said another delay period differs between consecutive symbols by a plurality of sample periods, so The number of sampling periods is equal to another constant multiplied by a varying value corresponding to the symbol position.

41. The transmitter of claim 38, wherein said second delay circuit provides said another delay period such that said another delay period differs between consecutive symbols relative to other consecutive symbols by A random number of sampling periods for .

42. The transmitter of claim 38, wherein the first delay circuit provides the delay period such that the delay period differs by f(x) sample periods between consecutive symbols, where f is function, x is a value that changes according to symbol position, and said second delay circuit provides said another delay period such that said another delay period differs by g(x) sample periods between consecutive symbols, where g is a different function than f, and x is a value that changes according to the position of the symbol.

43. The transmitter of claim 38, further comprising: a scaling circuit connected between said first delay circuit and said antenna; and another scaling circuit connected between said second delay circuit between the circuit and the other antenna.

44. The transmitter of claim 38, wherein the modulator modulates symbols using multiple carrier frequencies.

45. The transmitter of claim 38, further comprising: a cyclic prefix generator circuit connected between the modulator and the first delay circuit; and another cyclic prefix generator circuit connected between the between the modulator and the second delay circuit.