WO2007015494A1

WO2007015494A1 - Ofdm communication apparatus

Info

Publication number: WO2007015494A1
Application number: PCT/JP2006/315235
Authority: WO
Inventors: Haitao Li; Jifeng Li
Original assignee: Matsushita Electric Industrial Co., Ltd.
Priority date: 2005-08-02
Filing date: 2006-08-01
Publication date: 2007-02-08
Also published as: CN1909534A

Abstract

An OFDM Communication apparatus capable of improving the spectrum efficiency and data transmission efficiency of the OFDM system. In this apparatus, an IDFT part (201) generates OFDM symbols. A CP inserting part (202) inserts a cyclic prefix among the OFDM symbols. A transport RF part (203) modulates the OFDM symbols, among which the cyclic prefix has been inserted, into a radio frequency band. An antenna (204) transmits the modulated signal. A control part (205) controls the cyclic prefix length and the OFDM symbol length in accordance with the network quality. An antenna (206) receives the signal, which is then down-converted by a received RF part (207) to the baseband signal. A CP removing part (208) removes CP from the baseband signal. A DFT part (209) performs DFT to demodulate the data streams of the respective subcarriers. A network quality estimating part (210) uses a received pilot signal to estimate, as network qualities, the Doppler frequency shift and the delay spread RMS value of a channel. A control part (211) controls the processings of the CP removing part (208) and DFT part (209).

Description

Specification

OFDM communication equipment

Technical field

[0001] The present invention relates to an OFDM communication apparatus, and more particularly to a transmission system such as a wireless local area network, mobile communication, and terrestrial digital broadcasting! It relates to OFDM communication equipment that is used.

Background art

[0002] With the fusion of wireless networks, multimedia, and the Internet, demands for the quality of wireless communication are increasing. In order to meet the demand for wireless multimedia and data transmission at high transmission rates, it is necessary to develop a next generation wireless communication system. In the next generation wireless communication system, new technology will be adopted for its physical structure and media access control power to the network. As the new technology, for example, there is OFDM (urthogonal frequency division multiplexing) power.

[0003] In OFDM, a channel is divided into a plurality of mutually orthogonal subchannels in the frequency domain. Thus, the broadband selective channel becomes a relatively flat subchannel. Also, in OFDM, inter symbol interference (ISI) can be significantly reduced by inserting a CP (Cyclic Prefix) as a guard interval (GI) between each OFDM symbol. it can. For this reason, OFDM has already been successfully applied to systems such as ADSL, VDSL, DVB, and WLAN, which are already highly multi-pathable, and OFDM is the core technology for next-generation wireless transmission systems.

[0004] One of the important features in OFDM is that the ISI that occurs when a multipath channel is delayed and spread can be reduced using CP. In a typical OFDM system, the CP length (CP duration) is usually set to a fixed length that is larger than the maximum delay spread time of the channel and is 2 to 4 times the RMS (Root Mean Square) value of the delay spread. The

[0005] According to actual tests, the RMS value of delay spread is about 35 ns in an indoor environment such as an office, whereas the RMS value of delay spread is 300 ns in a factory environment. . Therefore, if the CP length is designed in advance assuming an indoor environment such as an office, when the user equipment (User Equipment: UE) enters a place such as a factory, the CP length becomes insufficient and severe ISI occurs. On the other hand, if the CP length is designed in advance in an environment such as a factory, when the user terminal enters a place such as an office, a surplus in the CP length occurs, and extra power is consumed.

[0006] As a conventional technique for solving the above-mentioned problem caused by the fixed CP length, for example, Patent Document 1 discloses adjusting the CP length based on a signal sample. Patent Document 2 discloses changing the CP length and the carrier period in an OFDM system that transmits a television signal. Patent Document 3 discloses that the CP length is determined based on the correlation value between the N samples at the CP end and the N samples at the end of the effective symbol. Patent Document 4 discloses that the minimum CP length for canceling ISI is calculated based on the channel quality situation.

FIG. 5 shows an example of the operation flow of the conventional OFDM system. In step S501, the receiving side estimates the channel quality from the received signal. In step S502, the minimum CP length for canceling ISI is calculated according to the channel quality. In step S503, the CP length is adjusted to the length calculated in step S502.

Patent Document 1: US Pat. No. 6,535,550 Specification

Patent Document 2: European Patent No. 1439679 Specification

Patent Document 3: Japanese Patent Laid-Open No. 2003-152670

Patent Document 4: European Patent No. 1014639

Disclosure of the invention

Problems to be solved by the invention

However, conventionally, the number of subcarriers for transmitting data even when the CP length is changed.

That is, since the OFDM symbol length is not changed, there is a problem that spectrum efficiency and data transmission efficiency are bad!

[0009] An object of the present invention is to improve spectral efficiency and data transmission efficiency.

To provide an FDM communication device.

Means for solving the problem

[0010] The OFDM communication apparatus of the present invention includes a generating means for generating an OFDM symbol, an OFDM symbol, It adopts a configuration comprising an inserting means for inserting a cyclic prefix between the symbols and a control means for controlling the cyclic prefix length and the OFDM symbol length according to the channel quality.

The invention's effect

[0011] According to the present invention, it is possible to improve the spectrum efficiency and data transmission efficiency of an OFDM system.

Brief Description of Drawings

[0012] [Fig.l] An example of an OFDM symbol

FIG. 2 is a block diagram showing an OFDM system according to an embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of an OFDM symbol used in an OFDM system according to an embodiment of the present invention.

FIG. 4 is an operation flow diagram of an OFDM system according to an embodiment of the present invention.

[Figure 5] Operation flow diagram of conventional OFDM system

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. It should be noted that the embodiments described here are not intended to limit the present invention, and the numbers described here are not limited, and those skilled in the art can appropriately change them as necessary.

[0014] When the user terminal moves, the channel environment always fluctuates, so the RMS value of the delay spread of the channel also changes.

[0015] Here, there are two channel environments, the first channel environment and the second channel environment. The maximum RMS value of the delay spread in the first channel environment is τ, and the maximum delay spread in the second channel environment is Assume that the RMS value is τ (where τ <<τ). When designing a CP based on τ, the CP length is 1, as shown in Fig. 1A. Meanwhile, τ

When designing a CP based on 1 1 max, the CP length is 1 as shown in Figure 1B. That is, 1

1

> 1. In this way, the CP length is adaptively adjusted according to changes in the channel environment, and the first

2

Data is transmitted when the CP length is changed to 1 in the second channel environment.

1 2

Subcarriers can be increased by 1 minute. Therefore, in the present invention, resources are wasted.

1 2

To prevent usage, the number of subcarriers that transmit data (number of data subcarriers) In other words, the OFDM symbol length is increased by the CP length reduction to improve the data transmission rate.

[0016] Specifically, the present invention proposes an OFDM system as shown in FIG. FIG. 2 is a block diagram of an OFDM system according to an embodiment of the present invention.

[0017] At the base station (transmission side), an IDFT (Inverse Discrete Fourier Transform) unit 201 inputs the input baseband data stream to a plurality of subcarriers and performs IDFT to generate an OFDM symbol. .

CP insertion section 202 inserts a CP as a GI between all OFDM symbols.

Transmission RF section 203 modulates the OFDM symbol after CP insertion into a radio frequency band and transmits it from antenna 204.

In the user terminal (reception side), the reception RF unit 207 down-converts the signal received via the antenna 206 into a baseband signal.

[0021] CP removing section 208 removes baseband signal power CP.

[0022] A DFT (Discrete Fourier Transform) unit 209 performs DFT on the OFDM symbol after CP removal, and demodulates the data stream of each subcarrier.

Channel quality estimation section 210 estimates the channel delay spread RMS value, user terminal transmission rate, and Doppler frequency shift as channel quality using the pilot signal of the received signal, The estimated information is output to the control unit 211 as control information.

Control unit 211 outputs this control information to CP removal unit 208 and also transmits it to control unit 205 on the base station side via a feedback channel. Further, the control unit 211 controls processing of the CP removal unit 208 and the DFT unit 209.

[0025] In the base station (transmission side), the control unit 205 adjusts the CP length according to the control information.

The number of data subcarriers, that is, the OFDM symbol length is adjusted.

[0026] It should be noted that the base station (transmitting side) can use an IFFT (Inverse Fast Fourier Transfer m) unit instead of the IDFT unit, and the user terminal (receiving side) can replace the DFT unit with an FFT (Fast t Fourier Transform) part can be used.

[0027] In the present invention, first, the CP length is set using the RMS value of the delay spread estimated in the stationary multipath channel environment. Below, each parameter of OFDM system We will explain the high spectral efficiency and OFDM system!

[0028] First, the following parameters are defined. Channel bandwidth is B, number of system subcarriers (total number of subcarriers) is N, number of modulation subcarriers (number of data subcarriers) of IFFT is N (mod

Because there is a possibility of zero filling when performing IFFT calculation, N ≤ N)

Number of carriers (number of CP subcarriers) is N, modulation constellation stage is C, subcarrier

CP

The period is T. Therefore, the OFDM symbol period (CP length + OFDM symbol length) is T

= NT, subcarrier frequency interval is Δ ί = 1 / T, CP length is Τ — Τ Τ. This c s CP CP

Equation (1) shows the spectral efficiency of the OFDM system. Equation (1) shows the transmission rate of one OFDM symbol block (CP + OFDM symbol).

[Number 1]

, lg, ₍₁₎

'N + N _CP BT

[0029] When bandwidth α = Ν Δ ί 導入 is introduced, the spectral efficiency can be expressed by equation (2).

[Equation 2]

"= \ + Ν. _Ρ / Ν ^{ΛΘ 2} _t ,

^CP (2)

log ₂ C

+ T _CP / T _S

[0030] Furthermore, if ζ = Τ 定義, equation (2) becomes equation (3).

CP s

[Equation 3]

[0031] When the bandwidth efficiency factor ex is fixed, ζ is made as small as possible in order to obtain the highest spectral efficiency. Therefore, when the user terminal moves to a channel with a small delay spread even with a large delay spread, the CP length is adaptively reduced. At the same time, the OFD M symbol period is constant and does not change! If the CP length decreases, the OFDM symbol length Increasing ζ decreases ζ and improves the spectral efficiency of the OFDM system. Then, assuming that Τ = (2-4) · τ, the OFDM symbol period can be expressed by Equation (4).

P max

Picture

T _s = 2 ~ 4 t / ζ (4)

[0032] Usually, ζ≤1Ζ4 is set to reduce the overhead of CP. For example, in the E802.11a standard, the CP length is 800 ns and the OFDM symbol period is 4 / z s, 1Z5, that is, 1/5.

[0033] The parameter setting in the stationary multipath channel environment has been described above.

[0034] Next, parameter adjustment in a multipath channel environment in a variable state will be described. In this case, it is necessary to consider Inter Carrier Interference (ICI) due to carrier frequency shift ε due to movement of user terminals. The upper limit of the ICI efficiency is the expression (£ »no (Hyunsoo Cheon, Daesik Hong, Effect of channel estimation erro r in OFDM-based WLAN", IEEE Communications Letters, vol.6, no.5, May 2002 p. (See 190-192.) From equation (5), the smaller the carrier frequency deviation ε, the lower the ICI.

[Number 5] Now (5)

[0035] As shown in Equation (6), when the maximum Doppler frequency shift is f, the maximum carrier frequency d

Deviation is ε = f

d ΖΔί c. Therefore, Τ = ε

s Zf d is obtained.

[Equation 6] e = f _d T _s (6)

[0036] The ε force, the OFDM performance is good, so the upper limit of T is T = ε / ί. S s s d

That is, it can be expressed by equation (7).

[Equation 7]

T _s ≤s / f _d (7) [0037] Based on Eqs. (4) and (7), the OFDM symbol period in the variable multipath channel environment satisfies the condition shown in Eq. (8).

[Equation 8]

(2-4) τ / _≤ (8)

[0038] As described above, by setting the OFDM symbol length in consideration of both channel parameters and system parameters, an OFDM system with high spectral efficiency and strong robustness can be realized.

[0039] FIGS. 3A and 3B are diagrams showing configurations of OFDM symbols in the OFDM system shown in FIG. As shown in FIGS. 3A and 3B, in the present invention, the OFDM symbol period (that is, the total number of subcarriers) is constant. The CP length (that is, the number of CP subcarriers) and the OFDM symbol length (that is, the data subcarriers). The number varies with the line quality.

FIG. 4 is a flowchart showing the operation of the OFDM system according to one embodiment of the present invention.

[0041] First, in step S401, the channel quality estimation unit 210 of the user terminal (receiving side) uses a pilot sequence transmitted from the base station (transmitting side) based on the prior art, for example, Osvaldo Simeone , Yeheskel Bar-Ness, Umberto Spagnolini, Pilot-based chan nel estimation for OFDM systems by tracking the delay- subspace ", IEEE Trans activations on Wireless Communications, vol.3, no.l, Jan 2004 pp.315—325, Also proposed in i¾ Pilot ton e selection for channel estimation in a mobile OFDM system “Negi, R .; shi loffi, J. Consumer Electronics, IEEE Transactions on Volume 44, Issue 3, Aug 1998 Page (s): 1122 -1128 In this way, the RMS value of the maximum delay spread of the multipath channel is estimated.

In step S 402, user terminal control section 211 and base station control section 205 set the CP length within a range of 2 to 4 times the RMS value of the maximum delay spread.

[0043] In step S403, the lower limit (equation (4)) of the OFDM symbol period is determined based on the ratio between the control unit 205 force CP length of the base station and the OFDM symbol period.

At the same time, in step S404, the channel quality estimation unit 210 of the user terminal (receiving side) Estimate the maximum Doppler frequency shift of the channel using a no- til sequence where the ground station (transmitter) force is also transmitted.

[0045] In step S405, the control unit 213 of the base station determines the upper limit of the OFDM symbol period (Equation (7)) based on the ratio of the maximum carrier frequency shift according to the system and the maximum Doppler frequency shift of the channel. ) Is confirmed.

[0046] In step S406, the control unit 213 of the base station determines the OFDM symbol period based on the upper and lower limits of the OFDM symbol period.

[0047] In step S407, the total number of subcarriers, the number of CP subcarriers, and the number of data subcarriers are calculated based on the control unit 213 power channel bandwidth, OFDM symbol period, and bandwidth efficiency coefficient of the base station.

[0048] Finally, in step S408, user terminal control section 211 and base station control section 20

5 outputs the control information of each parameter to the corresponding module and adjusts the parameter.

[0049] As described above, in the present invention, it is the RMS value of the maximum delay spread and the maximum Doppler frequency shift that are estimated as the channel quality. In the present invention, the number of CP subcarriers and the number of data subcarriers are adjusted based on the channel quality indicated by these parameters. Conventionally, the CP length (that is, the number of CP subcarriers) is set only based on the RMS value of the maximum delay spread of the channel in the stationary state, while the OFDM symbol length (that is, the number of data subcarriers) is set. However, in the present invention, the OFDM symbol length (that is, the number of data subcarriers) is also adjusted based on the channel quality.

[0050] Hereinafter, specific embodiments to which the OFDM system according to the present invention is applied will be described.

[0051] RMS value of maximum delay spread of channel is τ = 200 ns, operating carrier band is f = 5G max c

Hz, user terminal moving speed is v = 30mZs, maximum Doppler frequency shift is f = v /

d λ = ν · ί / c = 30 X 5 X 10 9/3 X 10 8 = 500Hz, channel bandwidth Β = 15ΜΗζ der is, shall use the QPSK modulation. Choosing ζ = 0.2, bandwidth efficiency factor α = 0.8, frequency shift ε ≤ 0.005 gives CP length Τ = 4 = 800 ns,

CP max

Bol period range 4 τ / ζ ≤ Ύ ≤ ε / ί = 800ns / 0. 2 ≤ Τ ≤ 0.005 / 500 = 4 s≤T≤ s is obtained. When T = 4μ s is selected, the OFDM symbol length is 4— 0

.8 = 3.2 ^ 5, modulation subcarrier number (data subcarrier number) is N = αΤΒ = 0.8Χ4

mod s

X10- ^⁶ X15X10 ⁶ = 48, the total number of sub-carrier becomes N = NZ α = 48 / 0.8 = 60.

mod

In order to make FFT easier, if 4 null subcarriers (data not transmitted !, subcarrier) are inserted and N = 64 is selected, the number of CP subcarriers becomes Equation (9) and takes 12. . Therefore, the spectral efficiency of the system is given by equation (10).

[Equation 9]

[Equation 10]

1-48-log ₂ 2

64 + 12 15xl0 ⁶ x4xl0- ^b / 64 0.67 bps / Hz (10) When the user terminal moves to a different environment, the RMS value of the maximum delay spread of the channel is τ

ma

= 50 ns, the CPCP length is T = 4 τ = 200 η χ CP max s, and the OFDM symbol period range is 4 τ / ζ = 200 ns / 0.2 ≤ T = 1 μ s ≤ T.

max s s

If T = 4 / z s is selected as in the above environment, the OFDM symbol length is 4 0.2 = 3.8

s

s. Therefore, the OFDM symbol length is increased by 0.6 μs compared to the above environment. Therefore, since this increased time can be used to increase the number of data carriers, a larger band efficiency coefficient α = 0.95 is selected. Thus, the modulation sub-carrier number (Detasa subcarriers number) is ^{^{Ν = αΤΒ = 0.95Χ4Χ10- 6 Χ15Χ10 6}} = 57, the total number of sub-carrier

mod s

Is Ν = Ν Ζα = 57Z0.95 = 60. 4 null mods to make FFT easier to implement

If subcarriers are inserted and N = 64 is selected, the number of CP subcarriers is given by equation (11), which is 4. Therefore, the spectral efficiency of the system is expressed by equation (12).

[Equation 11]

[Equation 12] ^{^{η ^ - ^ 7 - ° g22}} , 0.89 bps / Hz (1 2)

^{^{64 + 4 15xl0 6 x4xl0 _6 /}} 64

[0053] In this way, by adjusting the CP length and OFDM symbol length adaptively according to the channel environment, the spectral efficiency of the system is improved by 0.89—0.67 = 0.22bpsZHz, so the data transmission rate is 0.22X15 X106 = In addition to improving 3.3 Mbps, it can also meet the requirements to prevent ISI.

[0054] While typical embodiments have been described above, various changes, substitutions, and additions can be made without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited to the above-described embodiments, but is limited by the scope of the claims and the scope equivalent thereto.

[0055] This specification is based on Chinese Application No. 200510088291.5 filed on August 2, 2005. The contents are all included here.

Industrial applicability

The OFDM communication apparatus of the present invention is suitable for a communication system that requires high transmission efficiency in an environment in which channel quality changes, particularly a mobile communication system.

Claims

The scope of the claims

[1] generation means for generating OFDM symbols;

An insertion means for inserting a cyclic prefix between OFDM symbols;

A control means for controlling the cyclic prefix length and the OFDM symbol length according to the channel quality;

An OFDM communication apparatus comprising:

[2] The control means includes

When the cyclic prefix length is decreased, the OFDM symbol length is increased by the decrease,

The OFDM communication apparatus according to claim 1.

[3] The control means includes

Set cyclic prefix length and OFDM symbol length based on RMS value of maximum delay spread of channel and maximum Doppler frequency shift.

The OFDM communication apparatus according to claim 2.

[4] The control means includes

Set the upper limit of the OFDM symbol period based on the ratio between the maximum carrier frequency deviation and the maximum Doppler frequency shift.

The OFDM communication apparatus according to claim 2.

[5] The control means includes

The cyclic prefix length is set based on the RMS value of the maximum delay spread of the channel, and the lower limit of the OF DM symbol period is set based on the ratio between the set cyclic 'prefix length and the OFDM symbol period.

The OFDM communication apparatus according to claim 2.

[6] The control means includes

Set the cyclic prefix length to at least 2 to 4 times the RMS value of the maximum delay spread of the channel.

The OFDM communication apparatus according to claim 5.

[7] generating an OFDM symbol; Inserting a cyclic prefix between OFDM symbols;

A step for controlling the cyclic prefix length and the OFDM symbol length according to the channel quality;

An OFDM communication method comprising: