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AN APPARAUS AND METHOD FOR RECEIVING SIGNALS

IN MULTI-CARRIER MULTIPLE ACCESS SYSTEMS

Field of the Invention

The present invention relates to an apparatus and method for receiving signals

in multi-carrier multiple access systems, and more particularly, to an apparatus and

method for estimating wireless channels between a mobile station and base stations

during handover (or handoff) and combining signals using the estimations.

Discussion of the Related Art

hi multi-carrier multiple access cellular mobile communications systems, a

mobile station in downlink handover gets a macro diversity gain. In other words, the

mobile station in handover receives the same data from two or more nearby base

stations in the form of a combined signal through different communication channels.

There are two methods of getting the macro diversity gain, namely, Equal Gain

Combining (EGC) and Maximal Ratio Combining (MRC). The EGC method does not

compensate amplitude distortions, but only compensates phase distortions. Since the

amplitude gain is always 1, the channel estimation is performed by using only phase

distortion compensation. However, the MRC method has better performance than the

EGC method in most channel environments because the MRC method performs a

channel estimation by compensating both amplitude and phase distortions. In general,

if maintained synchronization between cells transmitting the same data during

handover, it is possible to effectively decode receiving signals using the EGC method

without additional signal processing. In that case, formats of the receiving signals such

as transmission type, pattern, location, etc. should be the same to get the macro

diversity gain. Even though the MRC method is a preferable method to increase the

combined gain, current multi-carrier multiple access systems have a problem for

increasing the macroscopic diversity gain using the MRC method because current

multi-carrier multiple access systems are difficult to perform channel estimations by

each communication link between each base station and the mobile station when the

same frequency band(s) or sub-carrier(s) in neighboring cells as those of a current

serving cell are assigned at the same time interval.

Therefore, it is highly desired to develop a technology which provides base

station differentiating codes to compensate channel distortions and to maximize macro

diversity gain.

SUMMARY OF THE INVENTION

"Accordingly, the present invention is directed to an apparatus and method form

receiving signals in multi-carrier multiple access systems that substantially obviate one

or more problems due to limitations and disadvantages of the related art.

An object of the present invention is to provide an apparatus and

method for a mobile station during handover to effectively combine and decode signals.

Another object of the present invention is to provide base station differentiating

codes to compensate channel distortions and to maximize macro diversity gain.

Additional advantages, objects, and features of the invention will be set forth in

part in the description which follows and in part will become apparent to those having

ordinary skill in the art upon examination of the following or may be learned from

practice of the invention. The objectives and other advantages of the invention may be

realized and attained by the structure particularly pointed out in the written description

and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with the

purpose of the invention, as embodied and broadly described herein, a method for

receiving signals in a multi-carrier multiple access system comprising receiving signals

from at least one base station wherein each signal has a base station identifier,

processing the signals to identify each base station by using the base station identifier,

performing a channel estimation using the processed signals, combining the processed

signals using an information obtained from the channel estimation and decoding the

combined signals.

hi another aspect of the present invention, an apparatus for

receiving signals in a multi-carrier multiple access system comprises a serial-to-

parallel converter for converting a serial signal including a base station identifier from

at least one base station to parallel signals, a Fast Fourier transformer for transforming

the parallel signals, a parallel-to-serial converter for converting the parallel signals to a

serial signal, a despreading processor for despreading frequency domain signals using

the base station identifier from at least one base station, a channel estimator for

compensating a channel variation between the at least one base station and a mobile

station, a signal combiner for combining signals from base stations and a decoder for

decoding the combined signal.

It is to be understood that both the foregoing general description and the

following detailed description of the present invention are exemplary and explanatory

and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further

understanding of the invention and are incorporated in and constitute a part of this

application, illustrate embodiment(s) of the invention and together with the description

serve to explain the principle of the invention. In the drawings;

FIG. 1 illustrates a method of receiving data in a mobile station during

handover;

FIG. 2 illustrates another method of receiving data in a mobile station during

handover;

FIG. 3 illustrates a code generation method to distinguish base stations during

handover; and

FIG. 4 illustrates a block diagram showing a receiver according to the present

invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of the

present invention, examples of which are illustrated in the accompanying drawings.

Wherever possible, the same reference numbers will be used throughout the

drawings to refer to the same or like parts.

An explanation is given, as an example, for the Orthogonal Frequency Division

Multiple Access (OFDMA) which is one of multi-carrier multiple access systems. The

OFDMA is a system in which a plurality of users performs multiple accesses using

OFDM.

FIG. 1 shows a method of receiving data in a mobile station during handover.

The mobile station during handover receives data from K number of base

stations in the same frame format. That is, the data received by the mobile station have

the same frame format since the data received in the mixed form are not

distinguishable by each base station.

Moreover, because channel estimations can not be done by each

base station, the received data are decoded using the EGC method. To obtain the macro

diversity gain using the EGC method, all base stations should be synchronized, thereby

the time- delays from each base station are within the guard interval. Otherwise, the

length of the guard interval should be extended. The guard interval can be generated by

repeating a part of the data to prevent an aliasing effect.

As shown in FIG. 1, however, it is likely that some, if not all, of base stations

are not synchronized. Also, since additional elements are required to maintain the

synchronization, it is desired to have a technology which could get the macro diversity

gain when not all base station is synchronized.

FIG. 2 shows another method of receiving data in a mobile station during

handover. In this figure, each data from each base station has a code to distinguish base

stations. In other words, each data has an orthogonal code or pseudo-orthogonal code

to distinguish each base station during soft handover in multi-carrier multiple access

wireless mobile communications systems. The orthogonal code includes Pseudo Noise

(PN) code, Orthogonal Variable Spreading Factor (OVSF) code and Walsh code. The

pseudo-orthogonal code includes Gold code. Especially, the OVSF code has better

performance when base stations are synchronized. Also, the codes have a fixed

Spreading Factor (SF) which can be easily implemented though a variable SF is also

possible to use.

FIG. 3 shows a code generation method to distinguish base stations during

handover. After determining a code and a SF to distinguish base stations, the

transmitted symbol is repeated by the SF until matched with the length of spreading

code. A frequency-time spreading code is assigned to each base station, hi the

frequency-time spreading codes, the unit of frequency domain is a subcarrier and the

unit of time domain is =a symbol duration.

The mobile station during handover receives data from K number of base

stations in the same frame format. That is, the data received by the mobile station have

the same frame format since the data received in the mixed form are not

distinguishable by each base station.

FIG. 3 shows when the SF is 8. In this case, the symbol is repeated 4 times in

the time domain and 2 times in frequency domain. The frequency-time spreading code

(C = {CI, ...,C8} ) is multiplied before sending to the mobile station.

The frequency-time SF can be expressed as:

SF = SFtime * SFfi-eq (Equation 1)

where SF is total spreading factor, SF _t i _me is a spreading factor in time domain

and SF _f ieq is a spreading factor in frequency domain.. When SF _t i _me = 1, data is spread

only over the frequency domain whereas when SF _f r _eq = 1, data is spread only over the

time domain. In general, if SFtime >1 and SFfreq >1, data is spread over both the time

and frequency domains. Therefore, the SF can be a two dimensional factor.

FIG. 3 is an example of Equation 1 when SF _t i _me - 2 and SF _f i _eq = 4. To generate

a frequency-time code of SF = 8, a various combination of SF _t i _me and SF _f r _eq can be

achieved. When the SF is set, SF _t i _me and SF _f r _eq can be varied according to the channel

condition.

If the data received at the mobile station can be distinguished by each base

station, the received data does not need to have the same frame format/structure,

transmission type, pattern, location, etc. hi other words, though all the base stations do

not have the same frame fβrmat, the mobile station can separate the data by each base

station.

Moreover, the mobile station performs channel estimations according to the

received data and decodes the data using the receiver employing the MRC method.

That is, the mobile station performs the channel estimations using a pilot signal from

each base station, compensates channel distortions by each channel link from each

base station to the mobile station and combines the received data using the MRC

method. Therefore, higher macro diversity gain is achieved when compared to non-

combining methods such as the EGC method.

According to the present invention, though it is not necessary to exactly

synchronize the data from base stations, it is recommended that a maximum delay

among data from base stations to the mobile station is within a guard interval. In other

words,, if the maximum delay is larger than the guard interval, it is highly likely that

OFDM symbols after performing FFT are severely destroyed.

FIG. 4 shows a receiver (400) during a handover which combines data from K

base stations and decodes the data. The mobile station during handover receives data

from K base stations.

The data from each base station can be expressed as:

^r i - ^u i ^i ^u i e (Equation 2)

where r,- is the received signal from ith base station, di is the transmitted data of

ith base station, si is a base station differentiating code assigned to the base station and

the channel coefficient!! _! = ^a ^ ^{' e '} has the ith channel gain (or amplitude) α j and

the ith channel phase shift θ i between ith base station and the mobile station. The

length of si is determined by the SF and the code si is designed to simultaneously

spread in the time domain and frequency domain according to the SF. Normally, 1-

dimensional spreading such as the time-domain spreading or the frequency-domain

spreading is preferred. However, 2-dimensional spreading such as the time-domain

and frequency-domain spreading is also possible. The main purpose of si is to

differentiate base stations.

The received signal r with some delays can be expressed as:

K i=i (Equation 3)

The S/P converter (41) converts the received signal r to parallel signals every N

sample, thereafter the parallel signals are Fourier transformed to signals on a frequency

axis by FFT (42). Moreover, the FFT converted signals are again converted to serial

signals by the P/S converter (43).

The serial signal from the P/S converter (43) can be expressed as:

K

R = ^H ₁ S ₁ D ₁ (Equation 4)

Z=I where D _t , Si and H ₂ - are Fourier transforms of dj, s _t and hi, respectively.

The despreading processor (44) identifies a signal from each base station using

the base station identifiers. After the dispreading processor (44), the signal can be

expressed as;

R ₁ = H ₁ (S ₁ ¹ S ₁ ) D ₁ = H ₁ D ₁ , v (S ₁ ^* S ₁ ) = 1 (Equation 5)

where i (= 1,..-.JKJAs the index of base stations. The above equation is

simplified for illustration purpose only and usually more complex.

The channel estimator (45) performs channel estimations by using the

differentiated signals. The channel estimator can be any type including a Minimum

Mean Square Error (MMSE) estimator, a Constrained Least Squares (CLS) estimator

or a Maximum Likelihood (ML) estimator. The channel estimator (45) calculates the

conjugates of the channel coefficients, that is, Hi ^* which can be obtained for all K base

stations or a specific base station. The channel estimator (45) also provides all the

necessary values (e.g., amplitude, phase, etc.) to perform the MRC method.

The signal combiner (46) performs the MRC method using the estimated

channel information, thereby compensating amplitude distortion in addition to the

phase distortion, and combines signals from base stations to maximize Signal to Noise

Interference Ratio (SNIR). The signal combiner (46) also can perform the MRC

method to maximize (or minimize) other ratios or parameters. In this way, soft

combining can be achieved. Moreover, a person having ordinary skill in the art also

can implement a hard combining of selecting only the best channel or several channels

using the present invention.

The combined signal after the signal combiner (46) can be expressed as:

R'= ∑H ^* H _t D ₁ = ∑D _t (Equation 6)

1=1 I=I

Finally, the decoder (47) decodes the combined signal.

It will be apparent to those skilled in the art that various modifications and

variations can be made in the ^{^} present invention without departing from the spirit or

scope of the inventions. Thus, it is intended that the present invention covers the

modifications and variations of this invention provided they come within the scope of

the appended claims and their equivalents.