United States Patent [19J
`Bar-Ness
`
`I lllll llllllll Ill lllll lllll lllll lllll lllll 111111111111111111111111111111111
`US006137785A
`[11] Patent Number:
`[45] Date of Patent:
`
`6,137,785
`Oct. 24, 2000
`
`[54] WIRELESS MOBILE STATION RECEIVER
`STRUCTURE WITH SMART ANTENNA
`
`[75]
`
`Inventor: Yeheskel Bar-Ness, Marlboro, N.J.
`
`[73] Assignee: New Jersey Institute of Technology,
`Newark, N.J.
`
`[21] Appl. No.: 09/042,948
`
`[22]
`
`Filed:
`
`Mar. 17, 1998
`
`[51]
`[52]
`[58]
`
`Int. Cl.7 ........................................................ H04J 3/14
`U.S. Cl. ............................. 370/328; 375/346; 455/63
`Field of Search ..................................... 375/346, 347,
`375/348, 349; 455/67.3, 67.1, 63, 524,
`525, 501, 517, 575, 132; 370/252, 241,
`334, 328, 342, 343, 345, 310
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`5,634,199
`5,659,584
`5,819,168
`5,905,721
`6,006,110
`
`5/1997 Gerlach et al. ........................... 455/63
`8/1997 Uesugi et al.
`.......................... 375/347
`10/1998 Golden et al. .......................... 455/303
`5/1999 Liu et al. ................................ 370/342
`12/1999 Raleigh ................................... 455/561
`
`Primary Examiner-Huy D. Vu
`Attorney, Agent, or Firm-Woodbridge & Associates, P.C.;
`Richard C. Woodbridge; Stuart H. Nissim
`
`[57]
`
`ABSTRACT
`
`This invention relates to a system and method utilizing a
`receiver architecture with a set of at least two antennae
`followed by a Rake demodulator at a mobile station for
`interference cancellation and diversity combining. Such a
`structure can work well only when the channel vector of
`desired signal is correctly estimated. The present invention
`makes use of the identifying spreading codes (as in IS-95 for
`example) to provide an adaptive channel vector estimate, to
`thereby cancel cochannel interference and improve the sys(cid:173)
`tem capacity.
`
`5,471,647 11/1995 Gerlach et al. ........................... 455/63
`
`11 Claims, 2 Drawing Sheets
`
`Ericsson v. IV II LLC
`Ex. 1024 / Page 1 of 6
`
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`
`Ex. 1024 / Page 2 of 6
`
`

`

`U.S. Patent
`
`Oct. 24, 2000
`
`Sheet 2 of 2
`
`6,137,785
`
`FIG. 3
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`0
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`
`ITERATION NUMBER: SYMBOL INTERVAL (Tsl
`
`FIG. 4
`
`.......... ONE ANTENNA
`0 - - 0 TWO ANTENNAS I USING MRC -----:---- ________ ; _________ _
`-
`SMART ANTENNA
`.
`.
`
`_2
`
`10
`
`PROBABILLITY OF
`BIT ERROR
`
`10-3
`
`0
`
`10
`
`20
`
`30
`
`40
`
`50
`
`60
`
`NUMBER OF USERS PER CALL
`
`Ex. 1024 / Page 3 of 6
`
`

`

`6,137,785
`
`1
`WIRELESS MOBILE STATION RECEIVER
`STRUCTURE WITH SMART ANTENNA
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`This invention relates to a system and method of inter(cid:173)
`ference cancellation for use with respect to a mobile station
`having a smart antenna and a Rake demodulator.
`2. Description of Related Art
`The capacity of wireless Code Division Multiple Access
`(CDMA) systems in the forward link direction (i.e., from a
`base station to a mobile receiver) is limited by both intra-cell
`and inter-cell cochannel interferences. In particular, when
`the mobile unit is close to a cell boundary, the desired signal 15
`is disturbed by relatively strong interference from neighbor(cid:173)
`ing base stations. Antenna arrays have been previously
`suggested for base stations of CDMA systems to improve
`the capacity in the reverse link through space diversity and
`interference cancellation. Less attention is given to the 20
`forward link due to the reliance on orthogonal spreading
`codes to handle the cochannel interference. The perfor(cid:173)
`mance in the forward link is limited, however, by multipath
`fading and inter-cell interference.
`In IS-95 CDMA, signals from the same base station and 25
`same path are separated by a set of orthogonal codes (Walsh
`codes), which eliminate the interference of other users'
`signals in the same signal path from the home cell (see, for
`example, J. D. Gibson, The Mobile Communications
`Handbook, Boca Raton, Fla., CRC Press, Inc., 1996 and T. 30
`S. Rappaport, Wireless Communications: Principles and
`Practice, Upper Saddle River, N.J., Prentice Hall PTR,
`1996). The other signal paths from a home base station,
`however, create self-interference. In addition, signals from
`different base stations are identified by a special short 35
`pseudo random code. Such base stations share the same
`short code, but with different shifts, and hence due to
`nonzero autocorrelation there exists inter-cell interference.
`The worst case occurs at the cell boundary point, where the
`desired signal is the weakest and the inter-cell interference 40
`is the strongest.
`Similar to its use in the reverse link, an antenna array at
`the mobile station can be used as a diversity combiner to
`maximize the signal-to-interference plus noise ratio (SINR).
`Due to packaging and cost considerations, however, such an 45
`array needs to be small. A dual antenna mobile station for
`wireless communications has been suggested and its imple(cid:173)
`mentation was studied in a paper by M. Lefevre, M. A
`Jensen, and M. D. Rice, ("Indoor measurements of handset
`dual-antenna diversity performance," in IEEE 4 7th Vehicular
`Technology Conference Proceedings, (Phoenix, Ariz.), pp.
`1763-1767, May 1997).
`The preferred embodiment of the present invention relates
`to a receiver with a two-element array, referred to as a smart
`antenna receiver. Adaptive arrays are employed to utilize the
`known direction of arrival and signal waveform structure of
`desired signal for interference cancellation in point-to-point
`communication (see, for example, S. P. Applebaum and D.
`J. Chapman, "Adaptive Arrays With Main Beam
`Constrains," IEEE Trans. Antennas Propagat., vol. 24, pp.
`650--662, September 1976 and J. R. T. Compton, "An
`Adaptive Array in Speed-Spectrum Communications,"
`Proc. IEEE, vol. 66, pp. 289-298, March 1978), wherein by
`using the pointing vector, the desired signal is co-phased and
`removed, prior to the application of weighting for interfer(cid:173)
`ence cancellation. To reduce sensitivity to pointing vector
`error in a point-to-point communication application, a self-
`
`2
`correcting loop was suggested to minimize the error by Y.
`Bar-Ness and F. Haber ("Self-Correcting Interference Can(cid:173)
`celling Processor for Point-to-Point Communications," in
`Proceedings of the 24th Midwest Symposium on Circuit and
`5 Systems, (Albuquerque, N.Mex.), pp. 663-665, June 1981).
`In multi-user wireless and other similar communication
`applications, such a pointing vector is not well defined, and
`hence cannot be used easily or accurately used by a mobile
`receiver for interference cancellation.
`
`10
`
`SUMMARY OF THE INVENTION
`
`Briefly described the invention comprises a novel receiver
`structure for a mobile station that simultaneously estimates
`the desired signal channel vector and adaptively controls the
`weights, thereby increasing the SINR at array output. Adap(cid:173)
`tive control continually corrects the channel vector making
`it a more meaningful parameter than the prior art's pointing
`vector in modeling the received signal at mobile stations of
`cellular CDMA systems.
`In a typical IS-95 CDMA mobile station receiver, for
`example, multiple paths are weighted and combined by a
`Rake demodulator to combat small-scale fading (ref. Gibson
`supra and A J. Viterbi, CDMA Principles of Spread Spec(cid:173)
`trum Communication, Reading, Mass.: Addison-Wesley
`Publishing Company, 1995). In the current invention, the use
`of a small antenna structure along with Rake demodulator
`provides higher SINR at the output for symbol detection,
`and improves capacity in the forward link.
`These and other features of the invention will be more
`fully understood by reference to the following drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 illustrates the smart antenna structure for BPSK
`demodulation at a mobile station according to the preferred
`embodiment of the present invention.
`FIG. 2 illustrates a Rake demodulator, according to the
`preferred embodiment of the present invention, wherein the
`demodulator has L parallel Rake fingers for the jth user.
`FIG. 3 is a graph of the SINR output versus iteration
`number for three different antenna systems.
`FIG. 4 is a graph of the probability of bit error versus the
`number of users per cell for three different antenna systems.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`During the course of this description, like numbers will be
`50 used to identify like elements according to different figures
`which illustrate the invention.
`This invention (10) relates to the receiver structure and
`the signal model used in the analysis of the proposed
`receiver scheme. The disclosed smart antenna receiver sys-
`55 tern of the present invention results in a dramatic capacity
`improvement over the receivers of the above disclosed prior
`art.
`Several assumptions are made in the development of the
`signal model. According to the preferred embodiment, the
`60 receiver consists of a small antenna array with two elements
`at a mobile station of a wireless cellular CDMA system. The
`disclosure assumes that there are one home base station
`(n=O) and N neighboring base stations, wherein each n (n=O,
`1, ... , N) base station serves Jn active users. The signal from
`65 each base station is composed of Jn users' information
`waveforms and one pilot waveform. There are L resolvable
`paths for each signal. The signal from different paths and
`
`Ex. 1024 / Page 4 of 6
`
`

`

`6,137,785
`
`3
`different base stations are assumed to independently undergo
`Rayleigh fading, while the Jn+l waveforms that arrive from
`the same path and the same base station at a given mobile
`receiver propagate over the same fading characteristics.
`According to the above model, the complex envelope of 5
`received signal at two antenna elements of the mobile station
`is given by a 2xl vector r(t):
`
`4
`signal at the first element. These two control signals can be
`expressed as
`
`Np
`Np
`h1(k)= ~ z(k,np)g'(k,np) and h2 (k)= ~ r 1(k,np)g'(k,np),
`np=l
`np=l
`
`L
`
`N
`
`r(t) =LL Cnt(t)
`
`n=O
`
`l=l
`
`where NP is the number of code chips per bit, and the
`superscript "*" denotes complex conjugate operation. In
`10 FIG. 2 items labeled 25 denote accumulators. The weight s1
`is then updated by complex weight controller (34) as:
`
`{t,,/Pnu ·dnj(l-TntJUnj(l-TntJ+~Pnp ·Unp(t-Tnt)}+v(t)
`
`15
`
`20
`
`where cnr(t) represents complex channel vectors, dn/t) are
`transmitted information bits, p nu and p np are, respectively,
`the received powers of the users' signals and pilot signals,
`un/t) and unit) are the spreading codes, v(t) is AWGN, i:n1
`are delays, and n=o refers to desired home base station. The
`following assumptions are adopted in the analysis:
`communication performance is examined for user 1 in cell
`0, for which the channel vector can be written as: 25
`c0'=[ c011 ,c012J" (t~l, .
`, L), where the superscript
`.
`.
`denotes transpose.
`slow fading is assumed for signals from all paths and all
`base stations.
`FIG. 1 shows the dual antenna receiver at mobile station. 30
`After down conversation (12) and matched filtering (14)
`(matched to the transmitting pulse), the signals received at
`two antennas are demodulated by L parallel demodulators
`(16) (Rake fingers). The output of L Rake fingers are
`combined for a symbol detection. In FIG. 2, all the param- 35
`eters which are referred to are used in ith Rake finger, hence
`the path index is ignored. When the weight s1 =c011/c012, the
`desired signal is co-phase combined at y b, and blocked at z.
`That is, the z signal output of antenna hybrid (18) consists
`of only interference as no desired signal is present. In this 40
`case, the weight s2 is used to estimate interference, which is
`subtracted from yb, and higher SINR can be obtained at the
`array output y and demodulated output brG). The weight s2
`can be updated, for example, by Direct Matrix Inversion
`(DMI) (20).
`When s 1 ;'C011/c 012 ,
`the channel vector estimate is
`erroneous, and s1 will not result in a null difference between
`the desired signals received at points x and r1 . Consequently,
`the residual desired signal contributions at z will be inter(cid:173)
`preted by the array as interference, and hence cancelled. This 50
`results in performance degradation of the canceller
`(reference J. R. T. Compton, "Pointing accuracy and
`dynamic range in steered beam array," IEEE Trans. Aero(cid:173)
`space and Electronic Systems., vol. 16, pp. 280-287, May
`1980). To overcome this effect, the preferred embodiment of 55
`the present invention uses the spreading code of the desired
`signal. As shown in the preferred embodiment depicting in
`FIG. 2, the processor at the mobile station, using the
`spreading code parameters and the correlator 22, despreads
`the array output y(t) (marked y) to yield y c This resulting 60
`signal, y c' is then accumulated over one symbol interval and
`that result, g0 is then respread by correlator 24 using the
`same respreading code to get a reference signal g. When the
`kth symbol of desired signal is received, the control signal
`hr(k) is generated by accumulating the multiplication of g(k, 65
`t) and z(k, t) over one symbol interval, and hik) is obtained
`similarly from the reference signal g(k, t) and the received
`
`It can be shown that when a pure reference signal is
`available at g, this algorithm gives an estimate of the channel
`vector error between two antenna elements and the weight
`s1=c011/c012 . Since y has a higher SINR than the array input,
`the matched filter (30) (matched to the channel attenuation
`and phase delay) estimated from y is more accurate than the
`one estimated from the array input r1 . The array output y is
`then despread using correlator 32 using the pilot and jth
`users' sequences to generate the Ith Rake finger's output
`brG).
`Based on these assumptions and analysis, simulation
`results were obtained in light of the following additional
`assumptions. The signal employed the same short code as in
`IS-95. It was also assumed that 20% of total transmitted
`power from each base station is used for pilot. Three paths
`for each signal are present, the relative delay between paths
`from same base stations is two chips. In each of the Rake
`fingers, there are one desired signal path from home base
`station, two interfering paths from home base station (self(cid:173)
`interference ), and three interfering paths from each of neigh-
`boring base stations. For comparison, three receiver models
`were examined:
`1. One antenna followed by Rake demodulator
`2. Two antennas with maximum ratio combining (MRS)
`followed by Rake demodulator
`3. The smart antenna of the preferred embodiment, fol(cid:173)
`lowed by the Rake demodulator
`In the data depicted in FIG. 3, 20 active users per cell is
`45 assumed, with the curves obtained from 1000 Monte Carlo
`runs. For receivers 1 and 2, the curves are also the average
`output SINR over bits 1 to 20. For a receiver with smart
`antenna, from bit 1 to 5, the initial beam steering weight
`s1 =1, the output SINR is averaged from bit 1 to 5. Starting
`from bit 6, the smart antenna uses the algorithm of the
`preferred embodiment to control the weight s1 , and the
`output SINR shown in FIG. 3 is averaged over bits 6 to 20.
`The curves show that after the weight (s1) correction starts,
`the receiver with the smart antenna of the preferred embodi(cid:173)
`ment achieved 1.5 dB and 3.5 dB higher output SINR
`compared to receivers 1 and 2, respectively.
`To see the capacity improvement due to proposed receiver
`of the preferred embodiment, FIG. 4 gives the curves of
`probability of bit error, Pe' versus number of users per cell.
`For performance requirement Pe=l0- 3
`, the system capacity
`is 24, 37 and 50 users per cell for receivers 1, 2 and 3,
`respectively. With the smart antenna of the preferred
`embodiment at the mobile station, the system capacity
`increases 108% and 35% compared to receivers 1 and 2,
`respectively. The proposed receiver structure of the pre(cid:173)
`ferred embodiment, therefore, can provide improved capac-
`ity over conventional receivers 1 and 2.
`
`Ex. 1024 / Page 5 of 6
`
`

`

`6,137,785
`
`5
`While the invention has been described with reference to
`the above preferred embodiment thereof, it will be appreci(cid:173)
`ated by those of ordinary skill in the art that various
`modifications can be made to the structure and function of
`the individual parts of the system without departing from the 5
`sprit and scope of the invention as a whole. In particular, the
`receiver model can be easily extended to the case of QPSK
`demodulation (rather than the BPSK demodulation depicted
`in FIGS. 1 and 2) as well as to the cases of Time Domain
`Multiple Access (TDMA) and Frequency Domain Multiple 10
`Access (FDMA) and to usage of polarization instead of
`spatial information in implementing the interference cancel(cid:173)
`lation.
`What is claimed is:
`1. An apparatus for reducing neighboring base station 15
`interference in a wireless mobile station which receives a
`signal transmitted from a home base station, said apparatus
`comprising:
`(a) at least two antennas;
`(b) a means for correction that continually corrects a 20
`channel vector model of the received signal by utilizing
`an antenna hybrid device and a pilot signal received
`from the home base station in order to process the
`output signal of each antenna;
`(c) an interference cancellation means for using the cor-
`rected channel vector model to adaptively control can(cid:173)
`cellation weights;
`(d) a despreading means for despreading a signal obtained
`from the output of the antenna hybrid device;
`( e) an accumulator means for accumulating the output of
`the despreading means over one symbol interval; and,
`(f) a respreading means to respread the accumulator
`results to yield a reference signal.
`2. The apparatus of claim 1 wherein said correction means 35
`further comprises a control signal generating means for
`generating a control signal using said reference signal.
`
`30
`
`25
`
`6
`3. The apparatus of claim 2 wherein said cancellation
`means comprises a complex weight controller using said
`control signal to update said cancellation weights.
`4. The apparatus of claim 3 wherein said correction means
`comprises a means for utilizing a spread code transmitted in
`a IS-95 CDMA signaling system.
`5. The apparatus of claim 3 wherein the received signal is
`a BPSK signal.
`6. The apparatus of claim 3 wherein the received signal is
`a QPSK signal.
`7. The apparatus of claim 3 wherein the received signal is
`a Time Domain Multiple Access (TDMA) signal.
`8. The apparatus of claim 3 wherein the received signal is
`a Frequency Domain Multiple Access (FDMA) signal.
`9. The apparatus of claim 3 wherein the interference
`cancellation means comprises a means for utilizing spatial
`information.
`10. The apparatus of claim 3 wherein the interference
`cancellation means comprises a means for utilizing polar(cid:173)
`ization information.
`11. An apparatus for reducing neighboring base station
`interference in a wireless mobile station receiving a signal
`transmitted from a home base station, said apparatus com(cid:173)
`prising:
`(a) at least two antennas;
`(b) means for using a pilot signal received from the home
`base station and an antenna hybrid device to continu(cid:173)
`ally correct a channel vector model of the received
`signal; and,
`(c) an interference cancellation means for using the cor(cid:173)
`rected channel vector model to adaptively control can(cid:173)
`cellation weights.
`
`* * * * *
`
`Ex. 1024 / Page 6 of 6
`
`