Adaptive array antenna and interference canceler in a mobile communications system

[0002] 1. Field of the Invention

[0003] The present invention relates to mobile communications systems and, in particular, to a device and method for eliminating an interference signal in a mobile communication system.

[0004] 2. Description of the Related Art

[0005] Recently, due to the rapidly growing demand for mobile communication, much attention has been directed to Code Division Multiple Access (CDMA) as a means for performing such communication. There are two types of CDMA systems. The first type is referred to as Direct Sequence CDMA or DS-CDMA, which is an application of spread spectrum technology. The second type is referred to as Frequency Hopping CDMA or FH-CDMA. In Korea, a digital DS-CDMA communication system has been developed and is currently in service. Additionally, DS-CDMA is currently being used in Korea for Personal Communications Services (PCS) systems.

[0006] In the above DS-CDMA system, diminished performance is primarily caused by multipath fading and multi-user interference (MUI), which, in turn, are caused by multipath propagation and the simultaneous use of the same channel by multiple users, respectively. Thus, stated alternatively, the performance of a DS-CDMA system can be improved by mitigating the effects of multipath fading and MUI.

[0007] Multi-user interference is generally reduced by the following two techniques: using an adaptive array antenna; and co-channel interference cancellation based on correlation. The use of an adaptive array antenna allows for the effective removal of an interference signal by appropriately controlling the antenna's directivity. However, this technique is limited in eliminating an undesired signal received at the same angle as that of a desired signal. On the other hand, the co-channel interference cancellation technique generally assures the removal of MUI, thereby improving system performance. However, mitigation of co-channel interference requires a PN (Pseudo Noise) code which exhibits excellent correlation characteristics, and an interference canceller. System hardware complexity increases with the number of users that simultaneously access the system because undesired spread spectrum signals from all users are demodulated, re-spread, and then extracted from a received signal in order to recover a desired signal using a digital filter.

[0008] To solve the problems of the above techniques, there is a need for a receiver having an adaptive array antenna and an interference canceller which are serially connected. An adaptive antenna and an interference canceller were used in the prior art as a way to increase DS-CDMA system capacity. However, such use was not employed in a CDMA environment where they should be serially connected, but rather, such use occurred in a general spread spectrum communication environment. In this context, it is difficult to make a direct application of the conventional devices to a CDMA system.

[0009] An object of the present invention is to provide a receiver and a receiving method in a radio communications system, which can increase reception performance by use of an adaptive array antenna and an interference canceller which are serially connected.

[0010] According to one aspect of the invention, a receiver in a mobile communications system is provided. The receiver includes: an adaptive array antenna having a plurality of antenna elements for forming a beam in the direction of a desired signal, each of the plurality of elements having an adaptive linear filter associated therewith; a real time adaptive receive processor for controlling beam patterns and directions of the plurality of antenna elements at a reception angle of the desired user; and an interference canceller operatively connected to said adaptive array antenna for detecting a multi-user interference signal from the desired signal and cancelling the multi-user signal.

[0011] According to another aspect of the invention, a receiving method in a mobile communications system is provided. The method includes the steps of: controlling beam patterns and directions of an adaptive array antenna to coincide with a reception direction of a desired signal; forming a beam in the reception direction of the desired signal; and eliminating an interference signal received at a different angle than the reception angle of the desired signal.

[0012] The above objects and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which:

[0013] FIG. 1 is a block diagram of a receiver according to an embodiment of the present invention in a radio communications system;

[0014] FIG. 2 is a block diagram of an adaptive array antenna shown in FIG. 1;

[0015] FIG. 3 is a block diagram of an interference canceller shown in FIG. 1;

[0016] FIG. 4 is a graph showing the error probability of a Direct Sequence Code Division Multiple Access Differential Phase-Shift-Keying (DS-CDMA/DPSK) cellular system employing an interference canceller according to an embodiment of the invention;

[0017] FIG. 5 is a graph showing the error probability of a DS-CDMA/DPSK cellular system employing an adaptive array antenna according to an embodiment of the is invention;

[0018] FIG. 6 is a graph showing the error probability of a DS-CDMA/DPSK cellular system employing both an adaptive array antenna and an interference canceller according to an embodiment of the invention; and

[0019] FIG. 7 is a graph showing the error probability of a DS-CDMA/DPSK cellular system employing both an adaptive array antenna and an interference canceller according to another embodiment of the invention.

[0020] FIG. 1 is a block diagram of an RF (Radio Frequency) transmitter and receiver in a radio communications system according to an embodiment of the present invention. An RF transmitter 10 includes: a modulator 11 for modulating transmit data TXD in the form of a transmission signal; and a mixer 12 for up-converting the frequency of the modulated signal received from modulator 11 into a desired transmission frequency. An RF receiver 20 includes: an adaptive array antenna 21 for adaptively receiving an RF signal; an interference canceller 22 for cancelling co-channel interference using correlation of the received RF signal; and a demodulator 23 for demodulating the signal received from interference canceller 22 to the original data and for generating receive data RXD. An adder 30 introduces an MUI signal which deteriorates the received RF signal and AWGN (Additive White Gaussian Noise). The RF receiver 20 of FIG. 1 is assumed to be ideal.

[0021] A description of the operation of receiver 20 according to an embodiment of the present invention will now be described. The adaptive array antenna 21, which is serially connected to interference canceller 22, eliminates an interference signal received at a different angle than that of a desired signal. The interference canceller 22 eliminates the remaining interference signal and also eliminates an interference signal received at the same angle as that of the desired signal, thereby more effectively removing MUI. Now, the extent to which the performance of a DS-CDMA/DPSK (Differential Phase-Shift-Keying) cellular system can be improved by the use of adaptive array antenna 21 and interference canceller 22 will be described hereinbelow.

[0022] In the embodiment of the present invention, it is assumed that data is transmitted on a reverse link in a mobile radio communications system and a channel model is a radio channel plagued by AWGN and MUI. As shown in FIG. 1, receiver 20 utilizes adaptive array antenna 21 and interference canceller 22 in a serially connected configuration in order to increase the reception performance of the radio communications system.

[0023] If power control is perfectly implemented in the mobile communications system between a base station and its associated mobile stations, all the mobile stations in a cell send signals of the same power level to the base station. Thus, an interference signal has the same power level and path loss as those of a reference signal. Let MUI from a different user be a Gaussian random parameter. Then, the degree to which an interference signal adds to MUI can be expressed as $\begin{array}{cc}\mathrm{MI}=\frac{2}{3N}{E}_b{a}^2& \left(1\right)\end{array}$

[0024] where the N is PN (Pseudo Noise) code length and a is the instantaneous amplitude of a transmission signal. The number of users that can be simultaneously served in a cell is calculated by $\begin{array}{cc}{K}_c=\frac{V_{m}K}{N_{\mathrm{secl}}}-1& \left(2\right)\end{array}$

[0025] where N_seclis the number of sectors in a cell, V_mis a voice activity factor given as ⅜, and K is the total number of users per cell.

[0026] Thus, interference I₁in the same cell can be expressed as $\begin{array}{cc}{I}_1=K_c\frac{2}{3N}{E}_{\mathrm{bn}}·a^2& \left(3\right)\end{array}$

[0027] and interference I₂from a mobile station in an adjacent cell can be expressed as $\begin{array}{cc}{I}_2=\left(K_c+1\right)\sum _{K=1}^C\frac{2}{3N}{E}_b·a_k^2& \left(4\right)\end{array}$

[0028] where C is the number of adjacent interfering cells and a_kis the instantaneous amplitude of a signal transmitted from a kth mobile station.

[0029] Assume that the MUI I₂from an adjacent cell is 2.2 times larger than the Rappaport interference (for an article discussing Rappaport interference see “Analytical Results for Capacity Improvements in CDMA”, by J. C. Liverti Jr. and T. S. Rappaport, appearing in IEEE Transactions on Vehicular Technology, Vol. No.3, August 1994, pp. 680-90). Then, a signal to noise ratio (SNR) involving MUI from an adjacent cell in the DS-CDMA system becomes $\begin{array}{cc}\begin{array}{c}\mathrm{SNR}=\frac{E_b·a^2}{N_1+N_0/2}=\frac{E_b·a^2}{I_1+I_2+N_0/2}\\ =\frac{E_{b·}{a}^2}{\frac{2K_c}{3N}{E}_b·a^2+\frac{2\left(K_c+1\right)}{3N}{E}_b·a^2·2.2+N_0/2}\end{array}& \left(5\right)\end{array}$

[0030] where N₀/2 is the power spectrum density of the AWGN. MUI can be modeled as described above.

[0031] The adaptive array antenna technique will now be explained. The adaptive array antenna technique is: defined as the detection of the direction of a signal by flexibly varying the direction of a beam according to the variable direction of the received signal, as compared to a conventional antenna technique wherein the antenna receives a signal in a fixed state. The adaptive array technique provides more effective reception of a desired signal and allows for the removal of an interference signal received from a different direction than that of the desired signal. As such, system performance is significantly improved. The adaptive array antenna 21 is composed of antenna array elements and an adaptive receive processor for processing a signal in real time. The adaptive processor, using a predetermined algorithm, automatically controls a weighting value which optimizes the SNR, so that the array elements form a beam in the direction of the desired signal. Also, the processor, by null-steering, reduces the reception gain of an interference signal received from a different direction than that of the desired signal to about −60 dB in order to remove the interference signal. However, a problem with the adaptive array antenna technique is its incapability in eliminating an interference signal received at the same angle as that of the desired signal.

[0032] FIG. 2 is block diagram of the adaptive array antenna 21 of FIG. 1 according to an illustrative embodiment of the present invention. Antenna 21 has M array elements. Each mobile station uses an omni-directional antenna and a base station forms a directional beam for each user in a cell. It is assumed that a beam pattern is provided in the direction of a desired user. This directional beam forming is implemented by the M array elements. With K users in a cell, each of the array elements has K adaptive linear filters (ALFs). Hence, MxK ALFs are provided to adaptive array antenna 21. If the ALFs are digitally implemented, a different ALF parameter can be used for each user. Therefore, a different beam pattern is provided to each user. In the illustrative embodiment of the invention, users are distributed uniformly in a cell and power control is perfectly performed between a base station and its associated mobile stations. In addition, a beam pattern is directed not vertically, but rather, horizontally at 360° so as to be provided in the direction of a desired user. Here, the regulated beam pattern G(θ) of the antenna is $\begin{array}{cc}G\left(\theta \right)\left[\mathrm{dB}\right]=10{\log }_{10}\frac{{\left|A\left(\theta \right)\right|}^2}{M^2}& \left(6\right)\end{array}$

[0033] where A(θ) is a beam pattern and M is the number of array elements. The directivity D of the antenna is. $\begin{array}{cc}D=\frac{2\pi }{{\int }_0^{2\pi }G\left(\theta \right)d\theta }& \left(7\right)\end{array}$

[0034] In the illustrative embodiment of the invention, the Rappaport interference in the cases of D=2.67 and D=3.0 are considered.

[0035] The adaptive array antenna 21 effectively receives a desired signal and eliminates an interference signal received from a different direction than that of the desired signal. Yet, it cannot eliminate an interference signal received from the same direction as that of the desired signal. To overcome this limitation of adaptive array antenna 21, interference canceller 22 is employed.

[0036] FIG. 3 is a block diagram of interference canceller 22 shown in FIG. 1. If the amplitude of a baseband signal is ±1 at an input terminal of interference canceller 22, the signal to noise ratio SNR_DS1of an input signal S_ris

[0037] SNR_DS1=1/[K_c·2/3N)+(N₀/E_b)]  (8)

[0038] If a bit error is not generated in signals S₂, S₃, . . . , S_k, which are included in signal S_r(r=2, 3, . . . , and k), then MUI is entirely eliminated. On the other hand, if an error is found during correlation detection in a signal from an ith user (I=2, 3, . . . , k), such signal being included in signal S_r, then a signal recovered by a PN code for the ith user PN_iwhich is different from the signal S_iis spread again by PN_iand added to the signal S_rdelayed by T. Here, since signal S_i(which includes an error) is added to input signal S_ras MUI, a signal S_chas twice the MUI voltage (four times the power) of signal S_r. As a result, the signal to noise ratio SNR_DS2between signals S_cand S_iis calculated by

[0039] SNR_DS2=1/[A_V·2/3N)+(N₀/2E_b)]SNR_DS2  (9)

[0040] =1/[(4K_c·Peb_DS1·2/3N)+(N₀·2E_b)]

[0041] where Peb_DS1is an error probability equation using SNR_DS1and A_vis 4K_cx Peb_DS1.

[0042] A description of the performance of a DS-CDMA/DPSK cellular system employing adaptive array antenna 21 and interference canceller 22 according to the invention will now be given.

[0043] In consideration of the MUI model, voice activity factor, and sectorization of an adjacent cell, an error probability equation for the DS-CDMA/DPSK system in an environment having AWGN and MUI is given by $P_{\mathrm{el}}=\frac{1}{2}\exp \left(-r_1\right)$

[0044] and $\begin{array}{cc}{r}_1=\frac{E_b·a^2}{\frac{2K_c}{3N}{E}_b·a^2+\frac{2\left(K_c+1\right)}{3N}{E}_b·a^2·2.2+N_0/2}& \left(10\right)\end{array}$

[0045] where K_cis the number of users causing MUI.

[0046] The adaptive array antenna 21 eliminates an interference signal received from a different direction than that of a desired signal by detecting the direction of the interference signal and providing a null to the interference signal. As a result, the performance of a received signal is increased. An error probability equation for a DS-CDMA/DPSK system employing adaptive array antenna 21, utilizing the directivity of the antenna, is $P_A=\frac{1}{2}\exp \left(-r_A\right)$

[0047] and $\begin{array}{cc}{\gamma }_A=\frac{E_b·a^2}{\frac{2K_c}{3\mathrm{ND}}{E}_b·a^2+\frac{2\left(K_c+1\right)}{3N}{E}_b·a^2·2.2+N_0/2}& \left(11\right)\end{array}$

[0048] Equation (11) is valid when power control is perfectly performed between a base station and its associated mobile stations, and when the beam pattern of an antenna in the base station is not directed vertically.

[0049] An error probability equation for a DS-CDMA/DPSK system employing interference canceller 22 in an environment having AWGN and MUI is calculated by $P_C=\frac{1}{2}\exp \left(-r_C\right)$

[0050] and $\begin{array}{cc}{r}_C=\frac{E_b·a^2}{4P_{\mathrm{el}}·\left[\frac{2K_c}{3N}{E}_b·a^2+\frac{2\left(K_c+1\right)}{3N}{E}_b·a^2·2.2\right]+N_0/2}& \left(12\right)\end{array}$

[0051] The adaptive array antenna 21, which is serially connected to interference canceller 22, forms a beam in the direction of a desired signal on the basis of a weighting value calculated in the adaptive receive processor which maximizes the reception gain of the desired signal. Antenna 21 also provides a null to an MUI signal received from a different direction than that of the desired signal in order to minimize the reception gain of the MUI signal. Thus, the MUI signal is eliminated. Here, the remaining MUI signal of antenna 21 and an MUI component received at the same angle as that of the desired signal are cancelled by relying on correlation of the received signal by interference canceller 22. An error probability equation for a DS-CDMA/DPSK system using the serially connected adaptive array antenna 21 and interference canceller 22 in an environment having AWGN and MUI is $P_{AC}=\frac{1}{2}\exp \left(-r_{AC}\right)$

[0052] and $\begin{array}{cc}{r}_{AC}=\frac{E_b·a^2}{4P_{\mathrm{el}}·\left[\frac{2K_c}{3\mathrm{ND}}{E}_b·a^2+\frac{2\left(K_c+1\right)}{3\mathrm{ND}}{E}_b·a^2·2.2\right]+N_0/2}& \left(13\right)\end{array}$

[0053] Equation (13) represents the case that the hardware of serially connected adaptive array antenna 21 and interference canceller 22 is ideally implemented.

[0054] In the following illustrative embodiments of the present invention, the performance improvements of a DS-CDMA/DPSK cellular system in the cases of implementing adaptive array antenna 21 and interference canceller 22 alone and in 15 serial connection are analyzed. FIGS. 4 to 7 illustrate numerical results from the error probability equations as functions of the number of users per cell K, PN code length N, the directivity D of adaptive array antenna 21, and the ratio of bit energy to noise power spectrun density E_b/N₀. In the Figures, the voice activity factor is ⅜, the cell is split into three sectors, and N is 127.

[0055] FIG. 4 illustrates the error probability of a DS-CDMA/DPSK cellular system implementing interference canceller 22 in an environment having AWGN and MUI. As shown, the error probability significantly improves when interference canceller 22 is used. In the case of E_b/N₀equal to 10 dB and K equal to 200, the error probability drops from 5.85×10⁻²to 5.40×10⁻⁴. In the case of E_b/N₀equal to 10 dB and K equal to 100, the error probability drops from 9.86×10⁻³to 1.66×10⁻⁸.

[0056] FIG. 5 illustrates the error probability with respect to D of a DS-CDMA/DPSK cellular system implementing adaptive array antenna 21 in an environment having to AWGN and MUI. In the case of E_b/N₀equal to 10 dB, K equal to 200, and D equal to 2.67, the error probability drops from 5.85×10⁻²to 3.83×10⁻³. In the case of E_b/N₀equal to 10 dB, K equal to 200, and D equal to 3.0, the error probability drops from 5.85×10⁻²to 2.46×10⁻³. Thus, as shown, system performance increases with directional gain. A comparison between the adaptive antenna technique and the interference cancellation technique reveals that the former increases system performance by less than the latter in both cases of D=2.67 and 3.0.

[0057] FIG. 6 is a graph showing the error probability of a DS-CDMA/DPSK cellular system implementing adaptive array antenna 21 serially connected to interference canceller 22 in an environment having AWGN and MUI, and with K=200. As shown, interference canceller 22 is superior to adaptive array antenna 21 in improving system performance when K=200 and D=2.67. On the other hand, adaptive array antenna 21 gains the advantage over interference canceller 22 in terms of system performance if K is increased (K>270) as shown in FIG. 7, because demodulation errors increase with K. The error probability with adaptive array antenna 21 serially connected to interference canceller 22 and with D=2.67 is proximate to the error probability in an environment having AWGN, and lower than the error probability when antenna 21 and canceller 22 are independently used.

[0058] FIG. 7 is a graph showing the error probability with respect to K of a DS-CDMA/DPSK cellular system implementing adaptive array antenna 21 serially connected to interference canceller 22 and with E_b/N₀=10 dB under the same conditions la as those associated with FIG. 6. In the case of K=200, the error probability with adaptive array antenna 21 and interference canceller 22 serially connected is lower than that when interference canceller 22 is used alone or when adaptive array antenna 21 is used alone, by about 4.74×10⁻⁴and 6.68×10⁻⁸, respectively. In conclusion, the use of adaptive array antenna 21 serially connected to interference canceller 22 is very effective in eliminating MUI.

[0059] As described above, there is a large difference between system performance depending on whether or not adaptive array antenna 21 and interference canceller 22 are serially connected or used independently. That is, except for the case where K is very large (i.e., K>270), interference canceller 22 is more effective than adaptive array antenna 21 in improving system performance, and the error probability with the serially connected adaptive array antenna 21 and interference canceller 21 is proximate to that in an environment having AWGN.

[0060] Since the performance of a mobile communications system is limited by MUI generated from the common use of the same channel by multiple users, the elimination of MUI is a major issue with respect to increasing system performance. Thus, according to the invention, a receiver in the mobile communications system employs: an adaptive array antenna for effectively receiving a desired signal and eliminating an interference signal received from a different direction than that of the desired signal; and an interference canceller for removing an interference signal received from the same direction as that of the desired signal. The use of the adaptive array antenna and interference canceller according to the invention significantly increase system performance. Due to its capability of reducing a bit error, the receiver according to the present invention is applicable to PCS systems as well as DS-CDMA systems in order to increase the performance of either such mobile communications system.

[0061] Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention.