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Reduction of cochannel interference in WCDMA cellular systems
Computers and Electrical Engineering 31 (2005) 422–430 www.elsevier.com/locate/compeleceng
Technical Communication
Reduction of cochannel interference in WCDMA cellular systems M.A. Salam
*
Department of Computer Science, Southern University, P.O. Box 9221, Baton Rouge, LA 70813, United States Received 27 April 2005; received in revised form 10 May 2005; accepted 14 May 2005 Available online 2 November 2005
Abstract In this paper, a novel cochannel interference reduction technique is proposed for wideband code division multiple access (WCDMA) cellular systems. Cochannel interference for the proposed cellular architecture is considered and analyzed. An analytic expression for the proposed method is derived. Simulation results demonstrate that the proposed method provides better signal-to-noise (S/N) ratio than the existing cochannel interference reduction methods. A significant reduction of cochannel interference is achieved compared to sectoring and omnidirectional architectures in the proposed microzoning architecture. In particular, it is shown here that the proposed architecture exhibits a larger number of users per cell while maintaining an adequate S/N ratio in comparison with other architectures. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Spread spectrum; Wideband code division multiple access (WCDMA); Cochannel interference; Microzoning; Omnidirectional antenna
1. Introduction Wideband code division multiple access schemes play a crucial role in the third-generation (3G) cellular systems. WCDMA is a digital cellular wireless technology that uses spread spectrum *
Tel.: +1 225 771 2060; fax: +1 225 771 4223. E-mail address: [email protected]
0045-7906/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compeleceng.2005.05.002
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techniques to scatter a digital radio signal across a wide range of frequencies [1,2]. Code division multiple access (CDMA) appears to be the most popular civilian applications that uses spread spectrum communications. In a code division multiple access system, users share time and frequency resources simultaneously. This occurs through assigning each user a distinct digital code. This code is added to the information data and modulated onto the carrier, using spread spectrum techniques. Since each user has a uniquely addressable code, privacy is inherent. WCDMA provides high data rate transmission over wireless and mobile channels [6]. In this paper, the effect of cochannel interference of WCDMA wireless systems, that utilizes microzoning architecture is examined and compared to sectoring and omnidirectional architectures. The proposed system can be generalized to other cellular CDMA systems.
2. Interferences in cellular radio systems Interference is the major limiting factor in the performance of wireless communication systems. In cellular radio systems, sources of interference include mobile units in the same cell, a call in progress in a neighboring cell, other base stations operating in the same frequency band, or any noncellular system which inadvertently leaks energy into the cellular frequency band [5]. Cochannel interference (CCI) arises from cellular frequency reuse and thus limits the quality and capacity (number of users) of wireless networks. CCI can be reduced by the use of microzoning or sectoring architectures. The smart antenna systems that utilize an array of antenna elements are also used to reduce the cochannel interference [9,10]. In this paper, the research is focused on the reduction of cochannel interference by utilizing the proposed cellular microzoning architecture.
3. Cochannel interference Frequency division multiple access and time division multiple access cellular systems rely on spatial attenuation to control intercell interference. As a result, neighboring cells need to be assigned different frequencies to protect against cochannel interference. In contrast, CDMA cellular system can apply a universal one-cell frequency reuse pattern [7]. Frequency reuse implies that in a given coverage of area there are several cells that use the same set of frequencies. These cells are called cochannel cells and the cochannel interference refers to the interference caused between two cells transmitting on the same frequency within a network. Since cochannel interference is caused by another cell transmitting the same frequency, we cannot simply filter out the interference by increasing the carrier power of the transmitter. This is because an increase in carrier transmit power increases the interference to neighboring cochannel cells. To reduce cochannel interference, cochannel cells need to be physically separated by a minimum distance to provide sufficient isolation due to propagation [5]. We can minimize the cochannel interference through the proposed architecture designed for cellular network. A cellular network must be designed to maximize the signal-to-interference (S/I) ratio. Here the S/I ratio is the signal-to-cochannel interference ratio. One of the ways to maximize the S/I ratio is to increase the frequency re-use distance, i.e. increase the distance between cells using the same set of transmission frequencies. The S/I ratio in part
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determines the frequency re-use distance of a cellular network. The generalized expression for the S/N ratio of either the forward or reverse link of a CDMA system can be expressed by [3]: " 1 1 #1 1 S Eb S S þ þ ð1Þ ¼ N I in-cell I CCI N0 where Eb/N0 is the S/N ratio due to additive white Gaussian noise (AWGN), N0 is the noise power, Eb = P0Tb is the average bit energy, Tb is bit duration, and P0 is the average transmitted power from the reference base station to the desired user in the reference cell for the forward link and is the average transmitted power from the reference mobile to the base station in the reference cell for the reverse link. For a WCDMA system utilizing asynchronous pseudo-noise (PN) codes for each user, the multi-user intra-cell interference term is represented by [3]: 1 K0 S 2 X Pk ¼ ð2Þ I in-cell 3N k¼1 P 0 where N is the system processing gain, K0 is the number of users in the reference cell, Pk is the average transmitted power from the reference base station to the kth user in the reference cell as received by the reference user for the forward link and is the average transmitted power from the kth user in the reference base station as received by the reference base station for the reverse link. 4. Proposed architecture for WCDMA cellular systems In wideband code division multiple access, the cochannel interferences from all surrounding cells is allowed in order to maximize efficiency but must be controlled. A variety of optimization techniques are used to mitigate the cochannel effects. Strategies for managing cochannel interference are complex but have been carefully developed within IS-95 deployments. The following strategies are deployed to manage the cochannel interference: antenna orientation and location in a cell, cell power adjustments, and the orientation of cell-specific information downloaded to mobile systems. In the following sections, we discuss various WCDMA cellular architectures with various antenna orientations and antenna location in a microzone. Section 4.1 described the proposed cellular network architecture. 4.1. Seven-microzone per cell architecture There are various cellular network architectures to reduce the cochannel interference [3,4]. The proposed cellular architecture is shown in Fig. 1. In the proposed architecture, each cell is represented by a circle and each hexagon represents a microzone. Microzoning is a term used to describe a cellular system where the cells have been divided into smaller zones. In the proposed architecture, a cell consists of seven microzones. The constellation shown in Fig. 1 has seven-cells and 49 hexagonal microzones. Since one-cell per cluster is considered, each cell is representing a cluster. The triangular unit in the reference center cell represents the location of mobile unit, where the cochannel interference is worst. Each microzone has a 60° directional transmitter which is represented by a semi-circle. We consider that antennas are mounted in a single tower for a particular location of the adjacent microzones. Side lobe canceller can be used to reduce the
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Cell C Cell D
C1
D1 Cell B
B1
Cell E O
x θ=120
E1
60
y
A1
R
Cell F
2.59R
Cell A
F1 Location of Mobile Unit
Fig. 1. Proposed seven-microzones per cell cellular architecture.
jamming interference. Each microzone transmitter is located at the outer edge of each cell except the center microzone for that cell. The center microzone transmitter radiates outward with respect to the reference center cell. The outer cell microzones transmitters radiate back towards the reference center cell microzone where the mobile unit is located. Out of 42 outer cells microzones only A1, B1, C1, D1, E1, and F1 microzones are causing interference to the mobile unit. The dotted lines represent the distance from the mobile unit to the interfering microzones. The distances between transmitters and mobile unit can be calculated based on the geometrical law of cosines. For example, the distance (oy) between the mobile unit located at the center cell at o and the transmitter in microzone (E1) at y can be computed as: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffi oy ¼ ðxyÞ2 þ ðoxÞ2 2ðxyÞðoxÞ cos h ¼ ð3RÞ2 þ ð5RÞ2 2ð3RÞð5RÞ cos 120 ¼ 49R Here R is the radius of a microzone. Multi-cell per cluster architecture reduces capacity as compared to one-cell per cluster architecture and is not being seriously considered for 3G wireless wideband CDMA systems. Consequently, in the proposed method, one-cell per cluster architecture is considered. Here we consider only the worst case scenario of the mobile unit. The interferences in other locations for the mobile unit can be computed in a similar way. For WCDMA systems with carrier stealing, the resulting first-tier cochannel interference at the location of the mobile unit can be obtained by the following equation [3]: 1 pffiffiffiffiffi nA1 pffiffiffiffiffi nB1 pffiffiffiffiffi nC1 S 2ð2:59RÞn0 h K A1 49R ¼ þ K B1 49R þ K C1 49R I CCI 3N pffiffiffiffiffi nD1 pffiffiffiffiffi nE1 pffiffiffiffiffi nF1 i þ K D1 49R þ K E1 49R þ K F1 49R ð3Þ
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where S is the desired signal power from the desired base station and I is the interference power. The subscripts KA1, KB1, KC1, KD1, KE1, and KF1 represent the number of users in the interfering microzones of the neighboring cells, as shown in Fig. 1. The respective propagation path loss exponents have the same subscript, i.e., the propagation path loss exponent for the signal transmitted from microzone A1 is nA1. The propagation path loss exponent for the reference microzone is n0. R is the radius of the microzone, 2.59R is the radius of a cell, and N is the processing gain.
5. Existing cellular WCDMA architectures 5.1. Microzoning Three microzones per cell cellular network architecture has been proposed by Mayer et al. [3]. MayerÕs microzoning architecture is shown in Fig. 2. Transmitters are located at the outer edge of each cell. The mobile unit is represented by the triangular unit in the reference center cell. Each microzone has a 120° directional transmitter which is represented by a semi-circle. The dotted lines represent the distance from the mobile unit to the interfering microzones. Only A1, A2, B1, C1, C2, D1, E1, E2, and F1 microzones are causing interference to the mobile unit. For WCDMA systems with carrier stealing, the first-tier cochannel interference for the microzoning architecture is obtained by [3]: 1 n pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffi S 2ð2RÞ 0 h ¼ K A1 ð 19RÞnA1 þ K A2 ð 19RÞnA2 þ K B1 ð5RÞnB1 þ K C1 ð 19RÞnC1 I CCI 3N i pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffi þ K C2 ð 19RÞnC2 þ K D1 ð5RÞnD1 þ K E1 ð 19RÞnE1 þ K E2 ð 19RÞnE2 þ K F1 ð5RÞnF1 ð4Þ
Cell D
D1
C2
Cell C
C1
E1
B1
E2
Cell B
Cell E
A2 Cell F
F1
A1
Cell A
Location of Mobile Unit
Fig. 2. MayerÕs three-microzones per cell architecture.
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5.2. Sectoring Sectoring is also used to reduce the cochannel interference. Here, the reference cell is the center hexagonal cell and it is surrounded by other six hexagonal cochannel cells. In this paper, one-cell per cluster WCDMA systems are considered. In this consideration, the center reference cell experiences interference from each of the cochannel cells. 5.2.1. Sixty degree sectoring In a 60° sectoring scheme, as shown in Fig. 3, it is assumed that only one-sixth of the total number of users per cell can be activated in a sector at one time. For WCDMA systems, the first-tier cochannel interference signal-to-interference ratio at the worst case location on the cell boundary is found by [3]: 1 i pffiffiffi pffiffiffi S 2Rn0 h ¼ K A ð 7RÞnA þ K B ð 7RÞnB þ K C ð2RÞnC þ K D ðRÞnD þ K E ðRÞnE þ K F ð2RÞnF I CCI 18N ð5Þ where KA through KF are the number of users in each of the six first-tier cochannel cells, nA through nF are the respective propagation path loss exponents, second-tier cochannel cells are assumed to have negligible effect. Antennas are placed at the center of each cell. All the neighboring cells (A, B, C, D, E, and F) are causing interference to the mobile unit.
C D B
E
A F
Location of Mobile Unit
Fig. 3. Sixty-degree sectoring architecture.
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5.2.2. One hundred and twenty degree sectoring In a 120° sectoring scheme, it is assumed that only one-third of the total number of users per cell can be activated in a sector at one time. For WCDMA systems, the first-tier cochannel interference signal-to-interference ratio at the worst case location on the cell boundary is found by the following [3]: 1 i pffiffiffi pffiffiffi S 2Rn0 h ¼ K A ð2RÞnA þ K B ð 7RÞnB þ K C ð 7RÞnC þ K D ð2RÞnD þ K E ðRÞnE þ K F ðRÞnF I CCI 9N ð6Þ where KA through KF is the number of users in each of the six first-tier cochannel cells. In 120° sectoring scheme, the cellular network architecture is similar to that of 60°, the only difference is that instead of using 60° directional antenna 120° directional antennas are used. 5.3. Omnidirectional In this architecture, an omnidirectional antenna is placed at the center of each cell. The first-tier cochannel interference signal-to-interference ratio for the mobile unit on its cell boundary is given by [3]: 1 i pffiffiffi n pffiffiffi n S 2Rn0 h n n n n K A ðRÞ A þ K B ð2RÞ B þ K C ð 7RÞ C þ K D ð 7RÞ D þ K E ð2RÞ E þ K F ðRÞ F ¼ I CCI 3N ð7Þ
6. Simulation results The simulation results shown in Fig. 4 are obtained for the forward link cochannel interference based on Eqs. (1)–(7). A comparison of the proposed cellular architecture with MayerÕs microzoning, 60° sectoring, 120° sectoring, and omnidirectional antenna architectures is shown in Fig. 4. The simulation is computed for a WCDMA system with a processing gain of 128, propagation path loss exponents of 5, and 24 users per cell. The proposed cellular architecture provides better gain in S/N ratio than other methods that reduce cochannel interference. For example, in Fig. 4, at Eb/N0 = 16 dB, the improvement in gain in S/N ratio over the microzoning, 60° sectoring, 120° sectoring, and omnidirectional antenna is approximately 0.33, 2.20, 3.69, and 7.04 dBs, respectively. Also at Eb/N0 = 25 dB, the improvement in S/N ratio compared to microzoning, 60° sectoring, 120° sectoring, and omnidirectional antenna is approximately 0.50, 3.04, 4.86, and 8.60 dBs, respectively. One of the most important issues in the WCDMA scheme is the number of admissible users per cell for a given available total bandwidth, for given radio propagation conditions, and for a required transmission quality [8,11–13]. The simulation results shown in Fig. 5 are obtained for the forward link cochannel interference based on Eqs. (1)–(7). In Fig. 5, the number of users per cell is plotted against S/N for different architectures where in each case the processing gain is 128, the propagation path loss exponents are taken to be 5, and Eb/N0 = 25 dB. As can be seen,
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16 14
S/N (dB)
12 10 8 6 4
Proposed Mayer 60-degree 120-degree Omnidirectional
2 0 10
12
14
16
18
20
22
24
Eb/No (dB)
Fig. 4. Comparison of CDMA architectures with a processing gain of 128, 24 users per cell, and propagation path loss exponents of 5. 35
30
S/N(dB)
25
20
15
10 Proposed Mayer 60-degree 120-degree Omnidirectional
5
0 5
15
25
35
45
55
Number of Users/Cell
Fig. 5. Comparison of CDMA architectures with a processing gain of 128 and propagation path loss exponents of 5.
the S/N ratio associated with the omnidirectional systems quickly falls below acceptable level. The proposed microzing, with the highest S/N ratio of all the architectures, accommodates the maximum number of users while maintaining an adequate S/N ratio.
7. Conclusions A comparative study has been provided for different cellular architectures. The simulation results were presented for the WCDMA cellular systems. The proposed method out-performed the existing microzoning, sectoring, and omnidirectional methods and achieved a better S/N ratio as compared to other methods. Further work will involve additional cellular architectures based on microzing and sectoring schemes that will further improve the overall signal-to-noise ratio.
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References [1] Pickholtz RL, Schilling DN, Milstein LB. Theory of spread spectrum communications—a tutorial. IEEE Trans Commun 1982;COM-30(5):855–84. [2] Pickholtz RL, Milstein LB, Schilling DN. Spread spectrum for mobile communications. IEEE Trans Veh Technol 1991;40(2):313–22. [3] Mayer T, Robertson C, Ha TT. Co-channel interference reduction on the forward channel of a wideband CDMA cellular system. In: Proc IEEE Military Commun Conf, 1999. [4] Salam MA, Al-Khatib MM, Alam MS. Reduction of cochannel interference on the forward link CDMA systems. In: Proc IEEE Radio Wireless Conf, 19–22 September 2004. p. 503–6. [5] Rappaport T. Wireless communications: principles and practice. Upper Saddle River, NJ: Prentice Hall; 2002. p. 57–104. [6] Milstein LB. Wideband code division multiple access. IEEE J Select Areas Commun 2000;18(8). [7] Kohno R, Meiden R, Milstein LB. Spread spectrum access methods for wireless communications. IEEE Commun Mag, January 1995. [8] Jung P, Baier PW, Steil A. Advantages of CDMA and spread spectrum techniques over FDMA and TDMA in cellular mobile radio applications. IEEE Trans Veh Technol 1993;42(3):357–64. [9] Hasna MO, Alouini MS, Bastami A, Ebbani ES. Performance analysis of cellular mobile systems with successive co-channel interference cancellation. IEEE Trans Wireless Commun 2003;2(1):29–40. [10] Tidestav C, Sternad M, Ahlen A. Reuse within a cell-interference rejection or multiuser detection. IEEE Trans Commun 1999;47(10):1511–22. [11] Choi W, Kim JY. Forward-link capacity of a DS/CDMA system with mixed multirate sources. IEEE Trans Veh Technol 2001;50(3):737–49. [12] Naguib AF, Paulraj A, Kailath T. Capacity improvement with base-station antenna arrays in cellular CDMA. IEEE Trans Veh Technol 1994;43(3):691–8. [13] Furukawa H. Downlink performance enhancements in CDMA cellular systems. In: Wang J, Ng TS, editors. Advances in 3G enchanced technologies for wireless communications. Artech House, Inc.; 2002.
Technical Communication
Reduction of cochannel interference in WCDMA cellular systems M.A. Salam
*
Department of Computer Science, Southern University, P.O. Box 9221, Baton Rouge, LA 70813, United States Received 27 April 2005; received in revised form 10 May 2005; accepted 14 May 2005 Available online 2 November 2005
Abstract In this paper, a novel cochannel interference reduction technique is proposed for wideband code division multiple access (WCDMA) cellular systems. Cochannel interference for the proposed cellular architecture is considered and analyzed. An analytic expression for the proposed method is derived. Simulation results demonstrate that the proposed method provides better signal-to-noise (S/N) ratio than the existing cochannel interference reduction methods. A significant reduction of cochannel interference is achieved compared to sectoring and omnidirectional architectures in the proposed microzoning architecture. In particular, it is shown here that the proposed architecture exhibits a larger number of users per cell while maintaining an adequate S/N ratio in comparison with other architectures. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Spread spectrum; Wideband code division multiple access (WCDMA); Cochannel interference; Microzoning; Omnidirectional antenna
1. Introduction Wideband code division multiple access schemes play a crucial role in the third-generation (3G) cellular systems. WCDMA is a digital cellular wireless technology that uses spread spectrum *
Tel.: +1 225 771 2060; fax: +1 225 771 4223. E-mail address: [email protected]
0045-7906/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compeleceng.2005.05.002
M.A. Salam / Computers and Electrical Engineering 31 (2005) 422–430
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techniques to scatter a digital radio signal across a wide range of frequencies [1,2]. Code division multiple access (CDMA) appears to be the most popular civilian applications that uses spread spectrum communications. In a code division multiple access system, users share time and frequency resources simultaneously. This occurs through assigning each user a distinct digital code. This code is added to the information data and modulated onto the carrier, using spread spectrum techniques. Since each user has a uniquely addressable code, privacy is inherent. WCDMA provides high data rate transmission over wireless and mobile channels [6]. In this paper, the effect of cochannel interference of WCDMA wireless systems, that utilizes microzoning architecture is examined and compared to sectoring and omnidirectional architectures. The proposed system can be generalized to other cellular CDMA systems.
2. Interferences in cellular radio systems Interference is the major limiting factor in the performance of wireless communication systems. In cellular radio systems, sources of interference include mobile units in the same cell, a call in progress in a neighboring cell, other base stations operating in the same frequency band, or any noncellular system which inadvertently leaks energy into the cellular frequency band [5]. Cochannel interference (CCI) arises from cellular frequency reuse and thus limits the quality and capacity (number of users) of wireless networks. CCI can be reduced by the use of microzoning or sectoring architectures. The smart antenna systems that utilize an array of antenna elements are also used to reduce the cochannel interference [9,10]. In this paper, the research is focused on the reduction of cochannel interference by utilizing the proposed cellular microzoning architecture.
3. Cochannel interference Frequency division multiple access and time division multiple access cellular systems rely on spatial attenuation to control intercell interference. As a result, neighboring cells need to be assigned different frequencies to protect against cochannel interference. In contrast, CDMA cellular system can apply a universal one-cell frequency reuse pattern [7]. Frequency reuse implies that in a given coverage of area there are several cells that use the same set of frequencies. These cells are called cochannel cells and the cochannel interference refers to the interference caused between two cells transmitting on the same frequency within a network. Since cochannel interference is caused by another cell transmitting the same frequency, we cannot simply filter out the interference by increasing the carrier power of the transmitter. This is because an increase in carrier transmit power increases the interference to neighboring cochannel cells. To reduce cochannel interference, cochannel cells need to be physically separated by a minimum distance to provide sufficient isolation due to propagation [5]. We can minimize the cochannel interference through the proposed architecture designed for cellular network. A cellular network must be designed to maximize the signal-to-interference (S/I) ratio. Here the S/I ratio is the signal-to-cochannel interference ratio. One of the ways to maximize the S/I ratio is to increase the frequency re-use distance, i.e. increase the distance between cells using the same set of transmission frequencies. The S/I ratio in part
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determines the frequency re-use distance of a cellular network. The generalized expression for the S/N ratio of either the forward or reverse link of a CDMA system can be expressed by [3]: " 1 1 #1 1 S Eb S S þ þ ð1Þ ¼ N I in-cell I CCI N0 where Eb/N0 is the S/N ratio due to additive white Gaussian noise (AWGN), N0 is the noise power, Eb = P0Tb is the average bit energy, Tb is bit duration, and P0 is the average transmitted power from the reference base station to the desired user in the reference cell for the forward link and is the average transmitted power from the reference mobile to the base station in the reference cell for the reverse link. For a WCDMA system utilizing asynchronous pseudo-noise (PN) codes for each user, the multi-user intra-cell interference term is represented by [3]: 1 K0 S 2 X Pk ¼ ð2Þ I in-cell 3N k¼1 P 0 where N is the system processing gain, K0 is the number of users in the reference cell, Pk is the average transmitted power from the reference base station to the kth user in the reference cell as received by the reference user for the forward link and is the average transmitted power from the kth user in the reference base station as received by the reference base station for the reverse link. 4. Proposed architecture for WCDMA cellular systems In wideband code division multiple access, the cochannel interferences from all surrounding cells is allowed in order to maximize efficiency but must be controlled. A variety of optimization techniques are used to mitigate the cochannel effects. Strategies for managing cochannel interference are complex but have been carefully developed within IS-95 deployments. The following strategies are deployed to manage the cochannel interference: antenna orientation and location in a cell, cell power adjustments, and the orientation of cell-specific information downloaded to mobile systems. In the following sections, we discuss various WCDMA cellular architectures with various antenna orientations and antenna location in a microzone. Section 4.1 described the proposed cellular network architecture. 4.1. Seven-microzone per cell architecture There are various cellular network architectures to reduce the cochannel interference [3,4]. The proposed cellular architecture is shown in Fig. 1. In the proposed architecture, each cell is represented by a circle and each hexagon represents a microzone. Microzoning is a term used to describe a cellular system where the cells have been divided into smaller zones. In the proposed architecture, a cell consists of seven microzones. The constellation shown in Fig. 1 has seven-cells and 49 hexagonal microzones. Since one-cell per cluster is considered, each cell is representing a cluster. The triangular unit in the reference center cell represents the location of mobile unit, where the cochannel interference is worst. Each microzone has a 60° directional transmitter which is represented by a semi-circle. We consider that antennas are mounted in a single tower for a particular location of the adjacent microzones. Side lobe canceller can be used to reduce the
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425
Cell C Cell D
C1
D1 Cell B
B1
Cell E O
x θ=120
E1
60
y
A1
R
Cell F
2.59R
Cell A
F1 Location of Mobile Unit
Fig. 1. Proposed seven-microzones per cell cellular architecture.
jamming interference. Each microzone transmitter is located at the outer edge of each cell except the center microzone for that cell. The center microzone transmitter radiates outward with respect to the reference center cell. The outer cell microzones transmitters radiate back towards the reference center cell microzone where the mobile unit is located. Out of 42 outer cells microzones only A1, B1, C1, D1, E1, and F1 microzones are causing interference to the mobile unit. The dotted lines represent the distance from the mobile unit to the interfering microzones. The distances between transmitters and mobile unit can be calculated based on the geometrical law of cosines. For example, the distance (oy) between the mobile unit located at the center cell at o and the transmitter in microzone (E1) at y can be computed as: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffi oy ¼ ðxyÞ2 þ ðoxÞ2 2ðxyÞðoxÞ cos h ¼ ð3RÞ2 þ ð5RÞ2 2ð3RÞð5RÞ cos 120 ¼ 49R Here R is the radius of a microzone. Multi-cell per cluster architecture reduces capacity as compared to one-cell per cluster architecture and is not being seriously considered for 3G wireless wideband CDMA systems. Consequently, in the proposed method, one-cell per cluster architecture is considered. Here we consider only the worst case scenario of the mobile unit. The interferences in other locations for the mobile unit can be computed in a similar way. For WCDMA systems with carrier stealing, the resulting first-tier cochannel interference at the location of the mobile unit can be obtained by the following equation [3]: 1 pffiffiffiffiffi nA1 pffiffiffiffiffi nB1 pffiffiffiffiffi nC1 S 2ð2:59RÞn0 h K A1 49R ¼ þ K B1 49R þ K C1 49R I CCI 3N pffiffiffiffiffi nD1 pffiffiffiffiffi nE1 pffiffiffiffiffi nF1 i þ K D1 49R þ K E1 49R þ K F1 49R ð3Þ
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where S is the desired signal power from the desired base station and I is the interference power. The subscripts KA1, KB1, KC1, KD1, KE1, and KF1 represent the number of users in the interfering microzones of the neighboring cells, as shown in Fig. 1. The respective propagation path loss exponents have the same subscript, i.e., the propagation path loss exponent for the signal transmitted from microzone A1 is nA1. The propagation path loss exponent for the reference microzone is n0. R is the radius of the microzone, 2.59R is the radius of a cell, and N is the processing gain.
5. Existing cellular WCDMA architectures 5.1. Microzoning Three microzones per cell cellular network architecture has been proposed by Mayer et al. [3]. MayerÕs microzoning architecture is shown in Fig. 2. Transmitters are located at the outer edge of each cell. The mobile unit is represented by the triangular unit in the reference center cell. Each microzone has a 120° directional transmitter which is represented by a semi-circle. The dotted lines represent the distance from the mobile unit to the interfering microzones. Only A1, A2, B1, C1, C2, D1, E1, E2, and F1 microzones are causing interference to the mobile unit. For WCDMA systems with carrier stealing, the first-tier cochannel interference for the microzoning architecture is obtained by [3]: 1 n pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffi S 2ð2RÞ 0 h ¼ K A1 ð 19RÞnA1 þ K A2 ð 19RÞnA2 þ K B1 ð5RÞnB1 þ K C1 ð 19RÞnC1 I CCI 3N i pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffi þ K C2 ð 19RÞnC2 þ K D1 ð5RÞnD1 þ K E1 ð 19RÞnE1 þ K E2 ð 19RÞnE2 þ K F1 ð5RÞnF1 ð4Þ
Cell D
D1
C2
Cell C
C1
E1
B1
E2
Cell B
Cell E
A2 Cell F
F1
A1
Cell A
Location of Mobile Unit
Fig. 2. MayerÕs three-microzones per cell architecture.
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5.2. Sectoring Sectoring is also used to reduce the cochannel interference. Here, the reference cell is the center hexagonal cell and it is surrounded by other six hexagonal cochannel cells. In this paper, one-cell per cluster WCDMA systems are considered. In this consideration, the center reference cell experiences interference from each of the cochannel cells. 5.2.1. Sixty degree sectoring In a 60° sectoring scheme, as shown in Fig. 3, it is assumed that only one-sixth of the total number of users per cell can be activated in a sector at one time. For WCDMA systems, the first-tier cochannel interference signal-to-interference ratio at the worst case location on the cell boundary is found by [3]: 1 i pffiffiffi pffiffiffi S 2Rn0 h ¼ K A ð 7RÞnA þ K B ð 7RÞnB þ K C ð2RÞnC þ K D ðRÞnD þ K E ðRÞnE þ K F ð2RÞnF I CCI 18N ð5Þ where KA through KF are the number of users in each of the six first-tier cochannel cells, nA through nF are the respective propagation path loss exponents, second-tier cochannel cells are assumed to have negligible effect. Antennas are placed at the center of each cell. All the neighboring cells (A, B, C, D, E, and F) are causing interference to the mobile unit.
C D B
E
A F
Location of Mobile Unit
Fig. 3. Sixty-degree sectoring architecture.
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5.2.2. One hundred and twenty degree sectoring In a 120° sectoring scheme, it is assumed that only one-third of the total number of users per cell can be activated in a sector at one time. For WCDMA systems, the first-tier cochannel interference signal-to-interference ratio at the worst case location on the cell boundary is found by the following [3]: 1 i pffiffiffi pffiffiffi S 2Rn0 h ¼ K A ð2RÞnA þ K B ð 7RÞnB þ K C ð 7RÞnC þ K D ð2RÞnD þ K E ðRÞnE þ K F ðRÞnF I CCI 9N ð6Þ where KA through KF is the number of users in each of the six first-tier cochannel cells. In 120° sectoring scheme, the cellular network architecture is similar to that of 60°, the only difference is that instead of using 60° directional antenna 120° directional antennas are used. 5.3. Omnidirectional In this architecture, an omnidirectional antenna is placed at the center of each cell. The first-tier cochannel interference signal-to-interference ratio for the mobile unit on its cell boundary is given by [3]: 1 i pffiffiffi n pffiffiffi n S 2Rn0 h n n n n K A ðRÞ A þ K B ð2RÞ B þ K C ð 7RÞ C þ K D ð 7RÞ D þ K E ð2RÞ E þ K F ðRÞ F ¼ I CCI 3N ð7Þ
6. Simulation results The simulation results shown in Fig. 4 are obtained for the forward link cochannel interference based on Eqs. (1)–(7). A comparison of the proposed cellular architecture with MayerÕs microzoning, 60° sectoring, 120° sectoring, and omnidirectional antenna architectures is shown in Fig. 4. The simulation is computed for a WCDMA system with a processing gain of 128, propagation path loss exponents of 5, and 24 users per cell. The proposed cellular architecture provides better gain in S/N ratio than other methods that reduce cochannel interference. For example, in Fig. 4, at Eb/N0 = 16 dB, the improvement in gain in S/N ratio over the microzoning, 60° sectoring, 120° sectoring, and omnidirectional antenna is approximately 0.33, 2.20, 3.69, and 7.04 dBs, respectively. Also at Eb/N0 = 25 dB, the improvement in S/N ratio compared to microzoning, 60° sectoring, 120° sectoring, and omnidirectional antenna is approximately 0.50, 3.04, 4.86, and 8.60 dBs, respectively. One of the most important issues in the WCDMA scheme is the number of admissible users per cell for a given available total bandwidth, for given radio propagation conditions, and for a required transmission quality [8,11–13]. The simulation results shown in Fig. 5 are obtained for the forward link cochannel interference based on Eqs. (1)–(7). In Fig. 5, the number of users per cell is plotted against S/N for different architectures where in each case the processing gain is 128, the propagation path loss exponents are taken to be 5, and Eb/N0 = 25 dB. As can be seen,
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16 14
S/N (dB)
12 10 8 6 4
Proposed Mayer 60-degree 120-degree Omnidirectional
2 0 10
12
14
16
18
20
22
24
Eb/No (dB)
Fig. 4. Comparison of CDMA architectures with a processing gain of 128, 24 users per cell, and propagation path loss exponents of 5. 35
30
S/N(dB)
25
20
15
10 Proposed Mayer 60-degree 120-degree Omnidirectional
5
0 5
15
25
35
45
55
Number of Users/Cell
Fig. 5. Comparison of CDMA architectures with a processing gain of 128 and propagation path loss exponents of 5.
the S/N ratio associated with the omnidirectional systems quickly falls below acceptable level. The proposed microzing, with the highest S/N ratio of all the architectures, accommodates the maximum number of users while maintaining an adequate S/N ratio.
7. Conclusions A comparative study has been provided for different cellular architectures. The simulation results were presented for the WCDMA cellular systems. The proposed method out-performed the existing microzoning, sectoring, and omnidirectional methods and achieved a better S/N ratio as compared to other methods. Further work will involve additional cellular architectures based on microzing and sectoring schemes that will further improve the overall signal-to-noise ratio.
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References [1] Pickholtz RL, Schilling DN, Milstein LB. Theory of spread spectrum communications—a tutorial. IEEE Trans Commun 1982;COM-30(5):855–84. [2] Pickholtz RL, Milstein LB, Schilling DN. Spread spectrum for mobile communications. IEEE Trans Veh Technol 1991;40(2):313–22. [3] Mayer T, Robertson C, Ha TT. Co-channel interference reduction on the forward channel of a wideband CDMA cellular system. In: Proc IEEE Military Commun Conf, 1999. [4] Salam MA, Al-Khatib MM, Alam MS. Reduction of cochannel interference on the forward link CDMA systems. In: Proc IEEE Radio Wireless Conf, 19–22 September 2004. p. 503–6. [5] Rappaport T. Wireless communications: principles and practice. Upper Saddle River, NJ: Prentice Hall; 2002. p. 57–104. [6] Milstein LB. Wideband code division multiple access. IEEE J Select Areas Commun 2000;18(8). [7] Kohno R, Meiden R, Milstein LB. Spread spectrum access methods for wireless communications. IEEE Commun Mag, January 1995. [8] Jung P, Baier PW, Steil A. Advantages of CDMA and spread spectrum techniques over FDMA and TDMA in cellular mobile radio applications. IEEE Trans Veh Technol 1993;42(3):357–64. [9] Hasna MO, Alouini MS, Bastami A, Ebbani ES. Performance analysis of cellular mobile systems with successive co-channel interference cancellation. IEEE Trans Wireless Commun 2003;2(1):29–40. [10] Tidestav C, Sternad M, Ahlen A. Reuse within a cell-interference rejection or multiuser detection. IEEE Trans Commun 1999;47(10):1511–22. [11] Choi W, Kim JY. Forward-link capacity of a DS/CDMA system with mixed multirate sources. IEEE Trans Veh Technol 2001;50(3):737–49. [12] Naguib AF, Paulraj A, Kailath T. Capacity improvement with base-station antenna arrays in cellular CDMA. IEEE Trans Veh Technol 1994;43(3):691–8. [13] Furukawa H. Downlink performance enhancements in CDMA cellular systems. In: Wang J, Ng TS, editors. Advances in 3G enchanced technologies for wireless communications. Artech House, Inc.; 2002.