CDMA Tutorial : Basics, Walsh & PN sequence codes, PHY layer

Introduction : Code Division Multiple Access (CDMA) is a popular digital communication technology that allows multiple users to share the same frequency band simultaneously by assigning unique codes to each user. Unlike other multiple access techniques like Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA), CDMA spreads the signal across a wide frequency spectrum, making it more resistant to interference and providing better spectral efficiency. Widely used in mobile networks and satellite communication, CDMA offers advantages in terms of capacity, security, and signal quality. This CDMA tutorial covers CDMA basics including PN Sequence codes, Walsh codes, CDMA Physical layer for Base Station and Mobile Subscriber.

Technical Specifications of CDMA Cellular System

Both base stations (BSs) and mobile subscribers (MSs) work together using CDMA technology, ensuring that multiple users can share the same frequency band with minimal interference while providing high-quality, secure communication. The base station in a CDMA cellular system is responsible for managing communications between mobile subscribers and the core network. It performs various functions, including signal transmission, reception, and power control. Mobile subscribers communicate with the base station using specific hardware and protocols that allow for mobile connectivity while maintaining call quality and data services. Following are the specifications of BS and MS.
1. Frequency Band:
• Uplink: 824–849 MHz (USA), 890–915 MHz (Europe)
• Downlink: 869–894 MHz (USA), 935–960 MHz (Europe)
2. Channel Bandwidth:
• 1.25 MHz (Standard for CDMA2000) or 5 MHz (for WCDMA in 3G systems)
3. Transmit Power:
• For Base Station : Between 20W to 50W depending on the cell size and location (urban/rural areas)
• For Mobile Subscriber : Between 200 mW to 1W for mobile handsets (Power control adjusts according to proximity to the base station)
4. Antenna Configuration:
• Multiple sector antennas for spatial diversity (e.g., 3-sector, 6-sector configurations)
• Support for MIMO (Multiple Input, Multiple Output) antennas in newer systems
5. Modulation Techniques:
• QPSK (Quadrature Phase Shift Keying), BPSK (Binary Phase Shift Keying)
6. Spread Spectrum Technique:
• Direct Sequence Spread Spectrum (DSSS) with unique pseudo-random codes for each user
• Spreading factor for Mobile Subscriber : 128 chips per symbol in CDMA2000, 256 chips per symbol for higher data rates in 3G systems
7. Power Control:
• Fast closed-loop power control to mitigate interference and optimize signal quality
8. Handoff Support:
• Soft handoff (simultaneous connection to multiple base stations)
• Hard handoff (switching from one base station to another without overlap)
9. Channel Access Method:
• Code Division Multiple Access (CDMA), using unique spreading codes for each user to avoid interference
10. Voice/Data Capacity:
• Up to 55 simultaneous voice users per sector (CDMA2000)
• Data rates up to 2 Mbps (EV-DO and CDMA2000 1xEV-DO for higher-speed data services)
11. Security Features:
• Authentication algorithms
• Encryption (A5/3 encryption for securing data transmission)

In CDMA Base Station (BTS) to Mobile Subscriber Stations path is called forward link and Mobile subscriber stations to BTS path is called reverse link. There are two main types of codes used in CDMA, PN sequence codes and walsh codes.

PN Sequence codes

PN sequence codes are used in reverse link. These codes differentiates various mobile subscribers at base station as they are unique to all mobile subscribers. There are access and traffic channels used from mobile to Base station in CDMA. PN sequence is a sequence of binary numbers which appears to be random but it is periodic in nature. For example, 3 bit PN sequence is generated with the LFSR(Linear Feedback Shift Register) shown below.

Say initial output bit pattern is '001' . The same pattern repeats after 2^3-1 or 7 patterns. There are two types of PN sequences used in CDMA, 15 bit pattern and 42 bit pattern.

PN sequence properties: 1. Difference of number of 0's and 1's always be equal to 1 in any PN sequence.
2. Correlation value of any two PN codes is determined by following equation.
(No. of like bits- No. of unlike bits) / (Total no. of bits)
Consider PN sequence of pattern 1110010, If both the patterns are same without any delay then correlation value is 7-0/7, leads to 1 i.e. maximum value.
For a bit change in this PN codes leads to bit pattern equal to 0111001 and it will give correlation value of 3-4/7 i.e. -1/7. Hence it is easy to get back the data of the Mobile/Base station if the PN code is known.
3. For N bit code, there will be N ones or zero runs. 1/2 of run will be of length 1, 1/4 of run will be of length 2, 1/8 of run will be of length 3 and so on.

Walsh codes

Walsh code is called Hadamard Transform. Few of the codes are mentioned below. Walsh codes are orthogonal to each other, Walsh codes differentiates each channel on forward link from Base station to Mobile Subscribers. They are unique for all the channels. There are pilot, sync, paging and traffic channels on this forward link path. W0 is used for pilot channel, W32 is used for sync channel, W1 to W7 are used for paging channel and rest of the 64 Walsh codes are used for traffic channels in IS 95 version of CDMA. There are 256 Walsh codes in CDMA-2000 version. Following figure is of CDMA Physical layer for Mobile Subscriber. Walsh code of length n will have n rows of n x n matrix.

For Example W2= [0 0
0 1]

W4=[0 0 0 0
0 1 0 1
0 0 1 1
0 1 1 0]

Walsh Codes properties 1. Cross correlation between two different Walsh codes is not zero; hence it cannot be used if good synchronization is not available. Instead PN sequence is used.
2. Walsh codes are orthogonal to each other as mentioned above.

CDMA Physical layer for Mobile Subscriber

Figure depicts all the modules used in CDMA Physical layer, as shown green modules are not used in access channel path, while the same are used in traffic channel path.

CDMA Physical layer for Base station

Figure depicts CDMA Physical layer used in CDMA Base station, as shown various channels follow different physical layer architecture.

Conclusion

CDMA stands as a robust and efficient communication technology that has significantly influenced wireless communications. Its ability to accommodate multiple users within the same frequency band, coupled with its resistance to interference, has made it a foundational technology for cellular systems and satellite networks. By understanding the basics of CDMA, including its spread spectrum techniques, advantages, and applications, one can appreciate its role in enabling high-quality, secure communication. As mobile networks continue to evolve, CDMA remains a crucial concept in both historical and future advancements in wireless technologies. This CDMA tutorial is very useful for beginners who would like to explore concept of Walsh codes and PN sequence codes in CDMA physical layer of base station and mobile subscribers (i.e. mobile phones).

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