CDMA- Code division multiple access, Capacity, Gain, Advantages
Table of Contents
CDMA full form- Code division multiple access
A type of multiplexing called Code Division Multiple Access (CDMA) makes it possible for various signals to share a single transmission channel. The use of the available bandwidth is optimized. Ultra-high-frequency (UHF) cellular telephone systems, which operate in the 800-MHz to 1.9-GHz frequency range, frequently employ the technology.
Time and frequency multiplexing is very different from code division multiple access. A user in this system has continuous access to the entire bandwidth. The fundamental idea is that different CDMA codes are used to differentiate between the various users. Direct sequence spread spectrum modulation (DS-CDMA), frequency hopping, or mixed CDMA detection (JDCDMA) are common techniques. Here, a signal that spans a large bandwidth is produced. This action is carried out using a code known as spreading code. In the presence of numerous other signals with various orthogonal codes, it is possible to choose one signal with a particular code by using a group of orthogonal codes.
How Does CDMA Work?
By processing each voice packet with two PN codes, CDMA allows up to 61 concurrent users on a 1.2288 MHz channel. To distinguish between calls and theoretical limits, 64 Walsh codes are available. The maximum number of calls will be a little bit lower than this value due to operational constraints and quality problems. In fact, multiple baseband “signals” with various spreading codes can be modulated on a single carrier to support a wide range of users. The interference between the signals is barely noticeable when using various orthogonal codes. On the other hand, because each mobile station has a different orthogonal spreading code, the base station can isolate each one when signals are received from multiple mobile stations.
The complexity of the CDMA system is depicted in the following diagram. We mixed all user signals together during propagation, but you still use the same code on the receiving side as you did on the sending side. Only each user’s signal can be removed.
The factors deciding the CDMA capacity are −
- Processing Gain
- Signal to Noise Ratio
- Voice Activity Factor
- Frequency Reuse Efficiency
In CDMA, all users are on each frequency and are divided by code, resulting in soft capacity. This means that interference and noise are present when CDMA is operating. Additionally, there is no reusing because nearby cells use the same frequencies. Thus, CDMA capacity calculations ought to be very straightforward. So A cell with no code channels multiplied by no cells. But it’s not quite that easy. 64 code channels are not currently available, but because the CDMA frequency is the same, it might not be possible to use them at once.
- The band used in CDMA is 824 MHz to 894 MHz (50 MHz + 20 MHz separation).
- Frequency channel is divided into code channels.
- 1.25 MHz of FDMA channel is divided into 64 code channels.
Spread spectrum technology includes CDMA. A code sequence spreads each data bit. So This translates to increased energy per bit. This indicates that we benefit from this.
P (gain) = 10log (W/R)
W is Spread Rate
R is Data Rate
For CDMA P (gain) = 10 log (1228800/9600) = 21dB
The gain factor and actual data propagation rate are these. So For an adequate voice quality under typical transmission conditions, a signal to noise ratio of 7 dB is typically needed. Signal must be five times stronger than noise, expressed as a ratio.
Actual processing gain = P (gain) – SNR
= 21 – 7 = 14dB
CDMA uses variable rate coder
The Voice Activity Factor of 0.4 is considered = -4dB.
CDMA has 100 percent frequency reuse as a result. Additional interference is created when nearby cells use the same frequency.
In CDMA frequency, reuse efficiency is 0.67 (70% eff.) = -1.73dB
The soft capacity of CDMA. The more codes there are, the more users there are. It offers the following benefits:
- Signal reception can be enhanced using rake receivers. Bit-level decisions can be made using delayed versions of the signal (multipath signals) that are time-delayed by a chip or more.
- Because CDMA experiences the near-far effect, it needs strict power management. In other words, a user transmitting at the same power close to the base station will drown the signal. At the receiver, all signals must have roughly equal power.
- Transfer can be flexible. Mobile base stations can change operators without changing. So The mobile receives signals from the two base stations and from the two base stations.
- Transmission Burst − reduces interference.
The disadvantages of using CDMA are as follows −
- Time synchronization is required.
- It’s important to choose the code length carefully. A long code length may result in interference or delay.
- A base station requires constant, precise power control because the total power it transmits and receives must be controlled. There may be several handovers as a result.
- Gradual transfer uses more radio resources and might cause capacity loss.