Why OFDM Over CDMA in LTE: Decoding the Sinc Signal Mystery

The transition from 3G to LTE brought about a radical shift in the modulation techniques used in mobile communication systems. One of the key changes was the move from CDMA to OFDM. While CDMA (Code Division Multiple Access) offers unique advantages, OFDM (Orthogonal Frequency Division Multiplexing) became the preferred choice for LTE due to its robustness and efficiency in handling multipath and wideband channels. This article will explore why OFDM is preferred over CDMA in LTE, delving into the role of sinc signals and their impact on multipath.

The Evolution from CDMA to OFDM

CDMA is a single-carrier technique that allows multiple users to communicate simultaneously by dividing the available bandwidth into narrow frequency channels. Each user is assigned a unique code to distinguish their signal from others. While CDMA can effectively utilize spectral efficiency and some aspects of multipath mitigation, it has limitations when it comes to handling wideband channels.

In comparison, OFDM uses multiple smaller sub-carriers that are orthogonal to each other, effectively dividing the bandwidth into parallel paths. This approach offers several advantages over CDMA, particularly in handling the challenges posed by multipath fading.

Multipath and Its Challenges

Multipath is a significant challenge in wireless communication systems. It occurs when signals travel different paths to the receiver, causing time delays and phase shifts. This can lead to loss of signal integrity and increase the complexity of the received signal.

While CDMA can handle some aspects of multipath through equalization techniques, it is limited by the bandwidth it can effectively utilize. In a 5MHz channel bandwidth, the presence of multipath can make it difficult to maintain a clear signal, leading to issues such as fading and signal degradation. This is where OFDM shines, as it uses multiple smaller sub-carriers, each with its own orthogonal frequency, which simplifies the equalization process.

OFDM and Sinc Signals

To better understand the benefits of OFDM, it is essential to explore the role of sinc signals. Sinc signals are functions that represent the frequency response of a system, particularly in the context of digital communication. In the frequency domain, sinc signals can be seen as the channels that a user can access with their cell phone.

The sinc signals are lined up in such a way that at certain frequency locations, the signals align perfectly, allowing for the cancellation of other frequencies. For example, consider a frequency location f0. At this location, multiple sinc functions (f1, f2, f3, etc.) overlap and sum up to produce a clear signal at f0, while other frequencies cancel out. This principle is illustrated using the emblem of Fry's Electronics, which features a series of sinc functions that align to form a clear, unambiguous signal at specific frequency points.

Mathematically, this can be represented as a summation problem. Imagine you need to sum up 4 numbers to get a result of 4. You are forced to use 4 numbers to do this. If you want one of those numbers to be 8, you can set up the equation: 8 x - y - z 4. By satisfying this equation, you force the other numbers to cancel out, leaving you with a clear signal at the desired frequency.

Implications and Advantages of OFDM

The use of sinc signals and the alignment of these signals in OFDM, similar to the emblem of Fry's Electronics, makes it advantageous in handling complex multipath environments. By breaking down the bandwidth into smaller sub-carriers, OFDM simplifies the equalization process, making it more robust and efficient. Moreover, the use of multiple parallel paths (MIMO) in OFDM further enhances its ability to handle wideband channels and improve data rates.

However, the complexity of implementing multiple transmit and receive carriers in OFDM can be a challenge. Fortunately, as computation processing power continues to increase due to Moore's Law, the challenges of implementing OFDM in hardware are diminishing, making it a more viable and preferred choice for modern wireless communication systems.

In conclusion, while CDMA offers unique advantages, OFDM's ability to handle multipath, wider bandwidth, and parallel paths makes it a more suitable choice for LTE systems. The use of sinc signals and the alignment of these signals in OFDM provide a powerful solution to the challenges of modern wireless communication.