3G Technology through the lens of CDMA and its Codes

3G Technology through the lens of CDMA and its Codes

"It's all about being one in a billion"

What is CDMA?

CDMA stands for "Code Division Multiple Access." CDMA is a wireless transmission technology that was developed during World War II by the English allies to avoid having their transmissions jammed. After the war ended, Qualcomm patented the technology and made it commercially available as a digital cellular technology. Now CDMA is a popular communications method used by many cell phone companies.

Unlike the GSM and TDMA technologies, CDMA transmits over the entire frequency range available. It does not assign a specific frequency to each user on the communications network. This method, called multiplexing, is what made the transmissions difficult to jam during World War II. Because CDMA does not limit each user's frequency range, there is more bandwidth available. This allows more users to communicate on the same network at one time than if each user was allotted a specific frequency range.

Because CDMA is a digital technology, analog audio signals must be digitized before being transmitted on the network. CDMA is used by 2G and 3G wireless communications and typically operates in the frequency range of 800 MHz to 1.9 GHz.

CDMA relies heavily on Codes so 3G also relies heavily on these codes. But what are these codes and why do we use them?

CDMA provides up to 10 times the calling capacity of earlier analog networks (AMPS) and up to five times the capacity of GSM systems.

3G spectrum

We have already seen that 3G is a spread spectrum technology. One user's data can be transmitted through multiple frequencies within a spectrum. The same happens with all of the users. In other words, with 3G, the spectrum is really a chaos of data fragments (frames or packets) and being unique is of utmost importance. This uniqueness is achieved by a string of 0s and 1s called a "Code".

Each user is provided a different code while performing wireless communication. These codes need to be unique to make sure that no two users get cross connection and the process may fail. Such codes are majorly divided into two categories:

1.      Pseudo Random Codes

2.      Mutually Orthogonal codes

Let's look at both of them one by one

1. Mutually Orthogonal Codes

Let's jump to high school mathematics a little. Vectors can be represented as sequence of numbers in a matrix. And two vectors whose dot product (SOP of corresponding numbers) is 0 are called mutually Orthogonal. In case of CDMA, generally a base station distributes 64 bit long mutually Orthogonal codes to the users and packs their data along with the code. So, every time the user receives a data-stream or packet it performs dot product of the initial 64 bits with its own code and if the result is 0, it ignores the information. That's how CDMA grants time and frequency to the users and separates them with codes instead. Science community calls these orthogonal vectors Walsh codes whereas communication committee sometimes also refers to them as chip codes.

 2. Pseudo Random Codes

Sometimes the users keep moving too much in which case they may keep hopping between two or more base stations. It becomes difficult for base stations to manage a common Orthogonal code so such users are treated with a different approach. 

They are provided a permanent (as long as the call lasts) PN (Pseudo Noise) or Pseudo Random Codes. The code is provided to respective base stations and the call remains intact. Such an arrangement is also useful when (rarely) base station runs out of mutually Orthogonal codes under high cellular traffic.

Base stations generate random code by adding difficult to predict intended noise in a smaller code signal. Such code seems random but is actually deterministic (pseudo). So, it won't be repeated by the BSc. One such popular PR or PN code is Gold code. Under critical applications like Military operations, Orthogonal codes are also replaced by PN codes.