Fundamentals of Wireless Communication Technology - 1G, 2G, 3G, 4G, 5G (GSM/CDMA/MCDMA/LTE/NR)

When we talk about ‘5G’, G refers to a generation of wireless technology. Each generation is capable of supporting more users and has better data transmission capabilities.
The first generation was analogue mobile phones, which are now obsolete. When carriers moved to 2G digital systems in the 1990s, they chose between several competing options; some of these have died out, but GSM is the surviving 2G camp for now.
Let's look at the development of mobile communications technology:

1. The origin of the cellular mobile communication principle GSM (Group Special Mobile) - Global System for Mobile Communications (GSM)

GSM originated in Europe, began as the European postal and telecommunications management joint mobile communications special group (Group Special Mobile) abbreviation, commonly known as GSM.
Due to its mature technology, it has been adopted by many countries in the world. 900MHz band was first adopted for GSM, but with the rapid growth of the number of users of GSM mobile communication network, the limited resources of GSM 900MHz band have been obviously difficult to meet the needs. 900MHz band was formally allocated to the GSM network only uplink and downlink of 25MHz each, and in the case of the rapid increase of GSM users, new frequency bands were introduced. In order to introduce a new frequency band instead of increasing base stations to meet the growth of GSM network capacity, DCS1800MHz band came into being. The propagation characteristics of 1800MHz and 900MHz bands are basically similar, and the use of 1800MHz band's more relaxed frequency resources and the adoption of GSM 900MHz/DCS 1800MHz dual-band operation can greatly alleviate the capacity of GSM 900MHz band and reduce the need of GSM 900MHz frequency band. The capacity pressure of GSM 900MHz band can be greatly relieved by adopting GSM 900MHz/DCS 1800MHz dual band operation. At the same time, due to the fact that 1800MHz and 900MHz systems are more consistent in terms of network configuration, engineering implementation, network maintenance and supported services, GSM900MHz/DCS 1800MHz dual band is adopted, which is also the reason why these two frequency bands still have the highest usage rate in the world.

2. CDMA (Code Division Multiple Access) originated from the United States, is a more advanced technology than GSM in the 3G era; it is a multi-access technology together with GSM, mainly using the frequency of 800MHZ (Band 20) or 850MHZ (Band 5). What we call CDMA is largely owned by chipmaker Qualcomm. Because of patents, the technology used to build CDMA is much more expensive than GSM equipment, and is gradually being shut down with the development of 4G and 5G. 
Verizon in the US (already shutting down in 2020), US Cellular and the old Sprint network (now owned by T-Mobile) and China Telecom (also shutting down CDMA in 2021) are among the most important operators in the world.
Most other operators around the world, such as China Mobile, AT&T and T-Mobile in the U.S., use GSM. CDMA technology is more advanced than GSM, but is not as widely available as GSM, because CDMA technology is limited by the patent protection of Qualcomm's chips, and because GSM's global popularity is due to the fact that it was made mandatory by law in Europe in 1987 and because of the fact that it is not as popular as GSM. In addition, GSM's global popularity is due to the fact that it was made mandatory by law in Europe in 1987, and because GSM comes from an industry consortium.

3. MCDMA (3G GSM)
GSM first appeared when it was a ‘time division’ system called TDMA (Time Division Multiple Access). Calls were rotated, your voice was converted into digital data and it was given a channel and a time slot so that three calls on one channel looked like this: 123123123123123. At the other end, the receiver would just listen to the allocated time slot and piece the call together. The pulsing of the time-slot signal created the infamous ‘GSM buzz’ that would occur whenever you placed your GSM phone near a speaker. Most of that is gone now, because 3G GSM (as I'll explain) is not a time division technology.
CDMA requires more processing power. It's a ‘code division’ system. Each call's data is encoded with a unique key, and then all calls are transmitted at once; if you have calls 1, 2 and 3 on a channel, that channel will just say 6666666666. each receiver has a unique key, which allows it to ‘divide’ the combined signals into separate calls.
Code division turns out to be a much more powerful and flexible technology, so ‘3G GSM’ is actually a CDMA technology called WCDMA (Wideband CDMA) or UMTS (Universal Mobile Telecommunications System).
 As the name implies, WCDMA requires a wider channel than the old CDMA system, but it has more data capacity. (GSM is actually just the official name for the 2G system. But the name is also widely used to refer to any technology on the ‘GSM path’ and is recognised by industry bodies, so I'm referring to WCDMA as 3G GSM so that people don't confuse it with 2G CDMA alone.)
Since its inception, GSM has evolved faster than CDMA. WCDMA is considered to be the 3G version of GSM technology. The 3GPP (the governing body of GSM) has released an extension called HSPA, which has increased the speed of GSM networks to 42Mbps, at least in theory. Meanwhile, our CDMA networks are stuck at 3.6Mbps. Faster CDMA technologies existed, but operators chose not to install them, instead moving to 4G LTE to be more compatible with global standards. This is why CDMA was eventually phased out
WCDMA mainly uses frequencies between 1900 and 2100 MHZ.

4. 4G LTE

The CDMA/GSM split is theoretically over, as communications operators around the world have been moving to LTE, a single global 4G standard, since 2010.

LTE, or Long Term Evolution, is the globally recognised 4G wireless standard. All carriers use it.
In the definition of ITU (International Telecommunication Union), any wireless data network system that reaches or exceeds 100 Mbps can be called 4G. 4G technology is a broadband wireless access method based on IP protocols, which is characterised by high data transmission rate, low latency, wide coverage, high capacity and high security, etc. LTE is a wireless communication technology based on the OFDM (Orthogonal Frequency Division Multiplexing) modulation method. LTE technology, like GSM and WCDMA, is based on the 3GPP standard, LTE is the ‘enhanced version’ of 3G at the beginning, technically known as 3.9G, with the later development of LTE gradually approaching the real 4G.

As you can see, 4G and LTE are not the same thing. LTE networks are able to meet the standards of 4G networks (100Mbps downlink), and the 4G era is dominated by LTE networks, so the two are often combined and collectively referred to as 4G LTE.
Like 4G, LTE is designed to provide high-speed data transmission.LTE introduces MIMO (Multiple Input Multiple Output) antenna technology, which allows data to be sent to multiple users at the same time, thereby increasing network capacity and throughput. In addition, LTE supports a wide range of network deployment scenarios, including macrocells, microcells, and hotspots.LTE technology also enables low-latency and highly reliable communication services, making the experience of using smartphones, tablets, and other mobile devices smoother.
Although what we now call 4G is usually LTE, they differ in many ways. For example, compared to 4G LTE is still slightly inferior in terms of data transfer speeds, and 4G can support the connection of more devices, while LTE can only support a certain number of end devices. In addition, 4G can provide better network quality and service experience, such as higher peak rate, lower latency, etc., while LTE needs to be adjusted according to different network environments and user needs.
Both 4G and LTE use packet switching technology for data transmission. In 4G, data is divided into small packets and allocated to different users using different frequencies; in LTE, these small packets are further divided into smaller sub-frames for better utilisation of spectrum resources. 4G and LTE technologies are both available in two different network standards, TD-LTE and FDD-LTE.

To add here.
II. TDD and FDD
TDD and FDD are two different types of duplexing. TDD (Time Division Duplex) refers to time division duplex, which is the way of dividing the channel by time; FDD (Frequency Division Duplex) is frequency division duplex, which is the way of using the bandwidths of the two carriers separately so as to achieve the bi-directional transmission of signals. Their main differences are as follows:
① Flexibility
TDD: Transmission and reception share a common RF frequency point, and the uplink and downlink use different time slots for communication.
FDD: The sender and receiver use different RF frequency points for communication.
FDD uplink and downlink frequency bands must be symmetrical, uplink free resources can not be used for downlink; while TDD can flexibly allocate uplink and downlink resources to achieve resource saving. In the present situation of tight frequency resources, TDD is obviously more advantageous.
Rate
In terms of rate, TDD is not as good as FDD, because under the TDD system, a certain time interval needs to be set aside to distinguish between uplink and downlink services, which is called the protection interval, and no data will be transmitted during the protection interval, thus reducing the data transmission rate. In addition, TDD uplink and downlink need to share the bandwidth, FDD is uplink and downlink exclusive bandwidth, in the case of the same bandwidth, FDD uplink and downlink rate are significantly higher than TDD.
Channel reciprocity
The BTS evaluates channel quality by channel reciprocity. Reciprocity means that when the channel quality of uplink and downlink is identical, the BTS only needs to assess the uplink channel quality (the downlink channel quality is the same as the uplink). tDD uplink and downlink are transmitted in the same frequency band, so the channel quality is naturally the same; FDD uplink and downlink are transmitted in two frequency bands, so it is not possible to assess the channel reciprocity. 5G needs to transmit data by means of beam foulement (Massive Mimo). 5G needs to transmit data by beam assignment (Massive MIMO), the more accurate the downlink channel quality information is obtained, the more accurate the assignment is, the advantage of TDD is shown.
Now, as most countries have completed the commercial promotion of TD-LTE and FDD-LTE 4G technologies, and started to phase out 2G/3G networks. Although the 5G era has come, 5G technology has not yet been popularised globally and has not yet reached full commercialisation, and the mainstream mobile communication terminals on the market are still dominated by 4G LTE --as of 2022.