Time Division Multiple Access, Frequency Division, Multiple Access,
and Orthogonal Frequency Division Multiplexing
NTC362
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Frequency Division Multiple Access
As the name implies, Frequency Division Multiple Access (FDMA), utilizes Frequency Division Modulation (FDM) to separate the given spectrum into individual analog bands or channels. A simplified view of how this is done can be seen in figure 1 below. Each of these channels is then able to facilitate communication for a single user. According to (Goleniewski, 2007) each FDMA cell is able to facilitate about 60 users, which is very a low number by current standards.
Figure 1. Goleniewski, L. (2007). Example of FDMA channel separation.
FDMA was short lived as a viable option for commercial use on its own; however, it was an important step in the right direction. I say this because it is now a critical part of multiple other techniques, where it is used to divide spectrum into channels before other modulation techniques are applied. Once such technology is the one explained next.
Time Division Multiple Access
Time Division Multiple Access or TDMA is a channel access that is shared out and is used on medium networks. This is also known as the 2G network that is slowly being phased out. What TDMA does is it allows multiple users to share the same channel within a network. This is accomplished by splitting a single channeling into two or three different time slots to maximize the data flow. The communication then can take place by allowing the users communication to pass through the channel at different intervals. When the communication is picked up on the other end the intervals are reconstructed as if each user was using a single line by themselves. This communication technology is used in the cell phone industry. “DMA is used by Digital-American Mobile Phone Service (D-AMPS), Global System for Mobile communications (GSM), and Personal Digital Cellular (PDC)” (Time division multiple). Since TDMA cannot be used in the exact same way these companies have to implement it in different ways. There are many advantages to using this system and here are a few of them:
- TDMA can easily adaptable
- TDMA provides users with an extended battery life
- TDMA is the most cost effective
Since TDMA is use a split channel it can be easily adapted to not only data transmissions, but voice communications as well. This is a great feature because anything that can do two things is going to be an asset. Having that ability to adapt on the fly without flipping a switch can save time and money for a company. Having the ability to adapt to either communication type is great, but adding that to a channel that can support two or three users id even better.
Another advantage that this provides is a longer battery life for user. This might seem a little strange, but since the channel is divided into two or three segments and communication takes place in intervals the devices is not in constant communication. The only time that the devices are used in when data or voice communication is taking place; this is why the battery life is extended. Now how much battery life it does save, is a different question.
The last advantage that is listed is the fact that the system is truly cost effective in terms of signal conversions. Having the ability to convert analog signals to digital signals is a plus because the technology already does so much. When taking into account everything that the technology can do you can see why it is truly cost effective. It is like a Swiss army knife by providing multiple functions to a cell phone network while maximizing the signal to its full capacity.
There are however disadvantages that this can bring a caller. Since users are not assigned a specific channel then can lose or drop calls. This happens because the slot time is not assigned to the user and when they go to roaming this may or may not allow the user to call out because the particular slot time is being occupied. This can really stink especially if you are trying to make an important call. The bottom line is that with new technology, comes better improvements with existing systems. These systems are not that bad because without them we would have nothing, but as we are always looking for the next best thing.
CDMA
Code Division Multiple Access (CDMA), is a technology based on spread spectrum: Direct Sequence Spread Spectrum (DSSS) to be precise. What makes it unique in relation to TDMA or FDMA is that spread spectrum allows users to operate on the same frequency simultaneously. Early CDMA operated at 1.25 MHz, with newer generations, also known as Wideband CDMA, operate at 3 MHz, 10 MHz, or 15 MHz (Goleniewski, 2007). The larger channels of the latter allow it to carry more calls and implement superior encryption. Compared to the analog technologies discussed in the previous two sections the spectral efficiency of first generation CDMA was 10 to 20 times greater. This translates into being able to squeeze 10 to 20 times the data given the same amount of spectrum.
As mentioned above CDMA allows users to share the same frequency. This is accomplished by using code to delineate users instead of frequency, which as the name implies is the heart of CDMA. This provides CDMA networks with a better resistance to interference and better security. This is how. DSSS converts each bit into micro pulses called chips. Chips are very fast pulses used to represent each bit as its own pseudorandom bit pattern (PN code). First generation CDMA utilizes a 64-bit (read chip) PN code. What this means is that each 1 bit is represented by a 64 chip pattern, and a 0 bit is represented by the exact inverse of the 1 bit. Consequently, even if some 1s and 0s are lost from the pattern, there are usually still enough intact for the receiver to determine the intended bit. The result is much higher quality, less error prone signal.
According to Langton (2002), the current standard of CDMA, CDMA 2000, performs modulation three times. Once for a code called Short Code, once for a code called Long Code, then the above mentioned 64-bit code called Walsh code.
Short Code
The short code serves the purpose of synchronization. It has a length of 215 – 1 resulting in 32,767 codes. This code is repeated every 26.666 milliseconds and 75 times every 2 seconds. When a user is ready to use the network, their device is able to search every base station in its network in 2 seconds.
Long Code
The long code is used for signal spread and encryption. It is 242 bits long and runs at a rate of 1.2288 Mb/s. At that rate the code is recycled every 41.2 days. CDMA uses Cellular Authentication and Voice Encryption (CAVE) protocol to shift the long code making it unique to each call.
Walsh Code
The 64-bit code mentioned above is used as the carrier code for CDMA and is known as a 64-bit Walsh code. Walsh codes are orthogonal to one another, so no code interferes with another. As one may have surmised, it is with this code that CDMA creates its channels. The CDMA 2000 uses a 256-bit code.
Figure 2. Langton (2002). Example of a 64 Bit Walsh code
CDMA is a very advanced technology that is still being honed and developed to keep up with future demands. One way this was done with the CDMA 2000 standard, was to combine it with another technology as part of a broader technology. Even though CDMA is capable of carrying both data and voice, it is now paired with EV-DO, which handles all of a user’s data needs when available. With its relatively high data rates, EV-DO gave CDMA a huge jump on competing technologies. According to Qualcomm (2007), EV-DO was designed to use the exact same frequency as CDMA 2000, and the EV-DO cards could be installed in current cabinets, which cut down on deployment cost and time. Indeed, advances in CDMA technology are far from seeing an end, with more promising revisions already on their way. Nonetheless, it may not be long before it needs to be redeployed from the ground up, as there are new technologies such as the one covered next, that are promising much more promise for future advancement.
Orthogonal Frequency Division Multiplexing
OFDM or Orthogonal Frequency Division Multiplexing is a technique that refers to modulation for wireless communications. OFDM patented in 1970 by Bell Labs and stood in Naval Communication system called Catherine. Like DMT Discrete MultiTone OFDM divides the data pieces into many RF channels, every one of which is sent over a frequency. DMT and other conventional techniques encode data symbols for a given data stream onto one radio frequency. In the OFDM system each tone is independent to the other tones. Multiple data symbols remain encoded concurrently onto multiple tones in a similar way. The signals to sound ratio of each of these frequencies remain cautiously observed toward ensuring the maximum performance. OFDM eliminates the requirement for guard bands to separate the frequencies and avoid interference from other RF channels; guard bands stay required around the edges of set tones. This creates efficiency around those tones of the allocated RF spectrum. OFDM remains founded on the mathematical concept that the FFT or Fast Fourier Transform will allow individual channels to maintain distance to the other channels. OFDM uses very narrowband tones that have frequency fading with multipath propagation degrades using only a small portion of the signal and has no effect on the remaining RF components.
Identify common frequency bands used in current RF communications
When it comes to using Radio Frequency or RF we use it in our everyday lives and fail to realize how important it has become. Some of the common RF’s in used today are:
- AM radio - 535 kilohertz to 1.7 megahertz
- Short wave radio - bands from 5.9 megahertz to 26.1 megahertz
- Citizens band (CB) radio - 26.96 megahertz to 27.41 megahertz
- Television stations - 54 to 88 megahertz for channels 2 through 6
- FM radio - 88 megahertz to 108 megahertz
- Television stations - 174 to 220 megahertz for channels 7 through 13
If you are a human living on the planet than you have at least heard of one of these list above. The AM is not a favorite because of the distortion that it has and how susceptible it is to interference from everything around it. FM radio provides a better signal because it operates at higher signal strengths then AM does. This is exactly how the band works the higher in strength the better quality of signal that it provides. The following band chart will provide the information need to see where each of the frequencies list above fall into.
After looking at the chart and where each on falls you can have a visual of when you move up the spectrum why the signal get better.
What this RF band brings is a new way of communication that is fairly new compared to the other ways we communicate over the web and across cell networks. All of cellular networks use the RF band. “TDMA systems operate in either the 800-MHz or 1900-MHz frequency bands” (Brian, Tyson & Layton), this may seem high however this is the old 2G network.
The RF band is very useful in the wireless market of today because this allows us to setup wireless networks for communication over the internet of other type of network. The standard wireless networks operate around the 2 GHz range, which are vital to most networks providing internet access. The 802.11A and 802.11B are the wireless networks that operate in this range as well. It is important to know the differences here because one provides a higher band width then the other. The 802.11A network runs at speeds of 54 mbps and is the most cost effective wireless network. The 802.11B only provides 11Mbps and is does not even come close to the other network type. The great thing is that there are so many advantages to this technology because it is ever expanding from small home Wi-Fi networks to the new WiMax that will potentially provide millions of people wireless access. This has come a very long way from the 2G networks of the past. Time will only tell what this will bring next to the communications field.
One of the major disadvantages to this network is since it emits signals they can be picked up by malicious users to gain access to a network. Wireless security should be the number one concern when implementing this technology on any network large or small. It is strange to thing that whenever something new is released it brings on a new challenge in security because it tends to carry a lot of potential vulnerabilities and threats. These threats will never go away and the only thing that can be done is knowing the risks and how to prevent attacks from happening.
Radio Frequency Transmission Characteristics
K (2007) “Radio frequency is the group of electromagnetic energy whose wavelengths are between audio and light range. These are usually transmitted between 500khz and 300 Ghz. The electromagnetic wave or voltage as it can be called is usually broadcast by velocity at the rate of 186,300 miles per second. Radio Frequency Identification is measured by identifying unique items using radio waves.” Using a reader, this joins by way of a tag that holds the digital information in a tiny chip form. When these are near the receiver, they will transmit information. The classic RF structure has two modules a transmitter and a receiver. The transmitter creates electrical fluctuations subsequent to the radio frequency titled carrier frequency; the frequency modulated toward contrast of the carrier signal. Frequency modulation transmits one pair of sidebands from modulation, this transforms into speech or other sounds. The effects that RF can have on different types of materials are unique. RF in a wireless environment can have dead spots without line of site as some materials the signals cannot penetrate.
Satellite Communications Options Considered
Satellites can be the perfect solution for a company’s end-to-end communication needs, as satellites offer many advantages compared to hardwired options: not the least of which being speed of deployment. Satellite communications, however, is not without its complications. For these reasons, when looking to deploy satellite as an end-to-end communication solution, it is important to weigh the advantages and disadvantages on a case by case basis.
One of the biggest barriers to satellite communication networks is the initial cost, which can be quite high. This is one area that satellites are quite unique, as the more locations being networked, the lower the installation cost compared to other solutions. Long term cost can also play a factor, and there are many considerations for cost with a satellite network other than just service cost. Depending on the orbit of the satellites being used for space transmission, the power requirement on the ground can fluctuate significantly. Below is a further review potential challenge for end-to-end satellite networks based on orbit and band.
GEO Satellites
Geosynchronous satellites (GEO) are deployed to 22,300 miles above the equator. Geo satellites remain stationary in relation to the earth’s service, giving the perception that they do not move. The following are the advantages and disadvantages of GEO satellites for end-to-end communications.
Advantages
- Data rates up to 155Mbps when using Ka-Band
- Largest coverage area or footprint
- Requires fewer relays for one-to-many networks
Disadvantages
- Requires a large amount of power or requires a larger dish
- Longest latency time of .25 seconds each way
- Using a hub station effectively doubles latency
MEO Satellites
Medium earth orbit (MEO) earth satellites are much closer to the earth than GEO satellites and range from 6,200 to 9,400 miles in elevation. The advantage and disadvantages to MEO satellites are:
Advantages
- Reduced latency compared to GEO satellites
- Maintains a footprint well suited for regional networks
Disadvantages
- Limited high speed offerings
- Still not the best solution for real-time applications
LEO Satellites
Low earth orbit (LEO) satellites orbit the earth anywhere between 400 to 1,000 miles above the surface. Because they are so low and move so fast in relation to a ground terminal, about 27 satellites are required to keep a constant line of site with a given point. Their relative proximity to the earth give LEO satellites some important advantages over the previous two types of satellites, as can be seen below.
Advantages
- Lowest latency at about .025 seconds each way
- Require little power to operate
- Can make use of non-direction antennas
- Best choice for real-time applications
Disadvantages