Here is a simple introduction to some aspects of 3G radio transmission technologies (RTTs). You will find the subjects covered in this section useful if you later consider the more detailed discussions in the sections on 3G Standards and 3G Spectrum.
Simplex vs. Duplex:
When people use walkie-talkie radios to communicate, only one person can talk at a time (the person doing the talking has to press a button). This is because walkie-talkie radios only use one communication frequency – a form of communication known as simplex:
Simplex: Using a walkie-talkie you have to push a button to talk one-way.
Of course, this is not how mobile phones work. Mobile phones allow simultaneous two-way transfer of data – a situation known as duplex (if more than two data streams can be transmitted, it is calledmultiplex):
Duplex: Allows simultaneous two-way data transfers.
The communication channel from the base station to the mobile device is called the downlink, and the communication from the mobile device back to the base station is called the uplink. How can duplex communication be achieved? Well, there are two possible methods which we will now consider: TDD and FDD.
TDD vs. FDD:
Wireless duplexing has been traditionally implemented by dedicating two separate frequency bands: one band for the uplink and one band for the downlink (this arrangement of frequency bands is calledpaired spectrum). This technique is called Frequency Division Duplex, or FDD. The two bands are separated by a “guard band” which provides isolation of the two signals:
FDD: Uses paired spectrum – one frequency band for the uplink, one frequency band for the downlink.
Duplex communications can also be achieved in time rather than by frequency. In this approach, the uplink and the downlink operate on the same frequency, but they are switched very rapidly: one moment the channel is sending the uplink signal, the next moment the channel is sending the downlink signal. Because this switching is performed very rapidly, it does appear that one channel is acting as both an uplink and a downlink at the same time. This is called Time Division Duplex, or TDD. TDD requires a guard time instead of a guard band between transmit and receive streams.
Symmetric Transmission vs. Asymmetric Transmission:
Data transmission is symmetric if the data in the downlink and the data in the uplink is transmitted at the same data rate. This will probably be the case for voice transmission – the same amount of data is sent both ways. However, for internet connections or broadcast data (e.g., streaming video), it is likely that more data will be sent from the server to the mobile device (the downlink).
FDD transmission is not so well suited for asymmetric applications as it uses equal frequency bands for the uplink and the downlink (a waste of valuable spectrum). On the other hand, TDD does not have this fixed structure, and its flexible bandwidth allocation is well-suited to asymmetric applications, e.g., the internet (see this PDF file for more details). For example, TDD can be configured to provide 384kbps for the downlink (the direction of the major data transfer), and 64kbps for the uplink (where the traffic largely comprises requests for information and acknowledgements). See this PDF file for more details.
Macro Cells, Micro Cells, and Pico Cells:
The 3G network might be divided up in hierarchical fashion:
- Macro cell – the area of largest coverage, e.g., an entire city.
- Micro cell – the area of intermediate coverage, e.g., a city centre.
- Pico cell – the area of smallest coverage, e.g., a “hot spot” in a hotel or airport.
Why is there this sub-division of regions? It is because smaller regions (shorter ranges) allow higher user density and faster transmission rates. This is why they are called “hot spots”.
TDD mode does not allow long range transmission (the delays incurred would cause interference between the uplink and the downlink). For this reason, TDD mode can only be used in environments where the propagation delay is small (pico cells). As was explained in the previous section on symmetric transmission vs. asymmetric transmission, TDD mode is highly efficient for transmission of internet data in pico cells.
TDMA vs. CDMA:
We have considered how a mobile phone can send and receive calls at the same time (via an uplink and a downlink). Now we will examine how many users can be multiplexed into the same channel (i.e., share the channel) without getting interference from other users, a capability called multiple access. For 3G technology, there are basically two competing technologies to achieve multiple access: TDMA and CDMA.
TDMA is Time Division Multiple Access. It works by dividing a single radio frequency into many small time slots. Each caller is assigned a specific time slot for transmission. Again, because of the rapid switching, each caller has the impression of having exclusive use of the channel.
CDMA is Code Division Multiple Access. CDMA works by giving each user a unique code. The signals from all the users can then be spread over a wide frequency band. The transmitting frequency for any one user is not fixed but is allowed to vary within the limits of the band. The receiver has knowledge of the sender’s unique code, and is therefore able to extract the correct signal no matter what the frequency.
This technique of spreading a signal over a wide frequency band is known as spread spectrum. The advantage of spread spectrum is that it is resistant to interference – if a source of interference blocks one frequency, the signal can still get through on another frequency. Spread spectrum signals are therefore difficult to jam, and it is not surprising that this technology was developed for military uses.
Finally, let’s consider another robust technology originally developed by the military which is finding application with 3G: packet switching.
Circuit Switching vs. Packet Switching:
Traditional connections for voice communications require a physical path connecting the users at the two ends of the line, and that path stays open until the conversation ends. This method of connecting a transmitter and receiver by giving them exclusive access to a direct connection is called circuit switching.
Most modern networking technology is radically different from this traditional model because it usespacket data. Packet data is information which is:
- chopped into pieces (packets),
- given a destination address,
- mixed with other data from other sources,
- transmitted over a line with all the other data,
- reconstituted at the other end.
Packet-switched networks chop the telephone conversation into discrete “packets” of data like pieces in a jigsaw puzzle, and those pieces are reassembled to recreate the original conversation. Packet data was originally developed as the technology behind the Internet.
The major part of a packet’s contents is reserved for the data to be transmitted. This part is called the payload. In general, the data to be transmitted is arbitrarily chopped-up into payloads of the same size. At the start of the packet is a smaller area called a header. The header is vital because the header contains the address of the packet’s intended recipient. This means that packets from many different phone users can be mixed into the same transmission channel, and correctly sorted at the other end. There is no longer a need for a constant, exclusive, direct channel between the sender and the receiver.
Packet data is added to the channel only when there is something to send, and the user is only charged for the amount of data sent. For example, when reading a small article, the user will only pay for what’s been sent or received. However, both the sender and the receiver get the impression of a communications channel which is “always on”.
On the downside, packets can only be added to the channel where there is an empty slot in the channel, leading to the fact that a guaranteed speed cannot be given. The resultant delays pose a problem for voice transmission over packet networks, and is the reason why internet pages can be slow to load.