Telephony and cellular telephony
For a century the telephone network was one copper pair per subscriber running to an exchange. Cellular telephony solved an apparently impossible problem —serving millions of users with only a few radio channels— with a simple idea: divide the territory into cells and reuse the frequencies.
01The fixed telephone network
| Element | Function |
|---|---|
| Subscriber pair | Two copper wires from the house to the exchange. It carries the voice and the power: the exchange supplies 48 V, which is why the landline phone kept working during a power outage. |
| Signaling | Going off-hook closes the loop and requests dial tone; DTMF sends each digit as the sum of two tones, one for the row and one for the column; the ringer signal is 75 V of alternating current at 25 Hz. |
| Exchange | It switches the call. From electromechanical to digital, and today to servers that handle voice over IP. |
| Bandwidth | The telephone channel occupies 300 to 3400 Hz. It is enough to understand speech and to recognize who is speaking, but not for music. |
| Digitization | Voice is sampled at 8 kHz with 8 bits per sample: 64 kbit/s per channel. It is the basis of the whole digital network, and from it come the 30-channel multiplexes. |
Sampling at 8 kHz a signal limited to 3400 Hz is a direct application of the sampling theorem —see signals and Fourier analysis— and the 8-bit quantization is the same one studied in A/D and D/A converters.
02The idea of the cell
A single powerful transmitter covering an entire city can only handle as many simultaneous calls as it has channels. By lowering the power and installing many small antennas, the same group of frequencies can be used again a few blocks away without interference.
- Adjustable capacity: where more traffic is needed, smaller cells are used. In the center of a city there are antennas every few blocks; on the highway, every several kilometers.
- Low power: a cell phone transmits between 0.1 and 2 W, and generally much less thanks to power control.
- Mobility: the network follows the user. The handover passes the call to the neighboring cell; roaming allows using another operator's network.
- Sectorization: each site usually has three 120° panel antennas, and each sector is a different cell: capacity is tripled with the same tower.
- Terminal with its SIM, which stores the subscriber's identity and keys. The device has its own identifier, the IMEI.
- Base station: antennas, radios, power supply and backup. It is the part that can be seen from the street.
- Controller: manages several base stations, the handovers and the radio resources.
- Core network: switches calls and data, authenticates, bills and connects with the other networks.
03How they share the air
Within a cell, many users have to coexist in the same spectrum. Multiple access techniques solve that sharing, and each generation used a different one.
| Method | How it divides | Where it was used |
|---|---|---|
| FDMA | Each user gets a frequency of their own for the whole call. | Analog 1G. Simple and very inefficient. |
| TDMA | Several users share one frequency, each in its own time slot. Eight per carrier in GSM. | 2G / GSM. |
| CDMA | Everyone transmits at the same time in the same band, each with a different code that the receiver uses to separate it. | 3G / UMTS. |
| OFDMA | The spectrum is divided into thousands of subcarriers and blocks of subcarriers and time are assigned to each user, according to what they need. | 4G / LTE and 5G. It is also the technique of modern WiFi. |
Because it allocates resources in blocks and in real time: someone downloading a file gets many subcarriers, someone who only has a messaging app open gets a few. Also, by splitting the channel into narrow subcarriers, each one sees an almost flat channel, and that makes it much simpler to correct the distortions of multipath.
04From 1G to 5G
| Gen. | What it introduced | Typical speed |
|---|---|---|
| 1G | Analog voice with frequency modulation. No encryption: anyone with a receiver could listen in. | — |
| 2G | Digital voice, encryption, SMS, the SIM card. Later GPRS and EDGE added data. | 9.6 to 384 kbit/s |
| 3G | Data considered from the design stage: mobile internet, video calls. | 0.4 to 42 Mbit/s |
| 4G | All IP, including voice. OFDMA and multiple antennas (MIMO). | 10 to 300 Mbit/s |
| 5G | More capacity, very low latency and device density: also designed for industry and sensors. | 100 Mbit/s to 1 Gbit/s |
- Bands in Argentina: 700 MHz, 850 MHz, 900 MHz, 1900 MHz, 2100 MHz, 2600 MHz and 3.5 GHz for 5G. Low bands penetrate buildings better and cover farther; high bands carry more data but reach less far.
- Power control: the network orders the mobile to transmit the minimum necessary. Near the antenna it transmits microwatts; at the cell edge, close to the maximum. This saves battery, reduces interference and increases capacity.
- Duplexing: FDD uses two frequencies, one for each direction; TDD uses the same one alternating in time.
- MIMO: several antennas at each end, transmitting different streams on the same frequency. It is the main reason for the jump in speed in 4G and 5G.
It sounds backwards but it is exact: in an area of poor coverage, power control demands that the mobile transmit at maximum and retry more times. That is why the phone heats up and the battery drains quickly right where there is little signal, and not where there is a lot.
05In the lab
With an app that shows the signal level in dBm and the cell the device is connected to, walk a route noting level and position. Plot it. You can see the level jumps when changing cells, and you can identify where the antenna serving each stretch is.
During a call, cover a long distance noting the cell identifier. Record at which points the handover occurs and what signal level there was at that moment. It is the direct way to see the mechanism that makes it possible to talk while moving.
Measure the signal level outside, inside the classroom, in a basement and inside an elevator. Compare a low band with a high one if the device allows forcing them. The drop inside metal structures shows the Faraday cage effect with your own numbers.
Record the keypad tones of a telephone and analyze them with a spectrum program. Identify the two tones of each digit and build the complete table of rows and columns. It is the most direct application of the spectral analysis seen in the previous topic.
06Common mistakes and misconceptions
| Misconception | What actually happens |
|---|---|
| “The signal bars indicate the data rate” | They indicate signal level. With an excellent signal and a congested cell, the speed can be awful. |
| “The more antennas nearby, the more radiation I receive” | The opposite: with the antenna nearby the mobile transmits much less, and it is the mobile that is held against the head. |
| “5G and WiFi's 5 GHz are the same thing” | They are unrelated: one is a generation of cellular telephony, the other a frequency band. |
| “The IMEI and the number are in the SIM” | The number and the keys are in the SIM; the IMEI identifies the device. |
| “With more power in the phone there would be more coverage” | The link is also limited in the other direction and by interference between cells: raising the power would degrade the network. |
| “The cell is a hexagon” | The hexagon is a drawing model. Real coverage depends on the terrain, the buildings and the antenna height. |
07Self-assessment
What is the central idea of the cellular system?
Reusing frequencies: dividing the territory into low-power cells and using the same group of channels again in non-neighboring cells. That way a few channels serve millions of users.
What is the handover?
The transfer of a call or data session from one cell to the neighboring one when the user moves, without the service being interrupted.
Difference between handover and roaming.
Handover is a change of cell within the same network. Roaming is using the network of another operator, typically in another country.
Why is the telephone channel limited to 3400 Hz?
Because it is enough for speech to be intelligible and recognizable, and limiting the bandwidth allows many more channels to be multiplexed over the same medium.
Where do the 64 kbit/s of a digital voice channel come from?
From sampling at 8000 samples per second with 8 bits each: 8000 × 8 = 64,000 bit/s.
How do TDMA and CDMA differ?
TDMA divides time: each user has their slot on the same frequency. CDMA lets everyone transmit at once in the same band, separating them by codes.
Why is OFDMA more efficient?
Because it dynamically allocates blocks of subcarriers and time according to what each user needs, and because narrow subcarriers simplify the handling of multipath.
What advantage do low bands have over high ones?
They penetrate buildings better and cover more area with the same power. In exchange, they offer less data capacity.
What is power control for?
So that each mobile transmits the minimum necessary: it saves battery, reduces interference toward the other cells and increases the total capacity of the system.
Why does the battery last less where there is little signal?
Because the mobile transmits at maximum power and repeats attempts. The consumption of the transmit stage is what dominates the drain under those conditions.