Duplex Operation
First Receive and Transmit Channels
How to share a radio channel
Signaling
Manual Call Setup
Automatic Call Reception
Mobile Telephone Service

Development of Radio Telephones

The times of World War II focused all technical progress on military developments. The focus was primarily on radar technology, which was of great importance. However, as described, there was also some progress in two-way radios. It was not until the early 1950s that research and development on civil and commercial technology started again. Especially on television and FM radio. But technology was also ready by now for a “radio telephone” as the pioneers of radio technology had always wanted.

Duplex Operation

But there was a big difference between the existing „walkie-talkie“ kind of communication and a true radio phone communication. With a walkie-talkie, all participants are on the same channel. Only one participant can talk while everyone else receives. It is not possible to arrange a private conversation between two participants. First, there had to be radio access to the telephone network, the PSTN (Public Switched Telephone Network). Second the radio telephone must be able to send and receive at the same time so that a natural conversation is possible similar to a normal phone.

Transmitters and receivers working in parallel are referred to as duplex operation. It is clear that transmitters and receivers cannot use the same frequency as with walkie talkie kind of radios. Therefore a radio telephone had to use two channels, which must have a sufficient frequency separation from each other so that they do not interfere. There is one channel from the base station to the radio telephone (downlink) and a channel from the radio telephone to the base station (uplink).

This was possible with a certain amount of effort. However it does not make sense to use two antennas. The antenna had to be shared. That was a critical element. About 10 W was generated on the antenna while transmitting. When receiving, a power that is orders of magnitude (x 1000) weaker is processed. It must therefore be ensured that practically none of the transmission power reaches the reception path. This is first of all guaranteed by building particularly good transmitters that only transmit in the prescribed frequency range. However, it cannot be completely avoided that interference signals still leak into the reception path. This can be prevented with a so-called duplex filter.

The challenge with a mobile phone is that it has to send and receive at the same time. This is a particular difficulty because sending always interferes with receiving.

A duplex filter is a filter that works in the radio frequency range at around 100 MHz. More precisely, it is a bandpass filter. It allows signals in a certain range to pass through and suppresses all frequencies to the right and left of this range. We have already discussed such a filter as an IF-filter for superheterodyne receivers. This means that any signals from the transmitter can be suppressed on the reception path, so that they do not interfere with reception. Technically, this is possible through complex and large structures, so-called resonators, which are made of metal and often have a volume of up to one liter. They not only make a radio telephone expensive, but also larger and heavier.

Another challenge with a radio telephone is that it requires two bands to operate. One to receive and one to send.

First Receive and Transmit Channels

In 1947 an ITU conference was held in Atlantic City, New Jersey. Necessary decisions for the release and use of new frequencies were dealt with. The new technologies that emerged during the war meant that frequencies for radar and aviation radio had to be set. Police, fire brigade and disaster control also needed frequency ranges for their tasks. And there was also a request for frequencies for public mobile telephone service. In the end, it was agreed to reserve two bands on the so-called 2 m band with frequencies around 150 MHz.

The first systems with the MTS (Mobile Telephone System) emerged in the United States. This was operated by Bell System in St. Luis, Missouri. However, only three channels were used which meant that only three radio conversations could be carried out in parallel.

In Germany, after the war, the frequency of 156 MHz to 174 MHz, well above the VHF range of FM radio (87.5 MHz to 108 MHz), was available for mobile communications. The so-called Öffentlich bewegter Landfunk (public mobile land radio) ÖbL was established to make public telephone conversations possible via radio. This began as early as 1950. However, there were different systems locally. There was a waterway radio, a train radio, a harbor radio, a city radio and a country road radio. The frequencies and radio channels were fixed. There were initially only 17 channels, later 36 with a distance of 50 kHz. Since duplex operation was required for land radio, the channels were actually always pairs, a receiving channel and a transmitting channel, separated by 4.5 MHz. The mobile transmitters worked with up to 10 W, the radio towers with up to 30 W.

How to share a radio channel

Now there were radio channels that could be used and soon there were devices that allowed to make radio calls via these channels. But there was a significant difference compared to the “normal phones”. A normal telephone has a line that only belongs to that one telephone. This guaranties:

• You can always establish a conversation (at least up to the control center)
• You can be identified (by the network and therefore receive an invoice)
• You can have a private conversation (no one else can listen in)

However, this is not possible with radio channels unless you get one radio channel per mobile phone. But then it would only be possible to have 38 telephones because there are only 38 channels. So you have to share the few radio channels with other participants. What do you need to share a channel? Let’s assume we want to make calls as a mobile subscriber.

• You have to know when a channel is free.
• If the channel is free, we have to occupy it in cooperation with the base station. This means it must be blocked for everyone else, otherwise the other participants would listen in.
• The base station must clearly identify the subscriber who wants to use the channel so that he or she will receive the bill later.
• The base station must be informed when the call has ended so that it can reactivate the channel.

On the other side, if somebody wants to call a mobile phone

• The base station must determine whether I, as a mobile subscriber, am in the network and can be reached.
• The base station must be able to identify and call me. So every mobile participant must be “identifiable”

In any case, you need something called signaling. Some technique of indicating a state. This happens with an acoustic tone that is sent or received.

Mobile telephony is not possible without a minimum of signaling.

Signaling

If a channel is free, the base station sends a tone with a frequency of 2280 Hz. This is received by all mobile subscribers. This does not mean that the participants hear this high-pitched tone, but the receiving electronics “hear” it. The receiver electronics has a filter that allows to identify this frequency tone. If this is the case, it turns on a green light indicating that the radio channel is available.

Tones can therefore be used to convey messages. Different tones can mean different things, just as the 2280 Hz tone means a radio channel is free. All that is required, is a tone detector that can detect and interpret the tones.

Manual Call Setup

If a user wants to make a call with his mobile telephone, he simply pick up the telephone receiver. This causes the transmitter to be switched on. The transmitter now sends a tone with a frequency of 1750 Hz to the base station. The base station is permanently on reception. If it now receives the tone at 1750 Hz, it stops sending out the 2280 Hz tone and all other telephones now “see” that the channel is blocked and the green light is switched off. The base station now establish a connection. The first thing that happens is that the operator (at the base station) speaks to the user, asking him to identify himself reading out the „telephone number“ which is written on the mobile telephone. After the subscriber is identified, he verbally communicates the number to which he want to be connected. The connection to the end user is created in the telephone network manually by the operator. Finally a connection to the radio channel is established and the actual telephone call can begin. The operator measures the time the conversation lasts. If the subscriber ends the call by hanging up the phone. The operator determines the costs and contacts the subscriber again to inform about of the costs. The channel will then be released again.

It can be seen that this is an expensive and complex process. Above all, the valuable channel is occupied for a long time. This means that only a very limited number of calls can be processed.

The whole thing is more complicated if you want to call a mobile subscriber from the telephone network. How can the telephone network or the operator determine whether the subscriber is ready and how is the connection established? One option that was actually used at the beginning was the “public call”. The operator asked all participants whether the participant with phone number xyz was present. If this was the case, it then established the connection. However, this was not a good practice. In the 1950s, people experimented with other ways to identify participants by sending out signal tones.

Automatic Call Reception

It was not until 1958 that a uniform and automatic system for identifying participants was established. In Germany the introduction of such a system was leading to the so call „A Netz“ (A Network). In Nuremberg, the TeKaDe company was commissioned to create such a uniform system, which became established in the early 1960s. This system was originally developed at Bell Labs (the good old development department of Alexander Graham Bell).

30 different tones were defined, all at a distance of 15 Hz from each other. Each participant was now assigned 4 different tone frequencies (4 out of 30). This resulted in thousands of possible participants. Each receiver now installed electronics that contained 4 filters that corresponded to the assigned four tones. When the channel was free, all receivers with their filters “listened” for their tones. If a tone with the corresponding frequency came, the corresponding filter signaled a “hit”. However, only when all four filters showed a hit, there was a “bingo”. The recipient’s phone “rang” and and the subscriber could take the phone call.

Mobile Telephone Service

Germany was one of the leading countries in the introduction of mobile phones. Of course they weren’t the only ones. Bell Communication, which developed the 4/30 process through Bell Labs, also launched systems in the United States that were called Mobile Telephone Service (MTS).

Mobile phones were not widely used. They were expensive and heavy. In addition, there was only a limited number of participants due to technology. In Germany in 1971 there were 11,000 participants, practically the “top ten thousand”.