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What happens in the last 10 metres?
Main Configuration
The following paragraphs will introduce the most basic idea about modulation & demodulation, transmission and information process in a Bluetooth chip. The task of Bluetooth is originally to transmit data from one device to another wirelessly, especially from Internet to handhold devices. All the transmission always happens 10 metres away from the end of any Network. Now, let's begin to dig into the detail of the last 10 metres!
Now, first thing first, a brief configuration of a simple Bluetooth Device is presented below. All the system looks almost the same with any other communication system, which is also hardware system controlled by software.:
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The most basic part is the Bluetooth Transmitter & Receiver, which plays the most important role here, generalizing RF signals and providing rude information for later process. Link Controller (LC for short) apparently controls the performance of Transmitter and Receiver, setting its operation modes, providing and receiving and processing information in digital form and so forth . LM (with full name Link Manager) is software. Downwards it programs the performance of LC, control the link setup, discrimination of different signals, attributes setting and other corresponding tasks. Upwards it communicates with other Bluetooth Device according to LMP (Link Manager Protocol). All these three components compose of the whole Bluetooth Module. Since they are most related to the our Principle of Communication course, we will emphasis on them, especially the Transmitter & Teceiver.
However, an independent Bluetooth Module can hardly be useful, it must obey the instruction from Host Controller, Mobile Phone and PC for examples. Application Layer translates instructions from HC, sends the information to the right part of Bluetooth Device, mainly LM, to control Bluetooth Module ultimately when to shut down, when to send a message and so on.
A further more graph is presented below to show how two Bluetooth Modules communicate, which is hardly beyond our expection:
Modulation
Now let¨s consider the basic modulation and demodulation techniques used in Transmitter & Receiver. Generally, Bluetooth employs a very common digital modulation, say GMFK. Here it is supposed that readers are all familiar with it, and beginners can click here to get to know it.
But it is strongly recommended that readers go through
the mathematical description first.
Mathematically, the equation for MSK can be expressed as:
Where
 is
the information in digital form
According to the equation above, we can easily understand the implementation below:
And Gaussion MSK is formed by prelocate a Gaussian LFP before MSK Module.
Demodulation
There are several ways to demodulate MSK, for GMSK as
well as GFSK, things are almost the same. From the
mathematical equation, the signal composes of two parts,
each orthogonal to the other. The most direct and
reasonable way is to employ cohere detector. For
convenience, equation for MSK is rewritten below:
The typical demodulation block diagram is shown here:
After the cohere carrier detector, two carrier waves are
generated,
 and
which are sent back to interfere with the input signal
correspondingly. And as the result, thesignal flowing
through the two direct paths will be changed to
 and
Because of the peculiar characteristics given below,
which has been talked about in the mathematical
description of GFSK:
and
when samples are taken alternatively from the two path,
original signal will be recovered.
From above we have got a impression about about how Transmitter & Receiver works. And the next graph
can help you know the whole Bluetooth Module, which is
usually integrated into IC by companies.
Frequency Hopping
What cannot be ignored here is the Frequency Hopping, which is used almost in Wireless Device. It is common sense that frequency 2.4GHz (ISM for short, namely industry, science and medicine frequency) is an open frequency range, which is noisily shared by many neighbours. Here is a list showing some of the them:
1. 2.4GHz wireless phone;
2. 802.11 Network;
3. HomeRF wireless Network;
4. Most urgency wireless communication between urban and suburb areas;
5. Social Communication Systems in Spain, France, Japan and other countries;
6. Some civil use, for instance, infant monitor-system and controller of garage;
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7. Microwave oven. It¨s very interesting and coincident that ISM frequency is the resonant frequency for water-molecule, which is the very design principle for Microwave oven. When Bluetooth used by Microwave oven, you will notice significantly that the capability declines.
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To solve the possible ＾block￣, Bluetooth uses Frequency Hopping technique. Transmitter & Receiver hop 1600 times per second between 79 frequency slots from 2.402GHz to 2.480GHz. 2.480GHz minus 2.402GHz equals 78MHz, which apparently means frequency interval is 1MHz. However, in Japan and some other countries whose ISM has been partly used Social Communication System have to limit the hopping frequencies. For example, in Japan only 2.471-2.497GHz can be used by Bluetooth.
TDD(Time Division Duplex)
By now, the Baseband transmission has been introduced. Nevertheless, to make communication possible, the techniques discussed above aren¨t sufficient, for they cannot make counterparts really communicate simultaneously. You have to wait to speak until the caller stops. To make fullduplex transmission possible, TDD (Time Division Duplex) should be employed. In this way, each carrier frequency is divided into several time slots, each generally 625ms lang. In one slot, master device transmits and slave device keeps ＾listening silently￣, and in the next slot vice versa. More sophisticatedly, to bring down interruption between two time slots, another protect time slots should be used, setting the transmitting time slots apart. Since the transient stay of mankinds¨ ears, such a quick interruption cannot be detected.
To make the principle more clearly understood, a graph is given below.
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