Pulse or digital communications – Transmitters
Reexamination Certificate
1999-08-11
2003-05-13
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Transmitters
C375S225000
Reexamination Certificate
active
06563884
ABSTRACT:
TECHNOLOGICAL FIELD
The invention concerns generally the technology required for changing the data rates during an active connection between a base station and a mobile station in a radio communications system. Especially the invention concerns a situation where the code rate and/or the modulation method is changed.
BACKGROUND OF THE INVENTION
EDGE or “Enhanced Datarates for GSM Evolution”, where GSM stands for “Global System for Mobile telecommunications”, is an international project where means are developed for providing users with higher data rates in a telecommunications system based on known GSM technology. The raw bit rate at the GSM air interface between a base station and a mobile station is 22.8 kbit/s. Channel coding and the overheads associated thereto lower the usable data rate so that data rates from 3.6 to 14.5 kbit/s are available to connected data applications. Even these rates carry a certain overhead on top of the available user data rate. At the time of filing this patent application EDGE aims at a raw air-interface bit rate of 69.2 kbit/s. The enhancement in data rates is largely due to the 8PSK (8-level Phase Shift Keying) modulation method which is employed in EDGE, in contrast to the GMSK (Gaussian Minimum Shift Keying) modulation used in GSM.
It is expected that EDGE connections will not be supported throughout a cellular network. EDGE connections will most probably be available only in central areas like office buildings and densely trafficated business centres. To take advantage of the full capability of the system a mobile station must therefore support both the advanced EDGE data rates and the conventional GSM data rates. A mechanism must thus be developed for changing between the two. In EDGE, the concept of link adaptation has been defined. It comprises all means of changing the radio interface data rate during an active connection by changing the modulation method or also by changing the amount of employed channel coding. The latter may be varied to compensate for changes in the quality of the radio environment. There exists a risk that link adaptation will cause perceivable artefacts, like clicks or silent periods in an audio communication or blank or distorted pixels or fields in a video communication. It is naturally desirable that a user will not be able to perceive an executed link adaptation command by just observing the results (sound, picture etc.) of the communication. A seamless change in the radio interface data rate is defined to be a link adaptation operation which goes totally unnoticed by the user.
As a background for the invention, the transmission chain used in the conventional GSM system will be briefly discussed with reference to FIG.
1
. The transmission of full-rate speech is used here as an example of a typical service requiring a circuit-switched connection. Speech recorded by a microphone
101
will first be encoded in a speech encoder
102
which converts an analogue speech signal into digital form and performs a group of encoding operations. The output signal of the speech encoder has a rate of 13 kbit/s and consists of blocks of 260 bits, the blocks following each other at an interval of 20ms. The channel encoder
103
introduces redundancy into this data flow, increasing its rate by adding into it information calculated from the contents of the blocks. The reason for channel coding is to allow the detection or even the correction of signal errors introduced later during transmission. The output of the channel encoder
103
consists of code words of 456 bits each. Exactly one code word is produced from each block of input information for the channel encoder.
The code words that come from the channel encoder
103
are input to the interleaver/burst formatter
104
for mixing up the bits of several code words in a predetermined fashion and organising them into bursts. The aim of interleaving is to decorrelate errors that will potentially occur in the transmission so that the resulting erraneous bits will be distributed into essentially randomised positions in several code words instead of corrupting a sequence of successive bits in a single code word. Most interleaving methods that are currently used are diagonal, meaning that bits from consecutive code words are cross-distributed so that certain bits of the later codeword come earlier in the interleaved data stream than certain other bits of the former codeword. In GSM, the bits from a certain code word are spread over a period of 22 bursts, and a single burst may contain bits from as many as five different code words. The interleaving details of GSM depend on the nature of the information to be interleaved (speech, data, access request etc.)
The burst formation part of the interleaver/burst formatter
104
takes a block of 116 interleaved bits and adds three zero bits (called the tail bits) at the beginning and end of the block as well as a so-called training sequence of 26 bits exactly in the middle of the block. At the output of the interleaver/burst formatter
104
the flow of information consists of formatted blocks known as bursts. For the description to be consistent throughout this patent application, the bits of a GSM burst will be called symbols in the following. Additionally the burst will be denominated as a digital burst while it is still in digital form. Each digital burst thus comprises 114 symbols of coded data and 2 so-called stealing flag symbols that indicate whether the coded data symbols include user data or signalling data belonging to a certain Fast Associated Control Channel (FACCH). Additionally each digital burst comprises the above-mentioned 3+3 tail symbols and the training sequence of 26 symbols.
The ciphering block
105
performs a logical exclusive-or operation between the coded data symbols of a digital burst and a certain pseudo-random bit sequence in order to impede the unauthorised reception of the transmitted data. The tail symbols, the stealing flag symbols and the training sequence are not ciphered. After ciphering the digital bursts are input into a modulator/upconverter
106
that transforms each digital burst into a sequence of a radio-frequency analogue oscillating signal, which is amplified in an amplifier
107
and conducted into an antenna
108
for transmission. Because of its close connection with the digital burst, the analogue signal sequence is also known as a burst; for clarity it can be further specified as a transmission burst. Several filtering operations take place inside the modulator/upconverter
106
and between it and the antenna
108
; for graphical clarity the respective filter blocks are omitted from FIG.
1
. In GSM a Time Division Multiple Access (TDMA) scheme is applied, in which each speech channel may use a single time slot in a cyclically repeated frame of eight consecutive time slots. The transmitter transmits one transmission burst in each time slot during the active connection.
A receiver chain for receiving, demodulating and decoding the data transmitted by the transmission chain of
FIG. 1
would consist of a receiving antenna for receiving the radio signal, some filters and amplifiers for filtering and amplifying the received signal, a downconverter/demodulator or an equalizer for converting the transmission burst into digital form on baseband frequency, a deciphering block for converting the ciphered bits into plain data, a burst deconstructing/de-interleaving block for exctracting the data bits and removing the interleaving, a channel decoder for removing the channel coding, and a speech decoder/D/A converter for converting the decoded digital signal into an analogue signal from which the original speech may be reproduced by a loudspeaker. The operation of the blocks in the receiver chain is approximately the inverse of that of the respective blocks in the transmitter chain.
Minor changes are required in the above-explained functions of the transmission and reception chain blocks for other transmission modes than full-rate speech. These changes are known to the person skilled in the art f
Corrielus Jean
Nokia Mobile Phones Limited
Perman & Green LLP
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