Các mô hình hệ thống chung tuân theo cấu trúc liên kết tương tự như mô tả trong hình 3.1, tức là một nguồnMT (s-MT) giao tiếp với một mục tiêu MT (t-MT) thông qua một số nhất định của chuyển tiếp MTs(r-MTs). R-MTs liền kề trong không gian được nhóm lại thành chuyển tiếp ảo ăng-ten mảng (rVAAs), cấu hình chính xác trong đó đã được giải thích kỹ lưỡng trong chương 3. Cáccấu hình hệ thống được mô tả đó trong đầy đủ chính xác để đối phó với năng lựccủa mạng lưới như vậy.Tuy nhiên, việc triển khai thực tế thu yêu cầu tiếp tục giải thích như thế nàoHệ thống như vậy sẽ làm việc trong thực tế. Nó do đó là mục đích của phần này để cung cấp này thiếuthông tin. Quan tâm ở đây là các truyãön và nhận được sử dụng, cũng như các hiện hànhKênh thông tin liên lạc.4.2.1 transceiver mô hìnhKhối chức năng thu hình thành nhiều giai đoạn phân phối MIMO chuyển tiếpmạng được mô tả trong hình 4.1. Đầu của con số 4.1 liên quan đến nguồn VAAcó s-MT; giữa liên quan đến một tier VAA tùy ý relaying; và phía dưới cùngliên quan đến mục tiêu VAA có t-MT. Trong hình, mỗi tầng VAA Hiển thịbao gồm ba thiết bị đầu cuối; đó là, Tuy nhiên, hiểu hợp lý rằng bất cứ sốthiết bị đầu cuối có thể được bố trí.Cụ thể, các nguồn thông tin đi thông tin đến một thu phát sóng hợp tác xã,đó chuyển tiếp dữ liệu để trong không gian bên cạnh r-MTs thuộc VAA cùng. Một lần nữa, điều nàyis assumed to happen over an air interface distinct from the interface used for inter-stagecommunication or an air interface not requiring any optimisation, and is not consideredfurther. It is also assumed that these cooperative links are error-free due to the short communication distances. Each of the terminals in the VAA perform distributed encoding ofthe information according to some prior specified rules. That information is then transmitted from the spatially distributed terminals after having been synchronised. Note that theproblem related to synchronisation is beyond the scope of this thesis.Any of the relaying VAA tiers functions as follows. First, each r-MT within that VAAreceives the data which is optionally decoded before being passed onto the cooperativetransceiver. Ideally, every terminal cooperates with every other terminal; however, anyamount of cooperation is feasible. If no decoding is performed, then an unprocessed ora sampled version of the received signal is exchanged with the other r-MTs. Note thatunprocessed relaying is equivalent to transparent relaying. After cooperation, appropriate decoding is performed. The obtained information is then re-encoded in a distributedmanner, synchronised and re-transmitted to the following relaying VAA tier.1411st Tier VAAsource-MTr-MT #2r-MT #1InformationSourceCooperativeTransceiverCooperativeTransceiverCooperativeTransceiver Synchr.Synchr.Synchr.TransmitterTransmitterTransmitterDistributedEncoderDistributedEncoderDistributedEncoderV-th Tier VAAtarget-MT...r-MT #2...r-MT #1...CooperativeTransceiverCooperativeTransceiverCooperativeTransceiverReceiverReceiverReceiverOptionalDecoderOptionalDecoderOptionalDecoderDistributedDecoderInformationSinkv-th Tier VAAr-MT #2r-MT #3r-MT #1CooperativeTransceiverCooperativeTransceiverCooperativeTransceiver Synchr.Synchr.Synchr.TransmitterTransmitterTransmitterReceiverReceiverReceiverOptionalDecoderOptionalDecoderOptionalDecoderDistributedTranscoderDistributedTranscoderDistributedTranscoderFigure 4.1: Functional blocks of the source VAA (top), the vth relaying VAA (middle) andthe target VAA (bottom).142As for the target VAA, the functional blocks are exactly the opposite to the source VAA.All terminals receive the information, possibly decode it, then pass it onto the cooperativetransceivers which relay the data to the target terminal. The data is processed and finallydelivered to the information sink.The functional blocks of the distributed transcoder, i.e. encoder and decoder, are nowelaborated on in more detail. To this end, the encoder and decoder are shown in Figure 4.2.Generally, the role of a channel encoder is to insert sufficient redundancy into the signalto mitigate the detrimental effects of noise and the fading channel. The insertion of redundancy decreases the data rate, where (with a good channel code) a decrease in rate comesalong with an increase in coding gain. Together with the additional complexity, these needto be traded-off to yield optimum performance in terms of the BER versus Eb/N0, whereEb is the information bit energy and N0 is the noise power spectral density.The channel code is traditionally accomplished by means of a convolutional code, which‘convolutes’ the redundancy into the original signal stream. Nowadays, it is considered tobe a low complexity code and is often found to be available within communication chip-sets.Another class of codes are the block codes. These generate the redundant information fromthe original data stream, after which it is inserted into it. A more complex class of codesare turbo codes, which were shown to operate near the Shannon capacity. For a properfunctioning and mathematical description of these codes, refer to [67].Cooperative EncoderBinaryInformation BitsChannelEncoderSpace-TimeEncoderEncodedInform. Symbolsto each AntennaControl #1 Control #2Cooperative Decoderdirect symbolsSpace-TimeDecoderChannelDecoderControl #3 Control #4cooperative symbols BinaryInformation BitsFigure 4.2: Distributed Encoder and Decoder.143The channel encoder may also consist of two or more concatenated codes, which arepreferably connected by interleavers which break long error sequences. For example, trelliscodes are known to produce a cluster of errors, which could then be corrected by appropriateblock codes.A channel encoder within a distributed encoder does not normally differ from a nondistributed encoder; however, it is generally possible to design channel codes which reflectthe distributed nature of the encoding process. Example trellis codes are introduced in [31],where the encoder requires some form of control as to decide which code to employ.The role of a space-time encoder is to utilise the additional spatial dimension created bysufficiently spaced antenna elements to increase the system performance. If each antennaelement is used to transmit independent data streams, then such spatial multiplexing technique is referred to as BLAST [30]. Clearly, the data rate of such a system increases linearlywith the number of transmit antennas; however, the lack of spatial redundancy makes itmore susceptible to noise and interference when compared to coding techniques describedbelow.If, instead, the additional spatial domain is used to provide redundant information, thensuch a spatial encoding technique is referred to as space-time coding. The computationallysimple space-time block codes (STBCs) have already been introduced in Chapter 2, wherethey were shown to orthogonalise the MIMO channel. More complex codes are space-timetrellis codes (STTCs), or space-time turbo codes. Note that space-time codes (STCs) can
also be concatenated with an outer channel code to yield additional performance gains as
described above.
The functionality of distributed space-time codes (STCs) differs from a traditional deployment because only a fraction of the entire space-time codeword is transmitted from any
of the spatially distributed terminals. The transmission across all terminals then yields the
complete space-time codeword. Therefore, a control signal to each distributed space-time
encoder is essential, as it tells each of them which fraction of the entire space-time codeword
to pass onto the transmitting antenna(s). This is indicated as Control #2 in Figure 4.2.
This control information is assumed to be available to the space-time encoder, and is thus
not discussed further in this thesis.
The cooperative decoder can be realised as the inversion of all processes at the cooperative transmitter. Here, the space-time decoder is fed with the signals directly received from
the available antenna(s), as well as the information received via the cooperative links from
adjacent terminals. Again, a control signal is needed which specifies the type of information
fed into the space-time decoder, to allow for optimum decoding. For example, the control
signal could inform the decoder that the relayed signals are a one bit representation of the
sampled soft information available at the respective cooperative relaying terminals.
144
After the space-time decoding process, the information is passed on to the channel
decoder which performs the inverse proces
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