Cooperative Communications (Foundations and Trends in Networking)

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Figure 3 illustrates the nodes and signal names used in the following derivation. Consider the initial source to relay transmission, where and are the carrier frequencies of the source and relay, respectively. The sequence of operations and corresponding signals are. As expected, the baseband signal received at the relay suffers a frequency offset due to the difference between the source and relay carrier frequencies.

Finally, we substitute the previous expression for :. Thus, the received baseband signal at the destination node suffers a frequency offset determined solely by the difference between and , independent of their respective offsets from. In other words, the relay's carrier frequency offset with respect to the source and destination nodes does not affect the final signal received at the destination.


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In order to substantiate the preceding analysis and to verify the impact of its inherent assumptions, we constructed an RF link which allows the direct observation of carrier frequency offsets. In this setup, one node acts as both the source and destination, while a second node acts as the relay.

The source generates a constant valued baseband signal, which after upconversion results in the transmission of a sinusoid at exactly. The relay node receives this sinusoid, downconverts it with its local carrier, and saves the samples at baseband. If the analysis is correct, these samples should be of a sinusoidal signal with frequency. The relay then transmits the same samples back to the first node. If our assumptions and analysis hold, the first node should receive a constant valued signal at baseband, showing no frequency offset as a result of amplification and retransmission by the relay.

Figure 6 shows the results of this experiment. Two trials are depicted here. In the first, the relay transmits its received signal after a short delay, approximately 10 milliseconds. In the second, the relay waits two minutes before retransmitting. The transmission in both directions happens over a wire to eliminate any channel effects.

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The top plots depict the phase of the signal received at the relay. The phase of this signal is increasing linearly in time, corresponding to a received sinusoid. This sinusoid is the direct result of carrier frequency offset between the two nodes. The bottom plots depict the phase of the signal received at the source node, after it is buffered and retransmitted by the relay. The complete lack of the saw wave pattern clearly illustrates the relay canceling its own carrier offset during retransmission.

In Figure 4 b , a very slight slope can be observed in the received signal's phase. This is the result of a minor drift in the node's local oscillator frequency. The WARP hardware utilized in this experiment uses temperature-compensated crystal oscillators for the carrier reference, which accounts for the very minor drift, even after two minutes. Cheaper oscillators, like those used in low-end commercial wireless hardware, could exhibit larger drifts over time. Experimental observation of carrier offset in a relay system.

This section describes the construction of an amplify and forward cooperative communications system which relies on the properties described in Section 2. This system is implemented on WARP [ 5 ], making heavy use of the custom hardware, physical layer designs, and other support packages provided by the platform.

Our system is built on the idea of distributed space-time coding [ 3 , 9 ], where multiple nodes cooperate to transmit a signal which approximates the transmission of a single, multiple-antenna node. Figure 5 illustrates the classic STBC configuration which the proposed cooperative scheme imitates. The signal names here correspond to the two spatial streams generated by a two-antenna Alamouti transmitter; these signals play a key role in the proposed cooperative version of this link. Equivalent Alamouti setup. Node configurations and activity in the amplify and forward system.

Time slot 1Time slot 2. Given two data symbols and , the code outputs the signals shown in Table 1. In each symbol period at the receiver, the superposition of the two streams is received after each passes through separate channels; the signals received in two symbol periods are represented by and. The receiver uses local channel estimates and the following combining rules to recover the original data symbols:. Much like other cooperative protocols for half-duplex radios, the proposed cooperative link operates in two time slots per packet.

Figure 6 illustrates the activity of each node in our scheme's two time slots. In the first slot, the source node transmits the full packet, encoded, using the Alamouti space-time block code. This signal matches that which would be sent from one antenna in a true two-antenna Alamouti transmission. The relay node receives this transmission and stores the raw samples in a buffer. In the second time slot, the source node transmits the other half of the Alamouti-encoded sequence, and the relay transmits its stored copy of the first transmission.

The destination node receives the superposition of these simultaneous transmissions. From the perspective of the destination, it receives a standard Alamouti-encoded packet, where each of the spatial components was exposed to an independent channel. In the proposed amplify and forward scheme, the timing of the two transmissions in the second time slot cannot be perfectly guaranteed. The offset between the arrival times of the source and relay's transmissions can be modeled as multipath.

This is analogous to the signals sent from a standard two-antenna Alamouti transmitter arriving at slightly different times at the receiver after passing through different channels. In order to cleanly handle this potential impairment, we chose OFDM as the underlying physical layer for our cooperative system. OFDM's inherent immunity to multipath makes it an ideal PHY for an amplify and forward system, as a delayed transmission which is treated as just another reflection in the channel.

Our cooperative physical layer uses the following frame format, partially inspired by IEEE The transmissions are composed of four components:. The composition of the transmissions in each time slot is illustrated in Figure 7. In the first time slot, the source node transmits a frame designed to trigger packet detection at the relay but avoids packet detection at the destination.

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This is achieved by omitting the long training symbols. If this correlation fails, the receiver assumes a false packet detection and resets. The relay node does not perform this check. In the second time slot, the LTS must be included to allow the destination to properly detect the packet and synchronize the receiver. It is critical as both the source and relay nodes send the LTS so that the destination can detect the packet based on either node's transmission.

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The relay node also sends a full preamble in the second time slot. The relay stores this preamble in a lookup table and sends it in place of the STS captured in the first slot. During the preamble, the source and relay are transmitting the same signal simultaneously.

This opens the possibility of unintended beamforming, where the identical signals constructively or destructively interfere at the destination. However, we did not observe this effect during our experiments. We attribute this to a number of phase variations between the source and relay transmissions. First, the relay generates its preamble locally, so its transmission occurs with a carrier frequency offset relative to the source. This offset does not exist for the buffered and retransmitted channel training and payload symbols, for the reasons discussed above.

Second, the source and relay have independent phase noise characteristics, as each node's carrier is generated from a local PLL. Finally, the source and relay use separate sampling clocks, introducing another source of independent jitter phase noise which will tend to blur their combined beam pattern. After sending their preambles, the nodes send channel training symbols in alternate symbol periods. This orthogonal-in-time training allows the destination to estimate the two channels separately.

Following the training symbols, both nodes transmit payload symbols simultaneously.

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These transmissions are entirely nonorthogonal, overlapping in frequency and time. The source transmits the second Alamouti spatial stream while the relay transmits the version of the first stream it captured during the first time slot. These simultaneous transmissions utilize the Alamouti space-time block code to avoid interfering with one another and exploit the delay spread tolerance of OFDM to combat small synchronization differences between the overlapping transmissions. We designed this scheme to allow successful packet detection and synchronization at the destination even if it receives just one of the two transmissions in the second time slot.

Both transmissions contain everything the destination needs to receive the packet—preamble, channel training symbols and the full payload. In an intuitive sense, this scheme preserves full diversity as it will fail only if two presumably independent channels are simultaneously in deep fades. In our setup, the relay node uses a wired synchronization signal from the source to initiate its buffering and retransmission processes.

The packet lengths are also fixed throughout our experiments and are known ahead of time by every node. The destination node implements autonomous packet detection. This system uses the RSSI received signal strength indicator signal from the RF transceiver to detect a spike in received energy indicating the start of a new packet. This is the same approach to packet detection and timing used in a noncooperative random access system. If the uncertainty of packet arrival times at the destination was eliminated, as in slotted systems like GSM or WiMAX, we expect that the system performance would improve.

Every node has independent sampling and radio reference clocks. Given the relatively short packets, we ignore sampling frequency offsets throughout. Offsets among the radio reference clocks result in carrier frequency offsets, the effects of which we explored in Section 2. Both the relay and destination nodes implement automatic gain control, which executes with each packet detection.

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Click here to subscribe. We use cookies to give you the best experience on our website. If you continue without changing your cookie settings, we assume that you consent to our use of cookies on this device. You can change your cookie settings at any time but if you do, you may lose some functionality on our website. Therefore, the successful completion of SA specification has laid a foundation for the overall launch of 5G industry.

With 5G commercialization entering the final sprint phase, Datang Telecom Group will continuously endeavor to promote 5G into global commercial success. Liu Guiqing said: "China Telecom is proud of being important part of the team for completing this historical 5G milestone. With this Rel SA specification frozen, China Telecom plans to lead the efforts on verifying the specified performance and optimizing the innovative features through field trials in many cities.

To get ready for commercialization, China Telecom will work closely with the vendors and partners to promote 5G ecosystem and to explore new applications and business cases. It is the result of wisdom and sweat of experts in communications, and carries high expectation and dreams of the whole society to form a more intelligent world.

The completion of specifications is a milestone for 5G commercialization. It can not only provide high speed wireless access, but also enable vertical industry applications.


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China Unicom will work together with the whole industry to build a good ecosystem and create a new dimension for 5G development, with the "five new" attitude". We appreciate the efforts and contributions from 3GPP colleagues and stakeholders. Chunghwa Telecom will continue cooperating with the whole industrial players to enter the 5G wonderland. We now look forward to continuing our cross-industry collaboration to accelerate the 5G ecosystem development and explore the applications of reduced latency and network slicing, so that we can bring the full benefits of 5G to our customers.

We look forward to contribute to future milestones which enable the full potential of 5G to address massive connectivity and solutions for other verticals beyond broadband. Erik Ekudden, Senior Vice President and Chief Technology Officer, Ericsson, says: "5G is fast approaching commercial reality thanks to the dedicated, industry-wide standardization work lead by 3GPP, we together have accelerated the delivery of the standard well ahead of time. Together with our ecosystem partners we will sustain this momentum and ensure that communication service providers can successfully launch 3GPP standard-based 5G networks.

The 5G network is surely the foundation for the secure exchange of information between trusted communities in the hyper-connected world. Fujitsu will strive to provide 5G network products and connected services to co-create human centric innovation through digital technologies with our customers and partners all over the world. Yang Chaobin, president of Huawei 5G product line, said: "With the development of 5G NR standardization, We are pleased to cooperate with global organizations to reach a great milestone that 3GPP 5G NR specification of Standalone SA have been completed, which is a critical step forward for the 5G standardization and industry ecosystem.

Huawei will positively invest on the research and development of 5G key technology and product, continue cooperating with global industry partner, and promote the implementation of 5G commercial deployment and health industry ecosystem worldwide. This new air interface is another major step in our journey to power this first wave of 5G. Intel, with our end to end network, cloud and client focus, and our partners are reimagining the network to deliver a true convergence of computing and communications.

Robert DiFazio, Vice President, InterDigital Labs: "The completion of the 5G NR standalone specification and indeed all the new technologies that will make up 5G are the culmination of many years of work from all the companies and hundreds of engineers at 3GPP, and InterDigital is proud to have worked alongside them. InterDigital has been committed to playing a leading role in mobile industry standards across multiple generations of wireless, and we are excited to continue contributing our best technology as 5G deploys and brings amazing connectivity and economic opportunity throughout the world.

Satish Dhanasekaran, senior vice president of Keysight Technologies, and president of the Communications Solutions Group CSG : "We are excited to enable the industry at a threshold of 5G acceleration and commercialization. The completion of the standalone SA 5G new radio NR specification marks a distinct milestone and offers a playbook for a connected ecosystem to move forward, in making 5G a reality and unlocking huge potential for society.

Keysight's is engaged with market leaders, contributions to the 3GPP standardization development and providing scalable 5G test and measurement solutions all the way from L1 to L7.

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With this successful completion of the 5G NR specification, KDDI will continue our collaboration with various business partners and prepare for the commercial 5G service launch in KT Corp. Kyocera believes the new technology will fundamentally change the efficiency of network resource utilization, deployment flexibility and support of various applications with different QoS requirements such as IoT and V2X, some of which require high efficiency, ultra-reliable, low latency operations. The game-changing 5G has extremely broad implications for the telecommunications as well as many other industries that affect the most exciting and promising fields of the modern society for years to come.

Lenovo has been working closely with key industry partners like China Mobile to accelerate the commercialization of 5G technologies to bring their full benefits to both our consumers and clients.

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Based on this standard, LG Uplus is preparing for the successful deployment and commercialization of 5G in We will contribute to the continued development of 5G NR. MediaTek is deeply involved in the standardization of 5G core technologies. Takashi Nishimura, executive officer in charge of Communication Systems Group, said: "It is our great pleasure to be a member of the group that has created a new 5G standard. Mitsubishi Electric Corporation wishes to contribute to the realization of a prosperous society that simultaneously achieves "sustainability" and "safety, security and comfort", making the best use of the strengths of our existing businesses as well as the opportunities presented by this recently developed 5G standard.

This is a significant milestone towards successful commercialization of 5G, which will enable new value and services through its secure and intelligent technologies. Going forward, NEC will continue to develop and provide innovative 5G solutions for society that enable advanced communications and a diversified range of sophisticated services.

Marcus Weldon, President of Bell Labs and Corporate Chief Technology Officer, Nokia, said: "Nokia is proud to have played a significant role in achieving this milestone with excellent global industry cooperation. It enables exciting possibilities for the digital transformation that 5G will bring us for a connected world.

A whole new horizon is available to create a digital economy with vertical industries beyond mobile broadband.



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