In the PDCCH region in DL radio frame, there can be many places where a specific PDCCH is located and UE searches all the possible locations. The possible location for a PDCCH differs depending on whether the PDCCH is UE-Specific or Common, and also depend on what aggregation level is used. All the possible location for PDCCH is called 'Search Space and each of the possible location is called 'PDCCH Candidates'.
The search space indicates the set of CCE locations where the UE may find its PDCCHs. Each PDCCH carries one DCI and is identified by RNTI. The RNTI is implicitly encoded in the CRC attachment of the DCI.
There are two types of search space : the common search space and the UE-specific search space. A UE is required to monitor both common and UE-specific search space. There might be overlap between common & UE-specific search spaces for a UE
- The common search space would carry the DCIs that are common for all UEs. For example, system information (using the SI-RNTI), paging (P-RNTI), PRACH responses (RA-RNTI), or UL TPC commands (TPC-PUCCH/PUSCH-RNTI). The UE monitors the common search space using aggregation level 4 and 8. Maximum number of CCEs present in common search space is 16.
- The UE-specific search space can carry DCIs for UE-specific allocations using the UE's assigned C-RNTI, semi-persistent scheduling (SPS C-RNTI),or initial allocation (temporary C-RNTI). The UE monitors the UE-specific search space at all aggregation levels (1, 2, 4, and 8).
A table from 36.213 shows these relationship as below.
TTI, Transmission Time Interval, is a parameter in UMTS (and other digital telecommunication networks) related to encapsulation of data from higher layers into frames for transmission on the radio link layer. TTI refers to the duration of a transmission on the radio link. The TTI is related to the size of the data blocks passed from the higher network layers to the radio link layer.
To combat errors due to fading and interference on the radio link, data is divided at the transmitter into blocks and then the bits within a block are encoded andinterleaved. The length of time required to transmit one such block determines the TTI. At the receiver all bits from a given block must be received before they can be deinterleaved and decoded.
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In radio, multiple-input and multiple-output, or MIMO (pronounced my-moh by some and me-moh by others), is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology. Note that the terms input andoutput refer to the radio channel carrying the signal, not to the devices having antennas.
MIMO technology has attracted attention in wireless communications, because it offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the same total transmit power over the antennas to achieve an array gainthat improves the spectral efficiency (more bits per second per hertz of bandwidth) and/or to achieve adiversity gain that improves the link reliability (reduced fading).
Function of MIMO
Precoding is multi-stream beamforming, in the narrowest definition. In more general terms, it is considered to be all spatial processing that occurs at the transmitter. In (single-stream) beamforming, the same signal is emitted from each of the transmit antennas with appropriate phase and gain weighting such that the signal power is maximized at the receiver input. The benefits of beamforming are to increase the received signal gain, by making signals emitted from different antennas add up constructively, and to reduce the multipath fading effect. In line-of-sight propagation, beamforming results in a well defined directional pattern. However, conventional beams are not a good analogy in cellular networks, which are mainly characterized by multipath propagation. When the receiver has multiple antennas, the transmit beamforming cannot simultaneously maximize the signal level at all of the receive antennas, and precoding with multiple streams is often beneficial. Note that precoding requires knowledge of channel state information (CSI) at the transmitter and the receiver.
In point-to-point systems, precoding means that multiple data streams are emitted from the transmit antennas with independent and appropriate weightings such that the link throughput is maximized at the receiver output.
Spatial multiplexing requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures and the receiver has accurate CSI, it can separate these streams into (almost) parallel channels. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher signal-to-noise ratios (SNR). The maximum number of spatial streams is limited by the lesser of the number of antennas at the transmitter or receiver. Spatial multiplexing can be used without CSI at the transmitter, but can be combined with precoding if CSI is available. Spatial multiplexing can also be used for simultaneous transmission to multiple receivers, known as space-division multiple access or multi-user MIMO, in which case CSI is required at the transmitter. The scheduling of receivers with different spatial signatures allows good separability.
Diversity Coding techniques are used when there is no channel knowledge at the transmitter. In diversity methods, a single stream (unlike multiple streams in spatial multiplexing) is transmitted, but the signal is coded using techniques called space-time coding. The signal is emitted from each of the transmit antennas with full or near orthogonal coding. Diversity coding exploits the independent fading in the multiple antenna links to enhance signal diversity. Because there is no channel knowledge, there is no beamforming or array gain from diversity coding. Diversity coding can be combined with spatial multiplexing when some channel knowledge is available at the transmitter.
In MIMO systems, a transmitter sends multiple streams by multiple transmit antennas. The transmit streams go through a matrix channel which consists of all paths between the transmit antennas at the transmitter and receive antennas at the receiver. Then, the receiver gets the received signal vectors by the multiple receive antennas and decodes the received signal vectors into the original information. A narrowband flat fading MIMO system is modelled as
where and are the receive and transmit vectors, respectively, and and are the channel matrix and the noise vector, respectively.
- A short tutorial on the significance of cyclic prefix in OFDM systems.
In telecommunications, the term cyclic prefix refers to the prefixing of a symbol with a repetition of the end. Although the receiver is typically configured to discard the cyclic prefix samples, the cyclic prefix serves two purposes.
- As a guard interval, it eliminates the intersymbol interference from the previous symbol.
- As a repetition of the end of the symbol, it allows the linear convolution of a frequency-selective multipath channel to be modelled as circular convolution, which in turn may be transformed to the frequency domain using a discrete Fourier transform. This approach allows for simple frequency-domain processing, such as channel estimation and equalization.
In order for the cyclic prefix to be effective (i.e. to serve its aforementioned objectives), the length of the cyclic prefix must be at least equal to the length of the multipath channel. Although the concept of cyclic prefix has been traditionally associated with OFDM systems, the cyclic prefix is now also used in single carriersystems to improve the robustness to multipath propagation.
Use in OFDM
The OFDM symbol is constructed by taking the inverse discrete Fourier transform (IDFT) of the message symbol, followed by a cyclic prefixing. Let the symbol obtained by the IDFT be denoted by
Prefixing it with a cyclic prefix of length , the OFDM symbol obtained is:
Assume that the channel is represented using
Then, after convolution with the channel, which happens as
where is the discrete Fourier transform of . Thus, a multipath channel is converted into scalar parallel sub-channels in frequency domain, thereby simplifying the receiver design considerably. The task of channel estimation is simplified, as we just need to estimate the scalar coefficients for each sub-channel and once the values of are estimated, for the duration in which the channel does not vary significantly, merely multiplying the received demodulated symbols by the inverse of yields the estimates of and hence, the estimate of actual symbols .
테트라셀에 대해 더 자세히 설명드리면
현재 LTE 기지국은 MIMO기술로 2개의 안테나를 사용해 신호를 주고 받고 있습니다. 다운로드와 업로드에 각각2개의 안테나를 쓴다고 ‘2X2' 로 표기한다고 합니다.
테트라셀의 핵심 기술의 원리는 바로 여기에 있는데요 다운로드에 쓰는 안테나 신호를 가상화 기술로 네개의 안테나를 더 쓰는 것과 같은 효과를 내도록 하는 기술이라고 해요 ^^ 즉 2X2인 기지국을 4X2로 쓰는 것이죠. 실제로 늘어난 것은 아니지만 2개의 새로운 다운로드 안테나 채널이 생긴 것과 같은 효과를 내는 것이라고 해요.
Cell reselection is the process of changing the mobile's serving cell (either in idle mode or while actively transmitting data). Cell reselections can be initiated by the mobile or network. When the network initiates a cell reselection, it sends a Packet Cell Change Order (GPRS/EGPRS) or a Cell Change Order (W-CDMA/HSPA), which provides the parameters necessary for the mobile to find and synchronize to the destination cell. If the mobile was actively transferring data at the time of the cell reselection, any subsequent allocation of traffic channel resources to continue the packet data transfer are handled by signaling between the mobile and destination cell, and does not involve the origination cell.
Handover refers to a cell transition that occurs when a circuit-switched (CS) connection is in place (such as CS voice, CS data, or Dual Transfer Mode). Handovers can only be initiated by the network. During a handover, the network sends the mobile a Handover command, which provides information about the destination cell, including the traffic channel configuration.
The procedure for mobility from LTE to another RAT supports both handover and Cell Change Order (CCO).The CCO procedure is applicable only for mobility to GERAN. In case of handover (as opposed to CCO), the source eNodeB requests the target RAN node to prepare for the handover. As part of the ‘handover preparation request’ the source eNodeB provides information about the applicable inter-RAT UE capabilities as well as information about the currently-established bearers. In response, the target RAN generates the ‘handover command’ and returns this to the source eNodeB.
What is Power Headroom ?
Power headroom indicates how much transmission power left for a UE to use in addition to the power being used by current transmission. Simply put, it can be described by a simple formula as below.
Power Headroom = UE Max Transmission Power - PUSCH Power = Pmax - P_pusch
If the Power Headroom value is (+), it indicates "I still have some space under the maximum power" implying "I can transmit more data if you allow".
If the power Headroom value is (-), it indicate "I am already transmitting the power greater than what I am allowed to transmit".
In computer networking, packet delay variation (PDV) is the difference in end-to-end one-way delay between selected packets in a flow with any lost packets being ignored. The effect is sometimes referred to as jitter, although the definition is an imprecise fit.
Terminology[edit source | edit]
The term PDV is defined in ITU-T Recommendation Y.1540, Internet protocol data communication service - IP packet transfer and availability performance parameters, section 6.2.
The variation in packet delay is sometimes called "jitter". This term, however, causes confusion because it is used in different ways by different groups of people. ... In this document we will avoid the term "jitter" whenever possible and stick to delay variation which is more precise.
Measurement of packet delay variation[edit source | edit]
The means of packet selection for measurement is not specified in RFC 3393, but could, for example, be the packets which had the largest variation in delay in a selected time period.
The delay is specified from the start of the packet being transmitted at the source to the start of the packet being received at the destination. A component of the delay which does not vary from packet to packet can be ignored, hence if the packet sizes are the same and packets always take the same time to be processed at the destination then the packet arrival time at the destination could be used instead of the time the end of the packet is received.
Instantaneous packet delay variation is the difference between successive packets—here RFC 3393 does specify the selection criteria—and this is usually what is loosely termed "jitter", although jitter is also sometimes the term used for the variance of the packet delay. As an example, say packets are transmitted every 20 ms. If the 2nd packet is received 30 ms after the 1st packet, IPDV = −10 ms. This is referred to as dispersion. If the 2nd packet is received 10 ms after the 1st packet, IPDV = +10 ms. This is referred to as clumping.
Limiting PDV or its effects[edit source | edit]
The effects of PDV in multimedia streams can be removed by a properly sized play-out buffer at the receiver, which may only cause a detectable delay before the start of media playback.
Computer networking with Internet protocols and technology
Because the packets between a given source and destination may vary in length, mayu take different routes, and may be subject to varying delays in the switches they encounter, the overall packet delay can vary substantially. This phenomenon, alled jitter, may not be desirable for some applications; for example, in real-time applications including telephone voice and real-time video.
Jitter: the magnitude of delay variation is a critical factor in real-time applications. The larger the allowable delay variation, the longer the real delay in delivering the data and the greater the size of the delay buffer requried at receivers. Real-time interactive applications, such as teleconferencing, may require a reasonable upper bound on jitter.
The variation in packet delay is sometimes called "jitter". This
term, however, causes confusion because it is used in different ways
by different groups of people.
"Jitter" commonly has two meanings: The first meaning is the
variation of a signal with respect to some clock signal, where the
arrival time of the signal is expected to coincide with the arrival
of the clock signal. This meaning is used with reference to
synchronous signals and might be used to measure the quality of
circuit emulation, for example. There is also a metric called
"wander" used in this context.
The second meaning has to do with the variation of a metric (e.g.,
delay) with respect to some reference metric (e.g., average delay or
minimum delay). This meaning is frequently used by computer
scientists and frequently (but not always) refers to variation in
In this document we will avoid the term "jitter" whenever possible
and stick to delay variation which is more precise.