tomh/ns-3-dev-spectrum-wifi: comparison src/lte/doc/source/lte-design.rst

equal deleted inserted replaced

-:e8595c3ac95b
+:9786bf474ea1
 .. _fig-epc-data-flow-dl:
 .. figure:: figures/epc-data-flow-dl.*
 :align: center
-Data flow in the dowlink between the internet and the UE
+Data flow in the downlink between the internet and the UE
 To begin with, we consider the case of the downlink, which is depicted
 in Figure :ref:`fig-epc-data-flow-dl`.
 Downlink Ipv4 packets are generated from a generic remote host, and
 addressed to one of the UE device. Internet routing will take care of
 Transmission Bandwidth Configuration in use.
 For certain bandwidth
 values not all the RBs are usable, since the
 group size is not a common divisor of the group. This is for instance the case
-when the bandwith is equal to 25 RBs, which results in a RBG size of 2 RBs, and
+when the bandwidth is equal to 25 RBs, which results in a RBG size of 2 RBs, and
 therefore 1 RB will result not addressable.
 In uplink the format of the DCIs is different, since only adjacent RBs
 can be used because of the SC-FDMA modulation. As a consequence, all
 RBs can be allocated by the eNB regardless of the bandwidth
 configuration.
 --------
 The physical layer model provided in this LTE simulator is based on
 the one described in [Piro2011]_, with the following modifications.  The model now includes the
 inter cell intereference calculation and the simulation of uplink traffic, including both packet transmission and CQI generation.
+.. only:: latex
+.. raw:: latex
+\clearpage
+Subframe Structure
+^^^^^^^^^^^^^^^^^^
+The subframe is divided into control and data part as described in Figure :ref:`fig-lte-subframe-structure`.
+.. _fig-lte-subframe-structure:
+.. figure:: figures/lte-subframe-structure.*
+:width: 50px
+Lte subframe division.
+Considering the granularity of the simulator based on RB, the control and the reference signaling have to be consequently modeled considering this constraint.  According to the standard [TS36.211]_, the downlink control frame starts at the beginning of each subframe and lasts up to three symbols across the whole system bandwidth, where the actual duration is provided by the Physical Control Format Indicator Channel (PCFICH). The information on the allocation are then mapped in the remaining resource up to the duration defined by the PCFICH, in the so called Physical Downlink Control Channel (PDCCH). A PDCCH transports a single message called Downlink Control Information (DCI) coming from the MAC layer, where the scheduler indicates the resource allocation for a specific user.
+The PCFICH and PDCCH are modeled with the transmission of the control frame of a fixed duration of 3/14 of milliseconds spanning in the whole available bandwidth, since the scheduler does not estimate the size of the control region. This implies that a single transmission block models the entire control frame with a fixed power (i.e., the one used for the PDSCH) across all the available RBs. According to this feature, this transmission represents also a valuable support for the Reference Signal (RS). This allows of having every TTI an evaluation of the interference scenario since all the eNB are transmitting (simultaneously) the control frame over the respective available bandwidths. We note that, the model does not include the power boosting since it does not reflect any improvement in the implemented model of the channel estimation.
+The Sounding Reference Signal (SRS) is modeled similar to the downlink control frame. The SRS is periodically placed in the last symbol of the subframe in the whole system bandwidth. The RRC module already includes an algorithm for dynamically assigning the periodicity as function of the actual number of UEs attached to a eNB according to the UE-specific procedure (see Section 8.2 of [TS36.213]_).
+.. only:: latex
+.. raw:: latex
+\clearpage
 MAC to Channel delay
 ^^^^^^^^^^^^^^^^^^^^
 example the spectrum licensing policies that are
 discussed in [Ofcom2.6GHz]_.
-PHY Error Model
+Data PHY Error Model
----------------
+--------------------
-The simulator includes an error model of the data plane (i.e., PDSCH) according to the standard link-to-system mapping (LSM) techniques. The choice is aligned with the standard system simulation methodology of OFDMA  radio transmission technology. Thanks to LSM we are able to maintain a good level of accuracy and at the same time limiting the computational complexity increase. It is based on the mapping of single link layer performance obtained by means of link level simulators to system (in our case network) simulators. In particular link the layer simulator is used for generating the performance of a single link from a PHY layer perspective, usually in terms of code block error rate (BLER), under specific static conditions. LSM allows the usage of these parameters in more complex scenarios, typical of system/network simulators, where we have more links, interference and "colored" channel propagation phenomena (e.g., frequency selective fading).
+The simulator includes an error model of the data plane (i.e., PDSCH and PUSCH) according to the standard link-to-system mapping (LSM) techniques. The choice is aligned with the standard system simulation methodology of OFDMA  radio transmission technology. Thanks to LSM we are able to maintain a good level of accuracy and at the same time limiting the computational complexity increase. It is based on the mapping of single link layer performance obtained by means of link level simulators to system (in our case network) simulators. In particular link the layer simulator is used for generating the performance of a single link from a PHY layer perspective, usually in terms of code block error rate (BLER), under specific static conditions. LSM allows the usage of these parameters in more complex scenarios, typical of system/network simulators, where we have more links, interference and "colored" channel propagation phenomena (e.g., frequency selective fading).
 To do this the Vienna LTE Simulator [Vienna]_ has been used for what concerns the extraction of link layer performance and the Mutual Information Based Effective SINR (MIESM) as LSM mapping function using part of the work recently published by the Signet Group of University of Padua [PaduaPEM]_.
 MIESM
 The model implemented uses the curves for the LSM of the recently LTE PHY Error Model released in the ns3 community by the Signet Group [PaduaPEM]_ and the new ones generated for different CB sizes. The ``LteSpectrumPhy`` class is in charge of evaluating the TB BLER thanks to the methods provided by the ``LteMiErrorModel`` class, which is in charge of evaluating the TB BLER according to the vector of the perceived SINR per RB, the MCS and the size in order to proper model the segmentation of the TB in CBs. In order to obtain the vector of the perceived SINR two instances of ``LtePemSinrChunkProcessor`` (child of ``LteSinrChunkProcessor`` dedicated to evaluate the SINR for obtaining physical error performance) have been attached to UE downlink and eNB uplink ``LteSpectrumPhy`` modules for evaluating the error model distribution respectively of PDSCH (UE side) and ULSCH (eNB side).
 The model can be disabled for working with a zero-losses channel by setting the ``PemEnabled`` attribute of the ``LteSpectrumPhy`` class (by default is active). This can be done according to the standard ns3 attribute system procedure, that is::
-Config::SetDefault ("ns3::LteSpectrumPhy::PemEnabled", BooleanValue (false));
+Config::SetDefault ("ns3::LteSpectrumPhy::DataErrorModelEnabled", BooleanValue (false));
+Control Channels PHY Error Model
+--------------------------------
+The simulator includes the error model for downlink control channels (PCFICH and PDCCH), while in uplink it is assumed and ideal error-free channel. The model is based on the MIESM approach presented before for considering the effects of the frequency selective channel since most of the control channels span the whole available bandwidth.
+PCFICH + PDCCH Error Model
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+The model adopted for the error distribution of these channels is based on an evaluation study carried out in the RAN4 of 3GPP, where different vendors investigated the demodulation performance of the PCFICH jointly with PDCCH. This is due to the fact that the PCFICH is the channel in charge of communicating to the UEs the actual dimension of the PDCCH (which spans between 1 and 3 symbols); therefore the correct decodification of the DCIs  depends on the correct interpretation of both ones. In 3GPP this problem have been evaluated for improving the cell-edge performance _[FujitsuWhitePaper], where the interference among neighboring cells can be relatively high due to signal degradation. A similar problem has been notices in femto-cell scenario and, more in general, in HetNet scenarios the bottleneck has been detected mainly as the PCFICH channel _[Bharucha2011], where in case of many eNBs are deployed in the same service area, this channel may collide in frequency, making impossible the correct detection of the PDCCH channel, too.
+In the simulator, the SINR perceived during the reception has been estimated according to the MIESM model presented above in order to evaluate the error distribution of PCFICH and PDCCH. In detail, the SINR samples of all the RBs are included in the evaluation of the MI associated to the control frame and, according to this values, the effective SINR (eSINR) is obtained by inverting the MI evaluation process. It has to be noted that, in case of MIMO transmission, both PCFICH and the PDCCH use always the transmit diversity mode as defined by the standard. According to the eSINR perceived the decodification error probability can be estimated as function of the results presented in _[R4-081920]. In case an error occur, the DCIs discarded and therefore the UE will be not able to receive the correspondent Tbs, therefore resulting lost.
 MIMO Model
 ----------
 * MIMO-OSIC-MMSE: :math:`\mu = 12.6` and :math:`\sigma = 15.5` [dB].
 * MIMO-ZF: :math:`\mu = 10.3` and :math:`\sigma = 12.6` [dB].
 Therefore the PHY layer implements the MIMO model as the gain perceived by the receiver when using a MIMO scheme respect to the one obtained using SISO one. We note that, these gains referred to a case where there is no correlation between the antennas in MIMO scheme; therefore do not model degradation due to paths correlation.
-.. only:: latex
-.. raw:: latex
-\clearpage
-Reference and Control Signaling
--------------------------------
-Considering the granularity of the simulator based on RB, the control and the reference signaling have to be consequently modeled considering this constraint. On this matter, the simulator splits the transmission of the data frame respect to the control one. According to the standard [TS36.211]_, the control frame starts at the beginning of each subframe and lasts up to three symbols. The actual duration is provided by the Physical Control Format Indicator Channel (PCFICH) by means of three different codewords of 32 bits in order to make it enough robust. These 32 bits are mapped to 16 resource elements (RE), a subcarrier per 1 symbol, using QPSK modulation. The information on the allocation are then mapped in the remaining resource up to the duration defined by the PCFICH, in the so called Physical Downlink Control Channel (PDCCH). A PDCCH transports a single message called Downlink Control Information (DCI) coming from the MAC layer, where the scheduler indicates the resource allocation for a specific user. Each PDCCH is transmitted in a Control Channel Element (CCE), which is defined as group of nine sets of four REs.
-According to the channel conditions the CCEs allocated can be 1, 2, 4 or 8, which correspond respectively to PDCCH format 0, 1, 2 and 3. Each PDCCH carries one of the possible 10 DCI formats configurations, modeling uplink and downlink assignment messages.
-PCFICH & PDDCH
-^^^^^^^^^^^^^^
-The PDCCH is modeled with the transmission of the control frame of a fixed duration of 3/14 of milliseconds spanning in the whole available bandwidth. This implies that a single block transmission models the entire control frame with a fixed power (i.e., the one used for the PDSCH) across all the available RBs. The SINR perceived during the reception of this channel has been estimated according to the MIESM model presented above in order to evaluate the error distribution of PCFICH and PDCCH. In detail, the SINR samples of all the RBs are included in the evaluation of the MI associated to the control frame and, according to this values, the effective SINR is obtained by inverting the MI evaluation process.
-The PCFICH error distribution is modeled considering the link level simulation curves of the performance of the PCFICH in AWGN channel presented in [Milos2012]_, by means of look-up tables according to the transmission mode used (e.g., SISO or MIMO) and the SINR evaluated as described before.
-While for what concern the PDCCH, the correct reception of all the PDCCHs is modeled with a configurable power threshold on the perceived SINR, where the default value is taken from Section 10.3.3 of [Sesia2009]_ where the performance of the convolutional turbo codes of PDCCH (i.e., with state tailbiting) are presented. This implies that all the DCIs can be correctly decoded only in case of the perceived SINR is above the threshold. In doing this, we are assuming that the DCIs are randomly placed in the control frame and therefore always interfere with the ones of other BSs. According to [Sesia2009]_, the number of bits of the DCIs are at most of 62 bits. Therefore, the default value is taken for a given PDCCH fixed dimension of 62 bits and a considering a rate of :math:`1/3`, which corresponds to 2.4 dB. According to this model, the control frame does not have any limitation in the PDCCHs that can be transmitted. Finally, It has to be noted that, in case of MIMO transmission, the PDCCH uses always the transmit diversity mode according to the standard.
-Reference Signal
-^^^^^^^^^^^^^^^^
-The Reference Signal (RS) is modeled according to the SINR perceived during the reception of the control frame. This allows of having every TTI an evaluation of the interference scenario since all the eNB are transmitting (simultaneously) the control frame over the respective available bandwidths. We note that, the model does not include the power boosting since it does not reflect any improvement in the implemented model of the channel estimation.
-PUCCH
-^^^^^
-The PUCCH is modeled with an error free channel.
-Sounding Reference Signal (SRS)
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-The SRS is modeled similar to the downlink control frame. The SRS is periodically placed in the last symbol of the subframe; therefore when a SRS signal has to be sent, the data frame will last on the 13th symbol in order to leave space for the SRS, which will be sent in the whole system bandwidth. The SINR can be evaluated according to MIESM model as done for RS. For what concern the periodicity, it can be defined as an integer multiple of TTIs; however we remark that the standard allows the following values: 2, 5, 10, 20, 40, 80, 160 or 320 TTIs.
-.. only:: latex
-.. raw:: latex
-\clearpage
 -----------------------
 Channel and Propagation
 -----------------------

changeset 9045	9786bf474ea1
parent 9034	6f40a766b37f
child 9047	284d30c8dd42