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26 This section provides an overview of the xRAN architecture.
33 The front haul interface, according to the ORAN Fronthaul specification,
34 performs communication between O-RAN Distributed Unit (O-DU) and O-RAN
35 Radio Unit (O-RU) and consists of multiple HW and SW components.
37 The logical representation of HW and SW components is shown in Figure 1.
39 .. image:: images/Architecture-Block-Diagram.jpg
41 :alt: Figure 1. Architecture Block Diagram
43 Figure 1. Architecture Block Diagram
48 From the hardware perspective, two networking ports are used to
49 communicate to the Front Haul and Back (Mid) Haul network as well as to
50 receive PTP synchronization. The system timer is used to provide a
51 “sense” of time to the gNB application.
53 From the software perspective, the following components are used:
55 * Linux PTP provides synchronization of system timer to GPS time:
57 - Ptp4l is used to synchronize oscillator on Network Interface
58 Controller (NIC) to PTP GM.
60 - Phc2sys is used to synchronize system timer to oscillator on NIC.
62 * DPDK to provide the interface to the Ethernet port.
64 * xRAN library is built on top of DPDK to perform U-plane and C-plane
65 functionality according to the ORAN Fronthaul specification.
67 * 5GNR reference PHY uses the xRAN library to access interface to O-RU.
68 The interface between the library and PHY is defined to communicate
69 TTI event, symbol time, C-plane information as well as IQ sample
72 * 5G NR PHY communicates with the L2 application using the set of
73 MAC/PHY APIs and the shared memory interface defined as WLS.
75 * L2, in turn, can use Back (Mid) Haul networking port to connect to
76 the CU unit in the context of 3GPP specification.
78 In this document, we focus on the details of the design and
79 implementation of the xRAN library with respect to providing Front Haul
80 functionality for both mmWave and Sub-6 scenarios.
82 The xRAN M-plane is not implemented and is outside of the scope of this
83 description. Configuration files are used to specify selected M-plane
89 ORAN FH Thread Performs:
91 - Symbol base “time event” to the rest of the system based on System
92 Clock synchronized to GPS time via PTP.
94 - Baseline polling mode driver performing TX and RX of Ethernet packets.
96 - Most of the packet processing such as Transport header, Application
97 header, Data section header and interactions with the rest of the PHY
100 - Polling of other call back function that was registered.
102 ORAN FH thread created the independent of usage of xRAN as an interface
105 Communication between L1 and xRAN layer is performed using a set of
106 callback functions where L1 assigned callback and xRAN layer executes
107 those functions at a particular event or time moment. Detailed
108 information on callback function options and setting as well as design,
109 can be found in the sections below.
111 Sample Application Thread Model
112 -------------------------------
114 Configuration of a sample application for both O-DU and O-RU follows the
115 model of 5G NR l1app application in the section of xRAN only. No BBU or
116 FEC related threads are needed as minimal xRAN functionality is used
119 .. image:: images/Sample-Application-Threads.jpg
121 :alt: Figure 3. Sample Application Threads
123 Figure 3. Sample Application Threads
128 In this scenario, the main thread is used only for initializing and
129 closing the application. No execution happens on core 0 during run time.
134 Figure 1 corresponds to the O-RU part of the xRAN split. Implementation
135 of the RU side of the xRAN protocol is not covered in this document.
137 .. image:: images/eNB-gNB-Architecture-with-O-DU-and-RU.jpg
139 :alt: Figure 4. eNB/gNB Architecture with O-DU and RU
141 Figure 4. eNB/gNB Architecture with O-DU and RU
146 More than one RU can be supported with the same implementation of the
147 xRAN library and depends on the configuration of gNB in general. In this
148 document, we address details of implementation for single O-DU – O-RU
151 The ORAN Fronthaul specification provides two categories of the split of
152 Layer 1 functionality between O-DU and O‑RU: Category A and Category B.
154 .. image:: images/Functional-Split.jpg
156 :alt: Figure 5. Functional Split
158 Figure 5. Functional Split
167 Table 3 lists the data flows supported for a single RU with a single
173 Table 3. Supported Data Flow
175 +---------+----+-----------------+-----------------+----------------+
176 | Plane | ID | Name | Contents | Periodicity |
177 +---------+----+-----------------+-----------------+----------------+
178 | U-Plane | 1a | DL Frequency | DL user data | symbol |
179 | | | Domain IQ Data | (PDSCH), | |
180 | | | | control channel | |
181 | | | | data (PDCCH, | |
183 +---------+----+-----------------+-----------------+----------------+
184 | | 1b | UL Frequency | UL user data | symbol |
185 | | | Domain IQ Data | (PUSCH), | |
186 | | | | control channel | |
187 | | | | data (PUCCH, | |
189 +---------+----+-----------------+-----------------+----------------+
190 | | 1c | PRACH Frequency | UL PRACH data | slot or symbol |
191 | | | Domain IQ Data | | |
192 +---------+----+-----------------+-----------------+----------------+
193 | C-Plane | 2a | Scheduling | Scheduling | ~ slot |
194 | | | Commands | information, | |
195 | | | | FFT size, CP | |
196 | | | (Beamforming is | length, | |
197 | | | not supported) | Subcarrier | |
198 | | | | spacing, UL | |
200 | | | | scheduling | |
201 +---------+----+-----------------+-----------------+----------------+
202 | S-Plane | S | Timing and | IEEE 1588 PTP | |
203 | | | Synchronization | packets | |
204 +---------+----+-----------------+-----------------+----------------+
209 .. image:: images/Data-Flows.jpg
211 :alt: Figure 6. Data Flows
218 Information on specific features of C-Plane and U-plane provided in
219 Section 6.0. Configuration of S-plane used on test setup for simulation
220 is provided in Appendix Appendix 2.
222 Data flow separation is based on VLAN (applicable when layer 2 or layer
223 3 is used for the C/U-plane transport.)
225 #. The mechanism for assigning VLAN ID to U-Plane and C-Plane is assumed
226 to be via the M-Plane.
228 VLAN Tag is configurable via the standard Linux IP tool (refer to
229 Appendix Appendix 1).
231 No Quality of Service (QoS) is supported.
236 .. image:: images/C-plane-and-U-plane-Packet-Exchange.jpg
238 :alt: Figure 7. C-plane and U-plane Packet Exchange
240 Figure 7. C-plane and U-plane Packet Exchange
245 Timing, Latency, and Synchronization to GPS
246 -------------------------------------------
248 The ORAN Fronthaul specification defines the latency model of the front
249 haul interface and interaction between O-DU and O-RU. This
250 implementation of the xRAN library supports only the category with fixed
251 timing advance and Defined Transport method. It determines O-DU transmit
252 and receive windows based on pre-defined transport network characteristics, and the delay characteristics of the RUs within the
255 Table 4 below provides default values used for the implementation of
256 O-DU – O-RU simulation with mmWave scenario. Table 5 and Table 6 below
257 provide default values used for the implementation of O-DU – O-RU
258 simulation with numerology 0 and numerology 1 for Sub6 scenarios.
259 Configuration can be adjusted via configuration files for sample |br|
260 application and reference PHY. However, simulation of the different
261 range of the settings was not performed, and additional implementation changes might be required as well as testing with actual O-RU. The
262 parameters for the front haul network are out of scope as a direct connection between O-DU and 0-RU is used for simulation.
267 Table 4. Front Haul Interface Latency (numerology 3 - mmWave)
269 +------+------------+-------------------+-------------------+----------------+------------+
270 | | Model | C-Plane | U-Plane | | |
271 | | Parameters | | | | |
272 +------+------------+-------------------+-------------------+----------------+------------+
273 | | | DL | UL | DL | UL |
274 +------+------------+-------------------+-------------------+----------------+------------+
275 | O-RU | T2amin | T2a_min_cp_dl=50 | T2a_min_cp_ul=50 | T2a_min_up=25 | NA |
276 +------+------------+-------------------+-------------------+----------------+------------+
277 | | T2amax | T2a_max_cp_dl=140 | T2a_max_cp_ul=140 | T2a_max_up=140 | NA |
278 +------+------------+-------------------+-------------------+----------------+------------+
279 | | | Tadv_cp_dl | NA | NA | NA |
280 +------+------------+-------------------+-------------------+----------------+------------+
281 | | Ta3min | NA | NA | NA | Ta3_min=20 |
282 +------+------------+-------------------+-------------------+----------------+------------+
283 | | Ta3max | NA | NA | NA | Ta3_max=32 |
284 +------+------------+-------------------+-------------------+----------------+------------+
285 | O-DU | T1amin | T1a_min_cp_dl=70 | T1a_min_cp_ul=60 | T1a_min_up=35 | NA |
286 +------+------------+-------------------+-------------------+----------------+------------+
287 | | T1amax | T1a_max_cp_dl=100 | T1a_max_cp_ul=70 | T1a_max_up=50 | NA |
288 +------+------------+-------------------+-------------------+----------------+------------+
289 | | Ta4min | NA | NA | NA | Ta4_min=0 |
290 +------+------------+-------------------+-------------------+----------------+------------+
291 | | Ta4max | NA | NA | NA | Ta4_max=45 |
292 +------+------------+-------------------+-------------------+----------------+------------+
298 Table 5. Front Haul Interface Latency (numerology 0 - Sub6)
300 +------+----------+----------+----------+----------+----------+
301 | | Model | C-Plane | U-Plane | | |
303 | | rameters | | | | |
304 +------+----------+----------+----------+----------+----------+
305 | | | DL | UL | DL | UL |
306 +------+----------+----------+----------+----------+----------+
307 | O-RU | T2amin | T | T | T2a_mi | NA |
308 | | | 2a_min_c | 2a_min_c | n_up=200 | |
309 | | | p_dl=400 | p_ul=400 | | |
310 +------+----------+----------+----------+----------+----------+
311 | | T2amax | T2 | T2 | T2a_max | NA |
312 | | | a_max_cp | a_max_cp | _up=1120 | |
313 | | | _dl=1120 | _ul=1120 | | |
314 +------+----------+----------+----------+----------+----------+
315 | | | Ta | NA | NA | NA |
316 | | | dv_cp_dl | | | |
317 +------+----------+----------+----------+----------+----------+
318 | | Ta3min | NA | NA | NA | Ta3 |
319 | | | | | | _min=160 |
320 +------+----------+----------+----------+----------+----------+
321 | | Ta3max | NA | NA | NA | Ta3 |
322 | | | | | | _max=256 |
323 +------+----------+----------+----------+----------+----------+
324 | O-DU | T1amin | T | T | T1a_mi | NA |
325 | | | 1a_min_c | 1a_min_c | n_up=280 | |
326 | | | p_dl=560 | p_ul=480 | | |
327 +------+----------+----------+----------+----------+----------+
328 | | T1amax | T | T | T1a_ma | NA |
329 | | | 1a_max_c | 1a_max_c | x_up=400 | |
330 | | | p_dl=800 | p_ul=560 | | |
331 +------+----------+----------+----------+----------+----------+
332 | | Ta4min | NA | NA | NA | T |
333 | | | | | | a4_min=0 |
334 +------+----------+----------+----------+----------+----------+
335 | | Ta4max | NA | NA | NA | Ta4 |
336 | | | | | | _max=360 |
337 +------+----------+----------+----------+----------+----------+
343 Table 6. Front Haul Interface Latency (numerology 1 - Sub6)
345 +------+------------+-------------------+-------------------+----------------+------------+
346 | | Model | C-Plane | U-Plane | | |
347 | | Parameters | | | | |
348 +------+------------+-------------------+-------------------+----------------+------------+
349 | | | DL | UL | DL | UL |
350 +------+------------+-------------------+-------------------+----------------+------------+
351 | O-RU | T2amin | T2a_min_cp_dl=285 | T2a_min_cp_ul=285 | T2a_min_up=71 | NA |
352 +------+------------+-------------------+-------------------+----------------+------------+
353 | | T2amax | T2a_max_cp_dl=429 | T2a_max_cp_ul=429 | T2a_max_up=428 | NA |
354 +------+------------+-------------------+-------------------+----------------+------------+
355 | | | Tadv_cp_dl | NA | NA | NA |
356 +------+------------+-------------------+-------------------+----------------+------------+
357 | | Ta3min | NA | NA | NA | Ta3_min=20 |
358 +------+------------+-------------------+-------------------+----------------+------------+
359 | | Ta3max | NA | NA | NA | Ta3_max=32 |
360 +------+------------+-------------------+-------------------+----------------+------------+
361 | O-DU | T1amin | T1a_min_cp_dl=285 | T1a_min_cp_ul=285 | T1a_min_up=96 | NA |
362 +------+------------+-------------------+-------------------+----------------+------------+
363 | | T1amax | T1a_max_cp_dl=429 | T1a_max_cp_ul=300 | T1a_max_up=196 | NA |
364 +------+------------+-------------------+-------------------+----------------+------------+
365 | | Ta4min | NA | NA | NA | Ta4_min=0 |
366 +------+------------+-------------------+-------------------+----------------+------------+
367 | | Ta4max | NA | NA | NA | Ta4_max=75 |
368 +------+------------+-------------------+-------------------+----------------+------------+
374 IEEE 1588 protocol and PTP for Linux\* implementations are used to
375 synchronize local time to GPS time. Details of the configuration used
376 are provided in Appendix Appendix 2. Local time is used to get Top of
377 the Second (ToS) as a 1pps event for SW implementation. Timing event is
378 obtained by performing polling of local time using clock_gettime(CLOCK_REALTIME,..)
380 All-time intervals are specified with respect to GPS time which
381 corresponds to OTA time.
387 Virtualization and Container-Based Usage
388 ----------------------------------------
390 xRAN implementation is deployment agnostic and does not require special
391 changes to be used in virtualized or |br|
392 container-based deployment options.
393 The only requirement is to provide one SRIOV base virtual port for
394 C-plane and one port for U-plane traffic per O-DU instance. This can be
395 achieved with the default Virtual Infrastructure Manager (VIM) as well
396 as using standard container networking.