Application design of the hottest relay mode in OF

Application design of relay mode in OFDMA system

Abstract: relay technology can enhance the coverage of the original base station, and 0fdma technology is the main multiple access method of the next generation mobile communication. Therefore, it is of great practical significance to study and design the relay scheme under the constraint of OFDMA technology. Based on the frame structure of LTE physical layer, the relay implementation of 0fdma modulation system is deeply analyzed. Combined with the flexible time-frequency resource allocation characteristics of OFDMA system, a multi hop/single hop resource allocation method for 0fdma is proposed. Finally, an implementation scheme of 0fdma relay for FDD mode is also proposed. The traditional concept that relay can only be used in TDD system is advanced

0 introduction

ngmn (next generation mobile network organization) first takes the introduction of wireless board bandwidth (WBB) as an important goal. Wireless access point AP is the key product to achieve this goal. AP has well realized the TCO optimization of mobile broadband data solution

the relay technology can enhance the coverage of the original base stations, especially the AP base stations, and effectively improve the throughput of the area, especially the cell edge. Therefore, it is of great practical significance to study and design the relay scheme under the constraints of LTE/OFDMA technology

firstly, this paper briefly introduces the frame structure of LTE physical layer, and then gives the air interface resource allocation method after the introduction of 0fdma system relay; Finally, an implementation scheme of 0fdma relay for FDD mode is proposed, which extends the traditional concept that relay can only be used in TDD system

l LTE frame structure

0fdma, as the most important and promising access scheme in the next few years, allows to split a wide frequency bandwidth into small segments to serve different terminals. At present, both LTE/UMB and WiMAX Systems take OFDMA as the basic modulation technology of the physical layer of the air interface

Figure 1 shows the basic frame structure of LTE, which is applicable to FDD and TDD modes. The basic frame is 10 ms long, which is divided into 20 0.5 ms subframes, and the two subframes form a 1 ms TTI. In FDD mode, 20 subframes are used for uplink and downlink respectively; In TDD mode, the uplink/downlink ratio can be configured (#0/5 subframes are used for downlink)

in the basic frame structure, when the short CP mode is adopted, there are 7 OFDM/SC FDMA symbols in each downlink/uplink subframe; When long CP mode is adopted to overcome larger multipath delay, each downlink/uplink subframe supports 6 OFDM/SC FDMA symbols

in the air interface resource representation of LTE, ndlbw represents the downlink bandwidth configuration and is represented by the number of downlink subcarriers; Nulbw refers to uplink bandwidth configuration, which is represented by the number of uplink virtual subcarriers; Ndlsymb represents the number of 0fdm symbols in a downlink time slot (subframe); Nulsymb represents the number of SC OFDM symbols in one uplink slot; Nrbbw represents the number of frequency domain resource blocks (with 12 subcarriers as the basic unit)

Figure 4 is a schematic diagram of the downlink resource grid based on OFDMA in LTE. The resource block based on user scheduling is defined as: the product of the number of continuous OFDM symbols in the time domain and the number of continuous subcarrier blocks in the frequency domain ndlsymb nrbbw. In uplink resource scheduling, resource block is defined as a subframe and parameters NTX and K0. These two parameters determine the transmission bandwidth and frequency hopping mode. NTX is also based on 12 virtual subcarriers

it can be seen that LTE can distinguish users in time domain and frequency domain respectively. Therefore, the following OFDMA based relay technology design can be directly applied to LTE

2 OFDMA based relay scheme

2.1 multi hop/single hop resource allocation method based on OFDMA

starting from the difference between single hop and multi hop connections, OFDMA technology can be used to split the available frequency band into two parts: one part for single hop communication and the other part for multi hop communication. It can be predicted that adjacent sub carriers are allocated to multi hop and single hop traffic respectively, resulting in two adjacent sub bands, one for multi hop and one for single hop. This means that the air port of the target system uses a complete frequency band. For example, 1 when the aircraft crashed into the right engine 00 MHz when landing, the whole frequency band is divided into two parts. As an example, figure 5 shows the allocation mode of NC subcarriers in the air interface based on 0fdma. MH area indicates that this part of subcarriers is used for multi hop communication, and sh area indicates that this part of subcarriers is used for single hop communication

2. Normal protection and maintenance of pendulum impact testing machine

by using the characteristics of OFDMA, the two sub bands are dynamically divided in a flexible way. OFDMA allows different subcarriers to be allocated to different users to form different connections. Here, it is suggested to allocate the subcarriers into two sub bands as needed. For example, the subcarriers of the high-order frequency band are allocated to the multi hop sub bands, and the remaining subcarriers are used for single hop. The number of subcarriers allocated to single hop and multi hop can be dynamically adjusted according to demand

depending on the frequency resource requirements of single hop and multi hop traffic, the sub-band Division will change. For example, if there is heavy local traffic between terminals in a multi hop fixed repeater area, and only a little bandwidth demand is used to transmit data with Internet, the multi hop sub-band will be reduced to a very small number of sub carriers. However, if multi hop requires more bandwidth, some carriers for single hop will be allocated to multi hop frequency bands. For example, in addition to adjusting according to the Convention, if each mobile node has a connection with the Internet, the multi hop sub-band will be increased to support the large traffic demand relayed through the fixed relay network

between the AP/relay station and the mobile node, as well as between the AP and the relay station, the uplink/downlink division is generally realized by TDD. However, FDD can also be implemented on single hop links through this concept, and a hybrid FDD method can be used for multi hop connections. Relatively speaking, the multi hop implementation of FDD is more complex than that of TDD, especially the hardware scheme. This section mainly takes TDD as an example

in Figure 6, the concept that subcarriers are dynamically allocated to multi hop and single hop traffic is simulated based on the deployment of two fixed relay stations. In this scenario, up to three hops are supported. The first two hops are realized by multi hop sub bands between AP and two fixed relay stations, and the third hop between relay station 2 and Mn3 (mobile node 3) is realized by single hop band 3 (SH3). In Figure 6, under this topology, different bandwidth allocations are marked. MHL band is used for bidirectional multi hop traffic between AP and fmhnl (shcommunication and mhcommlmication over fixedrelay stations, here refers to fixed relay stations), and shares subcarriers with single hop traffic in SHL area. Mobile nodes in SHL area are directly served by AP. The multi hop and single hop traffic in MH2 area and SH2 area also share subcarriers in a dynamic way. In SH3 area, traffic will monopolize all subcarriers, because there are no more multi hop connections

2.2 subband bandwidth design in MS OFDMA

as the downlink traffic from the AP to the mobile node is distributed to the single hop area, and the uplink traffic from the mobile node to the AP is converged, this leads to a gradual increase in the bandwidth demand towards the AP and Internet for multi hop connections. This requirement is considered by allocating a higher number of subcarriers to the MH link near the AP, such as the MHL link in Figure 6. However, other different subcarrier allocation methods are also possible. For example, when heavy local traffic or large traffic is transmitted between single hop areas through relay stations rather than APS, MHL will allocate fewer subcarriers than MH2

since we expect the traffic on MH link to be realized in Los environment through high gain antenna, the unit spectrum data rate is much higher than the last hop link between AP/fmhn and Mn under the same bandwidth condition; Therefore, assuming that all traffic comes from/goes to AP and Internet, the number of carriers allocated to multi hop links can be less than that required on a single hop link. In addition, mobile nodes served by the last relay, such as Mn3, will experience the highest number of hops to reach the Internet. The maximum allocated bandwidth for these mobile nodes, such as SH3 area, partially makes up for this deficiency and reduces the delay of different links to a certain extent

on the other hand, it is necessary to choose the cell size appropriately. Compared with the cells served by fmhn, the last cell (SH3 area) will become the largest cell (covering more mobile nodes). This cell planning can ensure that each user has a constant data rate in the whole deployment area, which is a research goal of the deployment scheme in the future mobile communication system

it can be seen that with the help of novel schemes, flexible resource allocation on end-to-end connections becomes possible. The multi hop scheme based on OFDMA introduces a logical segmentation method of multi hop traffic and single hop traffic, which will be served by different protocols based on the common physical layer and shared common frequency band. Compared with single hop communication, multi hop communication puts forward different requirements in protocol design, and can develop and deploy effective protocols to independently target different problem areas. At the same time, just like conventional solutions, there is no special requirement for frequency division, only one frequency band is required. At the same time, the guard band between single hop and multi hop frequency bands does not need to be similar to that in conventional FDMA, because OFDMA allows closer segmentation, and the subcarriers are orthogonal in the frequency domain

3 preliminary discussion on FDD relay scheme

at present, although the hardware implementation of relay technology in FDD mode is relatively complex and the cost is relatively high, FDD is the mobile communication mode with the largest traffic so far, and FDD mode occupies the most frequency band, and the coverage ability of the frequency band is also the most superior. Therefore, the combination of FDD mode and relay technology is very necessary. The following is a preliminary discussion on the two hop mode based on FDD and the logical architecture of base station transceiver

fdd communication mode, the uplink and downlink communication frequency bands are physically divided, and the uplink and downlink communication between the base station and the terminal can be carried out at the same time, that is, the uplink and downlink communication can be received and transmitted at the same time. The uplink occupied frequency band is in the low order, expressed in LB, and the center frequency is defined as FLB; The downlink occupied frequency band is in the high position, represented by Hb, and the center frequency is defined as FHB; The duplex interval between the two frequency bands is up to tens of megahertz fdup

in order to save costs, it is assumed that the relay station has only one set of transceiver, that is, it can only receive and transmit one channel of signals at the same time. Because the relay station needs to support four modes: br/Rb/RM/MR (base station transmit relay receive/relay transmit base station receive/relay transmit terminal receive/terminal transmit relay receive), it needs to schedule the transceiver resources of the relay station through time division, which is divided into two transceiver states: br/RB and RM/MR in the time domain

based on the basic method of MS OFDMA, the LB and Hb bands are divided into SH and MH sub bands respectively for single hop and multi hop communication. MHL/SHL refers to the division between base station and relay station and between base station and terminal