3rd Generation Partnership Project (3GPP) work item on the Long Term Evolution (LTE) is sometimes also referred to as Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN). In E-UTRAN Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink.
In the present context, the expression uplink denotes the communication from a user equipment (UE) to a base station, while the expression downlink denotes communication in the opposite direction, i.e. from the base station to the user equipment.
In both uplink and downlink the data transmission is split into several sub-streams, where each sub-stream is modulated on a separate sub-carrier. Hence in OFDMA based systems, the available bandwidth is sub-divided into several resource blocks. A resource block is defined in both time and frequency. According to the current assumptions, a resource block size is 180 KHz and 0.5 ms in frequency and time domains, respectively. The transmission bandwidth in uplink and downlink can be as large as 20 MHz in E-UTRAN release 8. The ongoing enhancements in E-UTRAN would extend transmission bandwidths up to 100 MHz and 40 MHz in the downlink and uplink respectively.
E-UTRA system may be deployed over a wide range of bandwidths, e.g. 1.4, 3, 5, 10, 15, 20 MHz, etc. As an example a 10 MHz bandwidth would contain 50 resource blocks. For data transmission the network can allocate variable number of resource blocks to the user equipment, both in the uplink and downlink. This allows more flexible use of channel bandwidth since it is allocated according to the amount of data to be transmitted, radio conditions, user equipment capability, scheduling scheme etc.
Another important consideration is that even in the same eNodeB, or base station, multiple cells may comprise different bandwidths. In addition, different eNodeBs may have different bandwidths, e.g. 10 MHz cells in one set of eNodeB and 15 MHz cells in another set of NodeB. Thus, adjacent eNodeBs in the border areas may support cells with different bandwidths.
Out of band emission requirements: The user equipment as well as the base station have to fulfil certain number of Out Of Band (OOB) emission requirements. Some of these are set by regulatory bodies such as e.g. ITU-R, FCC, ARIB and ETSI. These out of band emission requirements are also referred to as regulatory radio requirements. The objective of out of band emission requirements is to limit the interference caused by the transmitter from user equipment and/or base station, outside its operating bandwidth to the adjacent carriers. Eventually the out of band emission requirements may be specified in 3GPP specifications.
The OOB requirements typically comprise of: Adjacent Channel Leakage Ratio (ACLR), Spectrum Emission Mask (SEM) and spurious emissions, whose specific definition can vary from one system to another. Furthermore, the OOB emission requirements have to be fulfilled on slot basis in Wideband Code Division Multiple Access (WCDMA), and sub-frame basis in E-UTRA.
Both user equipment and base station have to fulfil the OOB emission requirements irrespective of their transmission power level. In case of user equipment to conserve its battery power the efficiency of the power amplifier is very critical. Therefore an efficient power amplifier will be typically designed for certain typical operating points or configurations e.g. modulation type, number of active resource block, in case of E-UTRA, number of physical channel/channelization codes/spreading factor, in case of UTRA, which is based on Code Division Multiple Access (CDMA) technology. However, the user equipment may have to transmit using any combination of modulation, resource blocks etc. Therefore, in some uplink transmission scenarios the user equipment power amplifier may not be able to operate in the linear zone, thereby causing OOB band emissions due to harmonics or inter-modulation products. To ensure that user equipment fulfils OOB requirements for all allowed uplink transmission configurations the user equipment is allowed to reduce its maximum uplink transmission power in some scenarios when it reaches its maximum power. This is called maximum power reduction (MPR) or user equipment power back-off in some literature. For instance a user equipment with nominal maximum output/transmit power of 24 dBm power class may reduce its maximum power from 24 dBm to 23 or 22 dBm depending upon the configuration. The base station may also have to perform maximum power reduction but this is not standardized. Secondly the base station can afford to have a power amplifier with larger operating range since its efficiency is less critical compared to that of user equipment.
The maximum power reduction values for different configurations are generally well specified in the standard. The user equipment uses these values to apply maximum power reduction when the conditions for the corresponding configurations are fulfilled. These maximum power reduction values are regarded as static in a sense that they are independent of resource block allocation and other deployment aspects.
In UTRA the maximum power reduction requirements for the user equipment are specified in release 5 of the 3GPP specifications for some configurations that contain HS-DPCCH transmission. Similarly, maximum power reduction is also specified for configurations using Enhanced Dedicated Channel (E-DCH) for Quadrature Phase Sift Keying (QPSK) and 16 Quadrature Amplitude Modulation (QAM). In UTRA the maximum power reduction can be as large as up to 3 dB or even more. But the actual value depends upon a particular uplink transmission configuration such as e.g. codes, spreading factor, modulations, physical channels and their gain factors etc. Further evolution of UTRA, for instance to downlink and/or uplink multicarrier transmission, may require increased level of maximum power reduction. In Table 1 quoted below is illustrated how maximum power reduction requirements are currently defined for UTRA user equipment. Note that Cubic Metric (CM) is the measure of the power contained in the third order harmonic.
TABLE 1UE transmit channel configurationCM (dB)MPR (dB)For all combinations of; DPDCH,0 ≦ CM ≦ 3.5MAX (CM-1, 0)DPCCH, HS-DPCCH, E-DPDCHand E-DPCCHNote 1:CM = 1 for βc/βd = 12/15, βhs/βc = 24/15. For all other combinations of DPDCH, DPCCH, HS-DPCCH, E-DPDCH and E-DPCCH the MPR is based on the relative CM difference.
In E-UTRA the maximum power reduction requirements for the user equipment are also being specified. There the maximum power reduction will depend upon factors such as transmission bandwidth, modulation and number of allocated resource blocks. In Table 2 below is illustrated how maximum power reduction requirements are currently defined for E-UTRA user equipment. The illustrated user equipment power class 3 in E-UTRA corresponds to user equipment nominal maximum output power of 23 dBm without maximum power reduction.
TABLE 2Channel bandwidth/Transmission bandwidthconfiguration (RB)1.43.0MPRModulationMHzMHz5 MHz10 MHz15 MHz20 MHz(dB)QPSK>5>4>8>12>16>18≦116 QAM≦5≦4≦8≦12≦16≦18≦116 QAM>5>4>8>12>16>18≦2
In E-UTRA an Additional Maximum Power Reduction (A-MPR) is also being specified on top of the normal maximum power reduction. The difference is that the former is not fully static. Instead it can vary between different cells, operating frequency bands and between cells belonging to different location areas. Additional spectrum emission requirement and additional maximum power reduction requirement are interchangeably used in literature.
The additional maximum power reduction includes all the remaining power reduction, on top of the normal maximum power reduction needed to account for factors such as: bandwidth, frequency band, resource block allocation to satisfy additional such as requirements set by regional regulatory bodies (FCC, ARIB etc). In Table 3 below is illustrated how additional maximum power reduction requirements are currently defined for E-UTRA user equipment in 3GPP TS 36.101: Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception
TABLE 3NetworkChannelSignallingRequirementsE-UTRAbandwidthResourcesA-MPRvalue(sub-clause)Band(MHz)Blocks(dB)NS_01NANANANANANS_036.6.2.2.12, 4, 10, 3>5≦135, 366.6.2.2.12, 4, 10, 5>6≦135, 366.6.2.2.12, 4, 10,10>6≦135, 366.6.2.2.12, 4, 10,15>8≦135, 366.6.2.2.12, 4, 10,20>10 ≦135, 36NS_046.6.2.2.2TBDTBDTBDNS_056.6.3.3.1 110, 15,≧50 for≦120QPSKNS_066.6.2.2.312, 13,1.4, 3,n/an/a14, 175, 10NS_076.6.2.2.31310TableTable6.6.3.3.26.2.4-26.2.4-2. . .NS_32—————
It is important to consider band 13 with respect to additional maximum power reduction requirements. Band 13 is one of the E-UTRAN FDD bands in the range of 700 MHz exclusively allocated in the USA. More specifically it operates for the uplink in the band from 777 MHz to 787 MHz and for the downlink in the band from 746 MHz to 756 MHz and entirely owned by one network operator.
One peculiar aspect of this band is its proximity to the Public Safety (PS) band. The public safety band is located on the left the of uplink part of band 13. According to FCC regulation the operation adjacent to the public safety band requires very tight emission requirements. This means the additional maximum power reduction requirements for band 13 are much tighter than those for the other bands. The E-UTRAN Physical Uplink Control Channels (PUCCH) are located at the edge of the bandwidth. Therefore the outer resource blocks of the bandwidth of cell operating in band 13 are required to maintain even tighter requirements, i.e. larger additional maximum power reduction requirements, compared to those located in the centre of the bandwidth.
Due to this reason the additional maximum power reduction requirements for band 13 are agreed to be split into 3 regions comprising of different set of contiguous resource blocks. The Table 4 below containing the split of bandwidth in 3 regions is reproduced. The Table 4 illustrates that the uplink cell bandwidth may be divided into 3 regions as described above. Each region is likely to have different additional maximum power reduction requirements.
RB start indicates the lowest resource block index of transmitted resource blocks and L_CRB is the length of a contiguous resource block allocation.
The Table 4 below is one proposal to attempt to specify the additional maximum power reduction figures. As can be seen from the Table 4, the additional maximum power reduction in 3GPP for band 13 in 3GPP TS 36.101: Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception can be very large, i.e. 6-14 dB. Furthermore the additional maximum power reduction value is dependent upon the part of the cell bandwidth. Hence prudent uplink resource allocation is required to avoid unnecessary additional maximum power reduction.
TABLE 4Region ARegion BRegion CRB_start10-1213-1819-4243-49L_CRB2 [RBs]6-81 to 5≧8≧18≦2and 9-50A-MPR [dB]8121263Note1RB_start indicates the lowest RB index of transmitted resource blocks2L_CRB is the length of a contiguous resource block allocation3 For intra-subframe frequency hopping between two regions, notes 1 and 2 apply on a per slot basis.4 For intra-subframe frequency hopping between two regions, the larger A-MPR value of the two regions may be applied for both slots in the subframe.
Due to variable bandwidth, varying number of resource block allocation, different bands in different parts of the networks etc, that the additional maximum power reduction need to fulfil, the regulatory requirements can vary from one eNodeB to another. Even if the deployment scenario, in terms of bands used, bandwidth size etc, is homogeneous in a large coverage area, there will always be border regions between these coverage areas. Indeed additional maximum power reduction is a cell specific value. Therefore, additional maximum power reduction is signalled to the user equipment via system information and via an user equipment specific channel. This will allow the user equipment to acquire this information when it camps on to a cell. The values will be used when it starts transmitting in the uplink.
In E-UTRAN system handover access takes place via Physical Random Access Channel (PRACH). The PRACH resources, i.e. resource blocks and sub-frames used for PRACH transmission are signalled to the user equipment via broadcast and/or user equipment specific channel.
The resource assignment for uplink and downlink transmission is done by the network in UTRAN and E-UTRAN. It is up to the network to use one or more available measurements for allocating the resources. For instance for generating the uplink scheduling grant or uplink resource allocation, the network can use one or more of the following state of the art reports: user equipment transmit power, user equipment power headroom, i.e. the difference between user equipment max power and user equipment estimated/measured power, user equipment buffer size, Happy bit, path loss and/or signal strength.
One or more of these measurements enable the network to decide the amount of resources the user equipment needs for uplink transmission.
Drawbacks of prior art solutions regarding maximum power reduction and/or additional maximum power reduction comprise inter alia:
The resource allocation node does not take into account the impact of maximum power reduction and/or additional maximum power reduction when issuing uplink grant or resource assignment. The desired uplink grant may not be fully used by user equipment in power limiting situation due to applied maximum power reduction and/or additional maximum power reduction. This will lead to the wastage of grant, which could otherwise be allocated to another prospective user equipment. Another implication is that the coverage loss may occur due to maximum power reduction and/or additional maximum power reduction depending upon the uplink grant.
The additional maximum power reduction can be as large as 15 dB in some portion of the bandwidth in certain bands e.g. band 13. Hence LTE user equipment of power class 23 dBm would transmit at maximum 8 dBm of output power. This means if resources are allocated without any regard to the expected maximum power reduction and/or additional maximum power reduction then there will be severe consequence of coverage less.
In case of PRACH transmission which is also used for the handover access if the resources, i.e. resource blocks in LTE are allocated in the part of bandwidth with larger maximum power reduction and/or additional maximum power reduction then there is high risk of call blocking, i.e. poor initial access, and call dropping, i.e. poor handover access. In LTE different PRACH formats are defined: formats 0, 1, 2, 3 and 4. This can be even more critical for format #4 since it is very short in time.