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When the designer turns attention to to move WiMAX apparatus, they know in the power amplifier (power amplifier soon, PA) Design respect have some who is special challenge. As to Wave 2 kinds of movement WiMAX products, the power amplifier must have high linear scales, can utilize 3.3 V direct-flow DC power to offer the output power of +23 dBm high-efficiently.

Until manufacturer design, launch, test and dispose to first generation, move power management in the WiMAX become the essential problem rapidly. It is that it cover the distance to move one of the challenges that WiMAX designs very long, the distance of cover of WiMAX network is generally every zone (cell) About 1 kilometer. For this reason, WiMAX must have optimal power to distribute: From the base station (base station) In order to drop to each assembly in the mobile equipment. In this case, the high transmitted power seems very important. Can reach transmitted power of but WiMAX how high,limit, limitations technological where regulator stipulate and last model what it is? The project personnel of designer and application PA of the power amplifier must find the best equilibrium between high-power and high efficiency, so as to ensure WiMAX its sane in interlinkage of apparatus and data rate high covering distance with to be enough.

Essence of WiMAX

As to designer, the challenge of WiMAX lies in it is a kind of access technology with a series of unique constraint conditions. So, are suitable for not can't be applied mechanically in WiMAX design briefly in honeycomb or Wi-Fi applied power amplifier circuiting, also unable to utilize through the simple modification.

WiMAX can be considered as a kind of hybrid technique in many aspects, because it has many characteristics of honeycomb and Wi-Fi network concurrently. Move WiMAX, very similar to honeycomb, original idea to used in high mobile equipment, and it use bring frequently authorize . In addition, it adopts the control technique of the transmitted power equally as CMDA honeycomb technology too. However, it is different from technology of the honeycomb, because its working data rate is more many (requirement for linear scaly high and stricter than the latter) ,And necessary simultaneous processing Internet's pronunciation (VoIP) , the transmission of data and video. The management of bandwidth and transmission priority of all kinds of services needs a service quality (QoS) The assembly, but the single movement pronunciation does not need QoS.

On the other hand, WiMAX is very similar to Wi-Fi again. For example, its data rate is very high, adopt the orthogonal frequency division to reply and use (OFDM) Technology, the modulation mode is from BPSK to 64 - QAM, and is likewise the whole IP network. But with the difference of Wi-Fi, what WiMAX is used is one kind totally on schedule (scheduled) Service,adopt based on conflict but Wi-Fi carrier sense multiple address visit (CSMA) Technology. This has offered an advantage of surmounting Wi-Fi greatly to WiMAX. In CSMA network, with the increase of user's number, the overall data rate will be apparent to drop, because send again in follow-up that every conflict needs. And utilize the service booked in advance, can manage each user's visit to the network effectively in because the base station, the total throughput of the network will not be influenced because of increase of user's number.

Distance of cover of WiMAX network

Each one of Wi-Fi network inserts some (access point, AP) Cover distance here generally or within several hectometres of several dozen; But each base station of WiMAX network can cover the range of about 1 kilometer. Want, reach the range, move the intersection of WiMAX and network, adopt include large transmitted power, sub channelization and adaptive modulation large quantities of technology, in order to realize the distance of more long cover.

In brief, the power of radio frequency (radio frequency, RF) Directly proportional to with covering the distance, so larger power is equal to the distance of more long cover. In order to get covering the distance long, the power level of transmission of WiMAX base station is in +43dBm (20W) Left and right sides; And the typical transmitted power of Wi-Fi AP is +18 dBm (60 mW) . The foot of foot of phase difference of two is more than 330 times! WiMAX moves the platform (mobile station, MS) Transmitted power so so +23 dBm (200mW) ,Wi-Fi only +18 dBm (60 mW) . Honeycomb (CDMA) The base station with the transmitted power of the mobile station and the WiMAX one is similar. However, in order to obtain more high throughput, the modulated stage that WiMAX adopts is much higher, so WiMAX need than (SNR) than the intersection of honeycomb and fine signal and noise much . For moving the projector, the high modulated stage demands to improve the linear scale of PA greatly, to cause PA to be designed is much more complicated than that of GSM or CDMA.

Power that you may notice that descent (from base station to mobile station) And upstream power (go from mobile station to the base station) Between have a long way to go,so it is with network up-chain limit strictly, the cellular network has this kind of situation too) . In other words, the mobile station is very apt to receive the signal of the base station, but the autogenous transmitted power of mobile station is lower, it is difficult to be intercept to by the base station.

One of the methods to solve this kind of mismatching problem is to utilize one to be known as sub channelization (subchannelization) Technology,it is getting more usable namely all for subchannel last several subset, be lasted each users particular respectively. In fact, the energy of every mobile station is concentrated within the range of a minor frequency, net signal gain turns into 10*log (Ntotal/Nused) ,Subcarrier number represented to users here Nused; But Ntotal represents the total number of usable subcarriers. For example, if a user obtains the assigned subchannel and includes 24 subcarriers, and base station (launch on all 841 assigned subcarriers) The associated net gain to noise temperature ratio is 10*log(841/24) =15.4 dB. Other subcarriers can be used in other users, and they but these subcarriers of simultaneously using.

Another technology of solving unbalance of periodic line is the adaptive modulation. In this case, the mobile station utilizes the modulated stage lower than the base station to launch. For example, the mobile station may transmit QPSK or 16QAM signal; But the base station adopts 64QAM technology to launch. Because that received QPSK or 16QAM required SNR than 64QAM is low, adopt lower modulated stage, can let mobile station and base station carry on communication (at this moment with lower transmitted power, because lower modulated stage makes emissive bit location of every subcarrier less, so the throughput of the up-chain will drop) . For example, QPSK-1/2 required SNR is 5 dB, 16QAM-1/2 is 10.5 dB, 64QAM-3/4 is 20 dB (the note 1) . If mobile station adopt QPSK modulation go on, take place, base station can admissible link loss take many 5.5 dB comparing with the time of adopting 16QAM.

If the sub channelization combines with modulation technique of the adaptation, the Network Operator can equilibrate the budgets of up-chain and downlink effectively, and network but two-way operation. But the shortcoming of this kind of comprehensive approach includes: The throughput of the up-chain will be less than the downlink; The sub channelization has been exercised restraint in moving and launching the usable subcarrier number; And lower modulated stage make each can use location less emissive bit at the subcarrier.

The power of moving WiMAX Residential Area is distributed

Have understood that after the above-mentioned explanation, let us see what if the transmitted power on WiMAX Residential Area is distributed. A general misunderstanding is that the mobile station just launches only on the edge of Residential Area with the power of maximum, and it is lower to be close to the power at the time of the base station. Actually this is not so, the mobile station is very high in the whole coverage inner transmitted power.

Where understand the reason, let us imagine a mobile equipment moves to the base station from edge of the Residential Area directly. At the intersection of Residential Area and end, path loss of it very large, mobile equipment adopt most sane modulation mode, go on, launch with power of maximum at this moment, so the upstream data rate is quite low. But because the mobile station is very tall in transmitted power, and it is sane to modulate, the base station can receive the signals of the mobile station, and the periodic line works well.

As the mobile equipment is close to the base station more and more, path loss reduces. Far greater than the noise base now because of the signal that is received, the signal rank of the base station increases, SNR becomes great. Accordingly, the base station may instruct the mobile equipment to begin to reduce the power (in order to reduce the interference between different mobile stations as much as possible) . However, once signal grade supports more high modulated stage, the base station will instruct the mobile equipment to switch over the modulation mode, in order to improve the network population size.

And then get back to the example that we compare QPSK / 16QAM, suppose a projector works under +23 dBm, and SNR of QPSK required 5 dB while just reaching and lying the edge range of Residential Area, when it nears to the base station, path loss drops, the base station may tell the mobile station to reduce the transmitted power. However, once path loss reduces 5.5 dB, because the mobile station can get SNR of 10.5 dB at this moment, so the base station will instruct the mobile station to switch to 16QAM-1/2 modulation mode, the transmitted power is replied to +23 dBm again. So, the mobile equipments are generally all launched with higher power, unless it depends on the base station, it is so near as to can transfer to adoption 16QAM way works (can even adopt 64QAM under a lot of situations) ,The power will drop at this moment. As shown in Fig. 1.

Fig. 1 is according to a WiMAX forum white paper (the note 2) The parameter had was drawn. It is a function to the distance of base station that it is modulated that expicity and achievable in the picture. We adopt the parameter on the white paper, suppose operating frequency as 2.5 GHz, channel bandwidth 10 MHz, 3 subchannels, penetrate and loss 10 dB, by the maximum likelihood path loss of this calculation. In the calculation of path loss, we adopt COST231 suburban model of 2.5 GHz, the height 32 m of the base station, height 1.2m of mobile station. This kind of analysis is supposed to have slowly (the constant state of logarithm) Dampen, store in, but make some to simplify, presume it is abundant that the fixing of 5.5 dB dampens me. Certainly, the course that it was one immediately to decay in reality, and could make use of loop-locked power control to lower its influence. But for analyze conclusion these significant, decay totally fuzzy to shed in different boundary line while modulating.

We need to pay attention to, QPSK-1/8 that red ring marks represents and changes and takes the place of the QPSK-1/2 modulation of 4 times. This is the sanest modulation scheme, and is really essential in the maximum distance place. In our analysis, we obtain according to calculating, when the transmitted power is +23 dBm, as to stand the intersection of 0.9 km and mobile equipment in 1.35km from group, the mobile station must adopt the QPSK-1/8 modulation. At nearer distance, the mobile station can adopt higher modulated stage, the network capacity increases. For example in stand from 0.45 km to place of 0.6 km from group, the mobile station can adopt the 16QAM-1/2 modulation. Because 16QAM-1/2 modulation will every symbol launch 2 the intersection of bit and location, and QPSK-1/8 every symbol only launch 0.5 pieces of bit location, so handling up 4 times in the red ring of the quantity relative ratio in green ring.

We have also estimated the required transmitted power, as the function of the distance. At edge of each one area of Fig. 1, the mobile station launches with maximum power. But as the mobile station moves to the base station, its transmitted power is dropping to enough power that can be realizing the next modulated stage constantly. At this moment, the transmitted power of the base station begins to increase again, in order to improve the capacity as much as possible. Fig. 2 indicates the transmitted power is the function of the distance, demonstrate the modulating influence of the adaptation. Can find out, have only realized that the highest and modulated grade (here is 64QAM-3/4) ,The transmitted power will drop greatly. Unless last 16QAM-3/4 if maximum modulated stage, when realizing this modulated stage,transmitted power can monotonicity decline.

What we should pay attention to is, the damped existence will cause this curve to show the change. In actual to dampen in the environment, need more Yu Yu to compensate the attenuating effect, it is much more less and the emissive situation of maximum power appears. However, the overall trend illustrated in Fig. 2 is accurate, can find out from it, only on the edge of Residential Area, even the place closer to the base station, the mobile stations are being all required high-power to launch, in order to realize higher modulated stage.

Heavy advantage

It is very apparent to move WiMAX terminal advantage greater in transmitted power. Just think from transmitted power +23 dBm (200mW) Improve by 40%, reach +24.5 dBm (281 mW) The influence produced, at first, it needs bigger PA. The loss after supposing PA is 1 dB, the output power of PA must be from 250 mW (+24 dBm) Rise to 355mW (+25.5 dBm) .

Larger transmitted power has two major advantages. First, launch and can improve maximum and cover the distance with larger output power. According to the parameter (the note 3) that offers of " moving WiMAX white paper " of WiMAX forum ,When the output power is improved from 23dBm to 24.5 dBm, mobile equipment and maximum distance of the base station rise to 1.5 kilometers from 1.35 kilometers, in this way, the overall cover area will be expanded by 23.5%. In principle, the base station that the Network Operator can reduce 23% because of this is disposed, thus save the cost. However, this dominant function is very limited, because a lot of networks design the magnitude of Residential Area according to upstream transmitted power of +23 dBm, so the magnitude of Residential Area may have been already fixed.

The second advantage is more important. If the mobile station can launch for more high power, when it is far away from the base station, can obtain higher SNR required in modulated stage. This will increase the total capacity of the network, thus will raise whole spectrum effectiveness.

Fig. 3 shows, under the transmitted power of +24.5 dBm, modulate the function that can be regarded as the distance of base station. In this picture, the function that we regard accessible modulation as the distance of base station again is drawn (distance under +23 dBm transmitted power that dash line represent Fig. 1, by reference) . The one that need to pay attention to here is, the maximum distance has arrived 1.5 kilometers from improvement of 1.35 kilometers, discuss as above. But even more should pay attention to, user can the maximum distance is 0.7 kilometers on (this more long distance, and +23 dBm is 0.6 kilometers hour) Adopt the 16QAM-1/2 modulation. Because the transmitted power is higher, users can realize more high modulated stage earlier, so every user can obtain more high throughput on more long distance, and the total capacity of the network increases correspondingly. Add, launch for more high power level by first user more while being every, the population size of the network can increase to some extent. We will understand, must be that all users can launch the cover area that can expand the Residential Area with higher power. The more high users of a transmitted power of every increase in the network, the population size of the network will become a little greater.

Finally, capacity increased as the transmitted power is improved from +23 dBm to +24.5 dBm to calculate simple relatively. We know how much bits of location can be launched each symbol of every kind of modulation scheme has; Know under two power level, what is every relative area that modulation scheme cover with. While adopting the information to calculate the relative capacity, we find when the transmitted power is improved from +23 dBm to +24.5 dBm, the capacity has increased by 24%. Even if the range of Residential Area of maximum is still stationary 1.35 kilometers, when the transmitted power is improved to +24.5 dBm (suppose the network directs against the apparatus of +23 dBm) ,If the apparatus can be launched with higher power, the capacity can still increase by 18%.

Restriction of the power

Now, we have already understood higher transmitted power has been very important why has been in WiMAX network, because it can improve the throughput of the whole network, and here " Newly-built " Arrangement in can obtain by Residential Area cover area on the ones larger,thus it is dispose by cost not to reduce. Then, why is not by launching more heavy-dutily? Have three factor limit emission ability of we when being more heavy-duty: PA efficiency, can spend the service voltage, and regulation requirement.

PA efficiency

In PA, efficiency defines that compares with direct-flow power input for RF output power. For example, if the efficiency of one PA is 10%, it is in +25.5dBm (355 mW) The transmitted power of hour is 3.55 W. If PA efficiency can double and reach 20%, then the power consumption of crest value is dropped to 1.7W. The most advanced WiMAX PA such as SE7262 of SiGe semiconductor at present, its operating efficiency exceeds 20% [see and supplement ].

PA efficiency has direct influence on battery life-span of the mobile equipment. Certainly, PA has not been in working state all the time, so the average power consumption is much lower than the power consumption of above-mentioned crest value. For example, as the firing data of mobile station, the transmission duty ratio of WiMAX apparatus is generally about 40%. So is 20% of the PA to efficiency, if launch with maximum power, the power consumption of average crest value is about 680mW. In addition, because often there are not data that wait to send, at this moment, the apparatus will be basically in leaving the state (basically unused, it only launches the news of range finding, in order to let the base station know it still within the range of Residential Area) .

However, after all, PA power consumption exerts a tremendous influence to battery life-span, it is very important to raise PA efficiency as much as possible.

Available service voltage

It is battery-driven to move WiMAX apparatus by mobile station directly, and the service voltage of the battery is changed greatly during using. While just fully charging, the working voltage of the battery is about 4.8V. With the discharging of the battery, the service voltage drops gradually, the minimum actual service voltage before the apparatus deenergization is generally 2.7V. Most manufacturer hope the as much as possible large operating range, stipulate power amplifier must offer, make the power (3.0V sometimes) fully really at 3.3V . Offer high-power to have some great challenges under these conditions. Just as most circuit designers know, the low service voltage needs heavy current, this means ultralow output impedence. So, it is very difficult to let low-impedance PA be exported and matched 50 ohms of aerials. If need more high output power, impedance becomes lower, should obtain good Broadband and match the more difficult to feel sleepy among PA and aerial.

Supervise the tube to require

The supervision of regulation requires the restriction on the thing that the power that PA can be offered has been made strictly too. An ideal linear PA should only utilize the input signal to produce the primitive frequency. But in reality, the non- linear scale of PA will pass intermodulation distortedly (intermodulation distortion, IMD) And introduce the new frequency, these out-of-band signals may interfere with the users in the adjacent channel (known as spectral hyperplasia or frequency spectrum letting out) .

The regulator makes rigorous norm to the out-of-band emissive power. For example, as to the mobile equipment of 2.5GHz frequency band, American federal communication committee member knows (FCC) Regulation (the note 4) ,In the apparatus the transmitted power that 5.5MHz measures must be small outside the assigned frequency band on - 25 dBm/MHz. Because this limit is an absolute power measured value, increase with the output power, need increasingly great out-of-band emission to inhibit, the linear scale of the power amplifier must be higher and higher. For example, channel bandwidth is 10 MHz, at the time of the transmitted power +23 dBm, should obtain - 25 dBm/MHz need 23-10log(10) +25 =The only inhibiting of 38 dB. 24.5 The transmitted power of dBm needs inhibition of 39.5 dB. So, with the increase of the output power, it becomes more and more difficult to meet the supervision requirement. In order to reduce IMD distortedly, the linear scale of PA must be improved, it is comes out that the efficiency of PA drops with improvement of the output power goal.

Importance weighed

Undoubtedly, to moving WiMAX network, higher transmitted power is very important. The network while disposing at present stipulates the minimum transmitted power is +23 dBm. Every transmitted power is big will raise the efficiency of the universe network to network to the users of +23 dBm. However, the higher the transmitted power is, the power consumption increases correspondingly. So, while adopting more high output power, the efficiency of the power amplifier becomes more important.

The note 1: The intersection of IEEE Std and the intersection of 802.16e-2005 and 8.4.13 section

The note 2: Mobile WiMAX chapter 1: Overview and Performance Table 11

The note 3: Mobile WiMAX chapter 1: 5.2nd of A Technical Overview and Performance Evaluation (publish by WiMAX forum)

The note 4: FCC 47/CFR 27.53(I.)

Supplement:

Why is PA efficiency of WiMAX so low?

The efficiency of most advanced WiMAX PA is 20%. A lot of designer, system of GSM, scorn to take even a glance at to the intersection of district and the intersection of district and 20% efficiency perhaps, because it is very much easy for GSM PA not to win over 50% of the efficiency. Why is efficiency of WiMAX PA so low? In that compared with WiMAX, the intersection of linear scale and requirement of GSM much, promote GSM PA may loud a lot in efforts of efficiency loosely. Moreover, OFDM modulation not adopted produce peak power than (PAPR) in the WiMAX It is a signal of 6 to 7 dB, and PAPR of GSM, for 0 dB (envelop permanently) . Certainly, OFDM has a large number of advantages too, including high spectrum effectiveness and outstanding one are antidamping, these characteristics are very important for large bandwidth moved such as WiMAX to move and employ.