Power control scheme for maximizing carrier signal-to-noise ratio in multicarrier transmitters

[0002] The present invention relates to the field of radio telecommunications, including cellular telecommunications, and more specifically, to maximizing the signal-to-noise ratio in multicarrier transmitters by controlling the power level of each of the carriers entering a digital-to-analog converter and by controlling overall power gain of the analog portion of the transmitter.

[0003] In conventional cellular systems, a base station is allocated a predetermined number of frequency channels for communication with mobile stations. In the base station a separate transmitter is employed for each frequency channel. However, the use of separate transmitters for each frequency channel results in a duplication of parts and an increase in cost due to the additional hardware required. Thereafter, it was realized that the hardware cost per channel could be reduced by using multicarrier transmitters in place of the plurality of single carrier transmitters to transmit a plurality of frequency channels. Since multicarrier transmitters transmit over a broad range of frequencies, they are also sometimes referred to in the art as wideband transmitters. However, for ease of discussion, the transmitters will be referred to herein as multicarrier transmitters.

[0004] FIG. 1 illustrates a conventional multicarrier transmitter 100 which might be used to transmit multiple frequency channels from a base station in a radiocommunication system. The conventional multicarrier transmitter 100 operates as follows. A number N of baseband frequency data signals BB₁. . . BB_Nare modulated by modulators Mod₁. . . Mod_N, respectively, where the bits associated with each data signal are symbol encoded for transmission, i.e., the modulator generates the corresponding baseband waveform. Each of the modulated data signals is forwarded to a corresponding digital power control module DPC₁. . . DPC_N, where each DPC adjusts the signal power level of the corresponding modulated data signal based on the commands provided by the Radio Control Unit 150. More specifically, the power level of each modulated data signal is adjusted such that the absolute power level of each carrier P_k,outat the transmitter is equal to the amount of power required for the carrier to reach a particular mobile station which is to receive the carrier, where k varies from 1 to N and identifies the corresponding baseband frequency data signals BB₁. . . BB_N.

[0005] The modulated data signals are then forwarded from the digital power control modules DPC₁. . . DPC₁N to multipliers Mult₁. . . Mult_N, respectively, where each modulated data signal is upconverted to a corresponding carrier frequency. The upconverted signals are then summed by adder 110. The compound signal produced by adder 110 is then forwarded to the digital-to-analog converter (DAC) 120. The resulting compound analog signal is then passed from DAC 120 through an analog transmitter chain which includes analog amplifier 160, upconverter (not shown), and filters (not shown). Analog amplifier 160 then amplifies the compound signal by a fixed gain G_i. For ease of discussion G_ihas been described as the gain of analog amplifier 160, however, one skilled in the art will recognize that G_irepresents the total gain of the analog section of the transmitter, including losses due to filters and upconverters. A more detailed discussion of multicarrier transmitters can be found in “Base-Station Technology Takes Software-Definable Approach” by Richard M. Lober, Wireless System Designs, February 1998, which is herein incorporated by reference.

[0006] Multicarrier transmitters are designed to handle a maximum number of simultaneous carriers N. In designing a multicarrier transmitter, care must be taken to ensure that the instantaneous in-phase sum, P_sum, of the N carriers does not exceed the full scale range of the DAC, i.e., the value associated with the greatest digital code that can be converted into an analog value. P_sumcan be calculated using equation (1) below, where C_Nrepresents the power of a specific user, N, in a specific time slot on a specific carrier frequency. Normally, C_Nis equal to the peak power within the specific time slot.

[0007] ({square root over (C₁)}+{square root over (C₂)}+ . . . +{square root over (C_N)})²=P_SUM  (1)

[0008] If the instantaneous sum of the N carriers exceeds the full scale range of the DAC, the DAC will clip the analog signal, i.e., prevent the analog signal from exceeding the amplitude corresponding to the full scale range of the DAC, which will affect the quality of the transmitted signal. However, one skilled in the art will recognize that in practical applications, a system might tolerate a power level which exceeds the DAC's full scale range by a small amount for short periods of time without suffering a decrease in system performance.

[0009] In a multicarrier transmitter with N carriers, the abovementioned “clipping” of the analog signal can be avoided by setting the full scale range of the DAC to 20*log(N) dB above the maximum allowed peak power level of any individual carrier 1 . . . N, since the full scale range set 20*log(N) dB above the maximum power level of any individual carrier represents the greatest power level attainable by the sum of the N carriers.

[0010] In FIG. 2, the maximum power level associated with each of the N carriers, during a given time slot in a TDMA based system, is C_max. Accordingly, the full scale range of the DAC is set to 20*log(N) dB above C_max. When the output noise of the DAC is dominated by quantization noise, the DAC noise floor level relative to the DAC full scale range, is constant for a given DAC resolution and sampling frequency. If all carriers are operating at maximum power, the signal-to-noise ratio for each individual carrier is at its greatest level.

[0011] Unlike FIG. 2, FIG. 3 illustrates a time slot where the N carriers have different power levels and not all of the N carriers are active. Assuming the DAC full scale range in FIG. 3 is the same as the DAC full scale range in FIG. 2, the noise floor levels of FIGS. 2 and 3 will also be the same. Since the power levels of the N carriers in FIG. 3 are less than the power levels of the N carriers in FIG. 2, while the noise floor level in FIGS. 2 and 3 is the same, the signal-to-noise ratio for each of the individual carriers in FIG. 3 will be lower than for the corresponding individual carriers in FIG. 2.

[0012] Accordingly, it would be desirable to maximize the signal-to-noise ratio for each carrier and time slot. Further, it would be desirable to maximize the signal-to-noise ratio for each individual carrier in a multicarrier transmitter when less than the maximum number of carriers are used and/or when not all carriers are operating at the maximum power level. In addition, it would be desirable to maximize the signal-to-noise ratio of each individual carrier in the multicarrier transmitter without the instantaneous sum of all the carriers exceeding the DAC's full scale range.

[0013] The power level of individual carriers is adjusted in the digital domain, by a command from a radio control unit, such that P_sumis equal to the full scale range of the DAC, or such that P_sumexceeds the full scale range of the DAC, though not enough to cause intolerable distortion to the signal. The radio control unit also commands an analog power control module to adjust the power level of all carriers simultaneously such that the transmitted actual absolute output power level of each carrier is set to the correct level for a corresponding mobile station to receive an acceptable signal.

[0014] It is an object of the present invention to improve the signal-to-noise ratio of a multicarrier signal which passes through a DAC.

[0015] It is also an object of the present invention to maximize the utilization of a DAC in a multicarrier transmitter system, thereby reducing the implementation cost of the transmitter chain in a radio base station.

[0016] It is a further object of the present invention to maximize the signal-to-noise ratio of each individual carrier in a multicarrier transmitter, thereby improving end user quality of service for all users.

[0017] The present invention will now be described with reference to the accompanying drawings in which:

[0018] FIG. 1 illustrates a conventional multicarrier transmitter;

[0019] FIG. 2 illustrates N carriers in one time slot at the maximum power level;

[0020] FIG. 3 illustrates a time slot where the N carriers are at different power levels and not all N carriers are active;

[0021] FIG. 4 illustrates an exemplary multicarrier transmitter for maximizing the signal-to-noise ratio;

[0022] FIG. 5 illustrates an exemplary method for power control;

[0023] FIG. 6 illustrates power control of a multicarrier transmitter according to an exemplary embodiment of the present invention; and

[0024] FIG. 7 illustrates a time slot where two carriers are active and one carrier's power level exceeds C_max.

[0025] In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, and techniques, in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, and circuits are omitted so as not to obscure the description of the present invention.

[0026] The exemplary radio communication systems discussed herein are described as using the time division multiple access (TDMA) protocol, in which communication between the base station and the mobile terminals is performed over a number of time slots. However, those skilled in the art will appreciate that the concepts disclosed herein find use in other access methods, including, but not limited to, frequency division multiple access (FDMA), code division multiple access (CDMA), or some hybrid of any of the above protocols. Likewise, some of the exemplary embodiments provide illustrative examples relating to the GSM system, however, the techniques described herein are equally applicable to radio base stations in any system.

[0027] FIG. 4 illustrates a multicarrier transmitter 400 according to exemplary embodiments of the present invention. Similar to multicarrier transmitter 100 of FIG. 1, each of a number of N baseband frequency data signals BB₁. . . BB_Nare forwarded to a corresponding modulator Mod₁. . . Mod_N. The modulated baseband signals BB₁. . . BB_Nare then forwarded to the digital power control modules DPC₁. . . DPC_N. The Radio Control Unit 450 individually sets the gain of each digital power control module DPC₁. . . DPC_N, such that P_sumis equal to the full scale range of the DAC 120, P_FS,DAC.

[0028] The signals generated by the digital power control modules DPC₁. . . DPC_Nare forwarded to multipliers Multi₁. . . Multi_Nwhere the signals are upconverted to a corresponding carrier frequency. The upconverted signals are then summed by adder 110. The resultant compound signal is converted from digital to analog by DAC 120. The analog signal is forwarded to an analog power control module 460 which is controlled by an analog power control table stored in the Radio Control Unit 450. Although analog power control module 460 is illustrated as a single element in FIG. 4, the functionality of the analog power control module 460 may be distributed over the analog transmitter chain, i.e., amplifiers, filters and upconverters, in order to minimize the effect on the noise figure.

[0029] In order to correctly adjust the absolute power level at the transmitter, P_k,out, for the carrier signals, the power level of the carrier signals are all adjusted by the same amount by the analog power control module 460. In order to adjust the gain of each digital power control module and the gain of the analog power control module 460, the Radio Control Unit 450 knows the required power level of each carrier at the output of the transmitter P_k,out, i.e., the amount of transmit power required to provide a corresponding mobile station with an acceptable signal. Since the full scale range of the DAC is determined at the time of installation of the DAC in the multicarrier transmitter, the full scale range P_FS,DACof the DAC 120 and the corresponding code can be programmed into the Radio Control Unit 450.

[0030] FIG. 5 is a flow chart illustrating an exemplary method for maximizing the signal-to-noise ratio of each carrier using the multicarrier transmitter illustrated in FIG. 4. Accordingly, in step 505, a vector is formed using the output power levels associated with each carrier in a given time slot, e.g., P=[P_1,outP_2,outP_3,out. . . P_N,out]. In step 510, using vector P, a matrix M is created where the elements are the ratios of all actual absolute power levels for the time slot. $\begin{array}{c}M=[\begin{array}{ccccc}{P}_1/P_1& P_1/P_2& P_1/P_3& \dots & P_1/P_N\\ P_2/P_1& P_2/P_2& P_2/P_3& \dots & P_2/P_N\\ P_3/P_1& P_3/P_2& P_3/P_3& \dots & P_3/P_N\\ \dots & \dots & \dots & \dots & \dots \\ P_N/P_1& P_N/P_2& P_N/P_3& \dots & P_N/P_N\end{array}]\\ =[\begin{array}{ccccc}{m}_{11}& 1/m_{21}& 1/m_{31}& \dots & 1/m_{\mathrm{N1}}\\ m_{21}& m_{22}& 1/m_{32}& \dots & 1/m_{\mathrm{N2}}\\ m_{31}& m_{32}& m_{33}& \dots & 1/m_{\mathrm{N3}}\\ \dots & \dots & \dots & \dots & \dots \\ m_{\mathrm{N1}}& m_{\mathrm{N2}}& m_{\mathrm{N3}}& \dots & m_{\mathrm{N4}}\end{array}]\end{array}$

[0031] If a carrier is not active then its power level will be zero. In order to avoid problems caused by a division by zero, all elements in both the row and column corresponding to non-active carriers are set to zero. e.g., if carrier 2 is not active (P_2,out=0) then both the second row and the second column of the matrix are set to zero.

[0032] In step 515, the power levels C₁. . . C_Nassociated with the active carriers are calculated. The power levels C₁. . . C_Ncorrespond to the power levels of the respective carriers at the DAC input. Given the number of carriers possible and the power requirements of the carriers and assuming that the DAC full scale range cannot be exceeded, the largest signal-to-noise ratio for each carrier and the sum of the carriers occurs when P_sumequals the DAC full scale range, as illustrated in equation (3) below.

[0033] P_FS,DAC=({square root over (C₁)}+{square root over (C₂)}+ . . . +{square root over (C)}_N)²=P_sum  (3)

[0034] Rearranging equation (3) above, each carrier power level C₁. . . C_Ncan be calculated using the following equations, wherein only active carriers are calculated in order to avoid division by zero: $\begin{array}{cc}{C}_1=\frac{P_{\mathrm{FS},\mathrm{DAC}}}{{\left(\sqrt{m_{11}}+\sqrt{m_{21}}+\dots +\sqrt{m_{\mathrm{N1}}}\right)}^2}& \text{(4-1)}\\ C_2=\frac{P_{\mathrm{FS},\mathrm{DAC}}}{{\left(\sqrt{m_{12}}+\sqrt{m_{22}}+\dots +\sqrt{m_{\mathrm{N2}}}\right)}^2}& \text{(4-2)}\\ C_N=\frac{P_{\mathrm{FS},\mathrm{DAC}}}{{\left(\sqrt{m_{1N}}+\sqrt{m_{2N}}+\dots +\sqrt{m_{\mathrm{NN}}}\right)}^2}& \text{(4-3)}\end{array}$

[0035] In step 520 the Radio Control Unit 450 determines the amount of gain required of the analog power control module 460 to adjust the power level of the individual carriers in the analog domain so that the absolute output power of the carriers is P_k,out, where k represents carriers 1 . . . N. Accordingly, the analog gain is calculated in step 520 in accordance with equation (5) shown below, where the analog gain of the analog power control module 460 is G_ana, P_k,outis the required absolute output power of a particular carrier, C_kis the power level of the corresponding carrier in the digital domain, and where G_ana, P_k,outand C_kare in decibels. For simplicity of description, G_anahas been described as the gain of power control module 460. However, one skilled in the art will recognize that G_anacan represent the total gain in the power control transmitter including the gain in the amplifier and any diminution of power caused by filters and upconverters in the analog section of the transmitter.

[0036] G_ana=P_k,out−C_k  (5)

[0037] In step 525 the power level of each individual carrier is adjusted. Further in accordance with step 525 and as illustrated in FIG. 4, once C₁. . . C_Nhave been calculated using equations (4-1) through (4-3) above, the Radio Control Unit 450 sets the gain of DPC₁. . . DPC_Nso that the output of multipliers Multi₁. . . Multi_Nhave a power level equal to C₁. . . C_N. According to equation (3), the in-phase sum of the carrier power levels C₁. . . C_Nwill be equal to the DAC full scale level P_FS,DAC. In step 530 the analog power level of a particular carrier P_k,outis adjusted using the calculated analog gain G_ana. Although steps 525 and 530 in FIG. 5 are illustrated as occurring sequentially in time, one skilled in the art will recognize that steps 525 and 530 can occur simultaneously.

[0038] FIG. 6 illustrates an exemplary multicarrier transmitter where two of the eight carriers are active; the full scale output power of the DAC is 1 milliwatt; the output power of carrier f₂is 1 Watt; and the output power of carrier f₈is 0.1 Watts. In accordance with step 505, the output power vector is P=[0 1 0 0 0 0 0 0.1]. Using the output power vector, the matrix M created according to step 510 becomes: $\begin{array}{c}M=[\begin{array}{ccccc}0& 0& 0& \dots & 0\\ 0& 1& 0& \dots & 10\\ 0& 0& 0& \dots & 0\\ \dots & \dots & \dots & \dots & \\ 0& {10}^{-1}& 0& \dots & 1\end{array}]\\ =[\begin{array}{ccccc}{m}_{11}& 1/m_{21}& 1/m_{31}& \dots & 1/m_{81}\\ m_{21}& m_{22}& 1/m_{32}& \dots & 1/m_{82}\\ m_{31}& m_{32}& m_{33}& \dots & 1/m_{83}\\ \dots & \dots & \dots & \dots & \dots \\ m_{81}& m_{82}& m_{83}& \dots & m_{88}\end{array}]\end{array}$

[0039] Since f₂and f₈are the only carriers, the power levels C₂and C₈are calculated according to step 515: $\begin{array}{c}{C}_2=\frac{P_{\mathrm{FS},\mathrm{DAC}}}{{\left(\sqrt{m_{12}}+\sqrt{m_{22}}+\dots +\sqrt{m_{82}}\right)}^2}\\ =\frac{P_{\mathrm{FS},\mathrm{DAC}}}{{\left(0+1+0+\dots +\sqrt{{10}^{-1}}\right)}^2}\\ =\frac{P_{\mathrm{FS},\mathrm{DAC}}}{1.732}\\ =\mathrm{.577}\end{array}$ $\begin{array}{c}{C}_8=\frac{P_{\mathrm{FS},\mathrm{DAC}}}{{\left(\sqrt{m_{18}}+\sqrt{m_{28}}+\dots +\sqrt{m_{88}}\right)}^2}\\ =\frac{P_{\mathrm{FS},\mathrm{DAC}}}{{\left(0+\sqrt{10}+0+\dots +1\right)}^2}\\ =\frac{P_{\mathrm{FS},\mathrm{DAC}}}{17.32}\\ =\mathrm{.057}\end{array}$

[0040] Referring now to FIG. 6, the gain of the Digital Power Control modules DPC₂and DPC₈are set such that the power levels of their output signals C₂and C₈are 0.577 mW and 0.057 mW, respectively.

[0041] The analog gain is calculated according to step 525 using the transmit power level and digital gain for the second carrier frequency f₂. Accordingly, P_2,outand C₂are converted from Watts to decibels, (i.e., P_2,out=1 Watt=10*log1000=30 dBm, and C₂=1/1.732 milliWatt=−2.4 dBm). In accordance with equation (5), the analog gain G_anais equal to 32.4 dB, i.e., G_ana=30−(−2.4)=32.4 dB. Alternatively, the analog gain can be calculated using the eighth carrier frequency f_g. Accordingly, P_g,outand C_gare converted from Watts to decibels, (i.e., P_8,out=0.1 Watt=10*log100=20 dBm, and C₈=1/17.32 milliWatt=−12.4 dBm). In accordance with equation (5), the analog gain G_anais equal to 32.4 dB, i.e., G_ana=20−(−12.4)=32.4 dB. Therefore, the Radio Control Unit 450 sets the gain of the analog power control module 460 in accordance with the analog gain calculated in accordance with equation (5).

[0042] FIG. 7 is a graph of the amplitude versus frequency of the two active carriers, f₂and f₈, described in the exemplary embodiment above. As stated above, the full scale power output of the DAC is equal to 1 milliwatt, or 0 dBm. Since there are two active carriers, C_maxwould be set at 6 dB, i.e., 20*log₁₀(2), below the DAC full scale level. Further, as stated above, the power level of the second carrier is −2.4 dBm and the power of the eighth carrier is −12.4 dBm. Accordingly, using the multicarrier transmitter as described above, results in the power level of the second carrier being approximately 3.6 dB above the C_maxpower level. In contrast, the prior multicarrier transmitters would limit the power levels of the carriers to C_max, which would result in a lower signal-to-noise ratio for the carriers.

[0043] Although the exemplary embodiments described above relate to a TDMA embodiment, one skilled in the art can see that the present invention is equally applicable in a CDMA embodiment. For instance, referring now to FIG. 4, the multipliers, multi₁. . . multi_Nwill be used to multiply the baseband data streams by a corresponding spreading code. The signals can be combined by the summer and forwarded to the DAC 120. Similar to the TDMA embodiment described above, in order to maximize the signal-to-noise ratio of each individual carrier and the sum of the carriers, digital power control modules DPC₁. . . DPC_Nadjust the power of the baseband data streams so that the sum of the baseband data streams is equal to the full scale range of the DAC. Once the summed signal has been converted into analog, a scrambling code may (optionally) be applied to the summed signal and then forwarded to the analog power control module 460 to adjust the signal power to the correct level required for the receiving mobile stations.

[0044] The present invention has been described by way of exemplary embodiments to which the invention is not limited. Modifications and changes will occur to those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.