GSM hardware in .NET framework

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A typical GSM mobile implementation consists of the following functional blocks: RF stages Baseband converters Timing controller Encryption unit DSP Microcontroller SIM interface Miscellaneous (LCD, keyboard, earpiece, microphone, battery charging circuitry).
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The original GSM frequency band has 124 channels, spaced 200 kHz apart. The mobile receive frequency is 935+0.2n MHz, the mobile transmit frequency is 890+0.2n MHz where n is the channel number between 1 and 124. On the receive side, the signal is ampli ed and downconverted using a frequency synthesizer for channel selection. A quadrature mixer is used, producing two baseband signal outputs I and Q (in-phase and quadrature components the result of mixing the received signal with two different carriers 90 out-of-phase). On the transmit side, the I and Q signals from the modulator are mixed up using another quadrature mixer to produce the nal RF frequency for feeding to the PA (power ampli er). The PA output level is adjustable by a separate DAC output. The baseband receive ADC samples both the I and Q receive channels. It may also contain some of the channel ltering (in which case it oversamples the I and Q signals and lters them digitally). The output of the baseband receive ADC is one I and one Q sample, of 10 bits each, every GSM bit period (3.69 s). Each receive burst will produce around 160 sample pairs (must receive more samples than there are bits to account for timing errors). The transmit modulator converts the bits to be transmitted into varying I and Q voltages according to the speci cation in the GSM standard (GMSK modulation). Essentially, a 1 bit causes the phase of the carrier to advance by 90 and a 0 bit causes the phase to be retarded by 90 ; however some low-pass ltering is applied so that the phase changes smoothly, with the previous 3 bits making a signi cant contribution to the carrier phase at any time. This is done to reduce the bandwidth of the transmitted signal. The modulation is done digitally and the output converted to I and Q signals by the baseband transmit DAC. At least three other baseband converters are required. A DAC is required to control the output power level while transmitting. A FIFO store is required to feed this DAC so that the power can be ramped up and down at the beginning and end of the transmit burst (one output sample per half bit-period). An ADC and DAC are required for the microphone and earpiece. These both work at a sampling rate of 8 kHz and resolution of 13 bits. A versatile timing controller is required, which can be programmed to a resolution of one-quarter bit period. This is used to switch various parts of the hardware on and off at the correct time, and to initiate receive and transmit operations at the correct time. The timing controller is synchronized with the received signal from the base station after FCCH and SCH receive. The DSP usually performs the following functions: Reading received data from the baseband receive ADC Frequency burst detection Sync and normal burst equalization
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Channel encoding Channel decoding Speech encoding Speech decoding Assembly of bursts for transmission Buffering of audio samples Generation of sidetone and miscellaneous GSM-speci ed tones. The DSP may also perform a certain amount of the low-level control and scheduling functions. The split of these between the DSP and microcontroller varies between implementations. It is implementation dependent whether the timing controller is con gured by the DSP or the microcontroller. The microcontroller performs the following tasks: Some of the layer 1 control functions (implementation-dependent split with DSP) Layers 2 and 3 of the GSM protocol stack Control of screen and keyboard, battery monitoring, and possibly charging User interface functions Control of the SIM interface Extra processing for data traf c channels. The SIM interface connects the SIM (subscriber identity module smart card) to the microcontroller. It is essentially a UART, but operates in a half-duplex mode. The SIM has only one data line and it is used for both reading and writing to the SIM. There is a speci ed protocol for this interface, which is usually implemented in hardware.
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