SYSTEM DESIGN CONSIDERATIONS

Due to the required system specifications and the trend of advanced CMOS technology, some system design parameters become the important challenges for millimeter-wave CMOS receiver front-end design.

4.1.1 Noise Figure

The noise performance of the receiver front-end defines the sensitivity of the receiver front-end by limiting the lowest input RF power that can be detected with a reasonable data error rate by the receiver. The noise factor (F) is defined as

where (S/N)input and (S/N)output are the signal-to-noise ratio (SNR) at a system of input and output, respectively; Gv is the available power gain; and Ninput and Noutput are the total noise at the input and output of a system, respectively.

In general, Noutput can be expressed as

N_output =G_aN_input +N_system (4.2)

where Nsystem is the total noise contribution from the system. It can be seen from (4.1) and (4.2) that F can be decreased by increasing the available power gain of the system Ga and decrease the noise contribution from the system.

The performance of a cascade system with total stage of M as shown in Fig. 4.1 can be calculated by the Frii’s formula. The total available power gain (Gtotal) and total noise factor (Ftotal) are expressed as

１２３

where Gn and Fn are the available power gain and F of the n-th stage, respectively.

It can be seen from (4.4) that the Ftotal can be minimized by either the increase of the available power gain G1 or the decrease of the F1. Therefore, the first stage of the receiver front-end is designed with minimum F and reasonable gain for lower system F and higher system sensitivity. For this reason, it is important to have a low noise amplifier as close to the antenna as possible.

Noise figure (NF) in dB can be expressed as

NF =10log(F) (4.5)

The minimum input signal strength needed to produce a good quality output signal is referred to as the receiver sensitivity. The sensitivity of the receiver can be written as [92]

P_sen =−174 dBm/Hz+10log

(

)

where BW is the channel bandwidth in hertz, NFRX is the NF of receiver front-end in dB, and SNRmin is the minimum SNR for digital section.

The possible channel for 60-GHz application is 1728 MHz as shown in Chapter 1. By substituting 1728 MHz to (4.6), we have

P_sen =−81.6 dBm/Hz+NF_RX +SNR_min (4.7)

It can be seen from (4.7) that the sensitivity for wide channel bandwidth can not be too low due to large channel bandwidth.

１２４

A simple for 60-GHz system is given as followed. By assuming that the modulation is QPSK with bit-error rate of 10^–6 with additive white Gaussian noise (AWGN) channel and the NF from receiver is 15 dB, it has the sensitivity of –55.6 dBm [= –81.6 + 15 +11].

4.1.2 P1dB and IIP3

Linearity is the criterion that defines the upper limit for detectable RF input power level of the receiver. The linearity performance of a RF system is usually determined by 1dB compression point (P1dB) and input-referred third-order intercept point (IIP3).

The P1dB value is defined as the input power level at which the power gain is decreased 1dB. The IIP3 is defined as the input power where the output powers of the fundamental and the third-order intermodulation are equal. In many circuits the IIP3 is beyond the allowed input range, thus the practical method to obtain the IIP3 is linear extrapolation on measured behavior for small input amplitude as shown in Fig.

4.2. In general, the IIP3 value is about 10 dB larger than the P1dB value for one stage circuit.

The IIP3 of a cascade system can expressed as [92]

= + + +Λ

where IIP3n and Gn are the IIP3 and power gain of the nth-stage, respectively.

For gains greater than one, the total receiver linearity is dominated by the linearity of latter stages. Thus, the linearity of the latter stages should be as large as possible to maximize the gain of the whole system.

As compared with (4.4) and (4.8), the power gain of first stage is increased to

１２５

reduce the NF of the total system. However, the linearity of the system is decreased due to the large gain of first stage. As a result, an adjustable or switched gain of first stage is designed to meet the NF and linearity.

The advanced CMOS technology is required for 60-GHz transceiver design.

However, the power supply voltage is decreased as the minimum channel length of CMOS technology decreases. In other words, the system linearity becomes worse while using advanced CMOS technology.

4.1.3 Link Budget Analysis

Link budget analysis is an important design issue for wireless communication system [93]. A link budget is the accounting of all of the gains and losses from the transmitter, communication medium in a telecommunication system.

The free-space path loss (LP) can be expressed as

⎟

where d is the distance between transmitter and receiver and λ is the wave length of the carrier frequency.

It can be seen from (4.9) that the path loss is increased as the increase of carrier frequency. For 60-GHz wireless communication system, the path losses are 68 and 88 dB for the distance of 1 and 10 m, respectively.

The power level at the receiver input can be written as

P_r =P_t+G_t +G_r −L_P (4.10)

where Pr is the power at the receiver input in dBm, Pt is the delivered power by the transmitter, and Gt and Gr are power gain of the transmitter and receiver antenna,

１２６

respectively.

Assume that the maximum transmit power at the transmitter output is 10 dBm, the antenna gain in the transmitter part is 9 dB, the shadowing loss is 10 dB, the receiver antenna gain is 9 dB, and the distance between receiver and transmitter is 1 m, the received power level at the receiver input is –50 dBm in a 60-GHz communication system.

From (4.6), the minimum power level that is detectable is –55.6 dBm if the modulation is QPSK with bit-error rate of 10^–6 and the NF from receiver is 15 dB are assumed. Detail system parameter is listed in Table 4.1. In summary, the available margin for SNR is only 5.6 dB. If the distance between transmitter and receiver is increased to 10 m, the signal level can not be detectable. Therefore, the secure communication in 60-GHz band is provided by large signal path loss.