A- ; - METHODS AND APPARATUS FOR SINGLE SIDEBAND MODULATION EMPLOYING A FREQUENCY SHIFT Field of the Invention The present invention relates generally to improvements to radio frequency identification (RFID) communications. More particularly, the invention relates to improved systems and techniques for generating a carrier signal that is modulated using a digital implementation of single sideband amplitude shift keying for downlink communication, with the digital implementation allowing for rapid switching between downlink and uplink frequencies performed before modulation of an actual physical carrier signal. Background of the Invention An RFID system typically operates by transmission of a carrier signal by an RFID reader. The carrier signal is modulated by an RFID tag, and the modulated signal is received at and interpreted by the reader. RFID systems may employ one of a number of different modulation techniques for communication. In communications with a passive RFID tag, an RFID reader transmits a carrier signal for uplink and downlink communication. In downlink communication, the reader is transmitting data to the tag, and modulates the carrier signal in order to communicate the data. In uplink communication, an RFID tag modulates the carrier signal transmitted by the reader, and the signal is returned to the reader in the form of modulated backscatter. During uplink communication, the carrier signal transmitted by the reader is i unmodulated. The carrier signal powers the tag and is modulated by the tag in order to furnish communication by the tag to the reader. The particular modulation technique used depends on a number of factors, such as the ^preferences of an organization and the radio frequency spectrum allocated to such use. The available spectrum, and the allocation of portions of the available spectrum to uplink and downlink communications depends on a number of factors, such as government regulations or industry standards. In many applications, a relatively narrow frequency spectrum is available. This condition is particularly prevalent in European applications, where a relatively narrow frequency range is reserved for RFID reader communications. In addition, installations employing multiple readers typically manage frequency allocations in such a way that the downlink communication of one reader does not overlap in frequency with the uplink communication of another reader. In such applications, single sideband amplitude shift keying is frequently used, because the frequency spectrum used by downlink communications, that is, modulated signals transmitted from the reader to an RFID tag, can be relatively narrow. In SSB- ASK communications, an RFID reader uses separate carrier frequencies for the carrier signal between downlink and uplink communications. Therefore, the reader must change carrier frequencies every time a switch is made between uplink and downlink communication. Prior art systems typically achieve the needed frequency changes in hardware. A fast frequency conversion requires a fast changing local oscillator. The use of a fast changing oscillator may lead to unstable operation and spurious out of band emissions. However, the use of a slow changing oscillator negatively affects performance, because the frequency change is relatively slow and communication cannot occur during a frequency change, but instead must wait until a carrier signal has stabilized at the new frequency. Summary of the Invention The present invention addresses such problems, as well as others, by performing modulation in such a way that implementation of the carrier shift between uplink and downlink , carrier signals is performed by digital processing of a data signal before the actual radio frequency modulation of the carrier signal takes place. The generation of the carrier signal, and the modulation of the carrier signal, when needed, may suitably be accomplished using digital signal processing techniques. A digital representation of a baseband signal is modulated onto a carrier signal having a negative frequency equal to a frequency shift between downlink and uplink carrier frequencies of the carrier signal transmitted by an RFID reader. This modulation creates a complex baseband signal having in-phase and quadrature components. This complex baseband signal then undergoes analog to digital conversion, and modulation. The in-phase and quadrature components are suitably converted to analog form and passed to an in- phase/quadrature modulator, which modulates the in-phase component with the carrier signal at the uplink frequency, and modulates the quadrature component with a 90 degree phase shift of the carrier signal at the uplink frequency. During uplink communication, a constant signal is imposed on the in-phase component. A zero signal, or no signal, is substituted for the quadrature component. During the uplink communication, therefore, the in-phase/quadrature modulator produces the carrier signal at an umodulated uplink frequency. A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following Detailed Description and the accompanying drawings. Brief Description of the Drawings Fig. 1 illustrates an RFID reader according to an aspect of the present invention; Fig. 2 illustrates a process of RFID communication according to an aspect of the present invention; Fig. 3 illustrates an RFID communication system according to an aspect of the present ..invention; and " Fig. 4 illustrates an exemplary frequency spectrum and signals employed in single sideband amplitude shift keying modulated communication according to an aspect of the present invention. Detailed Description Fig. 1 illustrates an RFID reader 100 according to an aspect of the present invention. The reader 100 comprises a data processing unit 102 and a communication unit 104, and may also include an external interface in order to receive data to be communicated to RFID tags and to relay data received from RFID tags. The communication unit 104 communicates with RFID tags using a carrier signal modulated using single sideband amplitude shift keying (SSB-ASK) for downlink communication, and unmodulated during uplink communication. In order to transmit data to a tag, such as the tag 106, the communication unit 104 receives data from the data processing unit 102, and uses the data to modulate a carrier signal used to communicate data to the tag 106 during a downlink communication. During the downlink communication, the carrier signal is transmitted at a chosen frequency, conveniently referred to as the downlink frequency. The communication unit 104 alternates between downlink transmissions to the tag 106 and uplink communications with the tag 106, during which the communication unit 104 receives data from the tag 106. During uplink, the carrier signal transmitted by the communication unit 104 to the tag 106 is unmodulated, and is at a frequency different from that of the carrier signal during downlink communication. The downlink frequency differs from the uplink frequency by a suitably negative frequency shift. During uplink communication, the carrier signal powers the tag 106 and is modulated by the tag 106 to communicate information to the communication unit 104 through modulated ♦ backscatter. In order to perform both uplink and downlink communications, the communication unit 104 must switch rapidly between the uplink frequency and the downlink frequency, with the carrier signal taking on the uplink frequency as soon as possible after the tag 106 has received the downlink transmission, and switching to the downlink frequency as soon as possible after the uplink communication has been received. In order to achieve a rapid switch between the uplink and downlink frequencies, the communication unit 104 employs digital signal processing techniques to create a signal, and implements the carrier frequency shift before radio frequency modulation of the carrier signal takes place. In order to perform SSB-ASK modulation, the communication unit 104 employs a digital signal processor (DSP) 108 to process a digital data signal produced by the data processing unit 102 and create a digital representation of a carrier signal. This digital representation is converted to analog form and further modulated in order to generate the signal transmitted by the communication unit 104. The digital signal processor separates a data signal produced by the data processing unit 102 into an in-phase, or I. component, and a quadrature, or Q, component, produced as I and Q outputs 109 and 110 of the digital signal processor 108. The I and Q outputs 109 and 110 are provided to digital to analog converters (D/A converters) 111 and 112, respectively, to convert the digital signals to analog signals. The analog signals produced by the D/A converters 111 and 112 are then provided to an I/Q modulator 114. The I/Q modulator 114 generates a radio frequency signal that is transmitted to the tag 106. During downlink communication, the carrier signal is modulated using single sideband amplitude shift keying modulation. This can be achieved by generating a double sideband modulated signal with suppressed carrier, and a double sideband suppressed carrier signal that is based on a 90 degrees phase shifted carrier signal. Steps for achieving this result can be described by the following definitions and operations: Sn (t) is a non-return to zero encoded base-band (data) signal consisting of a sequence of logical "1" and "0" data symbols represented by a positive number being a logical "1" and an equal magnitude negative number a logical "0". As illustrated here, Sn (/) is the output of the data processing unit 102. Sn (/) is a 90 degrees phase shifted (Hilbert transform) copy of Sn (t), where cos(/) is the -90 degrees shifter carrier signal. Cos(cot) is the modulating carrier signal. Sin(o)t) is a negative 90, or -90 degree shift of the carrier signal. Ss(t) is the transmitted single sideband radio frequency signal transmitted by the communication unit 104. That is. Ss(t) is the output of the I/Q modulator 114. In the complex domain. S,(t) can be represented as follows: Sx (I) = Re(/) x e '""), where Sc (t) = Sn (t) + jSn (t) and e is the complex carrier wave. In the example given, the physical output signal transmitted by the communication unit 104 is the real part of the complex multiplication and is implemented in hardware by the I/Q modulator 114. The DSP 108 receives the data signal Sn(Ofrom the data processing unit 102 and performs processing on the signal. The DSP 108 is illustrated here as including various processing elements for ease of illustration, but it will be recognized that the elements shown here represent functions implemented in the DSP 108 by suitable programming. Specifically, the DSP 108 first implements a filtering operation 116 in order to limit the signal spectrum that will be generated in accordance with applicable regulations and standards. The filtered signal is then split and subjected to a Hilbert transform 118 to generate the imaginary part of the complex expression above. The signal is also subjected to a delay operation 120, so that the real part of the signal will match the timing of the imaginary part, which is subjected to a delay generated by the Hilbert transform 118. The real and imaginary parts of the signal are then passed as the outputs 109 and 110 of the DSP to the D/A converters 111 and 112. The outputs of the D/A converters 111 and 112 are supplied as inputs to the I/Q modulator 114. The I/Q modulator 114 also receives the carrier signal cos(corl) as an input. The carrier signal is passed to a multiplier 122, and multiplied by the real output of the DSP 108. The carrier signal is also passed to a phase shifter 124 and subjected to a 90 degree phase shift, and this 90 degree phase shifted carrier signal is passed to a multiplier 126 and multiplied by the imaginary output of the DSP 108. The outputs of the multipliers 122 and 126 are then supplied to a summation unit 128 to generate the transmitted signal Sv(/) as an output of the I/Q modulator 114. The transmitted signal Sx(t) is transmitted to the tag 106 through an antenna 130. If the outputs of the Hilbert transform 118 and the delay operation 120 are supplied to the I/Q modulator 114 without further processing, a single sideband modulated signal is produced, requiring a local oscillator and additional hardware to switch between the modulated signal produced by the I/Q modulator 114 during downlink communication, and an umodulated signal generated by the reader 100 during uplink communication. This unmodulated signal is transmitted by the reader 100 during uplink communication in order to power RFID tags, such as " ' the tag 106, within range. The unmodulated signal is modulated by the RFID tags and returned to the reader in order to furnish communication between the tags and the reader 100. However, in order to avoid the local oscillator and additional hardware, the outputs of the Hilbert transform 118 and the delay operation 120 are subjected to additional processing by the DSP 108 as discussed below. The DSP 108 modulates the baseband signal Sn (/)with a negative carrier frequency equal to the frequency shift desired between the downlink transmission and the uplink transmission. If the desired carrier frequency is represented as a>, the unmodulated carrier frequency for the uplink may be represented as a>( and the frequency shift may be represented as (oR. This relationship may be represented as: (0 = 0) (--(0g. SL\1) is a complex representation, allowing the use of negative frequency shifts. Sc(t) = Sn(i) + /Hilbert(S,„ (/)). where Sn (l) is the output of the data processing unit 102 and /HilbertCS^f)) is the complex Hilbert transformation of Sn (t). The single sideband signal SJl), discussed above, can be represented as follows: . (t) x e}. Substituting the relationship a = o)r - a)s yields Ss (t) = Re{sc(t) x e "°J x }, which is equivalent to Ss(t) = Re{s,(0 x e~x e ""•' J. The single sideband carrier frequency shift can be implemented by first modulating the complex baseband signal in the DSP 108 with a negative carrier frequency shift, and then converting the complex baseband signal, thus modulated, to a physical signal for transmission by further modulating the complex baseband signal using an in-phase/quadrature modulator in order to modulate the complex baseband signal onto the unmodulated uplink signal. In-phase and • quadrature components of the complex baseband signal modulated with a negative carrier frequency are supplied to the I/Q modulator 114. As noted above, the DSP 108 first implements a filter operation 116 on the signal Sn (/) in order to limit the signal spectrum that will be generated in accordance with applicable regulations and standards. The filtered signal is then split and subjected to a Hilbert transform 118 to generate the imaginary part of Sc(i). The signal is also subjected to a delay operation 120, which produces as an output the real part of 5.(/). The delay operation 120 insures that the real part of the Sc(t) will match the timing of the imaginary part, which is subjected to a delay generated by the Hilbert transform 118. The next operation is a complex multiplication of the complex signal Sc(t) with a complex negative frequency signal e'""*'. This complex multiplication is equivalent to cos(-mst) + sin(-