About cdma20001

Introduction

Basic Concepts of cdma2000

Spreading Rate     

Radio Configuration      

Forward Link Air Interface     

Reverse Link Air Interface — HPSK     

Forward Link Power Control      

Introduction

One of the technologies meeting the IMT-2000 requirements for a third generation (3G) global wireless communications system is cdma2000, also known as IS-2000. The Third-Generation Partnership Project 2 (3GPP2) wrote the specification for this wideband code division multiple access (CDMA) system as a derivative of the IS-95-B CDMA system, also known as cdmaOne. The 3GPP2 organizational partners are the Japanese Association of Radio Industries and Businesses (ARIB), Telecommunication Technology Committee (TTC), Telecommunications Industries Association (TIA), and Korean Telecommunications Technology Association (TTA).

Basic Concepts of cdma2000

The main advantages that cdma2000 offers over other IMT-2000 proposals are backward compatibility with cdmaOne systems and a smooth migration from second-generation (2G) cdmaOne systems to 3G. Figure 1 shows the potential evolution path from cdmaOne to cdma2000 systems.

Figure 1. Evolution from cdmaOne to cdma2000

Spreading Rate

Spreading rate (SR) defines the final spread chip rate in terms of 1.2288 Mcps. The two spreading rates are SR1 and SR3.

SR1: An SR1 signal has a chip rate of 1.2288 Mcps and occupies the same bandwidth as cdmaOne signals. The SR1 system doubles the system capacity, therefore, it can be considered an improved cdmaOne system. The main differences from cdmaOne are:

SR3: An SR3 cdma2000 signal has a rate of 3.6864 Mcps (3 x 1.2288 Mcps) and occupies three times the bandwidth of cdmaOne. Originally, the SR3 system appeared to be viable. Upon further investigation the SR3 cdma2000 system was determined to not be viable and is no longer receiving any commercial attention at this time.

Radio Configuration

Radio configuration (RC) defines the physical channel configuration based upon a specific channel data rate. Each RC specifies a set of data rates based on either 9.6 or 14.4 kbps. These are the two existing data rates supported for cdmaOne. Each RC also specifies the spreading rate (either SR1 or SR3) and the physical coding. Currently there are nine radio configurations defined in the cdma2000 system for the forward link and six for the reverse link. Examples are:

Each base transceiver station (BTS) or MS must be capable of transmitting using different RCs at the same SR. Refer to [2] for detailed information on the different RCs.

Forward Link Air Interface

The forward link air interface for a cdma2000 SR1 channel is very similar to that of cdmaOne. In order to preserve compatibility, cdma2000 uses the same structure as cdmaOne for the forward pilot (F-Pilot), forward sync (F-Sync), and forward paging (F-Paging) channels.

In cdma2000, each user is assigned a forward traffic (F-Traffic) channel, which consists of:

The F-FCHs are used for voice and the F-FCCHs and F-SCHs are used for data. The BTS may also send zero or one F-DCCHs. An F-DCCH is associated with traffic channels (either FCH, SCH, or SCCH) and may carry signaling data and power control data.

One of the main differences between cdmaOne and cdma2000 is that the latter uses true quadrature phase shift keying (QPSK) modulation (as opposed to dual-BPSK) for all traffic channels from RC3 to RC9. As an example, Figure 2 shows the forward link structure for an RC4 F-FCH. The coding is identical to cdmaOne up through the long code scrambling of the voice data. The F-FCH is optionally punctured with the reverse link power control data bits. The data is then converted from a serial bit stream into a two-bit wide parallel data stream to produce true QPSK modulation. This reduces the data rate of each stream by a factor of two. Each branch is spread with a 128 Walsh code to generate a spreading rate of 1.2288 Mcps. In this case, the processing gain is doubled for each channel relative to cdmaOne. Each channel is transmitted at one-half the power used before, but there are now two of them for no apparent gain. The actual processing gain for each channel depends on its data rate and RC.

The outputs of the I and Q Walsh spreaders are then complex multiplied against the same I and Q channel short codes used in cdmaOne. Complex scrambling is used in the forward link instead of regular scrambling because it is a more robust scheme against interference.

Figure 2. Coding and air interface for a cdma2000 RC4 F-FCH

Reverse Link Air Interface — HPSK

The cdma2000 reverse link is very different from cdmaOne. The MS can transmit more than one code channel to accommodate the high data rates. The minimum configuration consists of a reverse pilot (R-Pilot) channel to allow the BTS to perform synchronous detection and a reverse fundamental channel (R-FCH) for voice. Additional channels, such as the reverse supplemental channels (R-SCHs) and the reverse dedicated control channel (R-DCCH) can be used to send data or signaling information, respectively.

The different channels are assigned to either the I or Q path. For example, for RC3 to RC6, the R-Pilot is assigned to I and R-FCH is assigned to Q (see Figure 3).

Figure 3. An example of channel summing and HPSK spreading in cdma2000 reverse link (SR1)

Channels can be at different rates and different power levels. Complex scrambling facilitates this by continuously phase rotating the constellation and thus distributing the power evenly between the axes.

Without scrambling, unequal channel powers would result in a rectangular four-quadrature amplitude modulation (QAM) constellation (assuming that only R-Pilot and R-FCH are active). With complex scrambling, the constellation for two channels generally has eight points distributed around a circle, with the angular distribution determined by the relative powers of the two channels. For example, an amplitude difference of 6 dB between the two channels results in the constellation shown in Figure 4, which is close to an 8-PSK (8-phase shift keying) constellation (an amplitude difference of 7.65 dB would result in a perfect 8-PSK constellation). If the amplitudes for the two channels are equal, then pairs of constellation points merge to give a QPSK-like constellation.

Figure 4. I/Q Measured Polar Vector Plot

(A reverse link cdma2000 SR1 signal with an R-Pilot and an R-FCH. The amplitude of the R-FCH is 6 dB lower than that of the R-Pilot.)

Basic complex scrambling applies a phase rotation of 0, ±π/2, or π radians to each chip. HPSK takes this idea a stage further and defines the complex scrambling so that for every second chip, the phase rotation is restricted to ±π/2. This constraint on the phase transitions entering the baseband pulse shaping filter reduces the peak-to-average ratio of the signal (about 1 to 1.5 dB) compared to regular complex scrambling (or regular QPSK). The HPSK technique continues to be advantageous even when the signal has more than two channels.

Forward Link Power Control

A key improvement in cdma2000 is forward link power control. The MS sends power control data back to the BTS by time multiplexing it with the R-Pilot channel. Like the existing reverse link closed loop power control of cdmaOne, the cdma2000 forward link closed loop power control sends 800 power control bits each second. These bits indicate whether the BTS should raise or lower its power in 1 dB, 0.5 dB, or 0.25 dB. The finer steps allow tighter power control for low mobility or stationary phones. Tighter control (less power ripple) lowers the average power and thus raises the capacity of the system.