An Overview of the QUALCOMM CDMA Digital Cellular Proposal

Transcription

1 An Overview of the QUALCOMM CDMA Digital Cellular Proposal Zeljko Zilic ELE 543S- Course Project Abstract.0 Introduction This paper describes a proposed Code Division Multiple Access (CDMA) digital cellular network by QUALCOMM. The main characteristics of the access method are described shortly and the simple capacity estimate is given. The power control mechanisms, the handoff algorithm and the privacy measures are described. The description also contains the layered description of communication services with the emphasis on the network control functions. The Code Division Multiple Access communication scheme [4], [2] poses a challenging alternative to existing digital mobile communication standard proposals. QUALCOMM Inc. from San Diego has designed one such system and implemented it in 989, in a breadboard version. In February 990, a total of three cell-site and few mobile prototype unites was used in an field trial in Manhattan. In June 990, QUALCOMM released the draft of the standard describing the CDMA digital cellular system and distributed it to the major subscriber equipment manufacturers. The current version of the standard [3] is upgraded by the suggestions received from the industry. We will enumerate here the main reasons for the belief that this standard will be accepted by industry as a de facto standard, leaving the detailed description of its main features for following sections. The following is a list of the main advantages of the QUALCOMM proposal: 5-20 times greater capacity than with the analog technology of 2

2 fewer cells needed, lower cost per subscriber exploitation of multipath diversity low transmission power, moderate processing power required high voice quality (variable rate vocoder) enhanced privacy easy introduction of new features This report is an attempt to summarize the main characteristics of the CDMA proposal from QUALCOMM. The basics of the CDMA access mechanism are given in Section 2, together with a simple capacity estimate. Section 3 contains a detailed description of forward (base to mobile) and reverse (mobile to base) channels. Short descriptions of handoff, power control and the authentication and encryption are given next, while the overall description of the system given in terms of communication protocol layers concludes the overview. 2.0 CDMA Access Method The multiple access is a main issue to be resolved when defining a new wireless system. Besides CDMA, the main methods to achieve this are FDMA, TDMA, and directed antennae. This standard uses a direct sequence spread spectrum (DS-SS) as its multiple access method. All the users use the same channel all of the time and the signal separation is done by using a set of orthogonal pseudorandom (PN) sequences for encoding the individual messages. This implementation of the DS-SS depends heavily on the channel used, so it will be described shortly. 2. The Channel Description The Federal Communications Commission (FCC) has allocated a total of 25 MHz for mobile-to-cell site communication and 25 MHz for traffic in the opposite direction. In order to discourage the monopolization by only one telecommunication equipment producer, this allocation is divided equally between two service providers. In Figure, these two frequency bands are denoted as block A and B. Each of these two blocks contains a certain number of 30 KHz channels that are compatible with basic telephone channel. Since both blocks occupy the noncontiguous areas, the limit in creating the spread spectrum is determined by the smallest sub-band. For that reason QUALCOMM defined.23 MHz as the basic spread spectrum channel width. A total of ten such channels can be used in the system. The basic system uses the PN code with a period of chips, produced by linear shift registers. The PN chip rate is.2288 MHz, or 28 times the basic 9600 bps information 2 of 2

3 Page 6-2 Proposed EIA/TIA Wideband Spread Spectrum Standard Table CDMA Channel Numbers and Corresponding Frequencies transmission rate. Since the QPSK modulation is used, two PN Transmitter Valid CDMA Analog CDMA codes are used. The frequency spacing Frequency is.23 MHz, and Channel the signals are Channel bandlimited Assignment by a digital (MHz) filter that guaran- Frequency System Assignments Count Number tees that the 3 db bandwidth is equal to.23 MHz. This number Mobile is also a multiple Base of 30 khz FIGURE. The Frequency ///////// Allocations 22 A" ( MHz) CDMA A CDMA 333 (0 MHz) B (0 MHz) CDMA A (.5 MHz) CDMA CDMA B Capacity Estimate CDMA 6 (2.5 MHz) The CDMA proposal provides a very effective frequency 777 utilization, by the following: the same spectrum ///////// is reused in all 22 the cells 799 voice inactivity periods are exploited Frequencies in shaded (///////) regions are not valid for CDMA frequency assignments. 3 of 2

4 7 better sectorization properties k4 lower fade margins 0.0% Since the same channel is used in each cell, a tight power k3 control must be maintained in order to minimize the interference, but also to conserve k2 the batteries 0.03% and battle the fading. k 0.2% Figure 2 depicts the interference effects in the case of perfect power control and uniform distribution of the station in each cell. When all the components are added, the interference 6% 00% increases for 50%, compared to a single-cell system. FIGURE 2. Interference Contribution from the Neighboring Cells We will now show the simple calculation of the channel capacity. The paper [] contains a more detailed capacity calculation, using the same approach. In order to estimate the channel capacity, we should recall that the processing gain, q, is equal to 28, when the data transmission rate of 9600 bps is utilized. Then, the signal-interference ratio C/I is equal to: C I = E b N 0 q (EQ ) In a multiple access system, the total interference is equal to C(N-), therefore the following defines the number of users in the system: 4 of 2

5 N = + q E b N 0 (EQ 2) In our example, up to 33 users can transmit simultaneously if the SNR is equal to 6 db. QUALCOMM uses few additional techniques to achieve the higher capacity. One of them is the exploitation of the silent periods. Duty cycle d is defined as the percentage of time during which the station is active. It is observed that the average user is active less than 40% of the time. The approximate equation for the number of users is then: N = + q E b N 0 d (EQ 3) Notice, however, that the effective number of users in multiple cell system includes the contribution from all the neighboring stations. In that case, the number of users in a cell will be divided approximately by F=.5. At the same time, the total number of users is multiplied by the number of cells present in the system. It is possible to divide cells into sectors in order to further increase the capacity. If we denote the number of sectors as G, the following capacity formula holds: N = + q E b N 0 d G F (EQ 4) 3.0 The Detailed Link Description Here, we give the detailed description of the CDMA system in terms of link channels. Since the forward (base to mobile) and reverse (mobile to base) channels are different, they will be described separately. For each of the links, the detailed description of the transmitter will accompany the link description. 3. The Forward Link Structure Figure 3 describes how the forward channels are organized. The channels are encoded using the orthogonal Walsh codes. There are 64 such channels and the corresponding codes are denoted by W0-W63 in the figure. 5 of 2

6 Introduction to CDMA and the CAI Page 33 W0 I-Channel Pilot PN Sequence FIGURE 3. Forward Link Channels Pilot Channel: All 0 s W32 Sync Channel Data 200 bps 4800 sps.2288 MHz Cover Wp Each.2 kbps base station User i Long transmits Long a pilot Code signal using one Pilot Channel. This signal is used as a Code Mask Generator Modulo 2 addition coherent carrier reference for demodulation. Scrambling It uses the zero Walsh function that consists of 64 zeroes and the mobile receiver can obtain synchronization Q-Channel without Pilot PN Sequence searching the entire length of the PN code. Different cell sites have a different phase offset and the mobile station Figure 4-4. Forward Link Channelization. Forward link channels share a common will track the pseudo-noise strongest signal. modulation, The Synchronization but are individually Channel uses identified the same by PN the sequence assignedas the pilot channel Walsh and functions can be demodulated (W0-W63). This whenever allows the flexible pilot signal allocation is traced. of channels It is used for to paging for traffic channels. the cell site identification, system time establishment and the proper signal power during the call initiation. The remaining 62 channels can be divided between the Traffic Channels and Paging Channels. Figure 4 shows how the signal is generated for each of the forward link channels. The pilot signal is transmitted using relatively more power: it consists simply of a pair of PN codes, used in quadrature modulation. The traffic and paging channel is modulated using the convolutional coding (half rate, constraint length is nine), block interleaving, user-dependent scrambling, Pilot Sync Walsh Paging orthogonal Paging coding Traffic and quadrature Traffic PN Traffic modulation. Traffic The Traffic scrambling is Chan Ch Chan Ch 7 Ch used to obtain the privacy; Up Ch N Ch 25 it uses a user-addressed long Up Ch 24 code PN sequence Up Ch 55 that is difficult to W0 W32 W to W7 W8 to W3 W33 to W63 break. 3.2 The Reverse Link Structure FIGURE 4. Paging Channel Data 9.6 kbps 4.8 kbps 2.4 kbps Forward Traffic Channel Data 9.6 kbps 4.8 kbps 2.4 kbps The structure of the reverse link is shown in Figure 5. There are up 32 Access Channels and up to 64 Traffic Channels in a link. The access channels are used for the paging traf- Figure 4-5. Forward CDMA Channels Transmitted by a Base Station. In addition to the pilot and sync channels, the forward link in each sector supports 62 channels that may be used for paging and traffic. Zero to seven channels are assigned to paging, the remainder are traffic channels. 6 of 2 Paging Channel p Long Code Mask W = Walsh Number Long Code Generator 9.2 ksps Power Control Bit 9.2 ksps MUX Cover Wi Cover.2288 MHz.2288 MHz FORWARD CDMA CHANNEL (.23 MHz radio channel transmitted by base station Traffic Data Mobile Power Control Sub-Channel

7 Introduction to CDMA and the CAI Page 33 W0 I-Channel Pilot PN Sequence fic purposes and are believed to be abandoned in the future. The QUALCOMM CDMA Pilot Channel: All 0 s standard anticipates such possibility. FIGURE 5. Sync Channel Data 200 bps Forward Link Channel Transmit Processes 4800 sps W MHz Cover Wp Paging Channel Data 9.6 kbps 4.8 kbps 2.4 kbps Paging Channel p Long Code Mask Long Code Generator 9.2 ksps Cover Wi.2288 MHz Forward Traffic Channel Data 9.6 kbps 4.8 kbps 2.4 kbps.2 kbps User i Long Code Mask Long Code Generator Power Control Bit 9.2 ksps Scrambling MUX Cover.2288 MHz Modulo 2 addition Q-Channel Pilot PN Sequence Figure 4-4. Forward Link Channelization. Forward link channels share a common pseudo-noise modulation, but are individually identified by the assigned Walsh functions (W0-W63). This allows flexible allocation of channels to paging for traffic channels. The reverse channel uses the same PN modulation as in forward channel, but the offset between the phases is fixed. In order to distinguish between the users, a very long (2 42 -) PN sequence is used. This ensures a high level of privacy in the system. Also, this provides a huge address space that can be needed in the future. Here, the Walsh function modulation is Pilot Sync Paging Paging Traffic Traffic Traffic Traffic Traffic used to obtain 64-ary modulation with coherence over two bit times. This ensures a high Chan Ch Chan Ch 7 Ch Up Ch N Ch 25 Up Ch 24 Up Ch 55 quality W0 link W32 when there W is tono possibility W7 W8to provide a pilot to signal. W3 The W33Walsh to function W63 chips are combined with the long and short PN codes and sent through a channel. W = Walsh Number The transmitted information is convolutionally encoded Mobile Power using rate /3 code of constraint Traffic Data Control length 9. The information is interleaved over a 20 Sub-Channel ms interval. This information is then grouped in six symbol groups that are used to select the Walsh code that is unique per channel. Figure 4-5. Forward 6 shows CDMA how the Channels modulation Transmitted on the reverse by a Base link Station. is done. In addition Figure to the pilot and sync channels, the forward link in each sector supports 62 channels that may be used for paging and traffic. Zero to seven channels are assigned to paging, the remainder are traffic channels. 7 of 2 FORWARD CDMA CHANNEL (.23 MHz radio channel transmitted by base station

8 Proposed EIA/TIA Wideband Spread Spectrum Standard Page 6-9 FIGURE 6. REVERSE CDMA CHANNEL (.23 MHz channel receive The Structure of the Reverse Link by base station) Access Access Ch Ch n Traffic Ch Traffic Ch m 2 3 Addressed by Long Code PN Figure Example of Logical Reverse CDMA Channels Received at a Base Reverse link Station 4 5 FIGURE 7. Reverse Link Modulation Process Information Bit 9.6 kbps 4.8 kbps 2.4 kbps.2 kbps r=/3 K=9 Long Code Mask Code 28.8 ksps Long Code Generator Code 28.8 ksps.2288 Mcps PN chip 64-ary Orthogonal Modulator Walsh chip kcps Data Burst Randomizer Zero-offset Pilot PN Sequence I-channel.2288 Mcps PN chip I Q Zero-offset Pilot PN Sequence Q-channel I Baseband Filter I(t) /2 PN chip Delay = ns Q D Baseband Filter Q(t) Σ : Modulo 2 Additio : Addition 4.0 The Power Control Figure Reverse CDMA Channel Modulation Process In order to maximize the channel capacity, a tight power control must be maintained. QUALCOMM standard proposes two different ways to achieve the control for forward and reverse channels of 2

9 4. Reverse Link Power Control The power transmitted from the mobile station must be tightly controlled because of the total impact to the system and because the mobile power consumption must be minimized. This is achieved by combining an open and closed loop control mechanisms. 4.. Open Loop Power Control Mobile station estimates the path loss from the cell site, by measuring the power level for both the pilot signal and the sum of all the other signals that are broadcast from the site. It is necessary to measure both values because the station can temporarily receive better signal from some other cell site for some signal path. Based on these measurements, the station can respond rapidly to a sudden improvement in signal level. However, when the signal level decreases, the respond will be delayed because the increased power can worsen the signal to noise ratio for all the other stations in a cell. The rate at which power increases is limited to the rate at which the closed loop mechanism can decrease the power Closed Loop Power Control It is obvious that the open loop control is not sufficient to maintain the tight power control. One reason is in the fact that the forward and reverse channels are separated by 45 MHz, therefore the measurements done for forward link will not be completely adequate; the Rayleigh fading characteristics will be different for these two channels. In order to maintain the control, each cell site will periodically inform the mobile station transmitter about the signal to noise ratio received. There is a special power adjustment command that is sent every.25 ms to every mobile transmitter. This command is combined with the information obtained in measuring the forward channel in the mobile station. 4.2 Forward Link Power Control The power transmitted from the cell site is also controlled because the mobile station can receive a poor signal at some locations. For example, when the signal level received by the cell site with which the mobile communicates is equal to the signal level received from some other two cell sites, the total interference is multiplied by three. In some locations the received signal can be stronger than necessary, and the cell site should decrease the transmitted power. 9 of 2

10 The forward link power control is maintained in closed loop manner. The cell site periodically reduces the power, and the correction is made when the mobile requires better quality of the signal. The adjustments are done in steps of 0.5 db, approximately each 20 ms. 5.0 The Handoff Algorithm The QUALCOMM CDMA handoff algorithm is tuned to exploit some peculiarities of the CDMA system, such as the common spectrum usage. During the transition, it is possible to have both the old and new cell communicating with the mobile. This scheme is called makebefore-break, as opposed to usual break-before-make handoff scheme in analog cellular systems. This makes the handoff transparent to the mobile station, although the complete procedure is initiated by it. The mobile station scans the neighboring cells to determine the strongest signal. After it is detected that some of neighboring stations has stronger signal than the original station, the mobile station transmits a control message with the information about the station with the strongest signal. The mobile station is then allowed to communicate with the new cell site, but the call is supported by both cells. This eliminates the ping-pong effect and decreases the control data traffic between cell sites. This scheme interacts with rake diversity multipath receiving scheme adopted in CDMA and together with the power control mechanisms, it provides very power-efficient system. 6.0 The Authentication, Encryption and Privacy This standard adopts the authentication approach taken in EIA IS-54-B standard. The authentication algorithm is the CAVE algorithm that works as follows. The mobile station keeps one key and a set of shared secret data (SSD). It can add the 8-bit authentication signature on the Access Channel. during different phases of the protocol. The base station will examine the signature before allowing the operation. Also, the base station can update SSD periodically. The message encryption procedure is identical to the CMEA algorithm defined in IS-54-B standard. Only the layer 3 messages are encrypted using this approach. At the lower levels, the privacy is ensured using the private long code masks. The private long code mask is used to ensure privacy, as defined in IS-54-B. 7.0 Service Layer Description 0 of 2

11 In order to structure the function of the QUALCOMM CDMA communication standard and to alleviate the software and hardware production, a layered description can be useful. Here, a short layered description is given. Note, however that very often the functions of various layers can be merged in one piece of hardware, especially in a mobile station. The standard uses three layers to describe all the functions that can be find in CDMA system. These are the physical layer, the link layer (layer 2) and the control process layer (layer 3). Figure 7 shows this division applied to the traffic, paging and sync channels. A multiplexer layer serves as an interface to various end-user application options. This allows the existence of service dependent versions of the link layer that can be implemented in future. The physical layer functions deal with the individual bit transmissions and the modulation details. Frames can be sent at 200, 2400, 4800 or 9600 bps, in a bit-oriented synchronous manner. The receiver detects the transmission speed and adjusts its receiving clock. FIGURE 8. Service Layers Upper Layers (Primary Traffic) Upper Layers (Secondary Traffic) Layer 3(Mobile Station Control Processes) Layer 2 Layer 2 (Primary Traffic) (Secondary Traffic) Multiplexer Layer (Traffic Subchannel) Layer 2 (Signaling) Layer 2 Layer 2 (Link Layer) (Paging &Access Channels) (Sync Channel) Layer (Physical Layer) The layer 2 functions for Synch Channel are trivial- base station transmits a frame that is synchronized with the pilot signal. This frame can contain only one message, the Sync Channel Message. The Paging Channel is used to transmit information from the base station to the mobile station. Four types of control messages are used for the link layer functions. The System Parameter Message describes the configuration of the Paging Channel, as well as the registration parameters and the information that helps acquire the pilot signal. The Access Parameter Message describes the Access Channel parameters. These parameters are mostly used in a closed loop control of some of the system parameters, such as mobile transmit rate. The Neighbor List Message is used in handoff handling, and it contains information such as pilot signal offset and neighbor station configuration. The Channel List Message contains the CDMA frequency assignments that are used for Paging Channels. The data information to of 2

12 the paging service users is transmitted using the Page Messages. These messages are usually sent when the base station receives a call. There is also a slotted mode of operation for paging channels. In this mode, all the messages are sent in predefined slots in time. The duration of these slots can be preprogrammed. The mobile station can turn its power down when the message is not expected, and the special protocol is used to define the slots used in this mode. The Access Channel Messages are used in call originations as well as for various response functions. There is one or more Access Channels associated with one Paging Channel. The base station responds to a message on a particular Access Channel by sending a message on a Paging Channel that is associated to the originating channel. The mobile station responds to a Paging Channel message by transmitting on some of associated Access Channels. Since the Access Channel is a random access channel, the higher number of stations that are active will pollute the channel. There is a limited channel control- it is accomplished by a set of Access Parameter Messages. It is possible, using these messages, to change the transmission rate for each of the users. That will lead to an improvement in total C/I ratio, according to Equation. The Traffic Channel can also dynamically change the data transmission rate. This receiver can direct the data rate on a 20 ms frame basis. There are four types of control messages on the Traffic Channels- they serve for the call control, handoff control, power control, security and for the special information transmission. The CDMA standard specifies in detail how these control messages are used and we have already explained the idea behind the security, handoff and power control. The multiplexer layer provides a socket into which various user applications (service options) can be plugged. One of such options is called Service Option - this is a variable rate vocoder. Other functions, such as FAX or advanced technology vocoder can be introduced into the system by connecting to the multiplexer sublayer. Two service options can be supported simultaneously, one of them will be the Primary Traffic, and the other will be the Secondary Traffic channel. These two data streams are prioritized. For example, the Service Option can be used as a Primary Traffic, while the packetized data can be used for the Secondary Traffic channel. 7. Acknowledgement: Qualcomm deserves credit for all the pictures. 2 of 2

13 8.0 References [] Gilhausen, K. S., et al., On the Capacity of a Cellular CDMA System, IEEE Transactions on Vehicular Technology, Vol. 40 No. 2, pp , May, 99. [2] An Overview of the Application of Code Division Multiple Access (CDMA) to Digital Cellular Systems and Personal Cellular Networks QUALCOMM, San Diego, California, 992. [3] Proposed EIA/TIA Interim Standard: Wideband Spread Spectrum Digital Cellular System Dual-Mode Mobile Station- Base Station Compatibility Standard QUALCOMM, San Diego, California, 992. [4] Salmasi, A., An Overview of Code Division Multiple Access (CDMA) Applied to the Design of Personal Communication Networks, Third Generation Wireless Information Networks, pages , Kluwer Academic Publishers, of 4